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// 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 (
        "errors"
        "io"
)

// 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, PutUvarint will panic.
func PutUvarint(buf []byte, x uint64) int {
        i := 0
        for x >= 0x80 {
                buf[i] = byte(x) | 0x80
                x >>= 7
                i++
        }
        buf[i] = byte(x)
        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, PutVarint will panic.
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
}

var overflow = errors.New("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, 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")
}

// ReadVarint reads an encoded unsigned integer from r and returns it as a uint64.
func ReadVarint(r io.ByteReader) (int64, 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
}