Add StyleRunes function for styling specific runes in a string

runes.go

@@ -0,0 +1,43 @@
+package lipgloss
+
+import (
+	"strings"
+)
+
+// StyleRunes applys a given style to runes at the given indicesin the string.
+// Note that you must provide styling options for both matched and unmatched
+// runes. Indices out of bounds will be ignored.
+func StyleRunes(str string, indices []int, matched, unmatched Style) string {
+	// Convert slice of indices to a map for easier lookups
+	m := make(map[int]struct{})
+	for _, i := range indices {
+		m[i] = struct{}{}
+	}
+
+	var (
+		out   strings.Builder
+		group strings.Builder
+		style Style
+		runes = []rune(str)
+	)
+
+	for i, r := range runes {
+		group.WriteRune(r)
+
+		_, matches := m[i]
+		_, nextMatches := m[i+1]
+
+		if matches != nextMatches || i == len(runes)-1 {
+			// Flush
+			if matches {
+				style = matched
+			} else {
+				style = unmatched
+			}
+			out.WriteString(style.Render(group.String()))
+			group.Reset()
+		}
+	}
+
+	return out.String()
+}

runes_test.go

@@ -0,0 +1,70 @@
+package lipgloss
+
+import (
+	"strings"
+	"testing"
+)
+
+func TestStyleRunes(t *testing.T) {
+	t.Parallel()
+
+	matchedStyle := NewStyle().Reverse(true)
+	unmatchedStyle := NewStyle()
+
+	tt := []struct {
+		input    string
+		indices  []int
+		expected string
+	}{
+		{
+			"hello",
+			[]int{0},
+			"\x1b[7mh\x1b[0mello",
+		},
+		{
+			"你好",
+			[]int{1},
+			"你\x1b[7m好\x1b[0m",
+		},
+		{
+			"hello 你好",
+			[]int{6, 7},
+			"hello \x1b[7m你好\x1b[0m",
+		},
+		{
+			"hello",
+			[]int{1, 3},
+			"h\x1b[7me\x1b[0ml\x1b[7ml\x1b[0mo",
+		},
+		{
+			"你好",
+			[]int{0, 1},
+			"\x1b[7m你好\x1b[0m",
+		},
+	}
+
+	fn := func(str string, indices []int) string {
+		return StyleRunes(str, indices, matchedStyle, unmatchedStyle)
+	}
+
+	for i, tc := range tt {
+		res := fn(tc.input, tc.indices)
+		if fn(tc.input, tc.indices) != tc.expected {
+			t.Errorf("Test %d, expected:\n\n`%s`\n`%s`\n\nActual Output:\n\n`%s`\n`%s`\n\n",
+				i, tc.expected, formatEscapes(tc.expected),
+				res, formatEscapes(res))
+		}
+	}
+}
+
+func formatEscapes(str string) string {
+	var b strings.Builder
+	for _, r := range str {
+		if r == '\x1b' {
+			b.WriteString("\\x1b")
+			continue
+		}
+		b.WriteRune(r)
+	}
+	return b.String()
+}