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// Copyright (C) 2019-2024 Algorand, Inc.
// This file is part of go-algorand
//
// go-algorand is free software: you can redistribute it and/or modify
// it under the terms of the GNU Affero General Public License as
// published by the Free Software Foundation, either version 3 of the
// License, or (at your option) any later version.
//
// go-algorand 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 Affero General Public License for more details.
//
// You should have received a copy of the GNU Affero General Public License
// along with go-algorand. If not, see <https://www.gnu.org/licenses/>.
package nibbles
import (
"bytes"
"errors"
)
// Nibbles are 4-bit values stored in an 8-bit byte arrays
type Nibbles []byte
const (
// oddIndicator for serialization when the last nibble in a byte array
// is not part of the nibble array.
oddIndicator = 0x01
// evenIndicator for when it is.
evenIndicator = 0x03
)
// Pack the nibble array into a byte array.
// Return the byte array and a bool indicating if the last byte is a full byte or
// only the high 4 bits are part of the encoding
// the last four bits of a oddLength byte encoding will always be zero.
// Allocates a new byte slice.
//
// [0x1, 0x2, 0x3] -> [0x12, 0x30], true
// [0x1, 0x2, 0x3, 0x4] -> [0x12, 0x34], false
// [0x1] -> [0x10], true
// [] -> [], false
func Pack(nyb Nibbles) ([]byte, bool) {
length := len(nyb)
data := make([]byte, length/2+length%2, length/2+length%2+1)
for i := 0; i < length; i++ {
if i%2 == 0 {
data[i/2] = nyb[i] << 4
} else {
data[i/2] = data[i/2] | nyb[i]
}
}
return data, length%2 != 0
}
// Equal returns true if the two nibble arrays are equal
// [0x1, 0x2, 0x3], [0x1, 0x2, 0x3] -> true
// [0x1, 0x2, 0x3], [0x1, 0x2, 0x4] -> false
// [0x1, 0x2, 0x3], [0x1] -> false
// [0x1, 0x2, 0x3], [0x1, 0x2, 0x3, 0x4] -> false
// [], [] -> true
// [], [0x1] -> false
func Equal(nyb1 Nibbles, nyb2 Nibbles) bool {
return bytes.Equal(nyb1, nyb2)
}
// ShiftLeft returns a slice of nyb1 that contains the Nibbles after the first
// numNibbles
func ShiftLeft(nyb1 Nibbles, numNibbles int) Nibbles {
if numNibbles <= 0 {
return nyb1
}
if numNibbles > len(nyb1) {
return nyb1[:0]
}
return nyb1[numNibbles:]
}
// SharedPrefix returns a slice from nyb1 that contains the shared prefix
// between nyb1 and nyb2
func SharedPrefix(nyb1 Nibbles, nyb2 Nibbles) Nibbles {
minLength := len(nyb1)
if len(nyb2) < minLength {
minLength = len(nyb2)
}
for i := 0; i < minLength; i++ {
if nyb1[i] != nyb2[i] {
return nyb1[:i]
}
}
return nyb1[:minLength]
}
// Serialize returns a byte array that represents the Nibbles
// an empty nibble array is serialized as a single byte with value 0x3
// as the empty nibble is considered to be full width
//
// [0x1, 0x2, 0x3] -> [0x12, 0x30, 0x01]
// [0x1, 0x2, 0x3, 0x4] -> [0x12, 0x34, 0x03]
// [] -> [0x03]
func Serialize(nyb Nibbles) (data []byte) {
p, h := Pack(nyb)
if h {
// 0x01 is the odd length indicator
return append(p, oddIndicator)
}
// 0x03 is the even length indicator
return append(p, evenIndicator)
}
// Deserialize returns a nibble array from the byte array.
func Deserialize(encoding []byte) (Nibbles, error) {
var ns Nibbles
length := len(encoding)
if length == 0 {
return nil, errors.New("invalid encoding")
}
if encoding[length-1] == oddIndicator {
if length == 1 {
return nil, errors.New("invalid encoding")
}
ns = makeNibbles(encoding[:length-1], true)
} else if encoding[length-1] == evenIndicator {
ns = makeNibbles(encoding[:length-1], false)
} else {
return nil, errors.New("invalid encoding")
}
return ns, nil
}
// makeNibbles returns a nibble array from the byte array. If oddLength is true,
// the last 4 bits of the last byte of the array are ignored.
//
// [0x12, 0x30], true -> [0x1, 0x2, 0x3]
// [0x12, 0x34], false -> [0x1, 0x2, 0x3, 0x4]
// [0x12, 0x34], true -> [0x1, 0x2, 0x3] <-- last byte last 4 bits ignored
// [], false -> []
// never to be called with [], true
// Allocates a new byte slice.
func makeNibbles(data []byte, oddLength bool) Nibbles {
length := len(data) * 2
if oddLength {
length = length - 1
}
ns := make([]byte, length)
j := 0
for i := 0; i < length; i++ {
if i%2 == 0 {
ns[i] = data[j] >> 4
} else {
ns[i] = data[j] & 0x0f
j++
}
}
return ns
}
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