package s3api

// the related code is copied and modified from minio source code

/*
 * Minio Cloud Storage, (C) 2016 Minio, Inc.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

import (
	"bufio"
	"bytes"
	"errors"
	"github.com/dustin/go-humanize"
	"io"
	"net/http"
)

const maxLineLength = 4 * humanize.KiByte // assumed <= bufio.defaultBufSize 4KiB

// lineTooLong is generated as chunk header is bigger than 4KiB.
var errLineTooLong = errors.New("header line too long")

// Malformed encoding is generated when chunk header is wrongly formed.
var errMalformedEncoding = errors.New("malformed chunked encoding")

// newSignV4ChunkedReader returns a new s3ChunkedReader that translates the data read from r
// out of HTTP "chunked" format before returning it.
// The s3ChunkedReader returns io.EOF when the final 0-length chunk is read.
func newSignV4ChunkedReader(req *http.Request) io.ReadCloser {
	return &s3ChunkedReader{
		reader: bufio.NewReader(req.Body),
		state:  readChunkHeader,
	}
}

// Represents the overall state that is required for decoding a
// AWS Signature V4 chunked reader.
type s3ChunkedReader struct {
	reader         *bufio.Reader
	state          chunkState
	lastChunk      bool
	chunkSignature string
	n              uint64 // Unread bytes in chunk
	err            error
}

// Read chunk reads the chunk token signature portion.
func (cr *s3ChunkedReader) readS3ChunkHeader() {
	// Read the first chunk line until CRLF.
	var hexChunkSize, hexChunkSignature []byte
	hexChunkSize, hexChunkSignature, cr.err = readChunkLine(cr.reader)
	if cr.err != nil {
		return
	}
	// <hex>;token=value - converts the hex into its uint64 form.
	cr.n, cr.err = parseHexUint(hexChunkSize)
	if cr.err != nil {
		return
	}
	if cr.n == 0 {
		cr.err = io.EOF
	}
	// Save the incoming chunk signature.
	cr.chunkSignature = string(hexChunkSignature)
}

type chunkState int

const (
	readChunkHeader chunkState = iota
	readChunkTrailer
	readChunk
	verifyChunk
	eofChunk
)

func (cs chunkState) String() string {
	stateString := ""
	switch cs {
	case readChunkHeader:
		stateString = "readChunkHeader"
	case readChunkTrailer:
		stateString = "readChunkTrailer"
	case readChunk:
		stateString = "readChunk"
	case verifyChunk:
		stateString = "verifyChunk"
	case eofChunk:
		stateString = "eofChunk"

	}
	return stateString
}

func (cr *s3ChunkedReader) Close() (err error) {
	return nil
}

// Read - implements `io.Reader`, which transparently decodes
// the incoming AWS Signature V4 streaming signature.
func (cr *s3ChunkedReader) Read(buf []byte) (n int, err error) {
	for {
		switch cr.state {
		case readChunkHeader:
			cr.readS3ChunkHeader()
			// If we're at the end of a chunk.
			if cr.n == 0 && cr.err == io.EOF {
				cr.state = readChunkTrailer
				cr.lastChunk = true
				continue
			}
			if cr.err != nil {
				return 0, cr.err
			}
			cr.state = readChunk
		case readChunkTrailer:
			cr.err = readCRLF(cr.reader)
			if cr.err != nil {
				return 0, errMalformedEncoding
			}
			cr.state = verifyChunk
		case readChunk:
			// There is no more space left in the request buffer.
			if len(buf) == 0 {
				return n, nil
			}
			rbuf := buf
			// The request buffer is larger than the current chunk size.
			// Read only the current chunk from the underlying reader.
			if uint64(len(rbuf)) > cr.n {
				rbuf = rbuf[:cr.n]
			}
			var n0 int
			n0, cr.err = cr.reader.Read(rbuf)
			if cr.err != nil {
				// We have lesser than chunk size advertised in chunkHeader, this is 'unexpected'.
				if cr.err == io.EOF {
					cr.err = io.ErrUnexpectedEOF
				}
				return 0, cr.err
			}

			// Update the bytes read into request buffer so far.
			n += n0
			buf = buf[n0:]
			// Update bytes to be read of the current chunk before verifying chunk's signature.
			cr.n -= uint64(n0)

			// If we're at the end of a chunk.
			if cr.n == 0 {
				cr.state = readChunkTrailer
				continue
			}
		case verifyChunk:
			if cr.lastChunk {
				cr.state = eofChunk
			} else {
				cr.state = readChunkHeader
			}
		case eofChunk:
			return n, io.EOF
		}
	}
}

// readCRLF - check if reader only has '\r\n' CRLF character.
// returns malformed encoding if it doesn't.
func readCRLF(reader io.Reader) error {
	buf := make([]byte, 2)
	_, err := io.ReadFull(reader, buf[:2])
	if err != nil {
		return err
	}
	if buf[0] != '\r' || buf[1] != '\n' {
		return errMalformedEncoding
	}
	return nil
}

// Read a line of bytes (up to \n) from b.
// Give up if the line exceeds maxLineLength.
// The returned bytes are owned by the bufio.Reader
// so they are only valid until the next bufio read.
func readChunkLine(b *bufio.Reader) ([]byte, []byte, error) {
	buf, err := b.ReadSlice('\n')
	if err != nil {
		// We always know when EOF is coming.
		// If the caller asked for a line, there should be a line.
		if err == io.EOF {
			err = io.ErrUnexpectedEOF
		} else if err == bufio.ErrBufferFull {
			err = errLineTooLong
		}
		return nil, nil, err
	}
	if len(buf) >= maxLineLength {
		return nil, nil, errLineTooLong
	}
	// Parse s3 specific chunk extension and fetch the values.
	hexChunkSize, hexChunkSignature := parseS3ChunkExtension(buf)
	return hexChunkSize, hexChunkSignature, nil
}

// trimTrailingWhitespace - trim trailing white space.
func trimTrailingWhitespace(b []byte) []byte {
	for len(b) > 0 && isASCIISpace(b[len(b)-1]) {
		b = b[:len(b)-1]
	}
	return b
}

// isASCIISpace - is ascii space?
func isASCIISpace(b byte) bool {
	return b == ' ' || b == '\t' || b == '\n' || b == '\r'
}

// Constant s3 chunk encoding signature.
const s3ChunkSignatureStr = ";chunk-signature="

// parses3ChunkExtension removes any s3 specific chunk-extension from buf.
// For example,
//     "10000;chunk-signature=..." => "10000", "chunk-signature=..."
func parseS3ChunkExtension(buf []byte) ([]byte, []byte) {
	buf = trimTrailingWhitespace(buf)
	semi := bytes.Index(buf, []byte(s3ChunkSignatureStr))
	// Chunk signature not found, return the whole buffer.
	if semi == -1 {
		return buf, nil
	}
	return buf[:semi], parseChunkSignature(buf[semi:])
}

// parseChunkSignature - parse chunk signature.
func parseChunkSignature(chunk []byte) []byte {
	chunkSplits := bytes.SplitN(chunk, []byte(s3ChunkSignatureStr), 2)
	return chunkSplits[1]
}

// parse hex to uint64.
func parseHexUint(v []byte) (n uint64, err error) {
	for i, b := range v {
		switch {
		case '0' <= b && b <= '9':
			b = b - '0'
		case 'a' <= b && b <= 'f':
			b = b - 'a' + 10
		case 'A' <= b && b <= 'F':
			b = b - 'A' + 10
		default:
			return 0, errors.New("invalid byte in chunk length")
		}
		if i == 16 {
			return 0, errors.New("http chunk length too large")
		}
		n <<= 4
		n |= uint64(b)
	}
	return
}