forked from blevesearch/bleve
/
int.go
176 lines (153 loc) · 4.96 KB
/
int.go
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// Copyright 2014 The Cockroach Authors.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or
// implied. See the License for the specific language governing
// permissions and limitations under the License.
// This code originated from:
// https://github.com/cockroachdb/cockroach/blob/2dd65dde5d90c157f4b93f92502ca1063b904e1d/pkg/util/encoding/encoding.go
// Modified to not use pkg/errors
package segment
import (
"errors"
"fmt"
)
const (
MaxVarintSize = 9
// IntMin is chosen such that the range of int tags does not overlap the
// ascii character set that is frequently used in testing.
IntMin = 0x80 // 128
intMaxWidth = 8
intZero = IntMin + intMaxWidth // 136
intSmall = IntMax - intZero - intMaxWidth // 109
// IntMax is the maximum int tag value.
IntMax = 0xfd // 253
)
// EncodeUvarintAscending encodes the uint64 value using a variable length
// (length-prefixed) representation. The length is encoded as a single
// byte indicating the number of encoded bytes (-8) to follow. See
// EncodeVarintAscending for rationale. The encoded bytes are appended to the
// supplied buffer and the final buffer is returned.
func EncodeUvarintAscending(b []byte, v uint64) []byte {
switch {
case v <= intSmall:
return append(b, intZero+byte(v))
case v <= 0xff:
return append(b, IntMax-7, byte(v))
case v <= 0xffff:
return append(b, IntMax-6, byte(v>>8), byte(v))
case v <= 0xffffff:
return append(b, IntMax-5, byte(v>>16), byte(v>>8), byte(v))
case v <= 0xffffffff:
return append(b, IntMax-4, byte(v>>24), byte(v>>16), byte(v>>8), byte(v))
case v <= 0xffffffffff:
return append(b, IntMax-3, byte(v>>32), byte(v>>24), byte(v>>16), byte(v>>8),
byte(v))
case v <= 0xffffffffffff:
return append(b, IntMax-2, byte(v>>40), byte(v>>32), byte(v>>24), byte(v>>16),
byte(v>>8), byte(v))
case v <= 0xffffffffffffff:
return append(b, IntMax-1, byte(v>>48), byte(v>>40), byte(v>>32), byte(v>>24),
byte(v>>16), byte(v>>8), byte(v))
default:
return append(b, IntMax, byte(v>>56), byte(v>>48), byte(v>>40), byte(v>>32),
byte(v>>24), byte(v>>16), byte(v>>8), byte(v))
}
}
// DecodeUvarintAscending decodes a varint encoded uint64 from the input
// buffer. The remainder of the input buffer and the decoded uint64
// are returned.
func DecodeUvarintAscending(b []byte) ([]byte, uint64, error) {
if len(b) == 0 {
return nil, 0, fmt.Errorf("insufficient bytes to decode uvarint value")
}
length := int(b[0]) - intZero
b = b[1:] // skip length byte
if length <= intSmall {
return b, uint64(length), nil
}
length -= intSmall
if length < 0 || length > 8 {
return nil, 0, fmt.Errorf("invalid uvarint length of %d", length)
} else if len(b) < length {
return nil, 0, fmt.Errorf("insufficient bytes to decode uvarint value: %q", b)
}
var v uint64
// It is faster to range over the elements in a slice than to index
// into the slice on each loop iteration.
for _, t := range b[:length] {
v = (v << 8) | uint64(t)
}
return b[length:], v, nil
}
// ------------------------------------------------------------
type MemUvarintReader struct {
C int // index of next byte to read from S
S []byte
}
func NewMemUvarintReader(s []byte) *MemUvarintReader {
return &MemUvarintReader{S: s}
}
// Len returns the number of unread bytes.
func (r *MemUvarintReader) Len() int {
n := len(r.S) - r.C
if n < 0 {
return 0
}
return n
}
var ErrMemUvarintReaderOverflow = errors.New("MemUvarintReader overflow")
// ReadUvarint reads an encoded uint64. The original code this was
// based on is at encoding/binary/ReadUvarint().
func (r *MemUvarintReader) ReadUvarint() (uint64, error) {
var x uint64
var s uint
var C = r.C
var S = r.S
for {
b := S[C]
C++
if b < 0x80 {
r.C = C
// why 63? The original code had an 'i += 1' loop var and
// checked for i > 9 || i == 9 ...; but, we no longer
// check for the i var, but instead check here for s,
// which is incremented by 7. So, 7*9 == 63.
//
// why the "extra" >= check? The normal case is that s <
// 63, so we check this single >= guard first so that we
// hit the normal, nil-error return pathway sooner.
if s >= 63 && (s > 63 || s == 63 && b > 1) {
return 0, ErrMemUvarintReaderOverflow
}
return x | uint64(b)<<s, nil
}
x |= uint64(b&0x7f) << s
s += 7
}
}
// SkipUvarint skips ahead one encoded uint64.
func (r *MemUvarintReader) SkipUvarint() {
for {
b := r.S[r.C]
r.C++
if b < 0x80 {
return
}
}
}
// SkipBytes skips a count number of bytes.
func (r *MemUvarintReader) SkipBytes(count int) {
r.C = r.C + count
}
func (r *MemUvarintReader) Reset(s []byte) {
r.C = 0
r.S = s
}