strings: add Builder

This is like a write-only subset of bytes.Buffer with an
allocation-free String method.

Fixes #18990.

Change-Id: Icdf7240f4309a52924dc3af04a39ecd737a210f4
Reviewed-on: https://go-review.googlesource.com/74931
Run-TryBot: Caleb Spare <cespare@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/strings/builder.go

+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package strings
+
+import (
+	"errors"
+	"io"
+	"unicode/utf8"
+	"unsafe"
+)
+
+// A Builder is used to efficiently build a string using Write methods.
+// It minimizes memory copying. The zero value is ready to use.
+type Builder struct {
+	buf []byte
+}
+
+// String returns the accumulated string.
+func (b *Builder) String() string {
+	return *(*string)(unsafe.Pointer(&b.buf))
+}
+
+// Len returns the number of accumulated bytes; b.Len() == len(b.String()).
+func (b *Builder) Len() int { return len(b.buf) }
+
+// Reset resets the Builder to be empty.
+func (b *Builder) Reset() { b.buf = nil }
+
+const maxInt = int(^uint(0) >> 1)
+
+// grow copies the buffer to a new, larger buffer so that there are at least n
+// bytes of capacity beyond len(b.buf).
+func (b *Builder) grow(n int) {
+	buf := make([]byte, len(b.buf), 2*cap(b.buf)+n)
+	copy(buf, b.buf)
+	b.buf = buf
+}
+
+// Grow grows b's capacity, if necessary, to guarantee space for
+// another n bytes. After Grow(n), at least n bytes can be written to b
+// without another allocation. If n is negative, Grow panics.
+func (b *Builder) Grow(n int) {
+	if n < 0 {
+		panic("strings.Builder.Grow: negative count")
+	}
+	if cap(b.buf)-len(b.buf) < n {
+		b.grow(n)
+	}
+}
+
+// Write appends the contents of p to b's buffer.
+// Write always returns len(p), nil.
+func (b *Builder) Write(p []byte) (int, error) {
+	b.buf = append(b.buf, p...)
+	return len(p), nil
+}
+
+// WriteByte appends the byte c to b's buffer.
+// The returned error is always nil.
+func (b *Builder) WriteByte(c byte) error {
+	b.buf = append(b.buf, c)
+	return nil
+}
+
+// WriteRune appends the UTF-8 encoding of Unicode code point r to b's buffer.
+// It returns the length of r and a nil error.
+func (b *Builder) WriteRune(r rune) (int, error) {
+	if r < utf8.RuneSelf {
+		b.buf = append(b.buf, byte(r))
+		return 1, nil
+	}
+	l := len(b.buf)
+	if cap(b.buf)-l < utf8.UTFMax {
+		b.grow(utf8.UTFMax)
+	}
+	n := utf8.EncodeRune(b.buf[l:l+utf8.UTFMax], r)
+	b.buf = b.buf[:l+n]
+	return n, nil
+}
+
+// WriteString appends the contents of s to b's buffer.
+// It returns the length of s and a nil error.
+func (b *Builder) WriteString(s string) (int, error) {
+	b.buf = append(b.buf, s...)
+	return len(s), nil
+}
+
+// minRead is the minimum slice passed to a Read call by Builder.ReadFrom.
+// It is the same as bytes.MinRead.
+const minRead = 512
+
+// errNegativeRead is the panic value if the reader passed to Builder.ReadFrom
+// returns a negative count.
+var errNegativeRead = errors.New("strings.Builder: reader returned negative count from Read")
+
+// ReadFrom reads data from r until EOF and appends it to b's buffer.
+// The return value n is the number of bytes read.
+// Any error except io.EOF encountered during the read is also returned.
+func (b *Builder) ReadFrom(r io.Reader) (n int64, err error) {
+	for {
+		l := len(b.buf)
+		if cap(b.buf)-l < minRead {
+			b.grow(minRead)
+		}
+		m, e := r.Read(b.buf[l:cap(b.buf)])
+		if m < 0 {
+			panic(errNegativeRead)
+		}
+		b.buf = b.buf[:l+m]
+		n += int64(m)
+		if e == io.EOF {
+			return n, nil
+		}
+		if e != nil {
+			return n, e
+		}
+	}
+}

src/strings/builder_test.go

+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package strings_test
+
+import (
+	"bytes"
+	"errors"
+	"io"
+	"runtime"
+	. "strings"
+	"testing"
+	"testing/iotest"
+)
+
+func check(t *testing.T, b *Builder, want string) {
+	t.Helper()
+	got := b.String()
+	if got != want {
+		t.Errorf("String: got %#q; want %#q", got, want)
+		return
+	}
+	if n := b.Len(); n != len(got) {
+		t.Errorf("Len: got %d; but len(String()) is %d", n, len(got))
+	}
+}
+
+func TestBuilder(t *testing.T) {
+	var b Builder
+	check(t, &b, "")
+	n, err := b.WriteString("hello")
+	if err != nil || n != 5 {
+		t.Errorf("WriteString: got %d,%s; want 5,nil", n, err)
+	}
+	check(t, &b, "hello")
+	if err = b.WriteByte(' '); err != nil {
+		t.Errorf("WriteByte: %s", err)
+	}
+	check(t, &b, "hello ")
+	n, err = b.WriteString("world")
+	if err != nil || n != 5 {
+		t.Errorf("WriteString: got %d,%s; want 5,nil", n, err)
+	}
+	check(t, &b, "hello world")
+}
+
+func TestBuilderString(t *testing.T) {
+	var b Builder
+	b.WriteString("alpha")
+	check(t, &b, "alpha")
+	s1 := b.String()
+	b.WriteString("beta")
+	check(t, &b, "alphabeta")
+	s2 := b.String()
+	b.WriteString("gamma")
+	check(t, &b, "alphabetagamma")
+	s3 := b.String()
+
+	// Check that subsequent operations didn't change the returned strings.
+	if want := "alpha"; s1 != want {
+		t.Errorf("first String result is now %q; want %q", s1, want)
+	}
+	if want := "alphabeta"; s2 != want {
+		t.Errorf("second String result is now %q; want %q", s2, want)
+	}
+	if want := "alphabetagamma"; s3 != want {
+		t.Errorf("third String result is now %q; want %q", s3, want)
+	}
+}
+
+func TestBuilderReset(t *testing.T) {
+	var b Builder
+	check(t, &b, "")
+	b.WriteString("aaa")
+	s := b.String()
+	check(t, &b, "aaa")
+	b.Reset()
+	check(t, &b, "")
+
+	// Ensure that writing after Reset doesn't alter
+	// previously returned strings.
+	b.WriteString("bbb")
+	check(t, &b, "bbb")
+	if want := "aaa"; s != want {
+		t.Errorf("previous String result changed after Reset: got %q; want %q", s, want)
+	}
+}
+
+func TestBuilderGrow(t *testing.T) {
+	for _, growLen := range []int{0, 100, 1000, 10000, 100000} {
+		var b Builder
+		b.Grow(growLen)
+		p := bytes.Repeat([]byte{'a'}, growLen)
+		allocs := numAllocs(func() { b.Write(p) })
+		if allocs > 0 {
+			t.Errorf("growLen=%d: allocation occurred during write", growLen)
+		}
+		if b.String() != string(p) {
+			t.Errorf("growLen=%d: bad data written after Grow", growLen)
+		}
+	}
+}
+
+func TestBuilderWrite2(t *testing.T) {
+	const s0 = "hello 世界"
+	for _, tt := range []struct {
+		name string
+		fn   func(b *Builder) (int, error)
+		n    int
+		want string
+	}{
+		{
+			"Write",
+			func(b *Builder) (int, error) { return b.Write([]byte(s0)) },
+			len(s0),
+			s0,
+		},
+		{
+			"WriteRune",
+			func(b *Builder) (int, error) { return b.WriteRune('a') },
+			1,
+			"a",
+		},
+		{
+			"WriteRuneWide",
+			func(b *Builder) (int, error) { return b.WriteRune('世') },
+			3,
+			"世",
+		},
+		{
+			"WriteString",
+			func(b *Builder) (int, error) { return b.WriteString(s0) },
+			len(s0),
+			s0,
+		},
+	} {
+		t.Run(tt.name, func(t *testing.T) {
+			var b Builder
+			n, err := tt.fn(&b)
+			if err != nil {
+				t.Fatalf("first call: got %s", err)
+			}
+			if n != tt.n {
+				t.Errorf("first call: got n=%d; want %d", n, tt.n)
+			}
+			check(t, &b, tt.want)
+
+			n, err = tt.fn(&b)
+			if err != nil {
+				t.Fatalf("second call: got %s", err)
+			}
+			if n != tt.n {
+				t.Errorf("second call: got n=%d; want %d", n, tt.n)
+			}
+			check(t, &b, tt.want+tt.want)
+		})
+	}
+}
+
+func TestBuilderWriteByte(t *testing.T) {
+	var b Builder
+	if err := b.WriteByte('a'); err != nil {
+		t.Error(err)
+	}
+	if err := b.WriteByte(0); err != nil {
+		t.Error(err)
+	}
+	check(t, &b, "a\x00")
+}
+
+func TestBuilderReadFrom(t *testing.T) {
+	for _, tt := range []struct {
+		name string
+		fn   func(io.Reader) io.Reader
+	}{
+		{"Reader", func(r io.Reader) io.Reader { return r }},
+		{"DataErrReader", iotest.DataErrReader},
+		{"OneByteReader", iotest.OneByteReader},
+	} {
+		t.Run(tt.name, func(t *testing.T) {
+			var b Builder
+
+			r := tt.fn(NewReader("hello"))
+			n, err := b.ReadFrom(r)
+			if err != nil {
+				t.Fatalf("first call: got %s", err)
+			}
+			if n != 5 {
+				t.Errorf("first call: got n=%d; want 5", n)
+			}
+			check(t, &b, "hello")
+
+			r = tt.fn(NewReader(" world"))
+			n, err = b.ReadFrom(r)
+			if err != nil {
+				t.Fatalf("first call: got %s", err)
+			}
+			if n != 6 {
+				t.Errorf("first call: got n=%d; want 6", n)
+			}
+			check(t, &b, "hello world")
+		})
+	}
+}
+
+var errRead = errors.New("boom")
+
+// errorReader sends reads to the underlying reader
+// but returns errRead instead of io.EOF.
+type errorReader struct {
+	r io.Reader
+}
+
+func (r errorReader) Read(b []byte) (int, error) {
+	n, err := r.r.Read(b)
+	if err == io.EOF {
+		err = errRead
+	}
+	return n, err
+}
+
+func TestBuilderReadFromError(t *testing.T) {
+	var b Builder
+	r := errorReader{NewReader("hello")}
+	n, err := b.ReadFrom(r)
+	if n != 5 {
+		t.Errorf("got n=%d; want 5", n)
+	}
+	if err != errRead {
+		t.Errorf("got err=%q; want %q", err, errRead)
+	}
+	check(t, &b, "hello")
+}
+
+type negativeReader struct{}
+
+func (r negativeReader) Read([]byte) (int, error) { return -1, nil }
+
+func TestBuilderReadFromNegativeReader(t *testing.T) {
+	var b Builder
+	defer func() {
+		switch err := recover().(type) {
+		case nil:
+			t.Fatal("ReadFrom didn't panic")
+		case error:
+			wantErr := "strings.Builder: reader returned negative count from Read"
+			if err.Error() != wantErr {
+				t.Fatalf("recovered panic: got %v; want %v", err.Error(), wantErr)
+			}
+		default:
+			t.Fatalf("unexpected panic value: %#v", err)
+		}
+	}()
+
+	b.ReadFrom(negativeReader{})
+}
+
+func TestBuilderAllocs(t *testing.T) {
+	var b Builder
+	b.Grow(5)
+	var s string
+	allocs := numAllocs(func() {
+		b.WriteString("hello")
+		s = b.String()
+	})
+	if want := "hello"; s != want {
+		t.Errorf("String: got %#q; want %#q", s, want)
+	}
+	if allocs > 0 {
+		t.Fatalf("got %d alloc(s); want 0", allocs)
+	}
+}
+
+func numAllocs(fn func()) uint64 {
+	defer runtime.GOMAXPROCS(runtime.GOMAXPROCS(1))
+	var m1, m2 runtime.MemStats
+	runtime.ReadMemStats(&m1)
+	fn()
+	runtime.ReadMemStats(&m2)
+	return m2.Mallocs - m1.Mallocs
+}

src/strings/example_test.go

@@ -351,3 +351,14 @@ func ExampleTrimRightFunc() {
 	}))
 	// Output: ¡¡¡Hello, Gophers
 }
+
+func ExampleBuilder() {
+	var b strings.Builder
+	for i := 3; i >= 1; i-- {
+		fmt.Fprintf(&b, "%d...", i)
+	}
+	b.WriteString("ignition")
+	fmt.Println(b.String())
+
+	// Output: 3...2...1...ignition
+}