From beee6ecd59ddb9fb56192e95f809f828d4262306 Mon Sep 17 00:00:00 2001
From: hksamm <htetsan206@gmail.com>
Date: Sun, 26 Jul 2026 03:45:11 +0900
Subject: [PATCH] Fix ZeroDivisionError in LinearColormap.to_step() with a
 single step

Step colors are sampled at points spread evenly across the colormap using
the weight i / (n - 1.0), which is 0 / 0 when there is only one step.

n is recomputed as len(index) - 1, so this affects every path that yields a
two-element index -- to_step(n=1), an explicit two-element index=, and the
data= variants for the linear, log and quantiles methods -- not just the
to_step(n=1) case in the report.

A single bin spans the whole range, so sample the middle of the colormap
for it rather than either endpoint. The n >= 2 branch is left byte-for-byte
identical: index[i + 1] * i / (n - 1.0) parses as
(index[i + 1] * i) / (n - 1.0), so hoisting the weight into a variable
changes the floating-point result for many n.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
---
 branca/colormap.py     | 20 ++++++++++++++------
 tests/test_colormap.py | 19 +++++++++++++++++++
 2 files changed, 33 insertions(+), 6 deletions(-)

diff --git a/branca/colormap.py b/branca/colormap.py
index 2e76ba5..957e4c1 100644
--- a/branca/colormap.py
+++ b/branca/colormap.py
@@ -445,12 +445,20 @@ def to_step(
         if round_method == "log10":
             index = [_base(x) for x in index]
 
-        colors = [
-            scaled_cm.rgba_floats_tuple(
-                index[i] * (1.0 - i / (n - 1.0)) + index[i + 1] * i / (n - 1.0),
-            )
-            for i in range(n)
-        ]
+        if n == 1:
+            # Steps are normally sampled at points spread evenly across the
+            # colormap, the first at `index[0]` and the last at `index[-1]`.
+            # A single step is the degenerate case of that spread (0 / 0), so
+            # sample the middle instead: the one bin covers the whole range
+            # and gets a color to match.
+            colors = [scaled_cm.rgba_floats_tuple((index[0] + index[1]) / 2.0)]
+        else:
+            colors = [
+                scaled_cm.rgba_floats_tuple(
+                    index[i] * (1.0 - i / (n - 1.0)) + index[i + 1] * i / (n - 1.0),
+                )
+                for i in range(n)
+            ]
 
         caption = self.caption
         text_color = self.text_color
diff --git a/tests/test_colormap.py b/tests/test_colormap.py
index cd30405..eebb263 100644
--- a/tests/test_colormap.py
+++ b/tests/test_colormap.py
@@ -89,6 +89,25 @@ def test_linear_to_step():
     lc.to_step(data=some_list, quantiles=[0, 0.3, 0.7, 1], round_method="log10")
 
 
+@pytest.mark.parametrize(
+    "kwargs",
+    [
+        {"n": 1},
+        {"index": [0, 1]},
+        {"data": [0, 0.5, 1], "n": 1},
+        {"data": [0.5, 0.75, 1], "n": 1, "method": "log"},
+        {"data": [0, 0.5, 1], "quantiles": [0, 1]},
+    ],
+)
+def test_linear_to_step_single_step(kwargs):
+    """A one-bin colormap is valid; every path here raised ZeroDivisionError."""
+    linear = cm.LinearColormap(["red", "blue"], vmin=0, vmax=1)
+    step = linear.to_step(**kwargs)
+    assert len(step.colors) == 1
+    # The single bin spans the whole range, so it takes the middle color.
+    assert step.colors[0] == linear.rgba_floats_tuple(0.5)
+
+
 def test_step_to_linear():
     step = cm.StepColormap(
         ["green", "yellow", "red"],