feat: allow rebinning by passing edges or a new axis

src/boost_histogram/histogram.py

@@ -881,7 +881,7 @@ def __getitem__(self: H, index: IndexingExpr) -> H | float | Accumulator:
         reduced: CppHistogram | None = None
 
         # Compute needed slices and projections
-        for i, ind in enumerate(indexes):
+        for i, ind in enumerate(indexes):  # pylint: disable=too-many-nested-blocks
             if isinstance(ind, SupportsIndex):
                 pick_each[i] = ind.__index__() + (
                     1 if self.axes[i].traits.underflow else 0
@@ -967,14 +967,26 @@ def __getitem__(self: H, index: IndexingExpr) -> H | float | Accumulator:
 
                     new_reduced = reduced.__class__(axes)
                     new_view = new_reduced.view(flow=True)
-
-                    j = 1
+                    j = 0
+                    new_j_base = 0
+                    if self.axes[i].traits.underflow:
+                        groups.insert(0, 1)
+                    else:
+                        new_j_base = 1
+                    if self.axes[i].traits.overflow:
+                        groups.append(1)
                     for new_j, group in enumerate(groups):
                         for _ in range(group):
                             pos = [slice(None)] * (i)
-                            new_view[(*pos, new_j + 1, ...)] += _to_view(
-                                reduced_view[(*pos, j, ...)]
-                            )
+                            if new_view.dtype.names:
+                                for field in new_view.dtype.names:
+                                    new_view[(*pos, new_j + new_j_base, ...)][
+                                        field
+                                    ] += reduced_view[(*pos, j, ...)][field]
+                            else:
+                                new_view[(*pos, new_j + new_j_base, ...)] += (
+                                    reduced_view[(*pos, j, ...)]
+                                )
                             j += 1
 
                     reduced = new_reduced

src/boost_histogram/tag.py

@@ -6,6 +6,8 @@
 from builtins import sum
 from typing import TYPE_CHECKING, Sequence, TypeVar
 
+import numpy as np
+
 if TYPE_CHECKING:
     from uhi.typing.plottable import PlottableAxis
 
@@ -112,26 +114,43 @@ def __call__(self, axis: AxisLike) -> int:  # noqa: ARG002
 
 class rebin:
     __slots__ = (
+        "axis",
+        "edges",
         "factor",
         "groups",
     )
 
     def __init__(
         self,
-        factor: int | None = None,
+        factor_or_axis: int | PlottableAxis | None = None,
+        /,
         *,
+        factor: int | None = None,
         groups: Sequence[int] | None = None,
+        edges: Sequence[int | float] | None = None,
+        axis: PlottableAxis | None = None,
     ) -> None:
-        if not sum(i is None for i in [factor, groups]) == 1:
-            raise ValueError("Exactly one, a factor or groups should be provided")
-        self.factor = factor
+        if (
+            sum(i is not None for i in [factor_or_axis, factor, groups, edges, axis])
+            != 1
+        ):
+            raise ValueError("Exactly one argument should be provided")
         self.groups = groups
+        self.edges = edges
+        self.axis = axis
+        self.factor = factor
+        if isinstance(factor_or_axis, int):
+            self.factor = factor_or_axis
+        elif factor_or_axis is not None:
+            self.axis = factor_or_axis
 
     def __repr__(self) -> str:
         repr_str = f"{self.__class__.__name__}"
-        args: dict[str, int | Sequence[int] | None] = {
+        args: dict[str, int | Sequence[int | float] | PlottableAxis | None] = {
             "factor": self.factor,
             "groups": self.groups,
+            "edges": self.edges,
+            "axis": self.axis,
         }
         for k, v in args.items():
             if v is not None:
@@ -147,4 +166,30 @@ def group_mapping(self, axis: PlottableAxis) -> Sequence[int]:
             return self.groups
         if self.factor is not None:
             return [self.factor] * len(axis)
+        if self.edges is not None or self.axis is not None:
+            newedges = None
+            if self.axis is not None and hasattr(self.axis, "edges"):
+                newedges = self.axis.edges
+            elif self.edges is not None:
+                newedges = self.edges
+
+            if newedges is not None and hasattr(axis, "edges"):
+                assert newedges[0] == axis.edges[0], "Edges must start at first bin"
+                assert newedges[-1] == axis.edges[-1], "Edges must end at last bin"
+                assert all(
+                    np.isclose(
+                        axis.edges[np.abs(axis.edges - edge).argmin()],
+                        edge,
+                    )
+                    for edge in newedges
+                ), "Edges must be in the axis"
+                matched_ixes = np.where(
+                    np.isin(
+                        axis.edges,
+                        newedges,
+                    )
+                )[0]
+                return [
+                    int(ix - matched_ixes[i]) for i, ix in enumerate(matched_ixes[1:])
+                ]
         raise ValueError("No rebinning factor or groups provided")

tests/test_histogram.py

@@ -650,6 +650,33 @@ def test_rebin_1d(metadata):
     assert_array_equal(hs.axes.edges[0], [1.0, 1.2, 1.6, 2.2, 5.0])
     assert h.axes[0].metadata is hs.axes[0].metadata
 
+    hs = h[bh.rebin(edges=[1.0, 1.2, 1.6, 2.2, 5.0])]
+    assert_array_equal(hs.view(), [1, 0, 0, 3])
+    assert_array_equal(hs.axes.edges[0], [1.0, 1.2, 1.6, 2.2, 5.0])
+
+    hs = h[bh.rebin(axis=bh.axis.Variable([1.0, 1.2, 1.6, 2.2, 5.0]))]
+    assert_array_equal(hs.view(), [1, 0, 0, 3])
+    assert_array_equal(hs.axes.edges[0], [1.0, 1.2, 1.6, 2.2, 5.0])
+
+
+def test_rebin_1d_flow():
+    h = bh.Histogram(bh.axis.Regular(5, 0, 5, underflow=True, overflow=True))
+    h.fill([-1, 1.1, 2.2, 3.3, 4.4, 5.5])
+    hs = h[bh.rebin(edges=[0, 3, 5.0])]
+    assert_array_equal(hs.view(), [2, 2])
+    assert_array_equal(hs.view(flow=True), [1, 2, 2, 1])
+    assert_array_equal(hs.axes.edges[0], [0.0, 3.0, 5.0])
+
+    h = bh.Histogram(bh.axis.Regular(5, 0, 5, underflow=False, overflow=False))
+    h.fill([-1, 1.1, 2.2, 3.3, 4.4, 5.5])
+    hs = h[bh.rebin(edges=[0, 3, 5.0])]
+    assert_array_equal(hs.view(flow=True), [0, 2, 2, 0])
+
+    h = bh.Histogram(bh.axis.Regular(5, 0, 5, underflow=True, overflow=False))
+    h.fill([-1, 1.1, 2.2, 3.3, 4.4, 5.5])
+    hs = h[bh.rebin(edges=[0, 3, 5.0])]
+    assert_array_equal(hs.view(flow=True), [1, 2, 2, 0])
+
 
 def test_shrink_rebin_1d():
     h = bh.Histogram(bh.axis.Regular(20, 0, 4))