gifLoop.c

// MIT License

// Copyright (c) 2021 Jack Hogan

// Permission is hereby granted, free of charge, to any person obtaining a copy
// of this software and associated documentation files (the "Software"), to deal
// in the Software without restriction, including without limitation the rights
// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the Software is
// furnished to do so, subject to the following conditions:

// The above copyright notice and this permission notice shall be included in all
// copies or substantial portions of the Software.

// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
// SOFTWARE.

// GIF Loop Configuration Utility
// Requires standard C compiler

#include <stdio.h>
#include <stdint.h>
#include <stdbool.h>
#include <stddef.h>
#include <stdlib.h>
#include <string.h>
#include <math.h>

void escapeSubBlocks(FILE *f) {
    // Find size bit and skip until its zero
    char nextSize;
    do {
        fread(&nextSize, 1, 1, f);
        fseek(f, nextSize, SEEK_CUR);
    }
    while (nextSize);
}

size_t nextColorTableResolution = 0;

void handleColorTable(FILE *f) {
    fseek(f, 3 * pow(2, nextColorTableResolution + 1), SEEK_CUR);
}

void handleCommentExtension(FILE *f) {
    escapeSubBlocks(f);
}

void handleApplicationExtension(FILE *f) {
    // Skip Application Block
    char toSkip;
    fread(&toSkip, 1, 1, f);
    fseek(f, toSkip, SEEK_CUR);

    // Make sure formatting is correct for looping
    char subBlock[2];
    fread(subBlock, 1, 2, f);
    if (subBlock[0] != 0x03 || subBlock[1] != 0x01) {
        printf("Invalid Application Extension block\n");
        fclose(f);
        exit(5);
    }
}

void handlePlainTextExtension(FILE *f) {
    // Skip required bytes
    char toSkip;
    fread(&toSkip, 1, 1, f);
    fseek(f, toSkip, SEEK_CUR);

    escapeSubBlocks(f);
}

void handleImageData(FILE *f) {
    // Seek past LZW
    fseek(f, 1, SEEK_CUR);

    escapeSubBlocks(f);
}

void handleImageDescriptor(FILE *f) {
    // Seek to packed field
    fseek(f, 8, SEEK_CUR);

    char packed;
    fread(&packed, 1, 1, f);
    // Is there a local color table?
    if (packed >> 7) {
        // Read size bits and seek past table
        fseek(f, 3 * pow(2, (packed & 0x7) + 1), SEEK_CUR);
    }
    handleImageData(f);
}

void handleGraphicsControlExtension(FILE *f) {
    // Seek out of extension
    fseek(f, 6, SEEK_CUR);
}

void handleLogicalScreenDescriptor(FILE *f) {
    // Seek to packed field
    fseek(f, 4, SEEK_CUR);

    char packed;
    fread(&packed, 1, 1, f);
    // Seek to next section
    fseek(f, 2, SEEK_CUR);
    // Is there a global color table?
    if (packed >> 7) {
        // Read size bits and seek past table
        fseek(f, 3 * pow(2, (packed & 0x7) + 1), SEEK_CUR);
    }
}

void handleHeader(FILE *f) {
    // Skip header field
    fseek(f, 6, SEEK_CUR);

    handleLogicalScreenDescriptor(f);
}

void findLoopBytes(FILE *f) {
    fseek(f, 0, SEEK_SET);
    handleHeader(f);
    char current;
    while (fread(&current, 1, 1, f)) {
        if (current == 0x2C) {
            handleImageDescriptor(f);
        }
        else if (current == 0x21) {
            char extension;
            fread(&extension, 1, 1, f);
            if (extension == 0x01) {
                handlePlainTextExtension(f);
            }
            // Be careful! 0xFF != (char)0xFF (overflow)
            else if (extension == (char)0xFF) {
                handleApplicationExtension(f);
                // Loop bytes found!
                return;
            }
            else if (extension == 0xFE) {
                handleCommentExtension(f);
            }
            else if (extension == 0xF9) {
                handleGraphicsControlExtension(f);
            }
        }
        else if (current == 0x3B) {
            // End of file reached
            printf("Application Extension block not found\n");
            fclose(f);
            exit(4);
        }
    }
}

void toggleGifLoop(FILE *f) {
    findLoopBytes(f);

    // Read current bytes
    char loopBytes[2];
    fread(&loopBytes, 1, 2, f);
    // Convert to integer
    uint16_t playCount = (loopBytes[1] << 8) + loopBytes[0];
    
    fseek(f, -2, SEEK_CUR);
    if (playCount == 0) {
        // Single play
        printf("Setting to play once\n");
        fputc(1, f);
        fputc(0, f);
    }
    else {
        // Enable infinite loop
        printf("Setting to play infinitely\n");
        fputc(0, f);
        fputc(0, f);
    }
}

void setGifLoop(FILE *f, uint16_t playCount) {
    findLoopBytes(f);

    // Needs to be here for Windows to work correctly
    fseek(f, 2, SEEK_CUR);
    fseek(f, -2, SEEK_CUR);

    fputc((int)(playCount & 0x00FF), f);
    fputc((int)(playCount >> 8), f);
}

bool verifyIsGif(FILE *f) {
    char version[7];
    fread(version, 1, 6, f);
    version[6] = '\0';
    return strcmp(version, "GIF89a") == 0;
}

void openFile(char *path, FILE **f) {
    (*f) = fopen(path, "rb+");
    printf("Reading file...\n");
    if (*f == NULL) {
        perror("Error reading file");
        exit(2);
    }

    printf("Verifying type...\n");
    if (!verifyIsGif(*f)) {
        printf("File is not a GIF or is not of standard GIF89a\n");
        fclose(f);
        exit(3);
    }

    printf("File is a GIF\n");
}

int main(int argc, char *argv[]) {
    printf("=== GIF Loop Configuration Utility ===\n");
    printf("Copyright 2021 Jack Hogan\n");
    printf("MIT License\n\n");

    FILE *f = NULL;

    if (argc == 1) {
        // No arguments supplied
        printf("Usage: gifLoop <file> [playTimes]\n\n");
        printf("playTimes: Optional integer from 0-65535 (0=infinite plays), leaving blank will toggle between infinite and single play mode\n\n");
        printf("Example Usage: gifLoop myGif.gif 20\n");
        printf("Example Usage: gifLoop myGif.gif\n");
    } 
    else if (argc > 3) {
        printf("Too many arguments given");
        return 1;
    } 
    else if (argc == 2) {
        // Toggle loop
        openFile(argv[1], &f);
        printf("Toggling GIF loop\n");
        toggleGifLoop(f);
    } 
    else {
        // Loop information given
        openFile(argv[1], &f);
        uint16_t count = (uint16_t)atoi(argv[2]);
        printf("Setting GIF play count to %d\n", count);
        setGifLoop(f, count);
    }

    if (f != NULL) {
        fclose(f);
    }
    return 0;
}