package alloc

import (
	"io"
	"sync"
)

const (
	defaultOffset = 16
)

// Buffer is a recyclable allocation of a byte array. Buffer.Release() recycles
// the buffer into an internal buffer pool, in order to recreate a buffer more
// quickly.
type Buffer struct {
	head   []byte
	pool   *bufferPool
	Value  []byte
	offset int
}

// Release recycles the buffer into an internal buffer pool.
func (b *Buffer) Release() {
	if b == nil {
		return
	}
	b.pool.free(b)
	b.head = nil
	b.Value = nil
	b.pool = nil
}

// Clear clears the content of the buffer, results an empty buffer with
// Len() = 0.
func (b *Buffer) Clear() *Buffer {
	b.offset = defaultOffset
	b.Value = b.head[b.offset:b.offset]
	return b
}

// AppendBytes appends one or more bytes to the end of the buffer.
func (b *Buffer) AppendBytes(bytes ...byte) *Buffer {
	b.Value = append(b.Value, bytes...)
	return b
}

// Append appends a byte array to the end of the buffer.
func (b *Buffer) Append(data []byte) *Buffer {
	b.Value = append(b.Value, data...)
	return b
}

// Prepend prepends bytes in front of the buffer. Caller must ensure total bytes prepended is
// no more than 16 bytes.
func (b *Buffer) Prepend(data []byte) *Buffer {
	b.SliceBack(len(data))
	copy(b.Value, data)
	return b
}

// Bytes returns the content bytes of this Buffer.
func (b *Buffer) Bytes() []byte {
	return b.Value
}

// Slice cuts the buffer at the given position.
func (b *Buffer) Slice(from, to int) *Buffer {
	b.Value = b.Value[from:to]
	return b
}

// SliceFrom cuts the buffer at the given position.
func (b *Buffer) SliceFrom(from int) *Buffer {
	b.Value = b.Value[from:]
	return b
}

// SliceBack extends the Buffer to its front by offset bytes.
// Caller must ensure cumulated offset is no more than 16.
func (b *Buffer) SliceBack(offset int) *Buffer {
	newoffset := b.offset - offset
	if newoffset < 0 {
		newoffset = 0
	}
	b.Value = b.head[newoffset : b.offset+len(b.Value)]
	b.offset = newoffset
	return b
}

// Len returns the length of the buffer content.
func (b *Buffer) Len() int {
	if b == nil {
		return 0
	}
	return len(b.Value)
}

// IsFull returns true if the buffer has no more room to grow.
func (b *Buffer) IsFull() bool {
	return len(b.Value) == cap(b.Value)
}

// Write implements Write method in io.Writer.
func (b *Buffer) Write(data []byte) (int, error) {
	b.Append(data)
	return len(data), nil
}

// Read implements io.Reader.Read().
func (b *Buffer) Read(data []byte) (int, error) {
	if b.Len() == 0 {
		return 0, io.EOF
	}
	nBytes := copy(data, b.Value)
	if nBytes == b.Len() {
		b.Value = b.Value[:0]
	} else {
		b.Value = b.Value[nBytes:]
	}
	return nBytes, nil
}

type bufferPool struct {
	chain     chan []byte
	allocator *sync.Pool
}

func newBufferPool(bufferSize, poolSize int) *bufferPool {
	pool := &bufferPool{
		chain: make(chan []byte, poolSize),
		allocator: &sync.Pool{
			New: func() interface{} { return make([]byte, bufferSize) },
		},
	}
	for i := 0; i < poolSize/2; i++ {
		pool.chain <- make([]byte, bufferSize)
	}
	return pool
}

func (p *bufferPool) allocate() *Buffer {
	var b []byte
	select {
	case b = <-p.chain:
	default:
		b = p.allocator.Get().([]byte)
	}
	return &Buffer{
		head:   b,
		pool:   p,
		Value:  b[defaultOffset:],
		offset: defaultOffset,
	}
}

func (p *bufferPool) free(buffer *Buffer) {
	select {
	case p.chain <- buffer.head:
	default:
		p.allocator.Put(buffer.head)
	}
}

var smallPool = newBufferPool(1024, 64)
var mediumPool = newBufferPool(8*1024, 128)
var largePool = newBufferPool(64*1024, 64)

// NewSmallBuffer creates a Buffer with 1K bytes of arbitrary content.
func NewSmallBuffer() *Buffer {
	return smallPool.allocate()
}

// NewBuffer creates a Buffer with 8K bytes of arbitrary content.
func NewBuffer() *Buffer {
	return mediumPool.allocate()
}

// NewLargeBuffer creates a Buffer with 64K bytes of arbitrary content.
func NewLargeBuffer() *Buffer {
	return largePool.allocate()
}