// Copyright 2014 The go-ethereum Authors
// This file is part of the go-ethereum library.
//
// The go-ethereum library is free software: you can redistribute it and/or modify
// it under the terms of the GNU Lesser General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.
//
// The go-ethereum library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
// GNU Lesser General Public License for more details.
//
// You should have received a copy of the GNU Lesser General Public License
// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.
package trie
// Trie keys are dealt with in three distinct encodings:
//
// KEYBYTES encoding contains the actual key and nothing else. This encoding is the
// input to most API functions.
//
// HEX encoding contains one byte for each nibble of the key and an optional trailing
// 'terminator' byte of value 0x10 which indicates whether or not the node at the key
// contains a value. Hex key encoding is used for nodes loaded in memory because it's
// convenient to access.
//
// COMPACT encoding is defined by the Ethereum Yellow Paper (it's called "hex prefix
// encoding" there) and contains the bytes of the key and a flag. The high nibble of the
// first byte contains the flag; the lowest bit encoding the oddness of the length and
// the second-lowest encoding whether the node at the key is a value node. The low nibble
// of the first byte is zero in the case of an even number of nibbles and the first nibble
// in the case of an odd number. All remaining nibbles (now an even number) fit properly
// into the remaining bytes. Compact encoding is used for nodes stored on disk.
func hexToCompact(hex []byte) []byte {
terminator := byte(0)
if hasTerm(hex) {
terminator = 1
hex = hex[:len(hex)-1]
}
buf := make([]byte, len(hex)/2+1)
buf[0] = terminator << 5 // the flag byte
if len(hex)&1 == 1 {
buf[0] |= 1 << 4 // odd flag
buf[0] |= hex[0] // first nibble is contained in the first byte
hex = hex[1:]
}
decodeNibbles(hex, buf[1:])
return buf
}
func compactToHex(compact []byte) []byte {
if len(compact) == 0 {
return compact
}
base := keybytesToHex(compact)
// delete terminator flag
if base[0] < 2 {
base = base[:len(base)-1]
}
// apply odd flag
chop := 2 - base[0]&1
return base[chop:]
}
func keybytesToHex(str []byte) []byte {
l := len(str)*2 + 1
var nibbles = make([]byte, l)
for i, b := range str {
nibbles[i*2] = b / 16
nibbles[i*2+1] = b % 16
}
nibbles[l-1] = 16
return nibbles
}
// hexToKeybytes turns hex nibbles into key bytes.
// This can only be used for keys of even length.
func hexToKeybytes(hex []byte) []byte {
if hasTerm(hex) {
hex = hex[:len(hex)-1]
}
if len(hex)&1 != 0 {
panic("can't convert hex key of odd length")
}
key := make([]byte, len(hex)/2)
decodeNibbles(hex, key)
return key
}
func decodeNibbles(nibbles []byte, bytes []byte) {
for bi, ni := 0, 0; ni < len(nibbles); bi, ni = bi+1, ni+2 {
bytes[bi] = nibbles[ni]<<4 | nibbles[ni+1]
}
}
// prefixLen returns the length of the common prefix of a and b.
func prefixLen(a, b []byte) int {
var i, length = 0, len(a)
if len(b) < length {
length = len(b)
}
for ; i < length; i++ {
if a[i] != b[i] {
break
}
}
return i
}
// hasTerm returns whether a hex key has the terminator flag.
func hasTerm(s []byte) bool {
return len(s) > 0 && s[len(s)-1] == 16
}
