perf: remove allocations from getFirstRuneAsString (#578)

Author: clipperhouse

borders.go

@@ -2,6 +2,7 @@ package lipgloss
 
 import (
 	"strings"
+	"unicode/utf8"
 
 	"github.com/charmbracelet/x/ansi"
 	"github.com/muesli/termenv"
@@ -485,6 +486,6 @@ func getFirstRuneAsString(str string) string {
 	if str == "" {
 		return str
 	}
-	r := []rune(str)
-	return string(r[0])
+	_, size := utf8.DecodeRuneInString(str)
+	return str[:size]
 }

borders_test.go

@@ -1,6 +1,8 @@
 package lipgloss
 
-import "testing"
+import (
+	"testing"
+)
 
 func TestStyle_GetBorderSizes(t *testing.T) {
 	tests := []struct {
@@ -94,3 +96,80 @@ func TestStyle_GetBorderSizes(t *testing.T) {
 		})
 	}
 }
+
+// Old implementation using rune slice conversion
+func getFirstRuneAsStringOld(str string) string {
+	if str == "" {
+		return str
+	}
+	r := []rune(str)
+	return string(r[0])
+}
+
+func TestGetFirstRuneAsString(t *testing.T) {
+	tests := []struct {
+		name  string
+		input string
+		want  string
+	}{
+		{"Empty", "", ""},
+		{"SingleASCII", "A", "A"},
+		{"SingleUnicode", "世", "世"},
+		{"ASCIIString", "Hello", "H"},
+		{"UnicodeString", "你好世界", "你"},
+		{"MixedASCIIFirst", "Hello世界", "H"},
+		{"MixedUnicodeFirst", "世界Hello", "世"},
+		{"Emoji", "😀Happy", "😀"},
+		{"MultiByteFirst", "ñoño", "ñ"},
+		{"LongString", "The quick brown fox jumps over the lazy dog", "T"},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := getFirstRuneAsString(tt.input)
+			if got != tt.want {
+				t.Errorf("getFirstRuneAsString(%q) = %q, want %q", tt.input, got, tt.want)
+			}
+
+			// Verify new implementation matches old implementation
+			old := getFirstRuneAsStringOld(tt.input)
+			if got != old {
+				t.Errorf("getFirstRuneAsString(%q) = %q, but old implementation returns %q", tt.input, got, old)
+			}
+		})
+	}
+}
+
+func BenchmarkGetFirstRuneAsString(b *testing.B) {
+	testCases := []struct {
+		name string
+		str  string
+	}{
+		{"ASCII", "Hello, World!"},
+		{"Unicode", "你好世界"},
+		{"Single", "A"},
+		{"Empty", ""},
+	}
+
+	b.Run("Old", func(b *testing.B) {
+		for _, tc := range testCases {
+			b.Run(tc.name, func(b *testing.B) {
+				b.ReportAllocs()
+				for i := 0; i < b.N; i++ {
+					_ = getFirstRuneAsStringOld(tc.str)
+				}
+			})
+		}
+	})
+
+	b.Run("New", func(b *testing.B) {
+		for _, tc := range testCases {
+			b.Run(tc.name, func(b *testing.B) {
+				b.ReportAllocs()
+				for i := 0; i < b.N; i++ {
+					_ = getFirstRuneAsString(tc.str)
+				}
+			})
+		}
+	})
+}