/*  Copyright (C) 2024-2025 P. David Buchan (pdbuchan@gmail.com)
  
    This program is free software: you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
    the Free Software Foundation, either version 3 of the License, or
    (at your option) any later version.
  
    This program is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU General Public License for more details.

    You should have received a copy of the GNU General Public License
    along with this program.  If not, see <http://www.gnu.org/licenses/>.
*/

// txt2srt.c - Take the timestamps from a SubRip (.srt) file and the text from a text file and create a new .srt file.
//             The text file must have the same number of subtitles as the SubRip file, and they must be separated by single blank lines.

// gcc -Wall txt2srt.c -o txt2srt

// Usage: ./txt2srt inputfilename.srt inputfilename.txt
// Output: out.srt

#include <stdio.h>
#include <stdlib.h>
#include <inttypes.h>  // uint8_t
#include <string.h>

// Definition of structs
typedef struct {
  int len;
  char *name;
  uint8_t *sequence;
} BOM;

// Function prototypes
int readline (FILE *, char *, int);
int byteordermark (char *, BOM *);
char *allocate_strmem (int);
char **allocate_strmemp (int);
BOM *allocate_bommem (int);

// Set some symbolic constants.
#define MAXLEN 256  // Maximum number of characters per line
#define MAXLINES 10  // Maximum number of lines of text per subtitle
#define MAXBOM 11  // Maximum number of Byte Order Mark (BOM) types

int
main (int argc, char **argv) {

  int type, allsrtlines, alltxtlines, nsrtlines, ntxtlines, line, srtline, txtline, nsubs, sub;
  char *temp, *srtfilename, *txtfilename, **srtinput, **txtinput;
  BOM *bom;
  FILE *fi1, *fi2, *fo;

  // Byte Order Mark (BOM) names and sequences.
  char name[MAXBOM][30] = {"UTF-8", "UTF-16 (BE)", "UTF-16 (LE)", "UTF-32 (BE)", "UTF-32 (LE)", "UTF-7", "UTF-1", "UTF-EBCDIC", "SCSU", "BOCU-1", "GB18030"};
  uint8_t utf8[3]       = {0xef, 0xbb, 0xbf};
  uint8_t utf16be[2]    = {0xfe, 0xff};
  uint8_t utf16le[2]    = {0xff, 0xfe};
  uint8_t utf32be[4]    = {0x00, 0x00, 0xfe, 0xff};
  uint8_t utf32le[4]    = {0xff, 0xfe, 0x00, 0x00};
  uint8_t utf7[3]       = {0x2b, 0x2f, 0x76};
  uint8_t utf1[3]       = {0xf7, 0x64, 0x4c};
  uint8_t utfebcdic[4]  = {0xdd, 0x73, 0x66, 0x73};
  uint8_t scsu[3]       = {0x0e, 0xfe, 0xff};
  uint8_t bocu1[3]      = {0xfb, 0xee, 0x28};
  uint8_t gb18030[4]    = {0x84, 0x31, 0x95, 0x33};

  // Allocate memory for various arrays.
  srtfilename = allocate_strmem (MAXLEN);
  txtfilename = allocate_strmem (MAXLEN);

  // Process the command line arguments, if any.
  if (argc == 3) {
    strncpy (srtfilename, argv[1], MAXLEN);
    strncpy (txtfilename, argv[2], MAXLEN);

  } else {
    fprintf (stdout, "\nUsage: ./txt2srt inputfilename.srt inputfilename.txt\n");
    fprintf (stdout, "       Output filename will be out.srt.\n\n");
    free (srtfilename);
    free (txtfilename);
    return (EXIT_SUCCESS);
  }

  // Allocate memory for various arrays.
  temp = allocate_strmem (MAXLEN);
  bom = allocate_bommem (MAXBOM); 

  // Populate array with Byte Order Mark data.
  bom[0].len = 3;    bom[0].name = name[0];    bom[0].sequence = utf8;
  bom[1].len = 2;    bom[1].name = name[1];    bom[1].sequence = utf16be;
  bom[2].len = 2;    bom[2].name = name[2];    bom[2].sequence = utf16le;
  bom[3].len = 4;    bom[3].name = name[3];    bom[3].sequence = utf32be;
  bom[4].len = 4;    bom[4].name = name[4];    bom[4].sequence = utf32le;
  bom[5].len = 3;    bom[5].name = name[5];    bom[5].sequence = utf7;
  bom[6].len = 3;    bom[6].name = name[6];    bom[6].sequence = utf1;
  bom[7].len = 4;    bom[7].name = name[7];    bom[7].sequence = utfebcdic;
  bom[8].len = 3;    bom[8].name = name[8];    bom[8].sequence = scsu;
  bom[9].len = 3;    bom[9].name = name[9];    bom[9].sequence = bocu1;
  bom[10].len = 4;   bom[10].name = name[10];  bom[10].sequence = gb18030;

  fprintf (stdout, "\nInput SubRip file (for timestamps): %s\n", srtfilename);

  // Open existing SubRip (.srt) file which contains the time stamps.
  fi1 = fopen (srtfilename, "r");
  if (fi1 == NULL) {
    fprintf (stderr, "ERROR: Unable to open input SubRip file %s.\n", srtfilename);
    exit (EXIT_FAILURE);
  }

  // Count lines of input SubRip file.
  allsrtlines = 0;  // Count of lines
  while (readline (fi1, temp, MAXLEN) != -1) {
    allsrtlines++;
  }
  fprintf (stdout, "\n%i lines found including any excess trailing line-feeds.\n", allsrtlines);
  rewind (fi1);

  // Allocate memory for array to hold input file.
  srtinput = allocate_strmemp (allsrtlines);
  for (line=0; line<allsrtlines; line++) {
    srtinput[line] = allocate_strmem (MAXLEN);
  }

  // Read input SubRip file into array input.
  for (line=0; line<allsrtlines; line++) {
    if (readline (fi1, srtinput[line], MAXLEN) == -1) {
      fprintf (stderr, "\nERROR: Cannot read line %i from input SubRip file %s.\n", line + 1, srtfilename);
      exit (EXIT_FAILURE);
    }
  }  // Next line

  // Close input file.
  fclose (fi1);

  // Remove excess line-feeds at end of array input.
  nsrtlines = allsrtlines;
  for (line=allsrtlines; line>1; line--) {
    if ((srtinput[line - 1][0] == '\n') && (srtinput[line - 2][0] == '\n')) {
      nsrtlines--;
    } else {
      break;
    }
  }
  fprintf (stdout, "%i lines found excluding trailing line-feeds.\n", nsrtlines);

  // Count number of subtitles in SubRip file; assume at least one.
  nsubs = 0;
  for (line=0; line<nsrtlines; line++) {

    nsubs++;

    // Advance through to next subtitle number, if there is one.
    // End of current subtitle is demarcated by a line containing only a line-feed.
    while (srtinput[line][0] != '\n') {
      line++;
      if (line == nsrtlines) break;
    }

  }  // Next sub
  fprintf (stdout, "\n%i subtitles found in %s. Assuming same in %s.\n\n", nsubs, srtfilename, txtfilename);

  fprintf (stdout, "Input text file (for subtitle texts): %s\n", txtfilename);

  // Open existing .txt file which contains the subtitle text, each sub separated by a blank line.
  fi2 = fopen (txtfilename, "r");
  if (fi2 == NULL) {
    fprintf (stderr, "ERROR: Unable to open input text file %s.\n", txtfilename);
    exit (EXIT_FAILURE);
  } 

  // Count lines of input text file.
  alltxtlines = 0;  // Count of lines
  while (readline (fi2, temp, MAXLEN) != -1) {
    alltxtlines++;
  }
  fprintf (stdout, "\n%i lines found including any excess trailing line-feeds.\n", alltxtlines);
  rewind (fi2);

  // Allocate memory for array to hold input file.
  txtinput = allocate_strmemp (alltxtlines);
  for (line=0; line<alltxtlines; line++) {
    txtinput[line] = allocate_strmem (MAXLEN);
  }

  // Read input SubRip file into array input.
  for (line=0; line<alltxtlines; line++) {
    if (readline (fi2, txtinput[line], MAXLEN) == -1) {
      fprintf (stderr, "\nERROR: Cannot read line %i from input text file %s.\n", line + 1, txtfilename);
      exit (EXIT_FAILURE);
    }
  }  // Next line

  // Close input file.
  fclose (fi2);

  // Remove excess line-feeds at end of array input.
  ntxtlines = alltxtlines;
  for (line=alltxtlines; line>1; line--) {
    if ((txtinput[line - 1][0] == '\n') && (txtinput[line - 2][0] == '\n')) {
      ntxtlines--;
    } else {
      break;
    }
  }
  fprintf (stdout, "%i lines found excluding trailing line-feeds.\n", ntxtlines);

  // Detect any Byte Order Mark (BOM) at beginning of first line.
  type = byteordermark (txtinput[0], bom);
  if (type < 0) {
    fprintf (stdout, "\nNo known Byte Order Mark (BOM) found in %s.\n", txtfilename);
  } else {
    fprintf (stdout, "\nByte Order Mark (BOM) detected for character encoding type: %s\n", bom[type].name);
  }

  // Remove existing Byte Order Mark (BOM) from first line of text file.
  if (type >= 0) {
    memset (temp, 0, MAXLEN * sizeof (char));
    strncpy (temp, txtinput[0] + bom[type].len, MAXLEN);
    memset (txtinput[0], 0, MAXLEN * sizeof (char));
    strncpy (txtinput[0], temp, MAXLEN);
  }

  // Open output file.
  fo = fopen ("out.srt", "r");
  if (fo != NULL) {
    fprintf (stderr, "ERROR: Output file out.srt already exists.\n");
    exit (EXIT_FAILURE);
  }
  fo = fopen ("out.srt", "w");
  if (fo == NULL) {
    fprintf (stderr, "ERROR: Unable to open output file out.srt.\n");
    exit (EXIT_FAILURE);
  }

  // Write Byte Order Mark (BOM) to output file if detected in input file.
  if (type != -1) {
    fwrite (bom[type].sequence, bom[type].len * sizeof (uint8_t), 1, fo);
  }

  // Loop through all lines in input file.
  sub = 0;
  txtline = 0;  // Line of text file
  srtline = 0;  // Line of srt file
  while (srtline < nsrtlines) {

    // Write sub number to output file.
    sub++;
    fprintf (fo, "%i\n", sub);
    srtline++;

    // Copy time stamps from srt file to output file.
    fprintf (fo, "%s", srtinput[srtline]);
    srtline++;

    // Copy text lines from input text file to output file.
    while (txtinput[txtline][0] != '\n') {
      fprintf (fo, "%s", txtinput[txtline]);
      txtline++;
      srtline++;
      if (txtline >= ntxtlines) break;
    }

    fprintf (fo, "\n");
    txtline++;
    srtline++;
    if (txtline >= ntxtlines) break;

  }  // Next sub
  fprintf (stdout, "\n");

  // Close output file.
  fclose (fo);

  // Free allocated memory.
  free (temp);
  free (bom);
  free (srtfilename);
  free (txtfilename);
  for (line=0; line<allsrtlines; line++) {
    free (srtinput[line]);
  }
  free (srtinput);
  for (line=0; line<alltxtlines; line++) {
    free (txtinput[line]);
  }
  free (txtinput);

  return (EXIT_SUCCESS);
}

// Read a single line of text from a text file.
// Returns -1 if EOF is encountered.
int
readline (FILE *fi, char *line, int limit) {

  int i, n;

  i = 0;  // i is pointer to byte in line.
  while (i < limit) {

    // Grab next byte from file.
    n = fgetc (fi);

    // End of file reached.
    // Tell calling function, by returning -1, that we're at end of file, so it won't call readline() again.
    if (n == EOF) {

      // If there's no end of line at the end of the file, add a line-feed.
      if (i > 0) {
        line[i] = '\n';
        return (0);
      }
      return (-1);
    }

    // Found a carriage return. Ignore it.
    if (n == '\r') {
      continue;
    }

    // Seems to be a valid character. Keep it.
    line[i] = n;
    i++;

    // Found a newline.
    // Break out of loop since this is the end of the current line.
    if (n == '\n') {
      return (0);
    }

  }

  // Advance to next line.
  n = 0;
  while ((n != '\n') && (n != EOF)) {
    n = fgetc (fi);
  }

  return (0);
}

// Detect Byte Order Mark (BOM), if it exists, at beginning of line.
// Return index of bom array corresponding to type of BOM detected,
// or return -1 if none (or unlisted type) detected.
int
byteordermark (char *text, BOM *bom) {

  int type, i, found;

  // Loop through all types of Byte Order Marks.
  for (type=0; type<MAXBOM; type++) {

    found = 1;  // Default to current type detected.
    for (i=0; i<bom[type].len; i++) {
      if ((uint8_t) text[i] != bom[type].sequence[i]) found = 0;
    }

    // We found a match.
    if (found) return (type);
  }

  // Failed to find a match.
  return (-1);
}

// Allocate memory for an array of chars.
char *
allocate_strmem (int len) {

  void *tmp;

  if (len <= 0) {
    fprintf (stderr, "ERROR: Cannot allocate memory because len = %i in allocate_strmem().\n", len);
    exit (EXIT_FAILURE);
  }

  tmp = (char *) malloc (len * sizeof (char));
  if (tmp != NULL) {
    memset (tmp, 0, len * sizeof (char));
    return (tmp);
  } else {
    fprintf (stderr, "ERROR: Cannot allocate memory for array in allocate_strmem().\n");
    exit (EXIT_FAILURE);
  }
}

// Allocate memory for an array of pointers to arrays of chars.
char **
allocate_strmemp (int len) {

  void *tmp;

  if (len <= 0) {
    fprintf (stderr, "ERROR: Cannot allocate memory because len = %i in allocate_strmemp().\n", len);
    exit (EXIT_FAILURE);
  }

  tmp = (char **) malloc (len * sizeof (char *));
  if (tmp != NULL) {
    memset (tmp, 0, len * sizeof (char *));
    return (tmp);
  } else {
    fprintf (stderr, "ERROR: Cannot allocate memory for array in allocate_strmemp().\n");
    exit (EXIT_FAILURE);
  }
}

// Allocate memory for an array of BOM (Byte Order Mark) structs.
BOM *
allocate_bommem (int len) {

  void *tmp;

  if (len <= 0) {
    fprintf (stderr, "ERROR: Cannot allocate memory because len = %i in allocate_bommem().\n", len);
    exit (EXIT_FAILURE);
  }

  tmp = (BOM *) malloc (len * sizeof (BOM));
  if (tmp != NULL) {
    memset (tmp, 0, len * sizeof (BOM));
    return (tmp);
  } else {
    fprintf (stderr, "ERROR: Cannot allocate memory for array in allocate_bommem().\n");
    exit (EXIT_FAILURE);
  }
}