encoding/binary: support for varint encoding
R=rsc, r, nigeltao, r, dsymonds
CC=golang-dev
https://golang.org/cl/5146048
diff --git a/src/pkg/encoding/binary/varint.go b/src/pkg/encoding/binary/varint.go
new file mode 100644
index 0000000..1439dd3
--- /dev/null
+++ b/src/pkg/encoding/binary/varint.go
@@ -0,0 +1,163 @@
+// Copyright 2011 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 binary
+
+// This file implements "varint" encoding of 64-bit integers.
+// The encoding is:
+// - unsigned integers are serialized 7 bits at a time, starting with the
+// least significant bits
+// - the most significant bit (msb) in each output byte indicates if there
+// is a continuation byte (msb = 1)
+// - signed integers are mapped to unsigned integers using "zig-zag"
+// encoding: Positive values x are written as 2*x + 0, negative values
+// are written as 2*(^x) + 1; that is, negative numbers are complemented
+// and whether to complement is encoded in bit 0.
+//
+// Design note:
+// At most 10 bytes are needed for 64-bit values. The encoding could
+// be more dense: a full 64-bit value needs an extra byte just to hold bit 63.
+// Instead, the msb of the previous byte could be used to hold bit 63 since we
+// know there can't be more than 64 bits. This is a trivial improvement and
+// would reduce the maximum encoding length to 9 bytes. However, it breaks the
+// invariant that the msb is always the "continuation bit" and thus makes the
+// format incompatible with a varint encoding for larger numbers (say 128-bit).
+
+import (
+ "io"
+ "os"
+)
+
+// MaxVarintLenN is the maximum length of a varint-encoded N-bit integer.
+const (
+ MaxVarintLen16 = 3
+ MaxVarintLen32 = 5
+ MaxVarintLen64 = 10
+)
+
+// PutUvarint encodes a uint64 into buf and returns the number of bytes written.
+// If the buffer is too small, the result is the negated number of bytes required
+// (that is, -PutUvarint(nil, x) is the number of bytes required to encode x).
+func PutUvarint(buf []byte, x uint64) int {
+ var i int
+ for i = range buf {
+ if x < 0x80 {
+ buf[i] = byte(x)
+ return i + 1
+ }
+ buf[i] = byte(x) | 0x80
+ x >>= 7
+ }
+ // buffer too small; compute number of bytes required
+ for x >= 0x4000 {
+ x >>= 2 * 7
+ i += 2
+ }
+ if x >= 0x80 {
+ i++
+ }
+ return -(i + 1)
+}
+
+// Uvarint decodes a uint64 from buf and returns that value and the
+// number of bytes read (> 0). If an error occurred, the value is 0
+// and the number of bytes n is <= 0 meaning:
+//
+// n == 0: buf too small
+// n < 0: value larger than 64 bits (overflow)
+// and -n is the number of bytes read
+//
+func Uvarint(buf []byte) (uint64, int) {
+ var x uint64
+ var s uint
+ for i, b := range buf {
+ if b < 0x80 {
+ if i > 9 || i == 9 && b > 1 {
+ return 0, -(i + 1) // overflow
+ }
+ return x | uint64(b)<<s, i + 1
+ }
+ x |= uint64(b&0x7f) << s
+ s += 7
+ }
+ return 0, 0
+}
+
+// PutVarint encodes an int64 into buf and returns the number of bytes written.
+// If the buffer is too small, the result is the negated number of bytes required
+// (that is, -PutVarint(nil, x) is the number of bytes required to encode x).
+func PutVarint(buf []byte, x int64) int {
+ ux := uint64(x) << 1
+ if x < 0 {
+ ux = ^ux
+ }
+ return PutUvarint(buf, ux)
+}
+
+// Varint decodes an int64 from buf and returns that value and the
+// number of bytes read (> 0). If an error occurred, the value is 0
+// and the number of bytes n is <= 0 with the following meaning:
+//
+// n == 0: buf too small
+// n < 0: value larger than 64 bits (overflow)
+// and -n is the number of bytes read
+//
+func Varint(buf []byte) (int64, int) {
+ ux, n := Uvarint(buf) // ok to continue in presence of error
+ x := int64(ux >> 1)
+ if ux&1 != 0 {
+ x = ^x
+ }
+ return x, n
+}
+
+// WriteUvarint encodes x and writes the result to w.
+func WriteUvarint(w io.Writer, x uint64) os.Error {
+ var buf [MaxVarintLen64]byte
+ n := PutUvarint(buf[:], x) // won't fail
+ _, err := w.Write(buf[0:n])
+ return err
+}
+
+var overflow = os.NewError("binary: varint overflows a 64-bit integer")
+
+// ReadUvarint reads an encoded unsigned integer from r and returns it as a uint64.
+func ReadUvarint(r io.ByteReader) (uint64, os.Error) {
+ var x uint64
+ var s uint
+ for i := 0; ; i++ {
+ b, err := r.ReadByte()
+ if err != nil {
+ return x, err
+ }
+ if b < 0x80 {
+ if i > 9 || i == 9 && b > 1 {
+ return x, overflow
+ }
+ return x | uint64(b)<<s, nil
+ }
+ x |= uint64(b&0x7f) << s
+ s += 7
+ }
+ panic("unreachable")
+}
+
+// WriteVarint encodes x and writes the result to w.
+func WriteVarint(w io.Writer, x int64) os.Error {
+ ux := uint64(x) << 1
+ if x < 0 {
+ ux = ^ux
+ }
+ return WriteUvarint(w, ux)
+}
+
+// ReadVarint reads an encoded unsigned integer from r and returns it as a uint64.
+func ReadVarint(r io.ByteReader) (int64, os.Error) {
+ ux, err := ReadUvarint(r) // ok to continue in presence of error
+ x := int64(ux >> 1)
+ if ux&1 != 0 {
+ x = ^x
+ }
+ return x, err
+}
