common.go

package zecutil

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

// binaryFreeList defines a concurrent safe free list of byte slices (up to the
// maximum number defined by the binaryFreeListMaxItems constant) that have a
// cap of 8 (thus it supports up to a uint64).  It is used to provide temporary
// buffers for serializing and deserializing primitive numbers to and from their
// binary encoding in order to greatly reduce the number of allocations
// required.
//
// For convenience, functions are provided for each of the primitive unsigned
// integers that automatically obtain a buffer from the free list, perform the
// necessary binary conversion, read from or write to the given io.Reader or
// io.Writer, and return the buffer to the free list.
type binaryFreeList chan []byte

// binaryFreeListMaxItems is the number of buffers to keep in the free
// list to use for binary serialization and deserialization.
const binaryFreeListMaxItems = 1024

var (
	// binarySerializer provides a free list of buffers to use for serializing and
	// deserializing primitive integer values to and from io.Readers and io.Writers.
	binarySerializer binaryFreeList = make(chan []byte, binaryFreeListMaxItems)

	// littleEndian is a convenience variable since binary.LittleEndian is
	// quite long.
	littleEndian = binary.LittleEndian
)

// Borrow returns a byte slice from the free list with a length of 8.  A new
// buffer is allocated if there are not any available on the free list.
func (l binaryFreeList) Borrow() []byte {
	var buf []byte
	select {
	case buf = <-l:
	default:
		buf = make([]byte, 8)
	}
	return buf[:8]
}

// Return puts the provided byte slice back on the free list.  The buffer MUST
// have been obtained via the Borrow function and therefore have a cap of 8.
func (l binaryFreeList) Return(buf []byte) {
	select {
	case l <- buf:
	default:
		// Let it go to the garbage collector.
	}
}

// PutUint8 copies the provided uint8 into a buffer from the free list and
// writes the resulting byte to the given writer.
func (l binaryFreeList) PutUint8(w io.Writer, val uint8) error {
	buf := l.Borrow()[:1]
	buf[0] = val
	_, err := w.Write(buf)
	l.Return(buf)
	return err
}

// PutUint16 serializes the provided uint16 using the given byte order into a
// buffer from the free list and writes the resulting two bytes to the given
// writer.
func (l binaryFreeList) PutUint16(w io.Writer, byteOrder binary.ByteOrder, val uint16) error {
	buf := l.Borrow()[:2]
	byteOrder.PutUint16(buf, val)
	_, err := w.Write(buf)
	l.Return(buf)
	return err
}

// PutUint32 serializes the provided uint32 using the given byte order into a
// buffer from the free list and writes the resulting four bytes to the given
// writer.
func (l binaryFreeList) PutUint32(w io.Writer, byteOrder binary.ByteOrder, val uint32) error {
	buf := l.Borrow()[:4]
	byteOrder.PutUint32(buf, val)
	_, err := w.Write(buf)
	l.Return(buf)
	return err
}

// PutUint64 serializes the provided uint64 using the given byte order into a
// buffer from the free list and writes the resulting eight bytes to the given
// writer.
func (l binaryFreeList) PutUint64(w io.Writer, byteOrder binary.ByteOrder, val uint64) error {
	buf := l.Borrow()[:8]
	byteOrder.PutUint64(buf, val)
	_, err := w.Write(buf)
	l.Return(buf)
	return err
}

// WriteVarInt serializes val to w using a variable number of bytes depending
// on its value.
func WriteVarInt(w io.Writer, pver uint32, val uint64) error {
	if val < 0xfd {
		return binarySerializer.PutUint8(w, uint8(val))
	}

	if val <= math.MaxUint16 {
		err := binarySerializer.PutUint8(w, 0xfd)
		if err != nil {
			return err
		}
		return binarySerializer.PutUint16(w, littleEndian, uint16(val))
	}

	if val <= math.MaxUint32 {
		err := binarySerializer.PutUint8(w, 0xfe)
		if err != nil {
			return err
		}
		return binarySerializer.PutUint32(w, littleEndian, uint32(val))
	}

	err := binarySerializer.PutUint8(w, 0xff)
	if err != nil {
		return err
	}
	return binarySerializer.PutUint64(w, littleEndian, val)
}

// WriteVarBytes serializes a variable length byte array to w as a varInt
// containing the number of bytes, followed by the bytes themselves.
func WriteVarBytes(w io.Writer, pver uint32, bytes []byte) error {
	slen := uint64(len(bytes))
	err := WriteVarInt(w, pver, slen)
	if err != nil {
		return err
	}

	_, err = w.Write(bytes)
	return err
}