unvis: improve performance by reducing allocations

By using a buffer, we can avoid a bunch of small allocations that the
previous implementation did. Based on a few small benchmarks, the
performance improvement is very stark (~3x faster for strings that don't
require any escaping, and ~20% faster for multi-byte utf8 strings):

  goos: linux
  goarch: amd64
  pkg: github.com/vbatts/go-mtree/pkg/govis
  cpu: AMD Ryzen 7 7840U w/ Radeon  780M Graphics
                    │    before    │                after                │
                    │    sec/op    │   sec/op     vs base                │
  Unvis/NoChange-16   1501.0n ± 0%   497.7n ± 1%  -66.84% (p=0.000 n=10)
  Unvis/Binary-16     1317.5n ± 3%   934.9n ± 9%  -29.04% (p=0.000 n=10)
  Unvis/ASCII-16      1325.5n ± 1%   616.8n ± 1%  -53.47% (p=0.000 n=10)
  Unvis/German-16     1884.5n ± 1%   986.9n ± 2%  -47.63% (p=0.000 n=10)
  Unvis/Russian-16     4.636µ ± 1%   3.796µ ± 1%  -18.11% (p=0.000 n=10)
  Unvis/Japanese-16    3.453µ ± 1%   2.867µ ± 1%  -16.99% (p=0.000 n=10)
  geomean              2.072µ        1.206µ       -41.77%

Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>

pkg/govis/unvis_test.go

@@ -19,6 +19,7 @@
 package govis
 
 import (
+	"crypto/rand"
 	"strconv"
 	"testing"
 
@@ -172,3 +173,56 @@ func TestUnvisUnicode(t *testing.T) {
 		})
 	}
 }
+
+func BenchmarkUnvis(b *testing.B) {
+	doBench := func(b *testing.B, text string) {
+		encoded, err := Vis(text, DefaultVisFlags)
+		require.NoErrorf(b, err, "vis(%q)", text)
+
+		decoded, err := Unvis(encoded, DefaultVisFlags)
+		require.NoErrorf(b, err, "unvis(vis(%q) = %q)", text, encoded)
+		require.Equalf(b, text, decoded, "unvis(vis(%q) = %q)", text, encoded)
+
+		for b.Loop() {
+			_, _ = Unvis(encoded, DefaultVisFlags)
+		}
+	}
+
+	b.Run("NoChange", func(b *testing.B) {
+		text := "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789"
+		doBench(b, text)
+	})
+
+	b.Run("Binary", func(b *testing.B) {
+		var data [32]byte
+		n, err := rand.Read(data[:])
+		require.NoError(b, err, "rand.Read")
+		require.Equal(b, len(data), n, "rand.Read len return")
+
+		text := string(data[:])
+		doBench(b, text)
+	})
+
+	// The rest of these test strings come from a set of test strings collated
+	// in <https://www.w3.org/2001/06/utf-8-test/quickbrown.html>.
+
+	b.Run("ASCII", func(b *testing.B) {
+		text := "The quick brown fox jumps over the lazy dog."
+		doBench(b, text)
+	})
+
+	b.Run("German", func(b *testing.B) {
+		text := "Falsches Üben von Xylophonmusik quält jeden größeren Zwerg"
+		doBench(b, text)
+	})
+
+	b.Run("Russian", func(b *testing.B) {
+		text := "В чащах юга жил бы цитрус? Да, но фальшивый экземпляр!"
+		doBench(b, text)
+	})
+
+	b.Run("Japanese", func(b *testing.B) {
+		text := "いろはにほへとちりぬるをイロハニホヘトチリヌルヲ"
+		doBench(b, text)
+	})
+}