// 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 zlib implements reading and writing of zlib format compressed data,
as specified in RFC 1950.

The implementation provides filters that uncompress during reading
and compress during writing.  For example, to write compressed data
to a buffer:

	var b bytes.Buffer
	w, err := zlib.NewWriter(&b)
	w.Write([]byte("hello, world\n"))
	w.Close()

and to read that data back:

	r, err := zlib.NewReader(&b)
	io.Copy(os.Stdout, r)
	r.Close()
*/
package zlib

import (
	"bufio"
	"compress/flate"
	"hash"
	"hash/adler32"
	"io"
	"os"
)

const zlibDeflate = 8

var ChecksumError = os.NewError("zlib checksum error")
var HeaderError = os.NewError("invalid zlib header")
var DictionaryError = os.NewError("invalid zlib dictionary")

type reader struct {
	r            flate.Reader
	decompressor io.ReadCloser
	digest       hash.Hash32
	err          os.Error
	scratch      [4]byte
}

// NewReader creates a new io.ReadCloser that satisfies reads by decompressing data read from r.
// The implementation buffers input and may read more data than necessary from r.
// It is the caller's responsibility to call Close on the ReadCloser when done.
func NewReader(r io.Reader) (io.ReadCloser, os.Error) {
	return NewReaderDict(r, nil)
}

// NewReaderDict is like NewReader but uses a preset dictionary.
// NewReaderDict ignores the dictionary if the compressed data does not refer to it.
func NewReaderDict(r io.Reader, dict []byte) (io.ReadCloser, os.Error) {
	z := new(reader)
	if fr, ok := r.(flate.Reader); ok {
		z.r = fr
	} else {
		z.r = bufio.NewReader(r)
	}
	_, err := io.ReadFull(z.r, z.scratch[0:2])
	if err != nil {
		return nil, err
	}
	h := uint(z.scratch[0])<<8 | uint(z.scratch[1])
	if (z.scratch[0]&0x0f != zlibDeflate) || (h%31 != 0) {
		return nil, HeaderError
	}
	if z.scratch[1]&0x20 != 0 {
		_, err = io.ReadFull(z.r, z.scratch[0:4])
		if err != nil {
			return nil, err
		}
		checksum := uint32(z.scratch[0])<<24 | uint32(z.scratch[1])<<16 | uint32(z.scratch[2])<<8 | uint32(z.scratch[3])
		if checksum != adler32.Checksum(dict) {
			return nil, DictionaryError
		}
		z.decompressor = flate.NewReaderDict(z.r, dict)
	} else {
		z.decompressor = flate.NewReader(z.r)
	}
	z.digest = adler32.New()
	return z, nil
}

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

	n, err = z.decompressor.Read(p)
	z.digest.Write(p[0:n])
	if n != 0 || err != os.EOF {
		z.err = err
		return
	}

	// Finished file; check checksum.
	if _, err := io.ReadFull(z.r, z.scratch[0:4]); err != nil {
		z.err = err
		return 0, err
	}
	// ZLIB (RFC 1950) is big-endian, unlike GZIP (RFC 1952).
	checksum := uint32(z.scratch[0])<<24 | uint32(z.scratch[1])<<16 | uint32(z.scratch[2])<<8 | uint32(z.scratch[3])
	if checksum != z.digest.Sum32() {
		z.err = ChecksumError
		return 0, z.err
	}
	return
}

// Calling Close does not close the wrapped io.Reader originally passed to NewReader.
func (z *reader) Close() os.Error {
	if z.err != nil {
		return z.err
	}
	z.err = z.decompressor.Close()
	return z.err
}