Fun with wav

I had to calculate the sum of the absolute value of datas of a .wav file. For efficiency (and fun) reasons, I had chosen C language.

It was a long time I didn’t used C. From my memory it was a pain to read and write to files. But I was really impressed by how clean the code is. And it is even more impressive knowing I used mostly low level functions.

A wav file has an header containing many meta-datas. This header was optimized to take the less space possible. Therefore, header is thinked with Bytes.

• The 4th first Bytes must contains RIFF in ASCII,
• the following 4th Bytes is an 32 bits integer giving the size of the file minus 8, etc…

Surprisingly, I believe read this kind of file with a higher level language would have more difficult than in C. Proof: I only have to search on the web the complete header format and write it in a struct.

struct wavfile
{
    char    id[4];          // should always contain "RIFF"
    int     totallength;    // total file length minus 8
    char    wavefmt[8];     // should be "WAVEfmt "
    int     format;         // 16 for PCM format
    short   pcm;            // 1 for PCM format
    short   channels;       // channels
    int     frequency;      // sampling frequency
    int     bytes_per_second;
    short   bytes_by_capture;
    short   bits_per_sample;
    char    data[4];        // should always contain "data"
    int     bytes_in_data;
};

If I had to read it in Ruby (for example), I believe I’d had to write a read entry for each bloc value. But in C I simply written:

fread(&header,sizeof(header),1,wav);

Only one step to fill my data structure. Magic!

Then, get an int value coded on two Bytes is also not a natural operation for high level language. In C, to read a sequence of 2 Bytes numbers I only had to write:

short value=0;
while( fread(&value,sizeof(value),1,wav) ) {
    // do something with value
}

Finally I ended with the following code. Remark I know the wav format (16 bit / 48000Hz):

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

struct wavfile
{
    char    id[4];          // should always contain "RIFF"
    int     totallength;    // total file length minus 8
    char    wavefmt[8];     // should be "WAVEfmt "
    int     format;         // 16 for PCM format
    short   pcm;            // 1 for PCM format
    short   channels;       // channels
    int     frequency;      // sampling frequency
    int     bytes_per_second;
    short   bytes_by_capture;
    short   bits_per_sample;
    char    data[4];        // should always contain "data"
    int     bytes_in_data;
};

int main(int argc, char *argv[]) {
    char *filename=argv[1];
    FILE *wav = fopen(filename,"rb");
    struct wavfile header;

    if ( wav == NULL ) {
        fprintf(stderr,"Can't open input file %s", filename);
        exit(1);
    }


    // read header
    if ( fread(&header,sizeof(header),1,wav) < sizeof(header) )
    {
        fprintf(stderr,"Can't read file header\n");
        exit(1);
    }
    if (    header.id[0] != 'R'
         || header.id[1] != 'I' 
         || header.id[2] != 'F' 
         || header.id[3] != 'F' ) { 
        fprintf(stderr,"ERROR: Not wav format\n"); 
        exit(1); 
    }

    fprintf(stderr,"wav format\n");

    // read data
    long sum=0;
    short value=0;
    while( fread(&value,sizeof(value),1,wav) ) {
        // fprintf(stderr,"%d\n", value);
        if (value<0) { value=-value; }
        sum += value;
    }
    printf("%ld\n",sum);
    exit(0);
}

Of course it is only a hack. But we can see how easy and clean it should be to improve. As I say often: the right tool for your need instead of the same tool for all your needs. Because here C is clearly far superior than Ruby to handle this simple tasks.

I am curious to know if somebody know a nice way to do this with Ruby or Python.

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