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path.go
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path.go
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// Copyright (C) 2019-2022, Ava Labs, Inc. All rights reserved.
// See the file LICENSE for licensing terms.
package merkledb
import (
"bytes"
"reflect"
"strings"
"unsafe"
)
const EmptyPath path = ""
// SerializedPath contains a path from the trie.
// The trie branch factor is 16, so the path may contain an odd number of nibbles.
// If it did contain an odd number of nibbles, the last 4 bits of the last byte should be discarded.
type SerializedPath struct {
NibbleLength int
Value []byte
}
func (s SerializedPath) hasOddLength() bool {
return s.NibbleLength&1 == 1
}
func (s SerializedPath) Equal(other SerializedPath) bool {
return s.NibbleLength == other.NibbleLength && bytes.Equal(s.Value, other.Value)
}
func (s SerializedPath) deserialize() path {
result := newPath(s.Value)
// trim the last nibble if the path has an odd length
return result[:len(result)-s.NibbleLength&1]
}
// Returns true iff [prefix] is a prefix of [s] or equal to it.
func (s SerializedPath) HasPrefix(prefix SerializedPath) bool {
if len(s.Value) < len(prefix.Value) {
return false
}
prefixValue := prefix.Value
if !prefix.hasOddLength() {
return bytes.HasPrefix(s.Value, prefixValue)
}
reducedSize := len(prefixValue) - 1
// grab the last nibble in the prefix and serialized path
prefixRemainder := prefixValue[reducedSize] >> 4
valueRemainder := s.Value[reducedSize] >> 4
prefixValue = prefixValue[:reducedSize]
return bytes.HasPrefix(s.Value, prefixValue) && valueRemainder == prefixRemainder
}
// Returns true iff [prefix] is a prefix of [s] but not equal to it.
func (s SerializedPath) HasStrictPrefix(prefix SerializedPath) bool {
return s.HasPrefix(prefix) && !s.Equal(prefix)
}
func (s SerializedPath) NibbleVal(nibbleIndex int) byte {
value := s.Value[nibbleIndex>>1]
isOdd := byte(nibbleIndex & 1)
isEven := (1 - isOdd)
// return value first(even index) or last 4(odd index) bits of the corresponding byte
return isEven*value>>4 + isOdd*(value&0x0F)
}
func (s SerializedPath) AppendNibble(nibble byte) SerializedPath {
// even is 1 if even, 0 if odd
even := 1 - s.NibbleLength&1
value := make([]byte, len(s.Value)+even)
copy(value, s.Value)
// shift the nibble 4 left if even, do nothing if odd
value[len(value)-1] += nibble << (4 * even)
return SerializedPath{Value: value, NibbleLength: s.NibbleLength + 1}
}
type path string
// Returns:
// * 0 if [p] == [other].
// * -1 if [p] < [other].
// * 1 if [p] > [other].
func (p path) Compare(other path) int {
return strings.Compare(string(p), string(other))
}
// Invariant: The returned value must not be modified.
func (p path) Bytes() []byte {
// avoid copying during the conversion
// "safe" because we never edit the value, only used as DB key
buf := *(*[]byte)(unsafe.Pointer(&p))
(*reflect.SliceHeader)(unsafe.Pointer(&buf)).Cap = len(p)
return buf
}
// Returns true iff [p] begins with [prefix].
func (p path) HasPrefix(prefix path) bool {
return strings.HasPrefix(string(p), string(prefix))
}
// Append [val] to [p].
func (p path) Append(val byte) path {
return p + path(val)
}
// Returns the serialized representation of [p].
func (p path) Serialize() SerializedPath {
// need half the number of bytes as nibbles
// add one so there is a byte for the odd nibble if it exists
// the extra nibble gets rounded down if even length
byteLength := (len(p) + 1) / 2
result := SerializedPath{
NibbleLength: len(p),
Value: make([]byte, byteLength),
}
// loop over the path's bytes
// if the length is odd, subtract 1 so we don't overflow on the p[pathIndex+1]
keyIndex := 0
lastIndex := len(p) - len(p)&1
for pathIndex := 0; pathIndex < lastIndex; pathIndex += 2 {
result.Value[keyIndex] = p[pathIndex]<<4 + p[pathIndex+1]
keyIndex++
}
// if there is was a odd number of nibbles, grab the last one
if result.hasOddLength() {
result.Value[keyIndex] = p[keyIndex<<1] << 4
}
return result
}
func newPath(p []byte) path {
// create new buffer with double the length of the input since each byte gets split into two nibbles
buffer := make([]byte, 2*len(p))
// first nibble gets shifted right 4 (divided by 16) to isolate the first nibble
// second nibble gets bitwise anded with 0x0F (1111) to isolate the second nibble
bufferIndex := 0
for _, currentByte := range p {
buffer[bufferIndex] = currentByte >> 4
buffer[bufferIndex+1] = currentByte & 0x0F
bufferIndex += 2
}
// avoid copying during the conversion
return *(*path)(unsafe.Pointer(&buffer))
}