zix99 · zix99 · May 7, 2025 · May 7, 2025 · May 7, 2025
diff --git a/pkg/color/coloring.go b/pkg/color/coloring.go
@@ -163,33 +163,42 @@ func LookupColorByName(s string) (ColorCode, bool) {
 }
 
 // UnderlineSingleRune is a special-use-case colorer for headers with a single character called out
-func HighlightSingleRune(word string, runeIndex int, base, highlight ColorCode) string {
+func WriteHighlightSingleRune(w io.StringWriter, word string, runeIndex int, base, highlight ColorCode) {
 	if !Enabled {
-		return word
-	}
-
-	if runeIndex >= 0 && runeIndex < len(word) {
-		var sb strings.Builder
-		sb.Grow(len(word) * 2)
-
-		sb.WriteString(string(base))
-		idx := 0
-		for _, r := range word {
-			if idx == runeIndex {
-				sb.WriteString(string(highlight))
-				sb.WriteRune(r)
-				sb.WriteString(string(Reset + base))
-			} else {
-				sb.WriteRune(r)
-			}
-			idx++
-		}
-		sb.WriteString(string(Reset))
+		w.WriteString(word)
+		return
+	}
 
-		return sb.String()
+	w.WriteString(string(base))
+
+	offset, width := byteIndexOfRune(word, runeIndex)
+	if offset >= 0 {
+		w.WriteString(word[:offset])
+		w.WriteString(string(highlight))
+		w.WriteString(word[offset : offset+width])
+		w.WriteString(Reset)
+		w.WriteString(string(base))
+		w.WriteString(word[offset+width:])
+	} else {
+		w.WriteString(word)
 	}
 
-	return Wrap(base, word)
+	w.WriteString(Reset)
+}
+
+// returns byte index of a rune index within a string
+//
+// this is an optimization to allow finding a position of a rune in a string
+// and then accessing it directly using string slices, preventing an alloc
+func byteIndexOfRune(s string, runeIndex int) (offset, width int) {
+	rIdx := 0
+	for bIdx, r := range s {
+		if rIdx == runeIndex {
+			return bIdx, utf8.RuneLen(r)
+		}
+		rIdx++
+	}
+	return -1, -1
 }
 
 // StrLen ignoring any color codes. If color disabled, returns len(s)

diff --git a/pkg/color/coloring_test.go b/pkg/color/coloring_test.go
@@ -154,6 +154,12 @@ func TestLookupColor(t *testing.T) {
 }
 
 func TestHighlightSingleRune(t *testing.T) {
+	HighlightSingleRune := func(word string, runeIndex int, base, highlight ColorCode) string {
+		var sb strings.Builder
+		WriteHighlightSingleRune(&sb, word, runeIndex, base, highlight)
+		return sb.String()
+	}
+
 	assert.Equal(t, "\x1b[34;1m\x1b[0m", HighlightSingleRune("", 0, BrightBlue, Underline))
 	assert.Equal(t, "\x1b[34;1m\x1b[4m\x1b[36;1ma\x1b[0m\x1b[34;1mbc\x1b[0m", HighlightSingleRune("abc", 0, BrightBlue, Underline+BrightCyan))
 	assert.Equal(t, "\x1b[34;1ma\x1b[4m\x1b[36;1mb\x1b[0m\x1b[34;1mc\x1b[0m", HighlightSingleRune("abc", 1, BrightBlue, Underline+BrightCyan))
@@ -166,6 +172,16 @@ func TestHighlightSingleRune(t *testing.T) {
 	Enabled = true
 }
 
+// BenchmarkSingleRune-4   	11593182	        96.75 ns/op	       0 B/op	       0 allocs/op
+func BenchmarkSingleRune(b *testing.B) {
+	buf := &bytes.Buffer{}
+
+	for range b.N {
+		WriteHighlightSingleRune(buf, "cantalope", 1, Red, Green)
+		buf.Reset()
+	}
+}
+
 func TestStringLength(t *testing.T) {
 	assert.Equal(t, 4, StrLen("test"))
 	assert.Equal(t, 4, StrLen(Wrap(Red, "test")))

diff --git a/pkg/multiterm/termrenderers/heatmap.go b/pkg/multiterm/termrenderers/heatmap.go
@@ -1,6 +1,7 @@
 package termrenderers
 
 import (
+	"io"
 	"rare/pkg/aggregation"
 	"rare/pkg/aggregation/sorting"
 	"rare/pkg/color"
@@ -129,11 +130,11 @@ func (s *Heatmap) WriteHeader(colNames ...string) (colCount int) {
 				writeRepeat(&sb, delim, indent)
 				i += indent
 			}
-			sb.WriteString(underlineHeaderChar(name, colCount-i-1))
+			underlineHeaderChar(&sb, name, colCount-i-1)
 			break
 		}
 
-		sb.WriteString(underlineHeaderChar(name, 0))
+		underlineHeaderChar(&sb, name, 0)
 		i += nameLen
 	}
 
@@ -180,6 +181,6 @@ func writeRepeat(sb *strings.Builder, r rune, count int) {
 	}
 }
 
-func underlineHeaderChar(word string, letter int) string {
-	return color.HighlightSingleRune(word, letter, color.BrightBlue, color.Underline+color.BrightCyan)
+func underlineHeaderChar(w io.StringWriter, word string, letter int) {
+	color.WriteHighlightSingleRune(w, word, letter, color.BrightBlue, color.Underline+color.BrightCyan)
 }