int.go

// 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 scorch

import "fmt"

const (
	// 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
}