// Copyright 2011 The Snappy-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 snappy

import (
	"encoding/binary"
	"errors"
	"io"
)

var (
	// ErrCorrupt reports that the input is invalid.
	ErrCorrupt = errors.New("snappy: corrupt input")
	// ErrTooLarge reports that the uncompressed length is too large.
	ErrTooLarge = errors.New("snappy: decoded block is too large")
	// ErrUnsupported reports that the input isn't supported.
	ErrUnsupported = errors.New("snappy: unsupported input")
)

// DecodedLen returns the length of the decoded block.
func DecodedLen(src []byte) (int, error) {
	v, _, err := decodedLen(src)
	return v, err
}

// decodedLen returns the length of the decoded block and the number of bytes
// that the length header occupied.
func decodedLen(src []byte) (blockLen, headerLen int, err error) {
	v, n := binary.Uvarint(src)
	if n <= 0 || v > 0xffffffff {
		return 0, 0, ErrCorrupt
	}

	const wordSize = 32 << (^uint(0) >> 32 & 1)
	if wordSize == 32 && v > 0x7fffffff {
		return 0, 0, ErrTooLarge
	}
	return int(v), n, nil
}

// Decode returns the decoded form of src. The returned slice may be a sub-
// slice of dst if dst was large enough to hold the entire decoded block.
// Otherwise, a newly allocated slice will be returned.
// It is valid to pass a nil dst.
func Decode(dst, src []byte) ([]byte, error) {
	dLen, s, err := decodedLen(src)
	if err != nil {
		return nil, err
	}
	if len(dst) < dLen {
		dst = make([]byte, dLen)
	}

	var d, offset, length int
	for s < len(src) {
		switch src[s] & 0x03 {
		case tagLiteral:
			x := uint(src[s] >> 2)
			switch {
			case x < 60:
				s++
			case x == 60:
				s += 2
				if s > len(src) {
					return nil, ErrCorrupt
				}
				x = uint(src[s-1])
			case x == 61:
				s += 3
				if s > len(src) {
					return nil, ErrCorrupt
				}
				x = uint(src[s-2]) | uint(src[s-1])<<8
			case x == 62:
				s += 4
				if s > len(src) {
					return nil, ErrCorrupt
				}
				x = uint(src[s-3]) | uint(src[s-2])<<8 | uint(src[s-1])<<16
			case x == 63:
				s += 5
				if s > len(src) {
					return nil, ErrCorrupt
				}
				x = uint(src[s-4]) | uint(src[s-3])<<8 | uint(src[s-2])<<16 | uint(src[s-1])<<24
			}
			length = int(x + 1)
			if length <= 0 {
				return nil, errors.New("snappy: unsupported literal length")
			}
			if length > len(dst)-d || length > len(src)-s {
				return nil, ErrCorrupt
			}
			copy(dst[d:], src[s:s+length])
			d += length
			s += length
			continue

		case tagCopy1:
			s += 2
			if s > len(src) {
				return nil, ErrCorrupt
			}
			length = 4 + int(src[s-2])>>2&0x7
			offset = int(src[s-2])&0xe0<<3 | int(src[s-1])

		case tagCopy2:
			s += 3
			if s > len(src) {
				return nil, ErrCorrupt
			}
			length = 1 + int(src[s-3])>>2
			offset = int(src[s-2]) | int(src[s-1])<<8

		case tagCopy4:
			return nil, errors.New("snappy: unsupported COPY_4 tag")
		}

		end := d + length
		if offset > d || end > len(dst) {
			return nil, ErrCorrupt
		}
		for ; d < end; d++ {
			dst[d] = dst[d-offset]
		}
	}
	if d != dLen {
		return nil, ErrCorrupt
	}
	return dst[:d], nil
}

// NewReader returns a new Reader that decompresses from r, using the framing
// format described at
// https://github.com/google/snappy/blob/master/framing_format.txt
func NewReader(r io.Reader) *Reader {
	return &Reader{
		r:       r,
		decoded: make([]byte, maxUncompressedChunkLen),
		buf:     make([]byte, MaxEncodedLen(maxUncompressedChunkLen)+checksumSize),
	}
}

// Reader is an io.Reader than can read Snappy-compressed bytes.
type Reader struct {
	r       io.Reader
	err     error
	decoded []byte
	buf     []byte
	// decoded[i:j] contains decoded bytes that have not yet been passed on.
	i, j       int
	readHeader bool
}

// Reset discards any buffered data, resets all state, and switches the Snappy
// reader to read from r. This permits reusing a Reader rather than allocating
// a new one.
func (r *Reader) Reset(reader io.Reader) {
	r.r = reader
	r.err = nil
	r.i = 0
	r.j = 0
	r.readHeader = false
}

func (r *Reader) readFull(p []byte) (ok bool) {
	if _, r.err = io.ReadFull(r.r, p); r.err != nil {
		if r.err == io.ErrUnexpectedEOF {
			r.err = ErrCorrupt
		}
		return false
	}
	return true
}

// Read satisfies the io.Reader interface.
func (r *Reader) Read(p []byte) (int, error) {
	if r.err != nil {
		return 0, r.err
	}
	for {
		if r.i < r.j {
			n := copy(p, r.decoded[r.i:r.j])
			r.i += n
			return n, nil
		}
		if !r.readFull(r.buf[:4]) {
			return 0, r.err
		}
		chunkType := r.buf[0]
		if !r.readHeader {
			if chunkType != chunkTypeStreamIdentifier {
				r.err = ErrCorrupt
				return 0, r.err
			}
			r.readHeader = true
		}
		chunkLen := int(r.buf[1]) | int(r.buf[2])<<8 | int(r.buf[3])<<16
		if chunkLen > len(r.buf) {
			r.err = ErrUnsupported
			return 0, r.err
		}

		// The chunk types are specified at
		// https://github.com/google/snappy/blob/master/framing_format.txt
		switch chunkType {
		case chunkTypeCompressedData:
			// Section 4.2. Compressed data (chunk type 0x00).
			if chunkLen < checksumSize {
				r.err = ErrCorrupt
				return 0, r.err
			}
			buf := r.buf[:chunkLen]
			if !r.readFull(buf) {
				return 0, r.err
			}
			checksum := uint32(buf[0]) | uint32(buf[1])<<8 | uint32(buf[2])<<16 | uint32(buf[3])<<24
			buf = buf[checksumSize:]

			n, err := DecodedLen(buf)
			if err != nil {
				r.err = err
				return 0, r.err
			}
			if n > len(r.decoded) {
				r.err = ErrCorrupt
				return 0, r.err
			}
			if _, err := Decode(r.decoded, buf); err != nil {
				r.err = err
				return 0, r.err
			}
			if crc(r.decoded[:n]) != checksum {
				r.err = ErrCorrupt
				return 0, r.err
			}
			r.i, r.j = 0, n
			continue

		case chunkTypeUncompressedData:
			// Section 4.3. Uncompressed data (chunk type 0x01).
			if chunkLen < checksumSize {
				r.err = ErrCorrupt
				return 0, r.err
			}
			buf := r.buf[:checksumSize]
			if !r.readFull(buf) {
				return 0, r.err
			}
			checksum := uint32(buf[0]) | uint32(buf[1])<<8 | uint32(buf[2])<<16 | uint32(buf[3])<<24
			// Read directly into r.decoded instead of via r.buf.
			n := chunkLen - checksumSize
			if !r.readFull(r.decoded[:n]) {
				return 0, r.err
			}
			if crc(r.decoded[:n]) != checksum {
				r.err = ErrCorrupt
				return 0, r.err
			}
			r.i, r.j = 0, n
			continue

		case chunkTypeStreamIdentifier:
			// Section 4.1. Stream identifier (chunk type 0xff).
			if chunkLen != len(magicBody) {
				r.err = ErrCorrupt
				return 0, r.err
			}
			if !r.readFull(r.buf[:len(magicBody)]) {
				return 0, r.err
			}
			for i := 0; i < len(magicBody); i++ {
				if r.buf[i] != magicBody[i] {
					r.err = ErrCorrupt
					return 0, r.err
				}
			}
			continue
		}

		if chunkType <= 0x7f {
			// Section 4.5. Reserved unskippable chunks (chunk types 0x02-0x7f).
			r.err = ErrUnsupported
			return 0, r.err
		}
		// Section 4.4 Padding (chunk type 0xfe).
		// Section 4.6. Reserved skippable chunks (chunk types 0x80-0xfd).
		if !r.readFull(r.buf[:chunkLen]) {
			return 0, r.err
		}
	}
}
