fix: DecodePaletteOrErr does not return an error when the number of colors is exceeded

DecodePaletteOrErr does not return an error when the number of colors is exceeded for images with RGB color mode. For indexed palette the behavior is correct.

Author: elgopher

internal/palette.go

@@ -15,7 +15,7 @@ type PaletteMaker[T ~uint32] struct { // generic type used instead of pi.RGB to
 	index            int
 }
 
-// Add returns true, when there is no space left
+// Add returns true, when the number of colors has been exceeded
 func (p *PaletteMaker[T]) Add(c color.Color) bool {
 	if p.colorAlreadyUsed == nil {
 		p.colorAlreadyUsed = map[color.Color]struct{}{}
@@ -24,6 +24,8 @@ func (p *PaletteMaker[T]) Add(c color.Color) bool {
 	_, found := p.colorAlreadyUsed[c]
 	if found {
 		return false
+	} else if p.index == maxColors {
+		return true
 	}
 
 	p.colorAlreadyUsed[c] = struct{}{}
@@ -36,7 +38,7 @@ func (p *PaletteMaker[T]) Add(c color.Color) bool {
 
 	p.index++
 
-	return p.index == maxColors
+	return false
 }
 
 func (p *PaletteMaker[T]) Palette() [maxColors]T {

internal/test/palette/indexed-64.png

@@ -125,6 +125,10 @@ func defaultPalette() PaletteArray {
 	}
 }
 
+var errToManyColors = fmt.Errorf(
+	"PNG file has too many colors in indexed palette. "+
+		"The maximum number is %d", MaxColors)
+
 // DecodePalette extracts a palette from a PNG file.
 //
 //   - If the file uses an indexed palette, colors are read directly
@@ -150,9 +154,7 @@ func DecodePaletteOrErr(pngFile []byte) (PaletteArray, error) {
 
 	if indexedPalette, ok := stdImage.ColorModel().(color.Palette); ok {
 		if len(indexedPalette) > MaxColors {
-			return PaletteArray{},
-				fmt.Errorf("PNG file has too many colors in indexed palette. "+
-					"The maximum number is %d", MaxColors)
+			return PaletteArray{}, errToManyColors
 		}
 		return convertIndexedPaletteToRGB(indexedPalette), nil
 	}
@@ -164,7 +166,7 @@ func DecodePaletteOrErr(pngFile []byte) (PaletteArray, error) {
 		for x := 0; x < bounds.Max.X; x++ {
 			c := stdImage.At(x, y)
 			if palette.Add(c) {
-				return palette.Palette(), nil
+				return PaletteArray{}, errToManyColors
 			}
 		}
 	}

internal/test/palette/indexed-65.png

@@ -0,0 +1,73 @@
+// Copyright 2025 Jacek Olszak
+// This code is licensed under MIT license (see LICENSE for details)
+
+package pi_test
+
+import (
+	_ "embed"
+	"github.com/elgopher/pi"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+	"testing"
+)
+
+var (
+	//go:embed internal/test/palette/indexed.png
+	indexedPalettePNG []byte
+	//go:embed internal/test/palette/rgb.png
+	rgbPalettePNG []byte
+
+	//go:embed internal/test/palette/indexed-65.png
+	indexed65PalettePNG []byte
+	//go:embed internal/test/palette/rgb-65.png
+	rgb65PalettePNG []byte
+
+	//go:embed internal/test/palette/indexed-64.png
+	indexed64PalettePNG []byte
+	//go:embed internal/test/palette/rgb-64.png
+	rgb64PalettePNG []byte
+)
+
+func TestDecodePaletteOrErr(t *testing.T) {
+	t.Run("should return error when palette has too many colors", func(t *testing.T) {
+		tests := map[string][]byte{
+			"indexed": indexed65PalettePNG,
+			"rgb":     rgb65PalettePNG,
+		}
+		for testName, file := range tests {
+			t.Run(testName, func(t *testing.T) {
+				palette, err := pi.DecodePaletteOrErr(file)
+				require.ErrorContains(t, err, "too many colors")
+				assert.Zero(t, palette)
+			})
+		}
+	})
+
+	t.Run("should not return error when palette has maximum number of colors", func(t *testing.T) {
+		tests := map[string][]byte{
+			"indexed": indexed64PalettePNG,
+			"rgb":     rgb64PalettePNG,
+		}
+		for testName, file := range tests {
+			t.Run(testName, func(t *testing.T) {
+				_, err := pi.DecodePaletteOrErr(file)
+				assert.NoError(t, err)
+			})
+		}
+	})
+
+	t.Run("should decode palette", func(t *testing.T) {
+		expected := pi.PaletteArray{0x000000, 0x1900ff, 0xff00b0, 0xff2600}
+		tests := map[string][]byte{
+			"indexed": indexedPalettePNG,
+			"rgb":     rgbPalettePNG,
+		}
+		for testName, file := range tests {
+			t.Run(testName, func(t *testing.T) {
+				palette, err := pi.DecodePaletteOrErr(file)
+				require.NoError(t, err)
+				assert.Equal(t, expected, palette)
+			})
+		}
+	})
+}