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// 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 tar

// TODO(dsymonds):
// - catch more errors (no first header, write after close, etc.)

import (
        "errors"
        "io"
        "strconv"
)

var (
        ErrWriteTooLong    = errors.New("write too long")
        ErrFieldTooLong    = errors.New("header field too long")
        ErrWriteAfterClose = errors.New("write after close")
)

// A Writer provides sequential writing of a tar archive in POSIX.1 format.
// A tar archive consists of a sequence of files.
// Call WriteHeader to begin a new file, and then call Write to supply that file's data,
// writing at most hdr.Size bytes in total.
//
// Example:
//      tw := tar.NewWriter(w)
//      hdr := new(Header)
//      hdr.Size = length of data in bytes
//      // populate other hdr fields as desired
//      if err := tw.WriteHeader(hdr); err != nil {
//              // handle error
//      }
//      io.Copy(tw, data)
//      tw.Close()
type Writer struct {
        w          io.Writer
        err        error
        nb         int64 // number of unwritten bytes for current file entry
        pad        int64 // amount of padding to write after current file entry
        closed     bool
        usedBinary bool // whether the binary numeric field extension was used
}

// NewWriter creates a new Writer writing to w.
func NewWriter(w io.Writer) *Writer { return &Writer{w: w} }

// Flush finishes writing the current file (optional).
func (tw *Writer) Flush() error {
        n := tw.nb + tw.pad
        for n > 0 && tw.err == nil {
                nr := n
                if nr > blockSize {
                        nr = blockSize
                }
                var nw int
                nw, tw.err = tw.w.Write(zeroBlock[0:nr])
                n -= int64(nw)
        }
        tw.nb = 0
        tw.pad = 0
        return tw.err
}

// Write s into b, terminating it with a NUL if there is room.
func (tw *Writer) cString(b []byte, s string) {
        if len(s) > len(b) {
                if tw.err == nil {
                        tw.err = ErrFieldTooLong
                }
                return
        }
        copy(b, s)
        if len(s) < len(b) {
                b[len(s)] = 0
        }
}

// Encode x as an octal ASCII string and write it into b with leading zeros.
func (tw *Writer) octal(b []byte, x int64) {
        s := strconv.FormatInt(x, 8)
        // leading zeros, but leave room for a NUL.
        for len(s)+1 < len(b) {
                s = "0" + s
        }
        tw.cString(b, s)
}

// Write x into b, either as octal or as binary (GNUtar/star extension).
func (tw *Writer) numeric(b []byte, x int64) {
        // Try octal first.
        s := strconv.FormatInt(x, 8)
        if len(s) < len(b) {
                tw.octal(b, x)
                return
        }
        // Too big: use binary (big-endian).
        tw.usedBinary = true
        for i := len(b) - 1; x > 0 && i >= 0; i-- {
                b[i] = byte(x)
                x >>= 8
        }
        b[0] |= 0x80 // highest bit indicates binary format
}

// WriteHeader writes hdr and prepares to accept the file's contents.
// WriteHeader calls Flush if it is not the first header.
// Calling after a Close will return ErrWriteAfterClose.
func (tw *Writer) WriteHeader(hdr *Header) error {
        if tw.closed {
                return ErrWriteAfterClose
        }
        if tw.err == nil {
                tw.Flush()
        }
        if tw.err != nil {
                return tw.err
        }

        tw.nb = int64(hdr.Size)
        tw.pad = -tw.nb & (blockSize - 1) // blockSize is a power of two

        header := make([]byte, blockSize)
        s := slicer(header)

        // TODO(dsymonds): handle names longer than 100 chars
        copy(s.next(100), []byte(hdr.Name))

        tw.octal(s.next(8), hdr.Mode)              // 100:108
        tw.numeric(s.next(8), int64(hdr.Uid))      // 108:116
        tw.numeric(s.next(8), int64(hdr.Gid))      // 116:124
        tw.numeric(s.next(12), hdr.Size)           // 124:136
        tw.numeric(s.next(12), hdr.ModTime.Unix()) // 136:148
        s.next(8)                                  // chksum (148:156)
        s.next(1)[0] = hdr.Typeflag                // 156:157
        tw.cString(s.next(100), hdr.Linkname)      // linkname (157:257)
        copy(s.next(8), []byte("ustar\x0000"))     // 257:265
        tw.cString(s.next(32), hdr.Uname)          // 265:297
        tw.cString(s.next(32), hdr.Gname)          // 297:329
        tw.numeric(s.next(8), hdr.Devmajor)        // 329:337
        tw.numeric(s.next(8), hdr.Devminor)        // 337:345

        // Use the GNU magic instead of POSIX magic if we used any GNU extensions.
        if tw.usedBinary {
                copy(header[257:265], []byte("ustar  \x00"))
        }

        // The chksum field is terminated by a NUL and a space.
        // This is different from the other octal fields.
        chksum, _ := checksum(header)
        tw.octal(header[148:155], chksum)
        header[155] = ' '

        if tw.err != nil {
                // problem with header; probably integer too big for a field.
                return tw.err
        }

        _, tw.err = tw.w.Write(header)

        return tw.err
}

// Write writes to the current entry in the tar archive.
// Write returns the error ErrWriteTooLong if more than
// hdr.Size bytes are written after WriteHeader.
func (tw *Writer) Write(b []byte) (n int, err error) {
        if tw.closed {
                err = ErrWriteTooLong
                return
        }
        overwrite := false
        if int64(len(b)) > tw.nb {
                b = b[0:tw.nb]
                overwrite = true
        }
        n, err = tw.w.Write(b)
        tw.nb -= int64(n)
        if err == nil && overwrite {
                err = ErrWriteTooLong
                return
        }
        tw.err = err
        return
}

// Close closes the tar archive, flushing any unwritten
// data to the underlying writer.
func (tw *Writer) Close() error {
        if tw.err != nil || tw.closed {
                return tw.err
        }
        tw.Flush()
        tw.closed = true

        // trailer: two zero blocks
        for i := 0; i < 2; i++ {
                _, tw.err = tw.w.Write(zeroBlock)
                if tw.err != nil {
                        break
                }
        }
        return tw.err
}