Source file src/pkg/encoding/git85/git.go

// Copyright 2009 The Go Authors.  All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

// Package git85 implements the radix 85 data encoding
// used in the Git version control system.
package git85

import (
    "bytes"
    "io"
    "os"
    "strconv"
)

type CorruptInputError int64

func (e CorruptInputError) String() string {
    return "illegal git85 data at input byte " + strconv.Itoa64(int64(e))
}

const encode = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz!#$%&()*+-;<=>?@^_`{|}~"

// The decodings are 1+ the actual value, so that the
// default zero value can be used to mean "not valid".
var decode = [256]uint8{
    '0': 1, 2, 3, 4, 5, 6, 7, 8, 9, 10,
    'A': 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23,
    24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36,
    'a': 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49,
    50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62,
    '!': 63,
    '#': 64, 65, 66, 67,
    '(': 68, 69, 70, 71,
    '-':    72,
    ';':    73,
    '<': 74, 75, 76, 77,
    '@': 78,
    '^': 79, 80, 81,
    '{': 82, 83, 84, 85,
}

// Encode encodes src into EncodedLen(len(src))
// bytes of dst.  As a convenience, it returns the number
// of bytes written to dst, but this value is always EncodedLen(len(src)).
// Encode implements the radix 85 encoding used in the
// Git version control tool.
//
// The encoding splits src into chunks of at most 52 bytes
// and encodes each chunk on its own line.
func Encode(dst, src []byte) int {
    ndst := 0
    for len(src) > 0 {
        n := len(src)
        if n > 52 {
            n = 52
        }
        if n <= 27 {
            dst[ndst] = byte('A' + n - 1)
        } else {
            dst[ndst] = byte('a' + n - 26 - 1)
        }
        ndst++
        for i := 0; i < n; i += 4 {
            var v uint32
            for j := 0; j < 4 && i+j < n; j++ {
                v |= uint32(src[i+j]) << uint(24-j*8)
            }
            for j := 4; j >= 0; j-- {
                dst[ndst+j] = encode[v%85]
                v /= 85
            }
            ndst += 5
        }
        dst[ndst] = '\n'
        ndst++
        src = src[n:]
    }
    return ndst
}

// EncodedLen returns the length of an encoding of n source bytes.
func EncodedLen(n int) int {
    if n == 0 {
        return 0
    }
    // 5 bytes per 4 bytes of input, rounded up.
    // 2 extra bytes for each line of 52 src bytes, rounded up.
    return (n+3)/4*5 + (n+51)/52*2
}

var newline = []byte{'\n'}

// Decode decodes src into at most MaxDecodedLen(len(src))
// bytes, returning the actual number of bytes written to dst.
//
// If Decode encounters invalid input, it returns a CorruptInputError.
//
func Decode(dst, src []byte) (n int, err os.Error) {
    ndst := 0
    nsrc := 0
    for nsrc < len(src) {
        var l int
        switch ch := int(src[nsrc]); {
        case 'A' <= ch && ch <= 'Z':
            l = ch - 'A' + 1
        case 'a' <= ch && ch <= 'z':
            l = ch - 'a' + 26 + 1
        default:
            return ndst, CorruptInputError(nsrc)
        }
        if nsrc+1+l > len(src) {
            return ndst, CorruptInputError(nsrc)
        }
        el := (l + 3) / 4 * 5 // encoded len
        if nsrc+1+el+1 > len(src) || src[nsrc+1+el] != '\n' {
            return ndst, CorruptInputError(nsrc)
        }
        line := src[nsrc+1 : nsrc+1+el]
        for i := 0; i < el; i += 5 {
            var v uint32
            for j := 0; j < 5; j++ {
                ch := decode[line[i+j]]
                if ch == 0 {
                    return ndst, CorruptInputError(nsrc + 1 + i + j)
                }
                v = v*85 + uint32(ch-1)
            }
            for j := 0; j < 4; j++ {
                dst[ndst] = byte(v >> 24)
                v <<= 8
                ndst++
            }
        }
        // Last fragment may have run too far (but there was room in dst).
        // Back up.
        if l%4 != 0 {
            ndst -= 4 - l%4
        }
        nsrc += 1 + el + 1
    }
    return ndst, nil
}

func MaxDecodedLen(n int) int { return n / 5 * 4 }

// NewEncoder returns a new Git base85 stream encoder.  Data written to
// the returned writer will be encoded and then written to w.
// The Git encoding operates on 52-byte blocks; when finished
// writing, the caller must Close the returned encoder to flush any
// partially written blocks.
func NewEncoder(w io.Writer) io.WriteCloser { return &encoder{w: w} }

type encoder struct {
    w    io.Writer
    err  os.Error
    buf  [52]byte
    nbuf int
    out  [1024]byte
    nout int
}

func (e *encoder) Write(p []byte) (n int, err os.Error) {
    if e.err != nil {
        return 0, e.err
    }

    // Leading fringe.
    if e.nbuf > 0 {
        var i int
        for i = 0; i < len(p) && e.nbuf < 52; i++ {
            e.buf[e.nbuf] = p[i]
            e.nbuf++
        }
        n += i
        p = p[i:]
        if e.nbuf < 52 {
            return
        }
        nout := Encode(&e.out, &e.buf)
        if _, e.err = e.w.Write(e.out[0:nout]); e.err != nil {
            return n, e.err
        }
        e.nbuf = 0
    }

    // Large interior chunks.
    for len(p) >= 52 {
        nn := len(e.out) / (1 + 52/4*5 + 1) * 52
        if nn > len(p) {
            nn = len(p) / 52 * 52
        }
        if nn > 0 {
            nout := Encode(&e.out, p[0:nn])
            if _, e.err = e.w.Write(e.out[0:nout]); e.err != nil {
                return n, e.err
            }
        }
        n += nn
        p = p[nn:]
    }

    // Trailing fringe.
    for i := 0; i < len(p); i++ {
        e.buf[i] = p[i]
    }
    e.nbuf = len(p)
    n += len(p)
    return
}

func (e *encoder) Close() os.Error {
    // If there's anything left in the buffer, flush it out
    if e.err == nil && e.nbuf > 0 {
        nout := Encode(&e.out, e.buf[0:e.nbuf])
        e.nbuf = 0
        _, e.err = e.w.Write(e.out[0:nout])
    }
    return e.err
}

// NewDecoder returns a new Git base85 stream decoder.
func NewDecoder(r io.Reader) io.Reader { return &decoder{r: r} }

type decoder struct {
    r       io.Reader
    err     os.Error
    readErr os.Error
    buf     [1024]byte
    nbuf    int
    out     []byte
    outbuf  [1024]byte
    off     int64
}

func (d *decoder) Read(p []byte) (n int, err os.Error) {
    if len(p) == 0 {
        return 0, nil
    }

    for {
        // Copy leftover output from last decode.
        if len(d.out) > 0 {
            n = copy(p, d.out)
            d.out = d.out[n:]
            return
        }

        // Out of decoded output.  Check errors.
        if d.err != nil {
            return 0, d.err
        }
        if d.readErr != nil {
            d.err = d.readErr
            return 0, d.err
        }

        // Read and decode more input.
        var nn int
        nn, d.readErr = d.r.Read(d.buf[d.nbuf:])
        d.nbuf += nn

        // Send complete lines to Decode.
        nl := bytes.LastIndex(d.buf[0:d.nbuf], newline)
        if nl < 0 {
            continue
        }
        nn, d.err = Decode(&d.outbuf, d.buf[0:nl+1])
        if e, ok := d.err.(CorruptInputError); ok {
            d.err = CorruptInputError(int64(e) + d.off)
        }
        d.out = d.outbuf[0:nn]
        d.nbuf = copy(&d.buf, d.buf[nl+1:d.nbuf])
        d.off += int64(nl + 1)
    }
    panic("unreacahable")
}