// Copyright 2018 Google 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.

// Package compressio provides parallel compression and decompression.
package compressio

import (
	"bytes"
	"compress/flate"
	"errors"
	"io"
	"runtime"
	"sync"

	"gvisor.googlesource.com/gvisor/pkg/binary"
)

var bufPool = sync.Pool{
	New: func() interface{} {
		return bytes.NewBuffer(nil)
	},
}

var chunkPool = sync.Pool{
	New: func() interface{} {
		return new(chunk)
	},
}

// chunk is a unit of work.
type chunk struct {
	// compressed is compressed data.
	//
	// This will always be returned to the bufPool directly when work has
	// finished (in schedule) and therefore must be allocated.
	compressed *bytes.Buffer

	// uncompressed is the uncompressed data.
	//
	// This is not returned to the bufPool automatically, since it may
	// correspond to a inline slice (provided directly to Read or Write).
	uncompressed *bytes.Buffer
}

// newChunk allocates a new chunk object (or pulls one from the pool). Buffers
// will be allocated if nil is provided for compressed or uncompressed.
func newChunk(compressed *bytes.Buffer, uncompressed *bytes.Buffer) *chunk {
	c := chunkPool.Get().(*chunk)
	if compressed != nil {
		c.compressed = compressed
	} else {
		c.compressed = bufPool.Get().(*bytes.Buffer)
	}
	if uncompressed != nil {
		c.uncompressed = uncompressed
	} else {
		c.uncompressed = bufPool.Get().(*bytes.Buffer)
	}
	return c
}

// result is the result of some work; it includes the original chunk.
type result struct {
	*chunk
	err error
}

// worker is a compression/decompression worker.
//
// The associated worker goroutine reads in uncompressed buffers from input and
// writes compressed buffers to its output. Alternatively, the worker reads
// compressed buffers from input and writes uncompressed buffers to its output.
//
// The goroutine will exit when input is closed, and the goroutine will close
// output.
type worker struct {
	input  chan *chunk
	output chan result
}

// work is the main work routine; see worker.
func (w *worker) work(compress bool, level int) {
	defer close(w.output)

	for c := range w.input {
		if compress {
			// Encode this slice.
			fw, err := flate.NewWriter(c.compressed, level)
			if err != nil {
				w.output <- result{c, err}
				continue
			}

			// Encode the input.
			if _, err := io.Copy(fw, c.uncompressed); err != nil {
				w.output <- result{c, err}
				continue
			}
			if err := fw.Close(); err != nil {
				w.output <- result{c, err}
				continue
			}
		} else {
			// Decode this slice.
			fr := flate.NewReader(c.compressed)

			// Decode the input.
			if _, err := io.Copy(c.uncompressed, fr); err != nil {
				w.output <- result{c, err}
				continue
			}
		}

		// Send the output.
		w.output <- result{c, nil}
	}
}

// pool is common functionality for reader/writers.
type pool struct {
	// workers are the compression/decompression workers.
	workers []worker

	// chunkSize is the chunk size. This is the first four bytes in the
	// stream and is shared across both the reader and writer.
	chunkSize uint32

	// mu protects below; it is generally the responsibility of users to
	// acquire this mutex before calling any methods on the pool.
	mu sync.Mutex

	// nextInput is the next worker for input (scheduling).
	nextInput int

	// nextOutput is the next worker for output (result).
	nextOutput int

	// buf is the current active buffer; the exact semantics of this buffer
	// depending on whether this is a reader or a writer.
	buf *bytes.Buffer
}

// init initializes the worker pool.
//
// This should only be called once.
func (p *pool) init(compress bool, level int) {
	p.workers = make([]worker, 1+runtime.GOMAXPROCS(0))
	for i := 0; i < len(p.workers); i++ {
		p.workers[i] = worker{
			input:  make(chan *chunk, 1),
			output: make(chan result, 1),
		}
		go p.workers[i].work(compress, level) // S/R-SAFE: In save path only.
	}
	runtime.SetFinalizer(p, (*pool).stop)
}

// stop stops all workers.
func (p *pool) stop() {
	for i := 0; i < len(p.workers); i++ {
		close(p.workers[i].input)
	}
	p.workers = nil
}

// handleResult calls the callback.
func handleResult(r result, callback func(*chunk) error) error {
	defer func() {
		r.chunk.compressed.Reset()
		bufPool.Put(r.chunk.compressed)
		chunkPool.Put(r.chunk)
	}()
	if r.err != nil {
		return r.err
	}
	if err := callback(r.chunk); err != nil {
		return err
	}
	return nil
}

// schedule schedules the given buffers.
//
// If c is non-nil, then it will return as soon as the chunk is scheduled. If c
// is nil, then it will return only when no more work is left to do.
//
// If no callback function is provided, then the output channel will be
// ignored.  You must be sure that the input is schedulable in this case.
func (p *pool) schedule(c *chunk, callback func(*chunk) error) error {
	for {
		var (
			inputChan  chan *chunk
			outputChan chan result
		)
		if c != nil {
			inputChan = p.workers[(p.nextInput+1)%len(p.workers)].input
		}
		if callback != nil && p.nextOutput != p.nextInput {
			outputChan = p.workers[(p.nextOutput+1)%len(p.workers)].output
		}
		if inputChan == nil && outputChan == nil {
			return nil
		}

		select {
		case inputChan <- c:
			p.nextInput++
			return nil
		case r := <-outputChan:
			p.nextOutput++
			if err := handleResult(r, callback); err != nil {
				return err
			}
		}
	}
}

// reader chunks reads and decompresses.
type reader struct {
	pool

	// in is the source.
	in io.Reader
}

// NewReader returns a new compressed reader.
func NewReader(in io.Reader) (io.Reader, error) {
	r := &reader{
		in: in,
	}
	r.init(false, 0)
	var err error
	if r.chunkSize, err = binary.ReadUint32(r.in, binary.BigEndian); err != nil {
		return nil, err
	}
	return r, nil
}

// errNewBuffer is returned when a new buffer is completed.
var errNewBuffer = errors.New("buffer ready")

// Read implements io.Reader.Read.
func (r *reader) Read(p []byte) (int, error) {
	r.mu.Lock()
	defer r.mu.Unlock()

	// Total bytes completed; this is declared up front because it must be
	// adjustable by the callback below.
	done := 0

	// Total bytes pending in the asynchronous workers for buffers. This is
	// used to process the proper regions of the input as inline buffers.
	var (
		pendingPre    = r.nextInput - r.nextOutput
		pendingInline = 0
	)

	// Define our callback for completed work.
	callback := func(c *chunk) error {
		// Check for an inline buffer.
		if pendingPre == 0 && pendingInline > 0 {
			pendingInline--
			done += c.uncompressed.Len()
			return nil
		}

		// Copy the resulting buffer to our intermediate one, and
		// return errNewBuffer to ensure that we aren't called a second
		// time. This error code is handled specially below.
		//
		// c.buf will be freed and return to the pool when it is done.
		if pendingPre > 0 {
			pendingPre--
		}
		r.buf = c.uncompressed
		return errNewBuffer
	}

	for done < len(p) {
		// Do we have buffered data available?
		if r.buf != nil {
			n, err := r.buf.Read(p[done:])
			done += n
			if err == io.EOF {
				// This is the uncompressed buffer, it can be
				// returned to the pool at this point.
				r.buf.Reset()
				bufPool.Put(r.buf)
				r.buf = nil
			} else if err != nil {
				// Should never happen.
				defer r.stop()
				return done, err
			}
			continue
		}

		// Read the length of the next chunk and reset the
		// reader. The length is used to limit the reader.
		//
		// See writer.flush.
		l, err := binary.ReadUint32(r.in, binary.BigEndian)
		if err != nil {
			// This is generally okay as long as there
			// are still buffers outstanding. We actually
			// just wait for completion of those buffers here
			// and continue our loop.
			if err := r.schedule(nil, callback); err == nil {
				// We've actually finished all buffers; this is
				// the normal EOF exit path.
				defer r.stop()
				return done, io.EOF
			} else if err == errNewBuffer {
				// A new buffer is now available.
				continue
			} else {
				// Some other error occurred; we cannot
				// process any further.
				defer r.stop()
				return done, err
			}
		}

		// Read this chunk and schedule decompression.
		compressed := bufPool.Get().(*bytes.Buffer)
		if _, err := io.Copy(compressed, &io.LimitedReader{
			R: r.in,
			N: int64(l),
		}); err != nil {
			// Some other error occurred; see above.
			return done, err
		}

		// Are we doing inline decoding?
		//
		// Note that we need to check the length here against
		// bytes.MinRead, since the bytes library will choose to grow
		// the slice if the available capacity is not at least
		// bytes.MinRead. This limits inline decoding to chunkSizes
		// that are at least bytes.MinRead (which is not unreasonable).
		var c *chunk
		start := done + ((pendingPre + pendingInline) * int(r.chunkSize))
		if len(p) >= start+int(r.chunkSize) && len(p) >= start+bytes.MinRead {
			c = newChunk(compressed, bytes.NewBuffer(p[start:start]))
			pendingInline++
		} else {
			c = newChunk(compressed, nil)
		}
		if err := r.schedule(c, callback); err == errNewBuffer {
			// A new buffer was completed while we were reading.
			// That's great, but we need to force schedule the
			// current buffer so that it does not get lost.
			//
			// It is safe to pass nil as an output function here,
			// because we know that we just freed up a slot above.
			r.schedule(c, nil)
		} else if err != nil {
			// Some other error occurred; see above.
			defer r.stop()
			return done, err
		}
	}

	// Make sure that everything has been decoded successfully, otherwise
	// parts of p may not actually have completed.
	for pendingInline > 0 {
		if err := r.schedule(nil, func(c *chunk) error {
			if err := callback(c); err != nil {
				return err
			}
			// The nil case means that an inline buffer has
			// completed. The callback will have already removed
			// the inline buffer from the map, so we just return an
			// error to check the top of the loop again.
			return errNewBuffer
		}); err != errNewBuffer {
			// Some other error occurred; see above.
			return done, err
		}
	}

	// Need to return done here, since it may have been adjusted by the
	// callback to compensation for partial reads on some inline buffer.
	return done, nil
}

// writer chunks and schedules writes.
type writer struct {
	pool

	// out is the underlying writer.
	out io.Writer

	// closed indicates whether the file has been closed.
	closed bool
}

// NewWriter returns a new compressed writer.
//
// The recommended chunkSize is on the order of 1M. Extra memory may be
// buffered (in the form of read-ahead, or buffered writes), and is limited to
// O(chunkSize * [1+GOMAXPROCS]).
func NewWriter(out io.Writer, chunkSize uint32, level int) (io.WriteCloser, error) {
	w := &writer{
		pool: pool{
			chunkSize: chunkSize,
			buf:       bufPool.Get().(*bytes.Buffer),
		},
		out: out,
	}
	w.init(true, level)
	if err := binary.WriteUint32(w.out, binary.BigEndian, chunkSize); err != nil {
		return nil, err
	}
	return w, nil
}

// flush writes a single buffer.
func (w *writer) flush(c *chunk) error {
	// Prefix each chunk with a length; this allows the reader to safely
	// limit reads while buffering.
	l := uint32(c.compressed.Len())
	if err := binary.WriteUint32(w.out, binary.BigEndian, l); err != nil {
		return err
	}

	// Write out to the stream.
	_, err := io.Copy(w.out, c.compressed)
	return err
}

// Write implements io.Writer.Write.
func (w *writer) Write(p []byte) (int, error) {
	w.mu.Lock()
	defer w.mu.Unlock()

	// Did we close already?
	if w.closed {
		return 0, io.ErrUnexpectedEOF
	}

	// See above; we need to track in the same way.
	var (
		pendingPre    = w.nextInput - w.nextOutput
		pendingInline = 0
	)
	callback := func(c *chunk) error {
		if pendingPre == 0 && pendingInline > 0 {
			pendingInline--
			return w.flush(c)
		}
		if pendingPre > 0 {
			pendingPre--
		}
		err := w.flush(c)
		c.uncompressed.Reset()
		bufPool.Put(c.uncompressed)
		return err
	}

	for done := 0; done < len(p); {
		// Construct an inline buffer if we're doing an inline
		// encoding; see above regarding the bytes.MinRead constraint.
		if w.buf.Len() == 0 && len(p) >= done+int(w.chunkSize) && len(p) >= done+bytes.MinRead {
			bufPool.Put(w.buf) // Return to the pool; never scheduled.
			w.buf = bytes.NewBuffer(p[done : done+int(w.chunkSize)])
			done += int(w.chunkSize)
			pendingInline++
		}

		// Do we need to flush w.buf? Note that this case should be hit
		// immediately following the inline case above.
		left := int(w.chunkSize) - w.buf.Len()
		if left == 0 {
			if err := w.schedule(newChunk(nil, w.buf), callback); err != nil {
				return done, err
			}
			// Reset the buffer, since this has now been scheduled
			// for compression. Note that this may be trampled
			// immediately by the bufPool.Put(w.buf) above if the
			// next buffer happens to be inline, but that's okay.
			w.buf = bufPool.Get().(*bytes.Buffer)
			continue
		}

		// Read from p into w.buf.
		toWrite := len(p) - done
		if toWrite > left {
			toWrite = left
		}
		n, err := w.buf.Write(p[done : done+toWrite])
		done += n
		if err != nil {
			return done, err
		}
	}

	// Make sure that everything has been flushed, we can't return until
	// all the contents from p have been used.
	for pendingInline > 0 {
		if err := w.schedule(nil, func(c *chunk) error {
			if err := callback(c); err != nil {
				return err
			}
			// The flush was successful, return errNewBuffer here
			// to break from the loop and check the condition
			// again.
			return errNewBuffer
		}); err != errNewBuffer {
			return len(p), err
		}
	}

	return len(p), nil
}

// Close implements io.Closer.Close.
func (w *writer) Close() error {
	w.mu.Lock()
	defer w.mu.Unlock()

	// Did we already close? After the call to Close, we always mark as
	// closed, regardless of whether the flush is successful.
	if w.closed {
		return io.ErrUnexpectedEOF
	}
	w.closed = true
	defer w.stop()

	// Schedule any remaining partial buffer; we pass w.flush directly here
	// because the final buffer is guaranteed to not be an inline buffer.
	if w.buf.Len() > 0 {
		if err := w.schedule(newChunk(nil, w.buf), w.flush); err != nil {
			return err
		}
	}

	// Flush all scheduled buffers; see above.
	if err := w.schedule(nil, w.flush); err != nil {
		return err
	}

	// Close the underlying writer (if necessary).
	if closer, ok := w.out.(io.Closer); ok {
		return closer.Close()
	}
	return nil
}