HDMI Morse code transmission using an animated GIF

/* Compile with: gcc -std=c11 -o gif gif.c -lgd */

#include <stdio.h>
#include <string.h>
#include <stdbool.h>
#include <err.h>
#include <gd.h>

const char * const morse_alpha[] = {
    ".-",   // A
    "-...", // B
    "-.-.", // C
    "-..",  // D
    ".",    // E
    "..-.", // F
    "--.",  // G
    "....", // H
    "..",   // I
    ".---", // J
    "-.-",  // K
    ".-..", // L
    "--",   // M
    "-.",   // N
    "---",  // O
    ".--.", // P
    "--.-", // Q
    ".-.",  // R
    "...",  // S
    "-",    // T
    "..-",  // U
    "...-", // V
    ".--",  // W
    "-..-", // X
    "-.--", // Y
    "--..", // Z
    "    " // space
};

const char * const morse_digit[] = {
    "-----", // 0
    ".----", // 1
    "..---", // 2
    "...--", // 3
    "....-", // 4
    ".....", // 5
    "-....", // 6
    "--...", // 7
    "---..", // 8
    "----.", // 9
};

int morse_encode(int *data, int data_len, const char *string)
{
    int data_idx = 0;

    void append_data(int value)
    {
    if(data_idx < data_len)
        data[data_idx++] = value;
    }

    char c;
    while(c = *string++) {
    int idx;
    char **table = NULL;

    if(c >= 'A' && c <= 'Z') {
        table = morse_alpha;
        idx = c - 'A';
    } else if(c >= 'a' && c <= 'z') {
        table = morse_alpha;
        idx = c - 'a';
    } else if(c >= '0' && c <= '9') {
        table = morse_digit;
        idx = c - '0';
    } else  if(c == ' ') {
        table = morse_alpha;
        idx = 26;
    } else {
        abort();
    }
    if(table == NULL)
        continue;

    const char *morse = table[idx];

    printf("%c -> %s\n", c, table[idx]);

    for(int i = 0; i < strlen(morse); i++) {
        char m = morse[i];
        switch(m) {
        case '.':
        append_data(1);
        break;
        case '-':
        append_data(1);
        append_data(1);
        append_data(1);
        break;
        case ' ':
        append_data(0);
        break;
        }

        append_data(0);
    }

    // space between letters is three units
    append_data(0);
    append_data(0);
    }

    return data_idx;
}


int main(int argc, char **argv)
{
    gdImagePtr im[2];
    FILE *out;

    int num_frames = 3;

    for(int i = 0; i < num_frames; i++) {
    gdImagePtr img;
    img = gdImageCreate(8, 8);

    int fg_val = 0x7f;
    int bg_val = 0x80;

    #if 0
    fg_val = 0xff;
    bg_val = 0x00;
    #endif

    #if 0
    int palette[256];

    for(int c = 0; c < 256; c++) {
        palette[c] = gdImageColorAllocate(img, c, c, c);
    }
    #endif

    int fg = gdImageColorAllocate(img, fg_val, fg_val, fg_val);
    int bg = gdImageColorAllocate(img, bg_val, bg_val, bg_val);

    for(int y = 0; y < gdImageSY(img); y++) {
        for(int x = 0; x < gdImageSX(img); x++) {
        bool tx_on = false;

        if(i >= 1) {
            if(1) { // y < (gdImageSY(img) / 2) ) {
            if((x >> 0) & 1)
                tx_on = !tx_on;
            if((y >> 0) & 1)
                tx_on = !tx_on;
            } else {
            if((x >> 1) & 1)
                tx_on = !tx_on;
            if((y >> 1) & 1)
                tx_on = !tx_on;
            }
        } else {
            if((x >> 1) & 1)
            tx_on = !tx_on;
            if((y >> 1) & 1)
            tx_on = !tx_on;
            tx_on = false;
        }

        if(i == 2) {
            if(y > (gdImageSY(img) / 2))
            tx_on = false;
        }
        gdImageSetPixel(img, x, y, tx_on ? fg : bg);
        }
    }

    im[i] = img;
    }

    out = fopen("out.gif", "w");
    if(out == NULL)
    err(1, "unable to open output file");

    int morse_array[16384];
    int len;
    len = morse_encode(morse_array, 16384, "HIDDEN MORSE TX IN WEB PAGE WTF  ");
    for(int i = 0; i < len; i++) {
    printf("%d", morse_array[i]);
    }
    printf("\n");

    gdImageGifAnimBegin(im[2], out, 1, 0);

    #if 0
    gdImageGifAnimAdd(im[2], out, 0, 0, 0, 0,
              gdDisposalRestorePrevious, NULL);
    #endif

    gdImagePtr prev = im[0];

    srand(1);
    int prev_data = morse_array[0];
    int im_prev_data = -1;
    int pos = 1;
    int num_periods = 0;
    while(pos <= len) {
    int data;
    if(pos < len)
        data = morse_array[pos];
    else
        data = -1;
    num_periods++;

    if(data != prev_data) {
        gdImageGifAnimAdd(im[prev_data], out, 0, 0, 0, num_periods * 10,
                  gdDisposalNone, NULL);
        num_periods = 0;
    }

    prev_data = data;
    pos++;
    }

    #if 0
    for(int i = 0; i < len; i++) {
    int idx = morse_array[i] & 1;
    gdImageGifAnimAdd(im[idx], out, 0, 0, 0, 10, gdDisposalRestorePrevious, NULL);
    }
    #endif
    gdImageGifAnimEnd(out);

    fclose(out);

    return 0;
}