Add a delay line to a s16le mono 44100 wav file.

delay_wav.c

/*
DO WHAT THE FUCK YOU WANT TO BUT IT'S NOT MY FAULT PUBLIC LICENSE
Version 1, October 2013

Copyright (c) 2020 Al-buharie Amjari <healer.harie@gmail.com>

Everyone is permitted to copy and distribute verbatim or modified copies
of this license document, and changing it is allowed as long as the name
is changed.
DO WHAT THE FUCK YOU WANT TO BUT IT'S NOT MY FAULT PUBLIC LICENSE TERMS
AND CONDITIONS FOR COPYING, DISTRIBUTION AND MODIFICATION:

0. You just DO WHAT THE FUCK YOU WANT TO.

1. Do not hold the author(s), creator(s), developer(s) or distributor(s)
liable for anything that happens or goes wrong with your use of the work.
*/

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#define DELAYLINE 22050 /* Half a second */

char wav_struct[] = {
  'R', 'I', 'F', 'F',
   0,   0,   0,   0,  /* chunksize */
  'W', 'A', 'V', 'E',
  'f', 'm', 't', ' ', /* subchunk1id */
   16,  0,   0,   0,  /* subchunk1size */
   1,   0,            /* audioformat */
   1,   0,            /* numchannels */
   68,  172, 0,   0,  /* samplerate 44100 */
   136, 88,  1,   0,  /* byterate 88200 */
   2,   0,            /* blockalign */
   16,  0,            /* bitspersample */
  'd', 'a', 't', 'a', /* subchunk2id */
   0,   0,   0,   0   /* subchunk2size */

};

int main(int argc, char *argv[]){
   if (argc < 3){
      fprintf(stderr, "usage: %s [wavfile] [decay]\n"
                      "example: %s file.wav 2\n", argv[0], argv[0]);
      return 1;
   }

   FILE *fpin = fopen(argv[1], "rb");
   if (!fpin){
      fprintf(stderr, "%s: No such file or directory\n", argv[1]);
      return 1;
   }

   char data_tag[4];

   /* seek to subchunk2size */
   fseek(fpin, 36, SEEK_SET);
   do {
      fread(data_tag, sizeof(char), 4, fpin);
      if (!strncmp(data_tag, "data", 4)) break;
      fseek(fpin, -3, SEEK_CUR);
   } while (1);

   uint32_t subchunk2size;
   uint32_t numsamples;
   fread(&subchunk2size, sizeof(uint32_t), 1, fpin);
   numsamples = (subchunk2size << 1);

   int16_t *pcm_in = malloc(sizeof(int16_t)*numsamples);
   if (pcm_in == NULL){
      fprintf(stderr, "Allocating failed\n");
      return 1;
   }

   fread(pcm_in, sizeof(int16_t), numsamples, fpin);
   fclose(fpin);

   FILE *fpout = fopen("write.wav", "wb");
   if (fpout == NULL) {
      fprintf(stderr, "Couldn't create write.wav\n");
      return 1;
   }

   fwrite(wav_struct, sizeof(wav_struct), 1, fpout);

   uint32_t delayline = DELAYLINE;

   int16_t s_temp;

   for (uint32_t i = 0; i < numsamples-delayline; i++){
      s_temp =
               pcm_in[i+delayline] += pcm_in[i]/atoi(argv[2]);

      fwrite(&s_temp, sizeof(int16_t), 1, fpout);
   }


   uint32_t length = 32+subchunk2size;

   fseek(fpout, 4, SEEK_SET);
   fwrite(&length, sizeof(uint32_t), 1, fpout);

   fseek(fpout, 40, SEEK_SET);
   fwrite(&subchunk2size, sizeof(uint32_t), 1, fpout);

   fclose(fpout);

   free(pcm_in);
   return 0;

}