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111 lines
2.6 KiB
111 lines
2.6 KiB
// Package modhex implements modhex encoding and decoding.
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package modhex
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import (
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"errors"
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"fmt"
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)
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const modhexTable = "cbdefghijklnrtuv"
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var modhexMap map[byte]byte
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func init() {
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modhexMap = make(map[byte]byte)
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for i, v := range []byte(modhexTable) {
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modhexMap[v] = byte(i)
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}
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}
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// EncodedLen returns the length of an encoding of n source bytes.
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// Specifically, it returns n * 2.
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func EncodedLen(n int) int {
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return n * 2
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}
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// Encode encodes src into EncodedLen(len(src)) bytes of dst.
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// It returns the number of bytes written to dst, but this is
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// always EncodedLen(len(src)).
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// Encode implements modhex encoding.
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func Encode(dst, src []byte) int {
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i := 0
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for _, v := range src {
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dst[i] = modhexTable[v>>4]
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dst[i+1] = modhexTable[v&0x0f]
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i += 2
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}
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return len(src) * 2
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}
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// EncodeToString returns the modhex encoding of src.
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func EncodeToString(src []byte) string {
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// make a new slice to hold the encoded bytes
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dst := make([]byte, EncodedLen(len(src)))
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// encode it!
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Encode(dst, src)
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// return the encoded bytes as a string
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return string(dst)
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}
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// DecodedLen returns the length of a decoding of n source bytes.
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// Specifically, it returns n / 2.
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func DecodedLen(n int) int {
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return n / 2
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}
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// Decode decodes src into DecodedLen(len(src)) bytes, returning
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// the actual number of bytes written to dst.
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//
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// Decode expects that src contains only modhex characters and that
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// src has an even length.
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// If the input is malformed, Decode returns the number
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// of bytes decoded before the error.
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func Decode(dst, src []byte) (int, error) {
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i, j := 0, 1
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for ; j < len(src); j += 2 {
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a, ok := modhexMap[src[j-1]]
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if !ok {
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return i, fmt.Errorf("modhex: invalid byte: %#U", rune(src[j-1]))
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}
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b, ok := modhexMap[src[j]]
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if !ok {
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return i, fmt.Errorf("modhex: invalid byte: %#U", rune(src[j]))
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}
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dst[i] = (a << 4) | b
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i++
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}
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if len(src)%2 == 1 {
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if _, ok := modhexMap[src[j-1]]; !ok {
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return i, fmt.Errorf("modhex: invalid byte: %#U", rune(src[j-1]))
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}
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return i, errors.New("modhex: odd length modhex string")
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}
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return i, nil
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}
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// DecodeString returns the bytes represesented by the modhex string s.
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//
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// Decode expects that s contains only modhex characters and that s
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// has an even length.
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// If the input is malformed, Decode returns the number
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// of bytes decoded before the error.
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func DecodeString(s string) ([]byte, error) {
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// convert the input string to an array of bytes
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src := []byte(s)
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// make a new slice to hold the decoded bytes
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dst := make([]byte, DecodedLen(len(src)))
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// decode it!
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n, err := Decode(dst, src)
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// return the decoded bytes along with any errors that may have occured
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return dst[:n], err
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}
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