Fixed palette index for zeroed color in FASTOCTREE quantize

Tests/test_image_quantize.py

@@ -85,3 +85,20 @@ def test_transparent_colors_equal():
     converted = im.quantize()
     converted_px = converted.load()
     assert converted_px[0, 0] == converted_px[0, 1]
+
+
+@pytest.mark.parametrize(
+    "method, color",
+    (
+        (Image.MEDIANCUT, (0, 0, 0)),
+        (Image.MAXCOVERAGE, (0, 0, 0)),
+        (Image.FASTOCTREE, (0, 0, 0)),
+        (Image.FASTOCTREE, (0, 0, 0, 0)),
+    ),
+)
+def test_palette(method, color):
+    im = Image.new("RGBA" if len(color) == 4 else "RGB", (1, 1), color)
+
+    converted = im.quantize(method=method)
+    converted_px = converted.load()
+    assert converted_px[0, 0] == converted.palette.colors[color]

src/libImaging/QuantOctree.c

@@ -317,7 +317,7 @@ void
 add_lookup_buckets(ColorCube cube, ColorBucket palette, long nColors, long offset) {
     long i;
     Pixel p;
-    for (i = offset; i < offset + nColors; i++) {
+    for (i = offset + nColors - 1; i >= offset; i--) {
         avg_color_from_color_bucket(&palette[i], &p);
         set_lookup_value(cube, &p, i);
     }