Refine ForegroundFunc style option

Refine new style option `ForegroundFunc` by finialising the function signature.

Fix wrong counting logic for new lines.

Add test cases for new style.

convenience.go

@@ -147,29 +147,35 @@ func Foreground(color colorful.Color) StyleOption {
 }
 
 // ForegroundFunc uses the provided function to set an individual foreground
-// color for each part of the text, based on the position in the text. The
-// function is given a float in the range between 0 and 1, normalized using the
-// longest row/line in the string. Each row is handled separately.
-func ForegroundFunc(f func(float64) colorful.Color) StyleOption {
+// color for each part of the text, which can be based on the position (x, y)
+// or the content (rune) in the text.
+func ForegroundFunc(f func(int, int, rune) *colorful.Color) StyleOption {
 	return StyleOption{
 		postProcess: func(s *String, flags map[string]struct{}) {
-			var maxLineLength = float64(s.lineLength())
+			_, blendColors := flags["blendColors"]
 
 			var x, y int
 			for i, c := range *s {
 				if c.Symbol == '\n' {
 					x = 0
 					y++
+					continue
 				}
 
-				color := f(float64(x) / maxLineLength)
-				r, g, b := color.RGB255()
+				if color := f(x, y, c.Symbol); color != nil {
+					r, g, b := color.RGB255()
+					if blendColors {
+						if fgColor := ((*s)[i].Settings >> 8 & 0xFFFFFF); fgColor != 0 {
+							r, g, b = blend(r, g, b, fgColor)
+						}
+					}
 
-				(*s)[i].Settings &= 0xFFFFFFFF000000FF
-				(*s)[i].Settings |= 1
-				(*s)[i].Settings |= uint64(r) << 8
-				(*s)[i].Settings |= uint64(g) << 16
-				(*s)[i].Settings |= uint64(b) << 24
+					(*s)[i].Settings &= 0xFFFFFFFF000000FF
+					(*s)[i].Settings |= 1
+					(*s)[i].Settings |= uint64(r) << 8
+					(*s)[i].Settings |= uint64(g) << 16
+					(*s)[i].Settings |= uint64(b) << 24
+				}
 
 				x++
 			}

convenience_test.go

@@ -165,5 +165,54 @@ var _ = Describe("convenience functions", func() {
 				BeEquivalentTo("\x1b[38;2;255;0;0mtext with \x1b[38;2;145;174;136mcolored parts\x1b[38;2;255;0;0m.\x1b[0m"),
 			)
 		})
+
+		It("should set a conditional foreground color based on the respective content", func() {
+			Expect(Style(
+				"foobar\nfOObAr",
+				ForegroundFunc(func(_, _ int, r rune) *colorful.Color {
+					switch r {
+					case 'o':
+						return &Green
+
+					case 'a':
+						return &Red
+					}
+
+					return nil
+				})),
+			).To(Equal("f\x1b[38;2;0;128;0moo\x1b[0mb\x1b[38;2;255;0;0ma\x1b[0mr\nfOObAr"))
+		})
+
+		It("should set a conditional foreground color based on the position in the content", func() {
+			Expect(Style(
+				"foobar\nfOObAr",
+				ForegroundFunc(func(x, _ int, _ rune) *colorful.Color {
+					switch x {
+					case 1, 2:
+						return &Green
+
+					case 4:
+						return &Red
+					}
+
+					return nil
+				})),
+			).To(Equal("f\x1b[38;2;0;128;0moo\x1b[0mb\x1b[38;2;255;0;0ma\x1b[0mr\nf\x1b[38;2;0;128;0mOO\x1b[0mb\x1b[38;2;255;0;0mA\x1b[0mr"))
+		})
+
+		It("should set a conditional foreground color by blending it with the current color", func() {
+			Expect(Style(
+				Sprintf("Lime{foobar}"),
+				Blend(),
+				ForegroundFunc(func(_, _ int, r rune) *colorful.Color {
+					switch r {
+					case 'o':
+						return &Red
+					}
+
+					return nil
+				})),
+			).To(Equal("\x1b[38;2;0;255;0mf\x1b[38;2;145;174;136moo\x1b[38;2;0;255;0mbar\x1b[0m"))
+		})
 	})
 })

model.go

@@ -20,11 +20,6 @@
 
 package bunt
 
-import (
-	"strings"
-	"unicode/utf8"
-)
-
 // String is a string with color information
 type String []ColoredRune
 
@@ -50,25 +45,3 @@ type ColoredRune struct {
 func (s *String) Substring(from, to int) {
 	*s = (*s)[from:to]
 }
-
-func (s *String) rawString() string {
-	var runes = make([]rune, len(*s))
-	for i, c := range *s {
-		runes[i] = c.Symbol
-	}
-
-	return string(runes)
-}
-
-func (s *String) lineLength() int {
-	var lines = strings.Split(s.rawString(), "\n")
-	var max = 0
-
-	for _, line := range lines {
-		if len := utf8.RuneCountInString(line); len > max {
-			max = len
-		}
-	}
-
-	return max
-}