imnodes.cpp

// the structure of this file:
//
// [SECTION] internal data structures
// [SECTION] global struct
// [SECTION] editor context definition
// [SECTION] ui state logic
// [SECTION] render helpers
// [SECTION] API implementation

#include "imnodes.h"

#include "imgui.h"
#define IMGUI_DEFINE_MATH_OPERATORS
#include "imgui_internal.h"

// Check minimum ImGui version
#define MINIMUM_COMPATIBLE_IMGUI_VERSION 16401
#if IMGUI_VERSION_NUM < MINIMUM_COMPATIBLE_IMGUI_VERSION
#error                                                                         \
    "Minimum ImGui version requirement not met -- please use a newer version!"
#endif

#include <assert.h>
#include <math.h>
#include <new>
#include <stdint.h>
#include <string.h> // strlen, strncmp
#include <stdio.h>  // for fwrite, ssprintf, sscanf
#include <stdlib.h>

namespace imnodes
{
namespace
{
bool initialized = false;

enum ScopeFlags
{
    Scope_None = 1,
    Scope_Editor = 1 << 1,
    Scope_Node = 1 << 2,
    Scope_Attribute = 1 << 3
};

enum Channels
{
    Channels_NodeBackground = 0,
    Channels_ImGui,
    Channels_Count
};

enum AttributeType
{
    AttributeType_None,
    AttributeType_Input,
    AttributeType_Output
};

// [SECTION] internal data structures

// The object T must have the following interface:
//
// struct T
// {
//     T();
//
//     int id;
// };
template<typename T>
struct ObjectPool
{
    ImVector<T> pool;
    ImVector<bool> in_use;
    ImVector<int> free_list;
    ImGuiStorage id_map;

    ObjectPool() : pool(), in_use(), free_list(), id_map() {}

    inline void update()
    {
        free_list.clear();
        for (int i = 0; i < in_use.size(); ++i)
        {
            if (!in_use[i])
            {
                id_map.SetInt(pool[i].id, -1);
                free_list.push_back(i);
            }
        }
        // set all values to false
        memset(in_use.Data, 0, sizeof(bool) * in_use.size());
    }

    inline int find_or_create_index_for(const int id)
    {
        int index = id_map.GetInt(static_cast<ImGuiID>(id), -1);
        if (index == -1)
        {
            if (free_list.empty())
            {
                index = pool.size();
                pool.push_back(T());
                in_use.push_back(true);
            }
            else
            {
                index = free_list.back();
                free_list.pop_back();
            }
            id_map.SetInt(static_cast<ImGuiID>(id), index);
        }
        in_use[index] = true;
        return index;
    }

    inline T& find_or_create_new(const int id)
    {
        const int index = find_or_create_index_for(id);
        return pool[index];
    }

    // Predicate must define operator()(const T& lhs, const T& operator) ->
    // bool.
    template<typename Predicate>
    inline bool contains(const T& v, Predicate predicate) const
    {
        for (int i = 0; i < pool.size(); ++i)
        {
            if (!in_use[i])
            {
                continue;
            }

            if (predicate(v, pool[i]))
            {
                return true;
            }
        }

        return false;
    }
};

// Emulates std::optional<int> using the sentinel value `invalid_index`.
struct OptionalIndex
{
    OptionalIndex() : m_index(invalid_index) {}
    OptionalIndex(const int value) : m_index(value) {}

    // Observers

    inline bool has_value() const { return m_index != invalid_index; }

    inline int value() const
    {
        assert(has_value());
        return m_index;
    }

    // Modifiers

    inline OptionalIndex& operator=(const int value)
    {
        m_index = value;
        return *this;
    }

    inline void reset() { m_index = invalid_index; }

    inline bool operator==(const OptionalIndex& rhs) const
    {
        return m_index == rhs.m_index;
    }

    inline bool operator==(const int rhs) const { return m_index == rhs; }

    static const int invalid_index = -1;

private:
    int m_index;
};

struct NodeData
{
    int id;
    ImVec2 origin; // The node origin is in editor space
    ImRect title_bar_content_rect;
    ImRect rect;

    struct
    {
        ImU32 background, background_hovered, background_selected, outline,
            titlebar, titlebar_hovered, titlebar_selected;
    } color_style;

    struct
    {
        float corner_rounding;
        ImVec2 padding;
    } layout_style;

    ImVector<int> pin_indices;
    bool draggable;

    NodeData()
        : id(0), origin(100.0f, 100.0f), title_bar_content_rect(),
          rect(ImVec2(0.0f, 0.0f), ImVec2(0.0f, 0.0f)), color_style(),
          layout_style(), pin_indices(), draggable(true)
    {
    }
};

struct PinData
{
    int id;
    int parent_node_idx;
    ImRect attribute_rect;
    AttributeType type;
    PinShape shape;

    struct
    {
        ImU32 background, hovered;
    } color_style;

    PinData()
        : id(), parent_node_idx(), attribute_rect(), type(AttributeType_None),
          shape(PinShape_CircleFilled), color_style()
    {
    }
};

struct LinkData
{
    int id;
    int start_pin_idx, end_pin_idx;

    struct
    {
        ImU32 base, hovered, selected;
    } color_style;

    LinkData() : id(), start_pin_idx(), end_pin_idx(), color_style() {}
};

struct LinkPredicate
{
    bool operator()(const LinkData& lhs, const LinkData& rhs) const
    {
        // Do a unique compare by sorting the pins' addresses.
        // This catches duplicate links, whether they are in the
        // same direction or not.
        // Sorting by pin index should have the uniqueness guarantees as sorting
        // by id -- each unique id will get one slot in the link pool array.

        int lhs_start = lhs.start_pin_idx;
        int lhs_end = lhs.end_pin_idx;
        int rhs_start = rhs.start_pin_idx;
        int rhs_end = rhs.end_pin_idx;

        if (lhs_start > lhs_end)
        {
            ImSwap(lhs_start, lhs_end);
        }

        if (rhs_start > rhs_end)
        {
            ImSwap(rhs_start, rhs_end);
        }

        return lhs_start == rhs_start && lhs_end == rhs_end;
    }
};

struct BezierCurve
{
    // the curve control points
    ImVec2 p0, p1, p2, p3;
};

struct LinkBezierData
{
    BezierCurve bezier;
    int num_segments;
};

enum ClickInteractionType
{
    ClickInteractionType_Node,
    ClickInteractionType_Link,
    ClickInteractionType_LinkCreation,
    ClickInteractionType_Panning,
    ClickInteractionType_BoxSelection,
    ClickInteractionType_None
};

struct ClickInteractionState
{
    struct
    {
        int start_pin_idx;
        int end_pin_idx;
    } link_creation;

    struct
    {
        ImRect rect;
    } box_selector;
};

struct ColorStyleElement
{
    ImU32 color;
    ColorStyle item;

    ColorStyleElement(const ImU32 c, const ColorStyle s) : color(c), item(s) {}
};

struct StyleElement
{
    StyleVar item;
    float value;

    StyleElement(const float value, const StyleVar variable)
        : item(variable), value(value)
    {
    }
};

// [SECTION] global struct
// this stores data which only lives for one frame
struct
{
    EditorContext* default_editor_ctx;
    EditorContext* editor_ctx;
    ImDrawList* canvas_draw_list;
    ImVec2 canvas_origin_screen_space;
    ImRect canvas_rect_screen_space;
    ScopeFlags current_scope;

    IO io;
    Style style;
    ImVector<ColorStyleElement> color_modifier_stack;
    ImVector<StyleElement> style_modifier_stack;
    ImGuiTextBuffer text_buffer;

    int current_node_idx;
    int current_pin_idx;

    OptionalIndex hovered_node_idx;
    OptionalIndex hovered_link_idx;
    OptionalIndex hovered_pin_idx;

    OptionalIndex active_pin_idx;
    OptionalIndex deleted_link_idx;

    bool link_created;
} g;

EditorContext& editor_context_get()
{
    assert(g.editor_ctx != NULL);
    return *g.editor_ctx;
}

inline bool is_mouse_hovering_near_point(const ImVec2& point, float radius)
{
    ImVec2 delta = ImGui::GetIO().MousePos - point;
    return (delta.x * delta.x + delta.y * delta.y) < (radius * radius);
}

inline ImVec2 eval_bezier(float t, const BezierCurve& bezier)
{
    // B(t) = (1-t)**3 p0 + 3(1 - t)**2 t P1 + 3(1-t)t**2 P2 + t**3 P3
    return ImVec2(
        (1 - t) * (1 - t) * (1 - t) * bezier.p0.x +
            3 * (1 - t) * (1 - t) * t * bezier.p1.x +
            3 * (1 - t) * t * t * bezier.p2.x + t * t * t * bezier.p3.x,
        (1 - t) * (1 - t) * (1 - t) * bezier.p0.y +
            3 * (1 - t) * (1 - t) * t * bezier.p1.y +
            3 * (1 - t) * t * t * bezier.p2.y + t * t * t * bezier.p3.y);
}

// Calculates the closest point along each bezier curve segment.
ImVec2 get_closest_point_on_cubic_bezier(
    const int num_segments,
    const ImVec2& p,
    const BezierCurve& bezier)
{
    IM_ASSERT(num_segments > 0);
    ImVec2 p_last = bezier.p0;
    ImVec2 p_closest;
    float p_closest_dist = FLT_MAX;
    float t_step = 1.0f / (float)num_segments;
    for (int i = 1; i <= num_segments; ++i)
    {
        ImVec2 p_current = eval_bezier(t_step * i, bezier);
        ImVec2 p_line = ImLineClosestPoint(p_last, p_current, p);
        float dist = ImLengthSqr(p - p_line);
        if (dist < p_closest_dist)
        {
            p_closest = p_line;
            p_closest_dist = dist;
        }
        p_last = p_current;
    }
    return p_closest;
}

inline float get_distance_to_cubic_bezier(
    const ImVec2& pos,
    const BezierCurve& bezier,
    const int num_segments)
{
    const ImVec2 point_on_curve =
        get_closest_point_on_cubic_bezier(num_segments, pos, bezier);

    const ImVec2 to_curve = point_on_curve - pos;
    return ImSqrt(ImLengthSqr(to_curve));
}

inline ImRect get_containing_rect_for_bezier_curve(const BezierCurve& bezier)
{
    const ImVec2 min = ImVec2(
        ImMin(bezier.p0.x, bezier.p3.x), ImMin(bezier.p0.y, bezier.p3.y));
    const ImVec2 max = ImVec2(
        ImMax(bezier.p0.x, bezier.p3.x), ImMax(bezier.p0.y, bezier.p3.y));

    const float hover_distance = g.style.link_hover_distance;

    ImRect rect(min, max);
    rect.Add(bezier.p1);
    rect.Add(bezier.p2);
    rect.Expand(ImVec2(hover_distance, hover_distance));

    return rect;
}

inline LinkBezierData get_link_renderable(
    ImVec2 start,
    ImVec2 end,
    const AttributeType start_type,
    const float line_segments_per_length)
{
    assert(
        (start_type == AttributeType_Input) ||
        (start_type == AttributeType_Output));
    if (start_type == AttributeType_Input)
    {
        ImSwap(start, end);
    }

    const float link_length = ImSqrt(ImLengthSqr(end - start));
    const ImVec2 offset = ImVec2(0.25f * link_length, 0.f);
    LinkBezierData link_data;
    link_data.bezier.p0 = start;
    link_data.bezier.p1 = start + offset;
    link_data.bezier.p2 = end - offset;
    link_data.bezier.p3 = end;
    link_data.num_segments =
        ImMax(static_cast<int>(link_length * line_segments_per_length), 1);
    return link_data;
}

inline bool is_mouse_hovering_near_link(
    const BezierCurve& bezier,
    const int num_segments)
{
    const ImVec2 mouse_pos = ImGui::GetIO().MousePos;

    // First, do a simple bounding box test against the box containing the link
    // to see whether calculating the distance to the link is worth doing.
    const ImRect link_rect = get_containing_rect_for_bezier_curve(bezier);

    if (link_rect.Contains(mouse_pos))
    {
        const float distance =
            get_distance_to_cubic_bezier(mouse_pos, bezier, num_segments);
        if (distance < g.style.link_hover_distance)
        {
            return true;
        }
    }

    return false;
}

inline float eval_implicit_line_eq(
    const ImVec2& p1,
    const ImVec2& p2,
    const ImVec2& p)
{
    return (p2.y - p1.y) * p.x + (p1.x - p2.x) * p.y +
           (p2.x * p1.y - p1.x * p2.y);
}

inline int sign(float val) { return int(val > 0.0f) - int(val < 0.0f); }

inline bool rectangle_overlaps_line_segment(
    const ImRect& rect,
    const ImVec2& p1,
    const ImVec2& p2)
{
    if (rect.Contains(p1) && rect.Contains(p2))
    {
        return true;
    }

    bool line_intersects_square = false;

    // First, test to see if the four corners are on different sides of the line
    // going through p1 and p2.

    {
        const int corner_signs[4] = {
            sign(eval_implicit_line_eq(p1, p2, rect.Min)),
            sign(eval_implicit_line_eq(p1, p2, ImVec2(rect.Max.x, rect.Min.y))),
            sign(eval_implicit_line_eq(p1, p2, ImVec2(rect.Min.x, rect.Max.y))),
            sign(eval_implicit_line_eq(p1, p2, rect.Max))};

        for (int i = 0, iprev = 3; i < 4; ++i, iprev = (iprev + 1) % 4)
        {
            const int s = corner_signs[i];
            const int s_prev = corner_signs[iprev];

            if (s == 0)
            {
                break;
            }

            // If the sign changes at any point, then the point is on another
            // side of the line than the previous point, and we know that there
            // is a possible intersection.

            if (s != s_prev)
            {
                line_intersects_square = true;
                break;
            }
        }
    }

    // See if the projections of the line segment and square overlap.
    if (line_intersects_square)
    {
        ImRect proj_rect = rect;
        if (proj_rect.Min.x > proj_rect.Max.x)
        {
            ImSwap(proj_rect.Min.x, proj_rect.Max.x);
        }

        if (proj_rect.Min.y > proj_rect.Max.y)
        {
            ImSwap(proj_rect.Min.y, proj_rect.Max.y);
        }

        if ((p1.x > proj_rect.Min.x && p1.x < proj_rect.Max.x) &&
            (p1.y > proj_rect.Min.y && p1.y < proj_rect.Max.y))
        {
            return true;
        }

        if ((p2.x > proj_rect.Min.x && p2.x < proj_rect.Max.x) &&
            (p2.y > proj_rect.Min.y && p2.y < proj_rect.Max.y))
        {
            return true;
        }
    }

    return false;
}

inline bool rectangle_overlaps_bezier(
    const ImRect& rectangle,
    const LinkBezierData& link_data)
{
    ImVec2 current = eval_bezier(0.f, link_data.bezier);
    const float dt = 1.0f / link_data.num_segments;
    for (int s = 0; s < link_data.num_segments; ++s)
    {
        ImVec2 next =
            eval_bezier(static_cast<float>((s + 1) * dt), link_data.bezier);
        if (rectangle_overlaps_line_segment(rectangle, current, next))
        {
            return true;
        }
        current = next;
    }
    return false;
}

inline bool rectangle_overlaps_link(
    const ImRect& rectangle,
    const ImVec2& start,
    const ImVec2& end,
    const AttributeType start_type)
{
    // First level: simple rejection test via rectangle overlap:

    ImRect lrect = ImRect(start, end);
    if (lrect.Min.x > lrect.Max.x)
    {
        ImSwap(lrect.Min.x, lrect.Max.x);
    }

    if (lrect.Min.y > lrect.Max.y)
    {
        ImSwap(lrect.Min.y, lrect.Max.y);
    }

    if (rectangle.Overlaps(lrect))
    {
        // First, check if either one or both endpoinds are trivially contained
        // in the rectangle

        if (rectangle.Contains(start) || rectangle.Contains(end))
        {
            return true;
        }

        // Second level of refinement: do a more expensive test against the
        // link

        const LinkBezierData link_data = get_link_renderable(
            start, end, start_type, g.style.link_line_segments_per_length);
        return rectangle_overlaps_bezier(rectangle, link_data);
    }

    return false;
}
} // namespace

// [SECTION] editor context definition

struct EditorContext
{
    ObjectPool<NodeData> nodes;
    ObjectPool<PinData> pins;
    ObjectPool<LinkData> links;

    // ui related fields
    ImVec2 panning;

    ImVector<int> selected_node_indices;
    ImVector<int> selected_link_indices;

    ClickInteractionType click_interaction_type;
    ClickInteractionState click_interaction_state;

    EditorContext()
        : nodes(), pins(), links(), panning(0.f, 0.f), selected_node_indices(),
          selected_link_indices(),
          click_interaction_type(ClickInteractionType_None),
          click_interaction_state()
    {
    }
};

namespace
{
// [SECTION] ui state logic

ImVec2 get_screen_space_pin_coordinates(
    const ImRect& node_rect,
    const ImRect& attribute_rect,
    const AttributeType type)
{
    assert(type == AttributeType_Input || type == AttributeType_Output);
    const float x = type == AttributeType_Input
                        ? (node_rect.Min.x - g.style.pin_offset)
                        : (node_rect.Max.x + g.style.pin_offset);
    return ImVec2(x, 0.5f * (attribute_rect.Min.y + attribute_rect.Max.y));
}

ImVec2 get_screen_space_pin_coordinates(
    const EditorContext& editor,
    const PinData& pin)
{
    const ImRect& parent_node_rect =
        editor.nodes.pool[pin.parent_node_idx].rect;
    return get_screen_space_pin_coordinates(
        parent_node_rect, pin.attribute_rect, pin.type);
}

// These functions are here, and not members of the BoxSelector struct, because
// implementing a C API in C++ is frustrating. EditorContext has a BoxSelector
// field, but the state changes depend on the editor. So, these are implemented
// as C-style free functions so that the code is not too much of a mish-mash of
// C functions and C++ method definitions.

void begin_node_selection(EditorContext& editor, const int node_idx)
{
    // Don't start selecting a node if we are e.g. already creating and dragging
    // a new link! New link creation can happen when the mouse is clicked over
    // a node, but within the hover radius of a pin.
    if (editor.click_interaction_type != ClickInteractionType_None)
    {
        return;
    }

    editor.click_interaction_type = ClickInteractionType_Node;
    // If the node is not already contained in the selection, then we want only
    // the interaction node to be selected, effective immediately.
    //
    // Otherwise, we want to allow for the possibility of multiple nodes to be
    // moved at once.
    if (!editor.selected_node_indices.contains(node_idx))
    {
        editor.selected_node_indices.clear();
        editor.selected_link_indices.clear();
        editor.selected_node_indices.push_back(node_idx);
    }
}

void begin_link_interaction(EditorContext& editor, const int link_idx)
{
    if (editor.click_interaction_type == ClickInteractionType_LinkCreation)
    {
        ClickInteractionState& state = editor.click_interaction_state;
        const LinkData& link = editor.links.pool[link_idx];
        state.link_creation.start_pin_idx =
            g.hovered_pin_idx == link.start_pin_idx ? link.end_pin_idx
                                                    : link.start_pin_idx;

        g.deleted_link_idx = link_idx;
    }
    else
    {
        editor.click_interaction_type = ClickInteractionType_Link;
    }

    // When a link is selected, clear all other selections, and insert the link
    // as the sole selection.
    editor.selected_node_indices.clear();
    editor.selected_link_indices.clear();
    editor.selected_link_indices.push_back(link_idx);
}

void begin_link_creation(EditorContext& editor, const int hovered_pin_idx)
{
    editor.click_interaction_type = ClickInteractionType_LinkCreation;
    editor.click_interaction_state.link_creation.start_pin_idx =
        hovered_pin_idx;
}

void begin_canvas_interaction(EditorContext& editor)
{
    const bool any_ui_element_hovered =
        g.hovered_node_idx.has_value() || g.hovered_link_idx.has_value() ||
        g.hovered_pin_idx.has_value() || ImGui::IsAnyItemHovered();

    const bool mouse_not_in_canvas =
        !(g.canvas_rect_screen_space.Contains(ImGui::GetMousePos()) &&
          ImGui::IsWindowHovered());

    if (any_ui_element_hovered || mouse_not_in_canvas)
    {
        return;
    }

    const bool left_mouse_clicked = ImGui::IsMouseClicked(0);
    const bool middle_mouse_clicked = ImGui::IsMouseClicked(2);

    const bool started_panning =
        g.io.emulate_three_button_mouse.enabled
            ? (left_mouse_clicked && *g.io.emulate_three_button_mouse.modifier)
            : middle_mouse_clicked;

    editor.click_interaction_type = started_panning
                                        ? ClickInteractionType_Panning
                                        : ClickInteractionType_BoxSelection;

    if (editor.click_interaction_type == ClickInteractionType_BoxSelection)
    {
        editor.click_interaction_state.box_selector.rect.Min =
            ImGui::GetIO().MousePos;
    }
}

void box_selector_update_selection(EditorContext& editor, ImRect box_rect)
{
    // Invert box selector coordinates as needed

    if (box_rect.Min.x > box_rect.Max.x)
    {
        ImSwap(box_rect.Min.x, box_rect.Max.x);
    }

    if (box_rect.Min.y > box_rect.Max.y)
    {
        ImSwap(box_rect.Min.y, box_rect.Max.y);
    }

    // Update node selection

    editor.selected_node_indices.clear();

    // Test for overlap against node rectangles

    for (int node_idx = 0; node_idx < editor.nodes.pool.size(); ++node_idx)
    {
        if (editor.nodes.in_use[node_idx])
        {
            NodeData& node = editor.nodes.pool[node_idx];
            if (box_rect.Overlaps(node.rect))
            {
                editor.selected_node_indices.push_back(node_idx);
            }
        }
    }

    // Update link selection

    editor.selected_link_indices.clear();

    // Test for overlap against links

    for (int link_idx = 0; link_idx < editor.links.pool.size(); ++link_idx)
    {
        if (editor.links.in_use[link_idx])
        {
            const LinkData& link = editor.links.pool[link_idx];

            const PinData& pin_start = editor.pins.pool[link.start_pin_idx];
            const PinData& pin_end = editor.pins.pool[link.end_pin_idx];
            const ImRect& node_start_rect =
                editor.nodes.pool[pin_start.parent_node_idx].rect;
            const ImRect& node_end_rect =
                editor.nodes.pool[pin_end.parent_node_idx].rect;

            const ImVec2 start = get_screen_space_pin_coordinates(
                node_start_rect, pin_start.attribute_rect, pin_start.type);
            const ImVec2 end = get_screen_space_pin_coordinates(
                node_end_rect, pin_end.attribute_rect, pin_end.type);

            // Test
            if (rectangle_overlaps_link(box_rect, start, end, pin_start.type))
            {
                editor.selected_link_indices.push_back(link_idx);
            }
        }
    }
}

void translate_selected_nodes(EditorContext& editor)
{
    if (ImGui::IsMouseDragging(0))
    {
        const ImGuiIO& io = ImGui::GetIO();
        for (int i = 0; i < editor.selected_node_indices.size(); ++i)
        {
            const int node_idx = editor.selected_node_indices[i];
            NodeData& node = editor.nodes.pool[node_idx];
            if (node.draggable)
            {
                node.origin += io.MouseDelta;
            }
        }
    }
}

bool finish_link_at_hovered_pin(
    EditorContext& editor,
    const OptionalIndex maybe_hovered_pin_idx)
{
    if (!maybe_hovered_pin_idx.has_value())
    {
        return false;
    }

    if (maybe_hovered_pin_idx ==
        editor.click_interaction_state.link_creation.start_pin_idx)
    {
        return false;
    }

    const int end_pin_idx = maybe_hovered_pin_idx.value();

    LinkData test_link;
    test_link.start_pin_idx =
        editor.click_interaction_state.link_creation.start_pin_idx;
    test_link.end_pin_idx = end_pin_idx;
    if (editor.links.contains(test_link, LinkPredicate()))
    {
        return false;
    }

    editor.click_interaction_state.link_creation.end_pin_idx = end_pin_idx;

    return true;
}

void click_interaction_update(EditorContext& editor)
{
    const bool left_mouse_released = ImGui::IsMouseReleased(0);

    switch (editor.click_interaction_type)
    {
    case ClickInteractionType_BoxSelection:
    {
        ImRect& box_rect = editor.click_interaction_state.box_selector.rect;
        box_rect.Max = ImGui::GetIO().MousePos;

        box_selector_update_selection(editor, box_rect);

        const ImU32 box_selector_color = g.style.colors[ColorStyle_BoxSelector];
        const ImU32 box_selector_outline =
            g.style.colors[ColorStyle_BoxSelectorOutline];
        g.canvas_draw_list->AddRectFilled(
            box_rect.Min, box_rect.Max, box_selector_color);
        g.canvas_draw_list->AddRect(
            box_rect.Min, box_rect.Max, box_selector_outline);

        if (left_mouse_released)
        {
            editor.click_interaction_type = ClickInteractionType_None;
        }
    }
    break;
    case ClickInteractionType_Node:
    {
        translate_selected_nodes(editor);

        if (left_mouse_released)
        {
            editor.click_interaction_type = ClickInteractionType_None;
        }
    }
    break;
    case ClickInteractionType_Link:
    {
        if (left_mouse_released)
        {
            editor.click_interaction_type = ClickInteractionType_None;
        }
    }
    break;
    case ClickInteractionType_LinkCreation:
    {
        const PinData& pin = editor.pins.pool[editor.click_interaction_state
                                                  .link_creation.start_pin_idx];

        const ImVec2 start_pos = get_screen_space_pin_coordinates(editor, pin);

        // If we are within the hover radius of a receiving pin, snap the link
        // endpoint to it
        const ImVec2 end_pos =
            g.hovered_pin_idx.has_value()
                ? get_screen_space_pin_coordinates(
                      editor, editor.pins.pool[g.hovered_pin_idx.value()])
                : ImGui::GetIO().MousePos;

        const LinkBezierData link_data = get_link_renderable(
            start_pos,
            end_pos,
            pin.type,
            g.style.link_line_segments_per_length);
        g.canvas_draw_list->AddBezierCurve(
            link_data.bezier.p0,
            link_data.bezier.p1,
            link_data.bezier.p2,
            link_data.bezier.p3,
            g.style.colors[ColorStyle_Link],
            g.style.link_thickness,
            link_data.num_segments);

        if (left_mouse_released)
        {
            g.link_created =
                finish_link_at_hovered_pin(editor, g.hovered_pin_idx);

            editor.click_interaction_type = ClickInteractionType_None;
        }
    }
    break;
    case ClickInteractionType_Panning:
    {
        const bool dragging =
            g.io.emulate_three_button_mouse.enabled
                ? (ImGui::IsMouseDragging(0, 0.f) &&
                   (*g.io.emulate_three_button_mouse.modifier))
                : ImGui::IsMouseDragging(2, 0.f);

        if (dragging)
        {
            editor.panning += ImGui::GetIO().MouseDelta;
        }
        else
        {
            editor.click_interaction_type = ClickInteractionType_None;
        }
    }
    break;
    case ClickInteractionType_None:
        break;
    default:
        assert(!"Unreachable code!");
        break;
    }
}

// [SECTION] render helpers

inline ImVec2 screen_space_to_grid_space(const ImVec2& v)
{
    const EditorContext& editor = editor_context_get();
    return v - g.canvas_origin_screen_space - editor.panning;
}

inline ImVec2 grid_space_to_editor_space(const ImVec2& v)
{
    const EditorContext& editor = editor_context_get();
    return v + editor.panning;
}

inline ImVec2 editor_space_to_screen_space(const ImVec2& v)
{
    return g.canvas_origin_screen_space + v;
}

inline ImRect get_item_rect()
{
    return ImRect(ImGui::GetItemRectMin(), ImGui::GetItemRectMax());
}

inline ImVec2 get_node_title_bar_origin(const NodeData& node)
{
    return node.origin + node.layout_style.padding;
}

inline ImVec2 get_node_content_origin(const NodeData& node)
{

    const ImVec2 title_bar_height = ImVec2(
        0.f,
        node.title_bar_content_rect.GetHeight() +
            2.0f * node.layout_style.padding.y);
    return node.origin + title_bar_height + node.layout_style.padding;
}

void draw_grid(EditorContext& editor, const ImVec2& canvas_size)
{
    const ImVec2 offset = editor.panning;

    for (float x = fmodf(offset.x, g.style.grid_spacing); x < canvas_size.x;
         x += g.style.grid_spacing)
    {
        g.canvas_draw_list->AddLine(
            editor_space_to_screen_space(ImVec2(x, 0.0f)),
            editor_space_to_screen_space(ImVec2(x, canvas_size.y)),
            g.style.colors[ColorStyle_GridLine]);
    }

    for (float y = fmodf(offset.y, g.style.grid_spacing); y < canvas_size.y;
         y += g.style.grid_spacing)
    {
        g.canvas_draw_list->AddLine(
            editor_space_to_screen_space(ImVec2(0.0f, y)),
            editor_space_to_screen_space(ImVec2(canvas_size.x, y)),
            g.style.colors[ColorStyle_GridLine]);
    }
}

struct QuadOffsets
{
    ImVec2 top_left, bottom_left, bottom_right, top_right;
};

QuadOffsets calculate_quad_offsets(const float side_length)
{
    const float half_side = 0.5f * side_length;

    QuadOffsets offset;

    offset.top_left = ImVec2(-half_side, half_side);
    offset.bottom_left = ImVec2(-half_side, -half_side);
    offset.bottom_right = ImVec2(half_side, -half_side);
    offset.top_right = ImVec2(half_side, half_side);

    return offset;
}

struct TriangleOffsets
{
    ImVec2 top_left, bottom_left, right;
};

TriangleOffsets calculate_triangle_offsets(const float side_length)
{
    // Calculates the Vec2 offsets from an equilateral triangle's midpoint to
    // its vertices. Here is how the left_offset and right_offset are
    // calculated.
    //
    // For an equilateral triangle of side length s, the
    // triangle's height, h, is h = s * sqrt(3) / 2.
    //
    // The length from the base to the midpoint is (1 / 3) * h. The length from
    // the midpoint to the triangle vertex is (2 / 3) * h.
    const float sqrt_3 = sqrtf(3.0f);
    const float left_offset = -0.1666666666667f * sqrt_3 * side_length;
    const float right_offset = 0.333333333333f * sqrt_3 * side_length;
    const float vertical_offset = 0.5f * side_length;

    TriangleOffsets offset;
    offset.top_left = ImVec2(left_offset, vertical_offset);
    offset.bottom_left = ImVec2(left_offset, -vertical_offset);
    offset.right = ImVec2(right_offset, 0.f);

    return offset;
}

void draw_pin_shape(
    const ImVec2& pin_pos,
    const PinData& pin,
    const ImU32 pin_color)
{
    static const int circle_num_segments = 8;

    switch (pin.shape)
    {
    case PinShape_Circle:
    {
        g.canvas_draw_list->AddCircle(
            pin_pos,
            g.style.pin_circle_radius,
            pin_color,
            circle_num_segments,
            g.style.pin_line_thickness);
    }
    break;
    case PinShape_CircleFilled:
    {
        g.canvas_draw_list->AddCircleFilled(
            pin_pos, g.style.pin_circle_radius, pin_color, circle_num_segments);
    }
    break;
    case PinShape_Quad:
    {
        const QuadOffsets offset =
            calculate_quad_offsets(g.style.pin_quad_side_length);
        g.canvas_draw_list->AddQuad(
            pin_pos + offset.top_left,
            pin_pos + offset.bottom_left,
            pin_pos + offset.bottom_right,
            pin_pos + offset.top_right,
            pin_color,
            g.style.pin_line_thickness);
    }
    break;
    case PinShape_QuadFilled:
    {
        const QuadOffsets offset =
            calculate_quad_offsets(g.style.pin_quad_side_length);
        g.canvas_draw_list->AddQuadFilled(
            pin_pos + offset.top_left,
            pin_pos + offset.bottom_left,
            pin_pos + offset.bottom_right,
            pin_pos + offset.top_right,
            pin_color);
    }
    break;
    case PinShape_Triangle:
    {
        const TriangleOffsets offset =
            calculate_triangle_offsets(g.style.pin_triangle_side_length);
        g.canvas_draw_list->AddTriangle(
            pin_pos + offset.top_left,
            pin_pos + offset.bottom_left,
            pin_pos + offset.right,
            pin_color,
            // NOTE: for some weird reason, the line drawn by AddTriangle is
            // much thinner than the lines drawn by AddCircle or AddQuad.
            // Multiplying the line thickness by two seemed to solve the
            // problem at a few different thickness values.
            2.f * g.style.pin_line_thickness);
    }
    break;
    case PinShape_TriangleFilled:
    {
        const TriangleOffsets offset =
            calculate_triangle_offsets(g.style.pin_triangle_side_length);
        g.canvas_draw_list->AddTriangleFilled(
            pin_pos + offset.top_left,
            pin_pos + offset.bottom_left,
            pin_pos + offset.right,
            pin_color);
    }
    break;
    default:
        assert(!"Invalid PinShape value!");
        break;
    }
}

void draw_pin(
    EditorContext& editor,
    const NodeData& node,
    const int pin_idx,
    const bool left_mouse_clicked)
{
    const PinData& pin = editor.pins.pool[pin_idx];
    const ImRect& parent_node_rect =
        editor.nodes.pool[pin.parent_node_idx].rect;
    const ImVec2 pin_pos = get_screen_space_pin_coordinates(
        parent_node_rect, pin.attribute_rect, pin.type);
    if (is_mouse_hovering_near_point(pin_pos, g.style.pin_hover_radius))
    {
        g.hovered_pin_idx = pin_idx;
        draw_pin_shape(pin_pos, pin, pin.color_style.hovered);

        if (left_mouse_clicked)
        {
            begin_link_creation(editor, pin_idx);
        }
    }
    else
    {
        draw_pin_shape(pin_pos, pin, pin.color_style.background);
    }
}

// TODO: It may be useful to make this an EditorContext method, since this uses
// a lot of editor state. Currently that is just not clear, since we don't pass
// the editor as a part of the function signature.
void draw_node(EditorContext& editor, const int node_idx)
{
    const NodeData& node = editor.nodes.pool[node_idx];
    ImGui::SetCursorPos(node.origin + editor.panning);
    // InvisibleButton's str_id can be left empty if we push our own
    // id on the stack.
    ImGui::PushID(node.id);
    ImGui::InvisibleButton("", node.rect.GetSize());
    ImGui::PopID();

    const bool item_hovered = ImGui::IsItemHovered();

    ImU32 node_background = node.color_style.background;
    ImU32 titlebar_background = node.color_style.titlebar;

    if (editor.selected_node_indices.contains(node_idx))
    {
        node_background = node.color_style.background_selected;
        titlebar_background = node.color_style.titlebar_selected;
    }
    else if (item_hovered)
    {
        node_background = node.color_style.background_hovered;
        titlebar_background = node.color_style.titlebar_hovered;
    }

    {
        // node base
        g.canvas_draw_list->AddRectFilled(
            node.rect.Min,
            node.rect.Max,
            node_background,
            node.layout_style.corner_rounding);

        // title bar:
        if (node.title_bar_content_rect.GetHeight() > 0.f)
        {
            ImRect expanded_title_rect = node.title_bar_content_rect;
            expanded_title_rect.Expand(node.layout_style.padding);

            ImRect title_bar_rect = ImRect(
                expanded_title_rect.Min,
                expanded_title_rect.Min + ImVec2(node.rect.GetWidth(), 0.f) +
                    ImVec2(0.f, expanded_title_rect.GetHeight()));

            g.canvas_draw_list->AddRectFilled(
                title_bar_rect.Min,
                title_bar_rect.Max,
                titlebar_background,
                node.layout_style.corner_rounding,
                ImDrawCornerFlags_Top);
        }

        if ((g.style.flags & StyleFlags_NodeOutline) != 0)
        {
            g.canvas_draw_list->AddRect(
                node.rect.Min,
                node.rect.Max,
                node.color_style.outline,
                node.layout_style.corner_rounding);
        }
    }

    // TODO: this could be done at the beginning of BeginNodeEditor and the
    // value could be stored in the global struct.
    const bool left_mouse_clicked = ImGui::IsMouseClicked(0);

    for (int i = 0; i < node.pin_indices.size(); ++i)
    {
        draw_pin(editor, node, node.pin_indices[i], left_mouse_clicked);
    }

    if (item_hovered)
    {
        g.hovered_node_idx = node_idx;
        if (left_mouse_clicked)
        {
            begin_node_selection(editor, node_idx);
        }
    }
}

void draw_link(EditorContext& editor, const int link_idx)
{
    const LinkData& link = editor.links.pool[link_idx];
    const PinData& start_pin = editor.pins.pool[link.start_pin_idx];
    const PinData& end_pin = editor.pins.pool[link.end_pin_idx];
    const ImRect& start_parent_node_rect =
        editor.nodes.pool[start_pin.parent_node_idx].rect;
    const ImRect& end_parent_node_rect =
        editor.nodes.pool[end_pin.parent_node_idx].rect;

    const ImVec2 start_pos = get_screen_space_pin_coordinates(
        start_parent_node_rect, start_pin.attribute_rect, start_pin.type);
    const ImVec2 end_pos = get_screen_space_pin_coordinates(
        end_parent_node_rect, end_pin.attribute_rect, end_pin.type);

    const LinkBezierData link_data = get_link_renderable(
        start_pos,
        end_pos,
        start_pin.type,
        g.style.link_line_segments_per_length);

    const bool is_hovered =
        is_mouse_hovering_near_link(link_data.bezier, link_data.num_segments);
    if (is_hovered)
    {
        g.hovered_link_idx = link_idx;
        if (ImGui::IsMouseClicked(0))
        {
            begin_link_interaction(editor, link_idx);
        }
    }

    // It's possible for a link to be deleted in begin_link_interaction. A user
    // may detach a link, resulting in the link wire snapping to the mouse
    // position.
    //
    // In other words, skip rendering the link if it was deleted.
    if (g.deleted_link_idx == link_idx)
    {
        return;
    }

    ImU32 link_color = link.color_style.base;
    if (editor.selected_link_indices.contains(link_idx))
    {
        link_color = link.color_style.selected;
    }
    else if (is_hovered)
    {
        link_color = link.color_style.hovered;
    }

    g.canvas_draw_list->AddBezierCurve(
        link_data.bezier.p0,
        link_data.bezier.p1,
        link_data.bezier.p2,
        link_data.bezier.p3,
        link_color,
        g.style.link_thickness,
        link_data.num_segments);
}

void begin_attribute(
    const int id,
    const AttributeType type,
    const PinShape shape,
    const int node_idx)
{
    // Make sure to call BeginNode() before calling
    // BeginAttribute()
    assert(g.current_scope == Scope_Node);
    g.current_scope = Scope_Attribute;

    ImGui::BeginGroup();
    ImGui::PushID(id);

    EditorContext& editor = editor_context_get();

    const int pin_idx = editor.pins.find_or_create_index_for(id);
    g.current_pin_idx = pin_idx;
    PinData& pin = editor.pins.pool[pin_idx];
    pin.id = id;
    pin.parent_node_idx = node_idx;
    pin.type = type;
    pin.shape = shape;
    pin.color_style.background = g.style.colors[ColorStyle_Pin];
    pin.color_style.hovered = g.style.colors[ColorStyle_PinHovered];
}
} // namespace

// [SECTION] API implementation

IO::EmulateThreeButtonMouse::EmulateThreeButtonMouse()
    : enabled(false), modifier(NULL)
{
}

IO::IO() : emulate_three_button_mouse() {}

Style::Style()
    : grid_spacing(32.f), node_corner_rounding(4.f),
      node_padding_horizontal(8.f), node_padding_vertical(8.f),
      link_thickness(3.f), link_line_segments_per_length(0.1f),
      link_hover_distance(10.f), pin_circle_radius(4.f),
      pin_quad_side_length(7.f), pin_triangle_side_length(9.5),
      pin_line_thickness(1.f), pin_hover_radius(10.f), pin_offset(0.f),
      flags(StyleFlags(StyleFlags_NodeOutline | StyleFlags_GridLines)), colors()
{
}

EditorContext* EditorContextCreate()
{
    void* mem = ImGui::MemAlloc(sizeof(EditorContext));
    new (mem) EditorContext();
    return (EditorContext*)mem;
}

void EditorContextFree(EditorContext* ctx)
{
    ctx->~EditorContext();
    ImGui::MemFree(ctx);
}

void EditorContextSet(EditorContext* ctx) { g.editor_ctx = ctx; }

ImVec2 EditorContextGetPanning()
{
    const EditorContext& editor = editor_context_get();
    return editor.panning;
}

void EditorContextResetPanning(const ImVec2& pos)
{
    EditorContext& editor = editor_context_get();
    editor.panning = pos;
}

void EditorContextMoveToNode(const int node_id)
{
    EditorContext& editor = editor_context_get();
    NodeData& node = editor.nodes.find_or_create_new(node_id);

    editor.panning.x = -node.origin.x;
    editor.panning.y = -node.origin.y;
}

void Initialize()
{
    assert(initialized == false);
    initialized = true;

    g.default_editor_ctx = NULL;
    g.editor_ctx = NULL;
    g.canvas_origin_screen_space = ImVec2(0.0f, 0.0f);
    g.canvas_rect_screen_space = ImRect(ImVec2(0.f, 0.f), ImVec2(0.f, 0.f));
    g.current_scope = Scope_None;

    g.default_editor_ctx = EditorContextCreate();
    EditorContextSet(g.default_editor_ctx);

    const ImGuiIO& io = ImGui::GetIO();

    g.io.emulate_three_button_mouse.modifier = &io.KeyAlt;

    StyleColorsDark();
}

void Shutdown() { EditorContextFree(g.default_editor_ctx); }

IO& GetIO() { return g.io; }

Style& GetStyle() { return g.style; }

void StyleColorsDark()
{
    g.style.colors[ColorStyle_NodeBackground] = IM_COL32(50, 50, 50, 255);
    g.style.colors[ColorStyle_NodeBackgroundHovered] =
        IM_COL32(75, 75, 75, 255);
    g.style.colors[ColorStyle_NodeBackgroundSelected] =
        IM_COL32(75, 75, 75, 255);
    g.style.colors[ColorStyle_NodeOutline] = IM_COL32(100, 100, 100, 255);
    // title bar colors match ImGui's titlebg colors
    g.style.colors[ColorStyle_TitleBar] = IM_COL32(41, 74, 122, 255);
    g.style.colors[ColorStyle_TitleBarHovered] = IM_COL32(66, 150, 250, 255);
    g.style.colors[ColorStyle_TitleBarSelected] = IM_COL32(66, 150, 250, 255);
    // link colors match ImGui's slider grab colors
    g.style.colors[ColorStyle_Link] = IM_COL32(61, 133, 224, 200);
    g.style.colors[ColorStyle_LinkHovered] = IM_COL32(66, 150, 250, 255);
    g.style.colors[ColorStyle_LinkSelected] = IM_COL32(66, 150, 250, 255);
    // pin colors match ImGui's button colors
    g.style.colors[ColorStyle_Pin] = IM_COL32(53, 150, 250, 180);
    g.style.colors[ColorStyle_PinHovered] = IM_COL32(53, 150, 250, 255);

    g.style.colors[ColorStyle_BoxSelector] = IM_COL32(61, 133, 224, 30);
    g.style.colors[ColorStyle_BoxSelectorOutline] = IM_COL32(61, 133, 224, 150);

    g.style.colors[ColorStyle_GridBackground] = IM_COL32(40, 40, 50, 200);
    g.style.colors[ColorStyle_GridLine] = IM_COL32(200, 200, 200, 40);
}

void StyleColorsClassic()
{
    g.style.colors[ColorStyle_NodeBackground] = IM_COL32(50, 50, 50, 255);
    g.style.colors[ColorStyle_NodeBackgroundHovered] =
        IM_COL32(75, 75, 75, 255);
    g.style.colors[ColorStyle_NodeBackgroundSelected] =
        IM_COL32(75, 75, 75, 255);
    g.style.colors[ColorStyle_NodeOutline] = IM_COL32(100, 100, 100, 255);
    g.style.colors[ColorStyle_TitleBar] = IM_COL32(69, 69, 138, 255);
    g.style.colors[ColorStyle_TitleBarHovered] = IM_COL32(82, 82, 161, 255);
    g.style.colors[ColorStyle_TitleBarSelected] = IM_COL32(82, 82, 161, 255);
    g.style.colors[ColorStyle_Link] = IM_COL32(255, 255, 255, 100);
    g.style.colors[ColorStyle_LinkHovered] = IM_COL32(105, 99, 204, 153);
    g.style.colors[ColorStyle_LinkSelected] = IM_COL32(105, 99, 204, 153);
    g.style.colors[ColorStyle_Pin] = IM_COL32(89, 102, 156, 170);
    g.style.colors[ColorStyle_PinHovered] = IM_COL32(102, 122, 179, 200);
    g.style.colors[ColorStyle_BoxSelector] = IM_COL32(82, 82, 161, 100);
    g.style.colors[ColorStyle_BoxSelectorOutline] = IM_COL32(82, 82, 161, 255);
    g.style.colors[ColorStyle_GridBackground] = IM_COL32(40, 40, 50, 200);
    g.style.colors[ColorStyle_GridLine] = IM_COL32(200, 200, 200, 40);
}

void StyleColorsLight()
{
    g.style.colors[ColorStyle_NodeBackground] = IM_COL32(240, 240, 240, 255);
    g.style.colors[ColorStyle_NodeBackgroundHovered] =
        IM_COL32(240, 240, 240, 255);
    g.style.colors[ColorStyle_NodeBackgroundSelected] =
        IM_COL32(240, 240, 240, 255);
    g.style.colors[ColorStyle_NodeOutline] = IM_COL32(100, 100, 100, 255);
    g.style.colors[ColorStyle_TitleBar] = IM_COL32(248, 248, 248, 255);
    g.style.colors[ColorStyle_TitleBarHovered] = IM_COL32(209, 209, 209, 255);
    g.style.colors[ColorStyle_TitleBarSelected] = IM_COL32(209, 209, 209, 255);
    // original imgui values: 66, 150, 250
    g.style.colors[ColorStyle_Link] = IM_COL32(66, 150, 250, 100);
    // original imgui values: 117, 138, 204
    g.style.colors[ColorStyle_LinkHovered] = IM_COL32(66, 150, 250, 242);
    g.style.colors[ColorStyle_LinkSelected] = IM_COL32(66, 150, 250, 242);
    // original imgui values: 66, 150, 250
    g.style.colors[ColorStyle_Pin] = IM_COL32(66, 150, 250, 160);
    g.style.colors[ColorStyle_PinHovered] = IM_COL32(66, 150, 250, 255);
    g.style.colors[ColorStyle_BoxSelector] = IM_COL32(90, 170, 250, 30);
    g.style.colors[ColorStyle_BoxSelectorOutline] = IM_COL32(90, 170, 250, 150);
    g.style.colors[ColorStyle_GridBackground] = IM_COL32(225, 225, 225, 255);
    g.style.colors[ColorStyle_GridLine] = IM_COL32(180, 180, 180, 100);
    g.style.flags = StyleFlags(StyleFlags_None);
}

void BeginNodeEditor()
{
    // Remember to call Initialize() before calling BeginNodeEditor()
    assert(initialized);
    assert(g.current_scope == Scope_None);
    g.current_scope = Scope_Editor;

    // Reset state from previous pass

    g.hovered_node_idx.reset();
    g.hovered_link_idx.reset();
    g.hovered_pin_idx.reset();
    g.active_pin_idx.reset();
    g.deleted_link_idx.reset();

    g.link_created = false;

    // reset ui content for the current editor
    EditorContext& editor = editor_context_get();
    editor.nodes.update();
    editor.pins.update();
    editor.links.update();

    ImGui::BeginGroup();
    {
        ImGui::PushStyleVar(ImGuiStyleVar_FramePadding, ImVec2(1.f, 1.f));
        ImGui::PushStyleVar(ImGuiStyleVar_WindowPadding, ImVec2(0.f, 0.f));
        ImGui::PushStyleColor(
            ImGuiCol_ChildBg, g.style.colors[ColorStyle_GridBackground]);
        ImGui::BeginChild(
            "scrolling_region",
            ImVec2(0.f, 0.f),
            true,
            ImGuiWindowFlags_NoScrollbar | ImGuiWindowFlags_NoMove |
                ImGuiWindowFlags_NoScrollWithMouse);
        g.canvas_origin_screen_space = ImGui::GetCursorScreenPos();

        // NOTE: we have to fetch the canvas draw list *after* we call
        // BeginChild(), otherwise the ImGui UI elements are going to be
        // rendered into the parent window draw list.
        g.canvas_draw_list = ImGui::GetWindowDrawList();

        {
            const ImVec2 canvas_size = ImGui::GetWindowSize();
            g.canvas_rect_screen_space = ImRect(
                editor_space_to_screen_space(ImVec2(0.f, 0.f)),
                editor_space_to_screen_space(canvas_size));

            if (g.style.flags & StyleFlags_GridLines)
            {
                draw_grid(editor, canvas_size);
            }
        }
    }
}

void EndNodeEditor()
{
    assert(g.current_scope == Scope_Editor);
    g.current_scope = Scope_None;

    EditorContext& editor = editor_context_get();

    const bool is_left_mouse_clicked = ImGui::IsMouseClicked(0);
    const bool is_middle_mouse_clicked = ImGui::IsMouseClicked(2);

    for (int link_idx = 0; link_idx < editor.links.pool.size(); ++link_idx)
    {
        if (editor.links.in_use[link_idx])
        {
            draw_link(editor, link_idx);
        }
    }

    if (is_left_mouse_clicked || is_middle_mouse_clicked)
    {
        begin_canvas_interaction(editor);
    }

    click_interaction_update(editor);

    // pop style
    ImGui::EndChild();      // end scrolling region
    ImGui::PopStyleColor(); // pop child window background color
    ImGui::PopStyleVar();   // pop window padding
    ImGui::PopStyleVar();   // pop frame padding
    ImGui::EndGroup();

    for (int idx = 0; idx < editor.nodes.pool.size(); idx++)
    {
        NodeData& node = editor.nodes.pool[idx];
        node.pin_indices.clear();
    }
}

void BeginNode(const int node_id)
{
    // Remember to call BeginNodeEditor before calling BeginNode
    assert(g.current_scope == Scope_Editor);
    g.current_scope = Scope_Node;

    EditorContext& editor = editor_context_get();

    const int node_idx = editor.nodes.find_or_create_index_for(node_id);
    g.current_node_idx = node_idx;

    NodeData& node = editor.nodes.pool[node_idx];
    node.id = node_id;
    node.color_style.background = g.style.colors[ColorStyle_NodeBackground];
    node.color_style.background_hovered =
        g.style.colors[ColorStyle_NodeBackgroundHovered];
    node.color_style.background_selected =
        g.style.colors[ColorStyle_NodeBackgroundSelected];
    node.color_style.outline = g.style.colors[ColorStyle_NodeOutline];
    node.color_style.titlebar = g.style.colors[ColorStyle_TitleBar];
    node.color_style.titlebar_hovered =
        g.style.colors[ColorStyle_TitleBarHovered];
    node.color_style.titlebar_selected =
        g.style.colors[ColorStyle_TitleBarSelected];
    node.layout_style.corner_rounding = g.style.node_corner_rounding;
    node.layout_style.padding =
        ImVec2(g.style.node_padding_horizontal, g.style.node_padding_vertical);

    // ImGui::SetCursorPos sets the cursor position, local to the current widget
    // (in this case, the child object started in BeginNodeEditor). Use
    // ImGui::SetCursorScreenPos to set the screen space coordinates directly.
    ImGui::SetCursorPos(
        grid_space_to_editor_space(get_node_title_bar_origin(node)));

    g.canvas_draw_list->ChannelsSplit(Channels_Count);
    g.canvas_draw_list->ChannelsSetCurrent(Channels_ImGui);

    ImGui::PushID(node.id);
    ImGui::BeginGroup();
}

void EndNode()
{
    assert(g.current_scope == Scope_Node);
    g.current_scope = Scope_Editor;

    EditorContext& editor = editor_context_get();

    // The node's rectangle depends on the ImGui UI group size.
    ImGui::EndGroup();
    ImGui::PopID();
    {
        NodeData& node = editor.nodes.pool[g.current_node_idx];
        node.rect = get_item_rect();
        node.rect.Expand(node.layout_style.padding);
    }

    g.canvas_draw_list->ChannelsSetCurrent(Channels_NodeBackground);
    draw_node(editor, g.current_node_idx);
    g.canvas_draw_list->ChannelsMerge();
}

void BeginNodeTitleBar()
{
    assert(g.current_scope == Scope_Node);
    ImGui::BeginGroup();
}

void EndNodeTitleBar()
{
    assert(g.current_scope == Scope_Node);
    ImGui::EndGroup();

    EditorContext& editor = editor_context_get();
    NodeData& node = editor.nodes.pool[g.current_node_idx];
    node.title_bar_content_rect = get_item_rect();

    ImGui::SetCursorPos(
        grid_space_to_editor_space(get_node_content_origin(node)));
}

void BeginInputAttribute(const int id, const PinShape shape)
{
    begin_attribute(id, AttributeType_Input, shape, g.current_node_idx);
}

void BeginOutputAttribute(const int id, const PinShape shape)
{
    begin_attribute(id, AttributeType_Output, shape, g.current_node_idx);
}

void EndAttribute()
{
    assert(g.current_scope == Scope_Attribute);
    g.current_scope = Scope_Node;

    ImGui::PopID();
    ImGui::EndGroup();

    if (ImGui::IsItemActive())
    {
        g.active_pin_idx = g.current_pin_idx;
    }

    EditorContext& editor = editor_context_get();
    PinData& pin = editor.pins.pool[g.current_pin_idx];
    NodeData& node = editor.nodes.pool[g.current_node_idx];
    pin.attribute_rect = get_item_rect();
    node.pin_indices.push_back(g.current_pin_idx);
}

void Link(int id, const int start_attr_id, const int end_attr_id)
{
    assert(g.current_scope == Scope_Editor);

    EditorContext& editor = editor_context_get();
    LinkData& link = editor.links.find_or_create_new(id);
    link.id = id;
    link.start_pin_idx = editor.pins.find_or_create_index_for(start_attr_id);
    link.end_pin_idx = editor.pins.find_or_create_index_for(end_attr_id);
    link.color_style.base = g.style.colors[ColorStyle_Link];
    link.color_style.hovered = g.style.colors[ColorStyle_LinkHovered];
    link.color_style.selected = g.style.colors[ColorStyle_LinkSelected];
}

void PushColorStyle(ColorStyle item, unsigned int color)
{
    // Remember to call Initialize() before using any other functions!
    assert(initialized);
    g.color_modifier_stack.push_back(
        ColorStyleElement(g.style.colors[item], item));
    g.style.colors[item] = color;
}

void PopColorStyle()
{
    assert(g.color_modifier_stack.size() > 0);
    const ColorStyleElement elem = g.color_modifier_stack.back();
    g.style.colors[elem.item] = elem.color;
    g.color_modifier_stack.pop_back();
}

float& lookup_style_var(const StyleVar item)
{
    // TODO: once the switch gets too big and unwieldy to work with, we could do
    // a byte-offset lookup into the Style struct, using the StyleVar as an
    // index. This is how ImGui does it.
    float* style_var = 0;
    switch (item)
    {
    case StyleVar_GridSpacing:
        style_var = &g.style.grid_spacing;
        break;
    case StyleVar_NodeCornerRounding:
        style_var = &g.style.node_corner_rounding;
        break;
    case StyleVar_NodePaddingHorizontal:
        style_var = &g.style.node_padding_horizontal;
        break;
    case StyleVar_NodePaddingVertical:
        style_var = &g.style.node_padding_vertical;
        break;
    default:
        assert(!"Invalid StyleVar value!");
    }

    return *style_var;
}

void PushStyleVar(const StyleVar item, const float value)
{
    float& style_var = lookup_style_var(item);
    g.style_modifier_stack.push_back(StyleElement(style_var, item));
    style_var = value;
}

void PopStyleVar()
{
    assert(g.style_modifier_stack.size() > 0);
    const StyleElement style_elem = g.style_modifier_stack.back();
    g.style_modifier_stack.pop_back();
    float& style_var = lookup_style_var(style_elem.item);
    style_var = style_elem.value;
}

void SetNodeScreenSpacePos(int node_id, const ImVec2& screen_space_pos)
{
    // Remember to call Initialize() before using any other functions!
    assert(initialized);
    EditorContext& editor = editor_context_get();
    NodeData& node = editor.nodes.find_or_create_new(node_id);
    node.origin = screen_space_to_grid_space(screen_space_pos);
}

void SetNodeGridSpacePos(int node_id, const ImVec2& grid_pos)
{
    // Remember to call Initialize() before using any other functions!
    assert(initialized);
    EditorContext& editor = editor_context_get();
    NodeData& node = editor.nodes.find_or_create_new(node_id);
    node.origin = grid_pos;
}

void SetNodeDraggable(int node_id, const bool draggable)
{
    assert(initialized);
    EditorContext& editor = editor_context_get();
    NodeData& node = editor.nodes.find_or_create_new(node_id);
    node.draggable = draggable;
}

bool IsEditorHovered()
{
    return g.canvas_rect_screen_space.Contains(ImGui::GetMousePos()) &&
           ImGui::IsWindowHovered();
}

bool IsNodeHovered(int* const node_id)
{
    assert(g.current_scope == Scope_None);
    assert(node_id != NULL);

    const bool is_hovered = g.hovered_node_idx.has_value();
    if (is_hovered)
    {
        const EditorContext& editor = editor_context_get();
        *node_id = editor.nodes.pool[g.hovered_node_idx.value()].id;
    }
    return is_hovered;
}

bool IsLinkHovered(int* const link_id)
{
    assert(g.current_scope == Scope_None);
    assert(link_id != NULL);

    const bool is_hovered = g.hovered_link_idx.has_value();
    if (is_hovered)
    {
        const EditorContext& editor = editor_context_get();
        *link_id = editor.links.pool[g.hovered_link_idx.value()].id;
    }
    return is_hovered;
}

bool IsPinHovered(int* const attr)
{
    assert(g.current_scope == Scope_None);
    assert(attr != NULL);

    const bool is_hovered = g.hovered_pin_idx.value();
    if (is_hovered)
    {
        const EditorContext& editor = editor_context_get();
        *attr = editor.pins.pool[g.hovered_pin_idx.value()].id;
    }
    return is_hovered;
}

int NumSelectedNodes()
{
    assert(g.current_scope == Scope_None);
    const EditorContext& editor = editor_context_get();
    return editor.selected_node_indices.size();
}

int NumSelectedLinks()
{
    assert(g.current_scope == Scope_None);
    const EditorContext& editor = editor_context_get();
    return editor.selected_link_indices.size();
}

void GetSelectedNodes(int* node_ids)
{
    assert(node_ids != NULL);

    const EditorContext& editor = editor_context_get();
    for (int i = 0; i < editor.selected_node_indices.size(); ++i)
    {
        const int node_idx = editor.selected_node_indices[i];
        node_ids[i] = editor.nodes.pool[node_idx].id;
    }
}

void GetSelectedLinks(int* link_ids)
{
    assert(link_ids != NULL);

    const EditorContext& editor = editor_context_get();
    for (int i = 0; i < editor.selected_link_indices.size(); ++i)
    {
        const int link_idx = editor.selected_link_indices[i];
        link_ids[i] = editor.links.pool[link_idx].id;
    }
}

bool IsAttributeActive()
{
    assert((g.current_scope & Scope_Node) != 0);

    if (!g.active_pin_idx.has_value())
    {
        return false;
    }

    return g.active_pin_idx == g.current_pin_idx;
}

bool IsAnyAttributeActive(int* const attribute_id)
{
    assert((g.current_scope & (Scope_Node | Scope_Attribute)) == 0);

    if (!g.active_pin_idx.has_value())
    {
        return false;
    }

    if (attribute_id != NULL)
    {
        const EditorContext& editor = editor_context_get();
        *attribute_id = editor.pins.pool[g.active_pin_idx.value()].id;
    }

    return true;
}

bool IsLinkCreated(int* const started_at_pin_id, int* const ended_at_pin_id)
{
    assert(g.current_scope == Scope_None);
    assert(started_at_pin_id != NULL);
    assert(ended_at_pin_id != NULL);

    if (g.link_created)
    {
        const EditorContext& editor = editor_context_get();
        const int start_idx =
            editor.click_interaction_state.link_creation.start_pin_idx;
        const int end_idx =
            editor.click_interaction_state.link_creation.end_pin_idx;
        *started_at_pin_id = editor.pins.pool[start_idx].id;
        *ended_at_pin_id = editor.pins.pool[end_idx].id;
    }

    return g.link_created;
}

bool IsLinkDestroyed(int* const link_id)
{
    assert(g.current_scope == Scope_None);

    const bool link_destroyed = g.deleted_link_idx.has_value();
    if (link_destroyed)
    {
        const EditorContext& editor = editor_context_get();
        const int link_idx = g.deleted_link_idx.value();
        *link_id = editor.links.pool[link_idx].id;
    }

    return link_destroyed;
}

namespace
{
void node_line_handler(EditorContext& editor, const char* line)
{
    int id;
    float x, y;
    if (sscanf(line, "[node.%i", &id) == 1)
    {
        const int node_idx = editor.nodes.find_or_create_index_for(id);
        g.current_node_idx = node_idx;
        NodeData& node = editor.nodes.pool[node_idx];
        node.id = id;
    }
    else if (sscanf(line, "origin=%f,%f", &x, &y) == 2)
    {
        NodeData& node = editor.nodes.pool[g.current_node_idx];
        node.origin = ImVec2(x, y);
    }
}

void editor_line_handler(EditorContext& editor, const char* line)
{
    sscanf(line, "panning=%f,%f", &editor.panning.x, &editor.panning.y);
}
} // namespace

const char* SaveCurrentEditorStateToIniString(size_t* const data_size)
{
    return SaveEditorStateToIniString(&editor_context_get(), data_size);
}

const char* SaveEditorStateToIniString(
    const EditorContext* const editor_ptr,
    size_t* const data_size)
{
    assert(editor_ptr != NULL);
    const EditorContext& editor = *editor_ptr;

    g.text_buffer.clear();
    // TODO: check to make sure that the estimate is the upper bound of element
    g.text_buffer.reserve(64 * editor.nodes.pool.size());

    g.text_buffer.appendf(
        "[editor]\npanning=%i,%i\n",
        (int)editor.panning.x,
        (int)editor.panning.y);

    for (int i = 0; i < editor.nodes.pool.size(); i++)
    {
        if (editor.nodes.in_use[i])
        {
            const NodeData& node = editor.nodes.pool[i];
            g.text_buffer.appendf("\n[node.%d]\n", node.id);
            g.text_buffer.appendf(
                "origin=%i,%i\n", (int)node.origin.x, (int)node.origin.y);
        }
    }

    if (data_size != NULL)
    {
        *data_size = g.text_buffer.size();
    }

    return g.text_buffer.c_str();
}

void LoadCurrentEditorStateFromIniString(
    const char* const data,
    const size_t data_size)
{
    LoadEditorStateFromIniString(&editor_context_get(), data, data_size);
}

void LoadEditorStateFromIniString(
    EditorContext* const editor_ptr,
    const char* const data,
    const size_t data_size)
{
    if (data_size == 0u)
    {
        return;
    }

    EditorContext& editor =
        editor_ptr == NULL ? editor_context_get() : *editor_ptr;

    char* buf = (char*)ImGui::MemAlloc(data_size + 1);
    const char* buf_end = buf + data_size;
    memcpy(buf, data, data_size);
    buf[data_size] = 0;

    void (*line_handler)(EditorContext&, const char*);
    line_handler = NULL;
    char* line_end = NULL;
    for (char* line = buf; line < buf_end; line = line_end + 1)
    {
        while (*line == '\n' || *line == '\r')
        {
            line++;
        }
        line_end = line;
        while (line_end < buf_end && *line_end != '\n' && *line_end != '\r')
        {
            line_end++;
        }
        line_end[0] = 0;

        if (*line == ';' || *line == '\0')
        {
            continue;
        }

        if (line[0] == '[' && line_end[-1] == ']')
        {
            line_end[-1] = 0;
            if (strncmp(line + 1, "node", 4) == 0)
            {
                line_handler = node_line_handler;
            }
            else if (strcmp(line + 1, "editor") == 0)
            {
                line_handler = editor_line_handler;
            }
        }

        if (line_handler != NULL)
        {
            line_handler(editor, line);
        }
    }
    ImGui::MemFree(buf);
}

void SaveCurrentEditorStateToIniFile(const char* const file_name)
{
    SaveEditorStateToIniFile(&editor_context_get(), file_name);
}

void SaveEditorStateToIniFile(
    const EditorContext* const editor,
    const char* const file_name)
{
    size_t data_size = 0u;
    const char* data = SaveEditorStateToIniString(editor, &data_size);
    FILE* file = ImFileOpen(file_name, "wt");
    if (!file)
    {
        return;
    }

    fwrite(data, sizeof(char), data_size, file);
    fclose(file);
}

void LoadCurrentEditorStateFromIniFile(const char* const file_name)
{
    LoadEditorStateFromIniFile(&editor_context_get(), file_name);
}

void LoadEditorStateFromIniFile(
    EditorContext* const editor,
    const char* const file_name)
{
    size_t data_size = 0u;
    char* file_data = (char*)ImFileLoadToMemory(file_name, "rb", &data_size);

    if (!file_data)
    {
        return;
    }

    LoadEditorStateFromIniString(editor, file_data, data_size);
    ImGui::MemFree(file_data);
}
} // namespace imnodes