From 512a804713488b79be0a553ffd795750ca1baf1f Mon Sep 17 00:00:00 2001
From: Kevin Vlaanderen <kevinvlaanderen@gmail.com>
Date: Wed, 13 Dec 2023 11:21:07 +0100
Subject: [PATCH] 2023 13-2

---
 2023/src/tasks/day13/tasks.go      | 161 ++++++++++++++++++++---------
 2023/src/tasks/day13/tasks_test.go |   7 ++
 2 files changed, 119 insertions(+), 49 deletions(-)

diff --git a/2023/src/tasks/day13/tasks.go b/2023/src/tasks/day13/tasks.go
index 236fc2d..515d9b6 100644
--- a/2023/src/tasks/day13/tasks.go
+++ b/2023/src/tasks/day13/tasks.go
@@ -3,72 +3,135 @@ package day13
 import (
 	"2023/src/framework"
 	"github.com/samber/lo"
+	lop "github.com/samber/lo/parallel"
 )
 
 func Task1(data string) (result framework.Result[int]) {
 	blocks := framework.LineBlocks(data)
 
-	horizontalIndices := make([]int, 0)
-	verticalIndices := make([]int, 0)
+	lop.ForEach(blocks, func(block []string, index int) {
+		direction, index := findReflection(block, false)
+		switch direction {
+		case Horizontal:
+			result.Value += index * 100
+		case Vertical:
+			result.Value += index
+		}
+	})
 
-Block:
-	for _, block := range blocks {
-		width, height := len(block[0]), len(block)
+	return
+}
 
-		rows := make([][]bool, height)
-		columns := make([][]bool, width)
+func Task2(data string) (result framework.Result[int]) {
+	blocks := framework.LineBlocks(data)
 
-		for y, line := range block {
-			if rows[y] == nil {
-				rows[y] = make([]bool, width)
-			}
-			for x, char := range line {
-				if columns[x] == nil {
-					columns[x] = make([]bool, height)
-				}
-				rows[y][x] = char == '#'
-				columns[x][y] = char == '#'
-			}
+	lop.ForEach(blocks, func(block []string, index int) {
+		direction, index := findReflection(block, true)
+		switch direction {
+		case Horizontal:
+			result.Value += index * 100
+		case Vertical:
+			result.Value += index
 		}
+	})
 
-		rowHashes := lo.Map(rows, func(row []bool, index int) framework.Hash {
-			return framework.ComputeHash(row)
-		})
-		columnHashes := lo.Map(columns, func(column []bool, index int) framework.Hash {
-			return framework.ComputeHash(column)
-		})
-
-	MidX:
-		for mid := 0; mid < width-1; mid++ {
-			checkN := min(mid+1, width-mid-1)
-			for i := 0; i < checkN; i++ {
-				if columnHashes[mid-i] != columnHashes[mid+i+1] {
-					continue MidX
-				}
+	return
+}
+
+func findReflection(block []string, checkVariants bool) (Direction, int) {
+	rows, columns := parseBlock(block)
+
+	rowHashes := calculateHashes(rows, checkVariants)
+	columnHashes := calculateHashes(columns, checkVariants)
+
+	if found, index := reflectionFound(columnHashes, checkVariants); found {
+		return Vertical, index
+	}
+	if found, index := reflectionFound(rowHashes, checkVariants); found {
+		return Horizontal, index
+	}
+	panic("no reflection found")
+}
+
+func parseBlock(block []string) ([][]bool, [][]bool) {
+	width, height := len(block[0]), len(block)
+
+	rows := make([][]bool, height)
+	columns := make([][]bool, width)
+
+	for y, line := range block {
+		if rows[y] == nil {
+			rows[y] = make([]bool, width)
+		}
+		for x, char := range line {
+			if columns[x] == nil {
+				columns[x] = make([]bool, height)
 			}
-			verticalIndices = append(verticalIndices, mid+1)
-			continue Block
+			rows[y][x] = char == '#'
+			columns[x][y] = char == '#'
+		}
+	}
+	return rows, columns
+}
+
+func calculateHashes(data [][]bool, checkVariants bool) []HashGroup {
+	return lo.Map(data, func(item []bool, index int) (hashGroup HashGroup) {
+		hashGroup.original = framework.ComputeHash(item)
+		if checkVariants {
+			hashGroup.variants = lo.Map(item, func(_ bool, index int) framework.Hash {
+				newRow := make([]bool, len(item))
+				copy(newRow, item)
+				newRow[index] = !item[index]
+				return framework.ComputeHash(newRow)
+			})
 		}
+		return
+	})
+}
+
+func reflectionFound(hashes []HashGroup, checkVariants bool) (bool, int) {
+	size := len(hashes)
+
+	for mid := 0; mid < size-1; mid++ {
+		checkN := min(mid+1, size-mid-1)
 
-	MidY:
-		for mid := 0; mid < height-1; mid++ {
-			checkN := min(mid+1, height-mid-1)
-			for i := 0; i < checkN; i++ {
-				if rowHashes[mid-i] != rowHashes[mid+i+1] {
-					continue MidY
+		matchFound := true
+		variantUsed := false
+
+		for offset := 0; offset < checkN; offset++ {
+			if hashes[mid-offset].original == hashes[mid+offset+1].original {
+				continue
+			}
+			if checkVariants && !variantUsed {
+				if compareLeftRight(mid-offset, mid+offset+1, hashes) ||
+					compareLeftRight(mid+offset+1, mid-offset, hashes) {
+					variantUsed = true
+					continue
 				}
 			}
-			horizontalIndices = append(horizontalIndices, mid+1)
-			continue Block
+			matchFound = false
+		}
+		if matchFound && (!checkVariants || variantUsed) {
+			return true, mid + 1
 		}
 	}
+	return false, 0
+}
 
-	for _, row := range horizontalIndices {
-		result.Value += row * 100
-	}
-	for _, column := range verticalIndices {
-		result.Value += column
-	}
+func compareLeftRight(originalIndex, variantIndex int, hashes []HashGroup) bool {
+	return lo.ContainsBy(hashes[variantIndex].variants, func(variant framework.Hash) bool {
+		return variant == hashes[originalIndex].original
+	})
+}
 
-	return
+type HashGroup struct {
+	original framework.Hash
+	variants []framework.Hash
 }
+
+type Direction uint8
+
+const (
+	Horizontal Direction = iota
+	Vertical
+)
diff --git a/2023/src/tasks/day13/tasks_test.go b/2023/src/tasks/day13/tasks_test.go
index dcd722c..d0305be 100644
--- a/2023/src/tasks/day13/tasks_test.go
+++ b/2023/src/tasks/day13/tasks_test.go
@@ -13,6 +13,13 @@ var taskDefinitions = []_testing.TaskDefinition[int]{
 			{"day13", 30802, _testing.RealData},
 		},
 	},
+	{
+		Task: Task2,
+		Tests: []_testing.TestDefinition[int]{
+			{"data", 400, _testing.TestData},
+			{"day13", 37876, _testing.RealData},
+		},
+	},
 }
 
 func TestDay13(t *testing.T) {