github.com/aigarnetwork/aigar@v0.0.0-20191115204914-d59a6eb70f8e/trie/encoding.go (about)

     1  //  Copyright 2018 The go-ethereum Authors
     2  //  Copyright 2019 The go-aigar Authors
     3  //  This file is part of the go-aigar library.
     4  //
     5  //  The go-aigar library is free software: you can redistribute it and/or modify
     6  //  it under the terms of the GNU Lesser General Public License as published by
     7  //  the Free Software Foundation, either version 3 of the License, or
     8  //  (at your option) any later version.
     9  //
    10  //  The go-aigar library is distributed in the hope that it will be useful,
    11  //  but WITHOUT ANY WARRANTY; without even the implied warranty of
    12  //  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
    13  //  GNU Lesser General Public License for more details.
    14  //
    15  //  You should have received a copy of the GNU Lesser General Public License
    16  //  along with the go-aigar library. If not, see <http://www.gnu.org/licenses/>.
    17  
    18  package trie
    19  
    20  // Trie keys are dealt with in three distinct encodings:
    21  //
    22  // KEYBYTES encoding contains the actual key and nothing else. This encoding is the
    23  // input to most API functions.
    24  //
    25  // HEX encoding contains one byte for each nibble of the key and an optional trailing
    26  // 'terminator' byte of value 0x10 which indicates whether or not the node at the key
    27  // contains a value. Hex key encoding is used for nodes loaded in memory because it's
    28  // convenient to access.
    29  //
    30  // COMPACT encoding is defined by the Ethereum Yellow Paper (it's called "hex prefix
    31  // encoding" there) and contains the bytes of the key and a flag. The high nibble of the
    32  // first byte contains the flag; the lowest bit encoding the oddness of the length and
    33  // the second-lowest encoding whether the node at the key is a value node. The low nibble
    34  // of the first byte is zero in the case of an even number of nibbles and the first nibble
    35  // in the case of an odd number. All remaining nibbles (now an even number) fit properly
    36  // into the remaining bytes. Compact encoding is used for nodes stored on disk.
    37  
    38  func hexToCompact(hex []byte) []byte {
    39  	terminator := byte(0)
    40  	if hasTerm(hex) {
    41  		terminator = 1
    42  		hex = hex[:len(hex)-1]
    43  	}
    44  	buf := make([]byte, len(hex)/2+1)
    45  	buf[0] = terminator << 5 // the flag byte
    46  	if len(hex)&1 == 1 {
    47  		buf[0] |= 1 << 4 // odd flag
    48  		buf[0] |= hex[0] // first nibble is contained in the first byte
    49  		hex = hex[1:]
    50  	}
    51  	decodeNibbles(hex, buf[1:])
    52  	return buf
    53  }
    54  
    55  func compactToHex(compact []byte) []byte {
    56  	if len(compact) == 0 {
    57  		return compact
    58  	}
    59  	base := keybytesToHex(compact)
    60  	// delete terminator flag
    61  	if base[0] < 2 {
    62  		base = base[:len(base)-1]
    63  	}
    64  	// apply odd flag
    65  	chop := 2 - base[0]&1
    66  	return base[chop:]
    67  }
    68  
    69  func keybytesToHex(str []byte) []byte {
    70  	l := len(str)*2 + 1
    71  	var nibbles = make([]byte, l)
    72  	for i, b := range str {
    73  		nibbles[i*2] = b / 16
    74  		nibbles[i*2+1] = b % 16
    75  	}
    76  	nibbles[l-1] = 16
    77  	return nibbles
    78  }
    79  
    80  // hexToKeybytes turns hex nibbles into key bytes.
    81  // This can only be used for keys of even length.
    82  func hexToKeybytes(hex []byte) []byte {
    83  	if hasTerm(hex) {
    84  		hex = hex[:len(hex)-1]
    85  	}
    86  	if len(hex)&1 != 0 {
    87  		panic("can't convert hex key of odd length")
    88  	}
    89  	key := make([]byte, len(hex)/2)
    90  	decodeNibbles(hex, key)
    91  	return key
    92  }
    93  
    94  func decodeNibbles(nibbles []byte, bytes []byte) {
    95  	for bi, ni := 0, 0; ni < len(nibbles); bi, ni = bi+1, ni+2 {
    96  		bytes[bi] = nibbles[ni]<<4 | nibbles[ni+1]
    97  	}
    98  }
    99  
   100  // prefixLen returns the length of the common prefix of a and b.
   101  func prefixLen(a, b []byte) int {
   102  	var i, length = 0, len(a)
   103  	if len(b) < length {
   104  		length = len(b)
   105  	}
   106  	for ; i < length; i++ {
   107  		if a[i] != b[i] {
   108  			break
   109  		}
   110  	}
   111  	return i
   112  }
   113  
   114  // hasTerm returns whether a hex key has the terminator flag.
   115  func hasTerm(s []byte) bool {
   116  	return len(s) > 0 && s[len(s)-1] == 16
   117  }