gif

gif.go

package main

import (
	"bytes"
	"encoding/binary"
	"fmt"
	"os"
)

func main() {

	if len(os.Args) != 3 {
		fmt.Println("need 2 args (ex: gif input output.gif")
		os.Exit(-1)
	}

	input_file_info, err := os.Stat(os.Args[1])
	if err != nil {
		fmt.Println("input file not found")
		os.Exit(-1)
	}
	var inputFileSize int64 = input_file_info.Size()
	inputFile, err := os.Open(os.Args[1])
	defer inputFile.Close()
	if err != nil {
		fmt.Println("input file error")
		os.Exit(-1)
	}

	outputFile, err := os.Create(os.Args[2])
	defer outputFile.Close()
	if err != nil {
		fmt.Println("input file error")
		os.Exit(-1)
	}

	initWrite(outputFile)
	fileDataWrite(inputFile, inputFileSize, outputFile)
	trailerWrite(outputFile)

}

func initWrite(outputFile *os.File) {
	buf := new(bytes.Buffer)

	// Signature
	binary.Write(buf, binary.LittleEndian, []byte("GIF"))

	// Version
	binary.Write(buf, binary.LittleEndian, []byte("89a"))

	// Width
	binary.Write(buf, binary.LittleEndian, uint16(0xFFFF))

	// Height
	binary.Write(buf, binary.LittleEndian, uint16(0xFFFF))

	// Color Datas
	binary.Write(buf, binary.LittleEndian, uint8(0))

	// Background Color Index
	binary.Write(buf, binary.LittleEndian, uint8(0))

	// Pixel Aspect Ratio
	binary.Write(buf, binary.LittleEndian, uint8(0))

	// Image Separator
	binary.Write(buf, binary.LittleEndian, uint8(0x2C))

	// Image Left Position
	binary.Write(buf, binary.LittleEndian, uint16(0))

	// Image Top Position
	binary.Write(buf, binary.LittleEndian, uint16(0))

	// Width
	binary.Write(buf, binary.LittleEndian, uint16(0xFFFF))

	// Height
	binary.Write(buf, binary.LittleEndian, uint16(0xFFFF))

	// Color Datas
	binary.Write(buf, binary.LittleEndian, uint8(0))

	// LZW Minimum Code Size
	binary.Write(buf, binary.LittleEndian, uint8(8))

	outputFile.Write(buf.Bytes())
}

func fileDataWrite(inputFile *os.File, intputFileSize int64, outputFile *os.File) {
	buf := new(bytes.Buffer)
	b255 := make([]byte, 255)
	for i := int64(0); i < (intputFileSize / 255); i++ {
		binary.Write(buf, binary.LittleEndian, uint8(255))
		inputFile.Read(b255)
		binary.Write(buf, binary.LittleEndian, b255)
		if i/int64(1024) == 0 {
			outputFile.Write(buf.Bytes())
			buf.Reset()
		}
	}

	remain := make([]byte, intputFileSize%255)
	binary.Write(buf, binary.LittleEndian, uint8(intputFileSize%255))
	inputFile.Read(remain)
	binary.Write(buf, binary.LittleEndian, remain)

	binary.Write(buf, binary.LittleEndian, uint8(0))
	outputFile.Write(buf.Bytes())
}

func trailerWrite(outputFile *os.File) {
	outputFile.Write([]byte{0x3B})
}