fix: Sheet.Print does not print text

pifont.Sheet.Print does not print text when the current color is equal to Sheet.BgColor

Author: elgopher

pifont/internal/test/font.png

@@ -17,7 +17,7 @@ type Sheet struct {
 	BgColor pi.Color // background color on sprites
 }
 
-var intermediateCanvas pi.Canvas // text is first rendered here to change its color
+var intermediateCanvas pi.Canvas // text is first rendered here to change its color from FgColor to selected color
 
 var prevFgColorTable [pi.MaxColors]pi.Color
 var prevBgColorTable [pi.MaxColors]pi.Color
@@ -33,27 +33,41 @@ func (s Sheet) Print(str string, x, y int) (currentX, currentY int) {
 
 	currentColor := pi.GetColor()
 
-	copy(prevFgColorTable[:], pi.ColorTables[0][s.FgColor][:])
-	copy(prevBgColorTable[:], pi.ColorTables[0][s.BgColor][:])
+	prevFgColorTable = pi.ColorTables[0][s.FgColor]
+	prevBgColorTable = pi.ColorTables[0][s.BgColor]
+
+	// create fake bg color to avoid a situation when fg and bg colors are the same
+	bgColor := (currentColor + 1) % pi.MaxColors
 	intermediateCanvas.Clear(s.BgColor)
 	pi.Pal(s.FgColor, currentColor)
-	pi.Palt(s.BgColor, true)
+	pi.Pal(s.BgColor, bgColor)
 	pi.SetDrawTarget(intermediateCanvas)
 
 	// first draw text in selected color on intermediateCanvas
 	currentX, currentY = s.PrintOriginal(str, x, y)
 
-	copy(pi.ColorTables[0][s.FgColor][:], prevFgColorTable[:])
+	// revert color tables
+	pi.ColorTables[0][s.FgColor] = prevFgColorTable
+	pi.ColorTables[0][s.BgColor] = prevBgColorTable
+
+	// make bgColor transparent
+	prevBgColorTable = pi.ColorTables[0][bgColor]
+	pi.Palt(bgColor, true)
 
 	// now copy text in target color on original draw target
 	coloredText := pi.Sprite{
-		Area:   pi.Area[int]{X: x - pi.Camera.X, Y: y - pi.Camera.Y, W: currentX - x, H: currentY - y + s.Height},
+		Area: pi.Area[int]{
+			X: x - pi.Camera.X,
+			Y: y - pi.Camera.Y,
+			W: currentX - x,
+			H: currentY - y + s.Height,
+		},
 		Source: intermediateCanvas,
 	}
 	pi.SetDrawTarget(originalDrawTarget)
 	pi.Spr(coloredText, x, y)
 
-	copy(pi.ColorTables[0][s.BgColor][:], prevBgColorTable[:])
+	pi.ColorTables[0][bgColor] = prevBgColorTable
 
 	return
 }

pifont/internal/test/text-color-different.png

@@ -4,12 +4,103 @@
 package pifont_test
 
 import (
+	_ "embed"
+	"github.com/elgopher/pi/pitest"
 	"testing"
 
 	"github.com/elgopher/pi"
 	"github.com/elgopher/pi/pifont"
 )
 
+//go:embed internal/test/font.png
+var fontPNG []byte
+
+var fontSheet pifont.Sheet
+
+func init() {
+	prevPalette := pi.Palette
+	defer func() {
+		pi.Palette = prevPalette
+	}()
+
+	pi.Palette = pi.DecodePalette(fontPNG)
+	fontCanvas := pi.DecodeCanvas(fontPNG)
+	fontSheet = pifont.Sheet{
+		Chars: map[rune]pi.Sprite{
+			'S': {
+				Area:   pi.Area[int]{X: 0, Y: 0, W: 8, H: 8},
+				Source: fontCanvas,
+			},
+			'T': {
+				Area:   pi.Area[int]{X: 8, Y: 0, W: 8, H: 8},
+				Source: fontCanvas,
+			},
+			'⬤': {
+				Area:   pi.Area[int]{X: 0, Y: 8, W: 8, H: 8},
+				Source: fontCanvas,
+			},
+			'❤': {
+				Area:   pi.Area[int]{X: 8, Y: 8, W: 8, H: 8},
+				Source: fontCanvas,
+			},
+		},
+		Height:  8,
+		FgColor: 1,
+		BgColor: 0,
+	}
+}
+
+var (
+	//go:embed internal/test/text-color-equal-to-bg.png
+	textColorEqualToBg []byte
+	//go:embed internal/test/text-color-equal-to-fg.png
+	textColorEqualToFg []byte
+	//go:embed internal/test/text-color-different.png
+	textColorDifferent []byte
+)
+
+func TestSheet_Print(t *testing.T) {
+	t.Run("should print with different colors", func(t *testing.T) {
+		tests := map[string]struct {
+			bgColor   pi.Color
+			textColor pi.Color
+			png       []byte
+		}{
+			"text color different than Bg and Fg": {
+				bgColor:   0,
+				textColor: 2,
+				png:       textColorDifferent,
+			},
+			"text color equal to Bg": {
+				bgColor:   2,
+				textColor: 0,
+				png:       textColorEqualToBg,
+			},
+			"text color equal to Fg": {
+				bgColor:   0,
+				textColor: 1,
+				png:       textColorEqualToFg,
+			},
+		}
+
+		for testName, testCase := range tests {
+			t.Run(testName, func(t *testing.T) {
+				pi.Palette = pi.DecodePalette(testCase.png)
+				expectedCanvas := pi.DecodeCanvas(testCase.png)
+				pi.SetScreenSize(8, 8)
+
+				pi.Screen().Clear(testCase.bgColor)
+				pi.SetColor(testCase.textColor)
+				pi.Palt(testCase.textColor, false)
+				// when
+				fontSheet.Print("S", 0, 0)
+				// then
+				pitest.AssertSurfaceEqual(t, expectedCanvas, pi.Screen())
+			})
+		}
+	})
+}
+
 func BenchmarkSheet_Print(b *testing.B) {
 	sheet := pifont.Sheet{
 		Chars: map[rune]pi.Sprite{