first cut at a string buffer.

can be made more efficient but this is reasonable.

R=rsc
DELTA=363  (363 added, 0 deleted, 0 changed)
OCL=34720
CL=34720

src/pkg/strings/Makefile

@@ -6,6 +6,7 @@ include $(GOROOT)/src/Make.$(GOARCH)
 
 TARG=strings
 GOFILES=\
+	buffer.go\
 	strings.go\
 
 include $(GOROOT)/src/Make.pkg

src/pkg/strings/buffer.go

+// Copyright 2009 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 "os"
+
+// Efficient construction of large strings.
+// Implements io.Reader and io.Writer.
+
+// A Buffer is a variable-sized buffer of strings
+// with Read and Write methods.  Appends (writes) are efficient.
+// The zero value for Buffer is an empty buffer ready to use.
+type Buffer struct {
+	str	[]string;
+	len	int;
+	byteBuf	[1]byte;
+}
+
+// Copy from string to byte array at offset doff.  Assume there's room.
+func copy(dst []byte, doff int, src string) {
+	for soff := 0; soff < len(src); soff++ {
+		dst[doff] = src[soff];
+		doff++;
+	}
+}
+
+// Bytes returns the contents of the unread portion of the buffer
+// as a byte array.
+func (b *Buffer) Bytes() []byte {
+	n := b.len;
+	bytes := make([]byte, n);
+	nbytes := 0;
+	for _, s := range b.str {
+		copy(bytes, nbytes, s);
+		nbytes += len(s);
+	}
+	return bytes;
+}
+
+// String returns the contents of the unread portion of the buffer
+// as a string.
+func (b *Buffer) String() string {
+	if len(b.str) == 1 {	// important special case
+		return b.str[0]
+	}
+	return string(b.Bytes())
+}
+
+// Len returns the number of bytes in the unread portion of the buffer;
+// b.Len() == len(b.Bytes()) == len(b.String()).
+func (b *Buffer) Len() (n int) {
+	return b.len
+}
+
+// Truncate discards all but the first n unread bytes from the buffer.
+func (b *Buffer) Truncate(n int) {
+	b.len = 0;	// recompute during scan.
+	for i, s := range b.str {
+		if n <= 0 {
+			b.str = b.str[0:i];
+			break;
+		}
+		if n < len(s) {
+			b.str[i] = s[0:n];
+			b.len += n;
+			n = 0;
+		} else {
+			b.len += len(s);
+			n -= len(s);
+		}
+	}
+}
+
+// Reset resets the buffer so it has no content.
+// b.Reset() is the same as b.Truncate(0).
+func (b *Buffer) Reset() {
+	b.str = b.str[0:0];
+	b.len = 0;
+}
+
+// Can n bytes be appended efficiently to the end of the final string?
+func (b *Buffer) canCombine(n int) bool {
+	return len(b.str) > 0 && n+len(b.str[len(b.str)-1]) <= 64
+}
+
+// WriteString appends string s to the buffer.  The return
+// value n is the length of s; err is always nil.
+func (b *Buffer) WriteString(s string) (n int, err os.Error) {
+	n = len(s);
+	b.len += n;
+	numStr := len(b.str);
+	// Special case: If the last string is short and this one is short,
+	// combine them and avoid growing the list.
+	if b.canCombine(n) {
+		b.str[numStr-1] += s;
+		return
+	}
+	if cap(b.str) == numStr {
+		nstr := make([]string, numStr, 3*(numStr+10)/2);
+		for i, s := range b.str {
+			nstr[i] = s;
+		}
+		b.str = nstr;
+	}
+	b.str = b.str[0:numStr+1];
+	b.str[numStr] = s;
+	return
+}
+
+// Write appends the contents of p to the buffer.  The return
+// value n is the length of p; err is always nil.
+func (b *Buffer) Write(p []byte) (n int, err os.Error) {
+	return b.WriteString(string(p))
+}
+
+// WriteByte appends the byte c to the buffer.
+// The returned error is always nil, but is included
+// to match bufio.Writer's WriteByte.
+func (b *Buffer) WriteByte(c byte) os.Error {
+	s := string(c);
+	// For WriteByte, canCombine is almost always true so it's worth
+	// doing here.
+	if b.canCombine(1) {
+		b.str[len(b.str)-1] += s;
+		b.len++;
+		return nil
+	}
+	b.WriteString(s);
+	return nil;
+}
+
+// Read reads the next len(p) bytes from the buffer or until the buffer
+// is drained.  The return value n is the number of bytes read.  If the
+// buffer has no data to return, err is os.EOF even if len(p) is zero;
+// otherwise it is nil.
+func (b *Buffer) Read(p []byte) (n int, err os.Error) {
+	if len(b.str) == 0 {
+		return 0, os.EOF
+	}
+	for len(b.str) > 0 {
+		s := b.str[0];
+		m := len(p) - n;
+		if m >= len(s) {
+			// consume all of this string.
+			copy(p, n, s);
+			n += len(s);
+			b.str = b.str[1:len(b.str)];
+		} else {
+			// consume some of this string; it's the last piece.
+			copy(p, n, s[0:m]);
+			n += m;
+			b.str[0] = s[m:len(s)];
+			break;
+		}
+	}
+	b.len -= n;
+	return
+}
+
+// ReadByte reads and returns the next byte from the buffer.
+// If no byte is available, it returns error os.EOF.
+func (b *Buffer) ReadByte() (c byte, err os.Error) {
+	if _, err := b.Read(&b.byteBuf); err != nil {
+		return 0, err
+	}
+	return b.byteBuf[0], nil
+}
+
+// NewBuffer creates and initializes a new Buffer
+// using str as its initial contents.
+func NewBuffer(str string) *Buffer {
+	b := new(Buffer);
+	b.str = make([]string, 1, 10);	// room to grow
+	b.str[0] = str;
+	b.len = len(str);
+	return b;
+}

src/pkg/strings/buffer_test.go

+// Copyright 2009 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 (
+	. "strings";
+	"rand";
+	"testing";
+)
+
+
+const N = 10000  // make this bigger for a larger (and slower) test
+var data string  // test data for write tests
+
+
+func init() {
+	bytes := make([]byte, N);
+	for i := 0; i < N; i++ {
+		bytes[i] = 'a' + byte(i % 26)
+	}
+	data = string(bytes);
+}
+
+// Verify that contents of buf match the string s.
+func check(t *testing.T, testname string, buf *Buffer, s string) {
+	bytes := buf.Bytes();
+	str := buf.String();
+	if buf.Len() != len(bytes) {
+		t.Errorf("%s: buf.Len() == %d, len(buf.Bytes()) == %d\n", testname, buf.Len(), len(bytes))
+	}
+
+	if buf.Len() != len(str) {
+		t.Errorf("%s: buf.Len() == %d, len(buf.String()) == %d\n", testname, buf.Len(), len(str))
+	}
+
+	if buf.Len() != len(s) {
+		t.Errorf("%s: buf.Len() == %d, len(s) == %d\n", testname, buf.Len(), len(s))
+	}
+
+	if string(bytes) != s {
+		t.Errorf("%s: string(buf.Bytes()) == %q, s == %q\n", testname, string(bytes), s)
+	}
+}
+
+
+// Fill buf through n writes of fus.
+// The initial contents of buf corresponds to the string s;
+// the result is the final contents of buf returned as a string.
+func fill(t *testing.T, testname string, buf *Buffer, s string, n int, fus string) string {
+	check(t, testname + " (fill 1)", buf, s);
+	for ; n > 0; n-- {
+		m, err := buf.WriteString(fus);
+		if m != len(fus) {
+			t.Errorf(testname + " (fill 2): m == %d, expected %d\n", m, len(fus));
+		}
+		if err != nil {
+			t.Errorf(testname + " (fill 3): err should always be nil, found err == %s\n", err);
+		}
+		s += fus;
+		check(t, testname + " (fill 4)", buf, s);
+	}
+	return s;
+}
+
+
+func TestNewBuffer(t *testing.T) {
+	buf := NewBuffer(data);
+	check(t, "NewBuffer", buf, data);
+}
+
+
+// Empty buf through repeated reads into fub.
+// The initial contents of buf corresponds to the string s.
+func empty(t *testing.T, testname string, buf *Buffer, s string, fub []byte) {
+	check(t, testname + " (empty 1)", buf, s);
+
+	for {
+		n, err := buf.Read(fub);
+		if n == 0 {
+			break;
+		}
+		if err != nil {
+			t.Errorf(testname + " (empty 2): err should always be nil, found err == %s\n", err);
+		}
+		s = s[n : len(s)];
+		check(t, testname + " (empty 3)", buf, s);
+	}
+
+	check(t, testname + " (empty 4)", buf, "");
+}
+
+
+func TestBasicOperations(t *testing.T) {
+	var buf Buffer;
+
+	for i := 0; i < 5; i++ {
+		check(t, "TestBasicOperations (1)", &buf, "");
+
+		buf.Reset();
+		check(t, "TestBasicOperations (2)", &buf, "");
+
+		buf.Truncate(0);
+		check(t, "TestBasicOperations (3)", &buf, "");
+
+		n, err := buf.Write(Bytes(data[0 : 1]));
+		if n != 1 {
+			t.Errorf("wrote 1 byte, but n == %d\n", n);
+		}
+		if err != nil {
+			t.Errorf("err should always be nil, but err == %s\n", err);
+		}
+		check(t, "TestBasicOperations (4)", &buf, "a");
+
+		buf.WriteByte(data[1]);
+		check(t, "TestBasicOperations (5)", &buf, "ab");
+
+		n, err = buf.Write(Bytes(data[2 : 26]));
+		if n != 24 {
+			t.Errorf("wrote 25 bytes, but n == %d\n", n);
+		}
+		check(t, "TestBasicOperations (6)", &buf, string(data[0 : 26]));
+
+		buf.Truncate(26);
+		check(t, "TestBasicOperations (7)", &buf, string(data[0 : 26]));
+
+		buf.Truncate(20);
+		check(t, "TestBasicOperations (8)", &buf, string(data[0 : 20]));
+
+		empty(t, "TestBasicOperations (9)", &buf, string(data[0 : 20]), make([]byte, 5));
+		empty(t, "TestBasicOperations (10)", &buf, "", make([]byte, 100));
+
+		buf.WriteByte(data[1]);
+		c, err := buf.ReadByte();
+		if err != nil {
+			t.Errorf("ReadByte unexpected eof\n");
+		}
+		if c != data[1] {
+			t.Errorf("ReadByte wrong value c=%v\n", c);
+		}
+		c, err = buf.ReadByte();
+		if err == nil {
+			t.Errorf("ReadByte unexpected not eof\n");
+		}
+	}
+}
+
+
+func TestLargeWrites(t *testing.T) {
+	var buf Buffer;
+	for i := 3; i < 30; i += 3 {
+		s := fill(t, "TestLargeWrites (1)", &buf, "", 5, data);
+		empty(t, "TestLargeWrites (2)", &buf, s, make([]byte, len(data)/i));
+	}
+	check(t, "TestLargeWrites (3)", &buf, "");
+}
+
+
+func TestLargeReads(t *testing.T) {
+	var buf Buffer;
+	for i := 3; i < 30; i += 3 {
+		s := fill(t, "TestLargeReads (1)", &buf, "", 5, data[0 : len(data)/i]);
+		empty(t, "TestLargeReads (2)", &buf, s, make([]byte, len(data)));
+	}
+	check(t, "TestLargeReads (3)", &buf, "");
+}
+
+
+func TestMixedReadsAndWrites(t *testing.T) {
+	var buf Buffer;
+	s := "";
+	for i := 0; i < 50; i++ {
+		wlen := rand.Intn(len(data));
+		s = fill(t, "TestMixedReadsAndWrites (1)", &buf, s, 1, data[0 : wlen]);
+
+		rlen := rand.Intn(len(data));
+		fub := make([]byte, rlen);
+		n, _ := buf.Read(fub);
+		s = s[n : len(s)];
+	}
+	empty(t, "TestMixedReadsAndWrites (2)", &buf, s, make([]byte, buf.Len()));
+}