gif-extractor-go/main.go

package main

import (
	"bytes"
	"compress/lzw"
	"encoding/binary"
	"fmt"
	"io"
	"math"
	"os"
	"strings"
)

func readNextBytes(file *os.File) byte {
	nextBytes := make([]byte, 1)
	file.Read(nextBytes)
	file.Seek(-1, os.SEEK_CUR)

	return nextBytes[0]
}

func (v Palette) UnmarshalBinary(data []byte) error {
	if len(v)*3 != len(data) {
		return fmt.Errorf("len is not valid. required: %d, actual: %d", len(v)*3, len(data))
	}
	for i := 0; i < len(v); i++ {
		v[i].Red = data[i*3]
		v[i].Green = data[i*3+1]
		v[i].Blue = data[i*3+2]
	}
	return nil
}

func (v Palette) MarshalBinary() []byte {
	data := make([]byte, len(v)*3)

	for i := 0; i < len(v); i++ {
		data[i*3] = v[i].Red
		data[i*3+1] = v[i].Green
		data[i*3+2] = v[i].Blue
	}

	return data
}

func deinterlace() {
	// TODO:
}

func main() {
	inputFile := os.Args[1]

	file, err := os.OpenFile(inputFile, os.O_RDONLY, os.FileMode(0755))
	if err != nil {
		panic(err)
	}

	header := Header{}
	headerData := make([]byte, 13)
	file.Read(headerData)
	binary.Read(bytes.NewBuffer(headerData), binary.LittleEndian, &header)

	if strings.ToUpper(string(header.Signature[:])) != "GIF" {
		panic("Not a valid gif file")
	}

	fmt.Print("Beginning extraction of gif frames\n\n")

	fmt.Printf("GIF version is: %v\n", string(header.Version[:]))
	if string(header.Version[:]) == "89a" {
		fmt.Print("WARNING: some features are unimplemented for this version. Some images may not extract correctly\n\n")
	}
	fmt.Printf("GIF aspect ratio is: %v\n", header.AspectRatio)
	fmt.Printf("GIF background color index is: %v\n", header.BackgroundColor)
	fmt.Printf("GIF height is: %v\n", header.ScreenHeight)
	fmt.Printf("GIF width is: %v\n", header.ScreenWidth)

	GlobalColorTableFlag := (header.Packed & 0x80) >> 7
	GlobalColorTableEntries := 1 << ((header.Packed & 7) + 1)
	GlobalColorTableSize := 3 * GlobalColorTableEntries // number of entries * 3 colors (r, g, b)

	fmt.Printf("\n")
	fmt.Printf("Global color table flag: %v\n", GlobalColorTableFlag)
	fmt.Printf("Global color table size: %v\n", GlobalColorTableSize)
	fmt.Printf("global color table entries: %v\n", GlobalColorTableEntries)

	st, err := file.Stat()
	if err != nil {
		panic(err)
	}
	dirName := strings.Split(st.Name(), ".gif")[0]

	os.Mkdir(dirName, os.FileMode(0755))

	var globalPalette Palette
	var currentPalette Palette
	transparencyIndex := -1

	if GlobalColorTableFlag == 1 { // global color table exists, use it
		globalColorTable := make([]byte, GlobalColorTableSize)
		file.Read(globalColorTable)

		globalPalette = make(Palette, GlobalColorTableEntries)
		globalPalette.UnmarshalBinary(globalColorTable)

		currentPalette = globalPalette
	}

	counter := 1

	for readNextBytes(file) != TRAILER {
		nextByte := make([]byte, 1)
		file.Read(nextByte)

		switch nextByte[0] {
		case EXTENSION_BLOCK:
			{
				nextSecondByte := make([]byte, 1)
				file.Read(nextSecondByte)

				switch nextSecondByte[0] {
				case PLAINTEXT_BLOCK:
					{
						file.Seek(PLAINTEXT_BLOCK_SIZE+1, os.SEEK_CUR)
						plaintextDataSize := make([]byte, 1)
						file.Read(plaintextDataSize)
						file.Seek(int64(plaintextDataSize[0]), os.SEEK_CUR)

						// TODO: read plaintext data and draw it to the frame (somehow idk ???)

						break
					}
				case GRAPHICS_CONTROL_BLOCK:
					{
						// skip the length byte
						file.Seek(1, os.SEEK_CUR)

						// TODO: use the delayTime for something ???

						block := GraphicsControlBlock{}
						data := make([]byte, GRAPHICS_CONTROL_BLOCK_SIZE)

						file.Read(data)

						binary.Read(bytes.NewBuffer(data), binary.LittleEndian, &block)

						transparencyFlag := block.Packed & 1

						if transparencyFlag == 1 {
							transparencyIndex = int(block.TransparentColorIndex)
						}

						break
					}
				case APPLICATION_BLOCK:
					{
						file.Seek(APPLICATION_BLOCK_SIZE+1, os.SEEK_CUR)
						applicationDataSize := make([]byte, 1)
						file.Read(applicationDataSize)
						file.Seek(int64(applicationDataSize[0]), os.SEEK_CUR)

						// TODO: read application data and do something with it ???

						break
					}
				case COMMENT_BLOCK:
					{
						commentDataSize := make([]byte, 1)
						file.Read(commentDataSize)
						file.Seek(int64(commentDataSize[0]), os.SEEK_CUR)

						// TODO: read comment data and output it to file ???

						break
					}
				}
				break
			}
		case IMAGE_DESCRIPTOR:
			{
				imageDescriptor := ImageDescriptor{}
				imageDescriptorData := make([]byte, 9)
				file.Read(imageDescriptorData)
				binary.Read(bytes.NewBuffer(imageDescriptorData), binary.LittleEndian, &imageDescriptor)

				decompressedFrameSize := uint64(int64(imageDescriptor.Height) * int64(imageDescriptor.Width))

				LocalColorTableFlag := (imageDescriptor.Packed & 0x80) >> 7
				InterlaceFlag := (imageDescriptor.Packed & 0x40) >> 6
				SortFlag := (imageDescriptor.Packed & 0x20) >> 5
				LocalColorTableEntries := math.Pow(2, float64((imageDescriptor.Packed&7)+1))

				// TODO: use offsets when writing image ??

				// local color table comes first
				var localPalette Palette
				if LocalColorTableFlag == 1 {
					tempLocalColorTable := make([]byte, int(LocalColorTableEntries*3))
					file.Read(tempLocalColorTable)

					localPalette = make(Palette, int(LocalColorTableEntries))
					localPalette.UnmarshalBinary(tempLocalColorTable)

					currentPalette = localPalette
				}

				if InterlaceFlag == 1 {
					// TODO: implement deinterlacing
					deinterlace()
				}

				if SortFlag == 1 {
					// TODO: implement sort flag thingy ???
				}

				LZWCodeSize := make([]byte, 1)
				file.Read(LZWCodeSize)

				reader := lzw.NewReader(newBlockReader(file), lzw.LSB, int(LZWCodeSize[0]))
				defer reader.Close()

				decompressedFrameData := make([]byte, decompressedFrameSize)

				_, err = io.ReadFull(reader, decompressedFrameData)
				if err != nil {
					panic(err)
				}

				fileName := fmt.Sprintf("./%s/%s-%v.png", dirName, dirName, counter)
				WriteToPNG(decompressedFrameData, currentPalette, fileName, imageDescriptor.Width, imageDescriptor.Height, transparencyIndex)

				counter++
				break
			}
		}
		currentPalette = globalPalette

		// skip the terminator byte
		file.Seek(1, os.SEEK_CUR)
	}

	fmt.Print("Extracted frames from gif successfully!\n")

	file.Close()
}