// Chooses random intervals from a file

#include <time.h>
#include <sys/time.h>
#include <sys/stat.h>
#include <stdio.h>
#include <string.h>
#include <errno.h>

static int Seed;
#define ACMa 16807
#define ACMm 2147483647
#define ACMq 127773         
#define ACMr 2836
#define hi (Seed / ACMq)
#define lo (Seed % ACMq)
  
static int fst=1;

	/* returns a random integer in 0..top-1 */

int aleat(top)

   { long test;
     struct timeval t;
     if (fst)
        { gettimeofday (&t,NULL);
          Seed = t.tv_sec*t.tv_usec;
          fst=0;
        }
       { Seed = ((test = ACMa * lo - ACMr * hi) > 0) ? test : test + ACMm;
         return ((double)Seed) * top/ACMm;
       }
   }

main (int argc, char **argv)

   { int n,m,J,t;
     struct stat sdata;
     FILE *ifile,*ofile;
     unsigned char *buff;

     if (argc < 5)
	{ fprintf (stderr,
"Usage: %s <file> <length> <number> <intervals file>\n"
"  randomly extracts <number> intervals of length <length> from <file>.\n"
"  The output file, <intervals file> has a first line of the form:\n"
"    # number=<number> length=<length> file=<file>\n"
"  and then <number> lines of the form <from>,<to>.\n",argv[0]
);
	  exit(1);
	}

     if (stat (argv[1],&sdata) != 0)
	{ fprintf (stderr,"Error: cannot stat file %s\n",argv[1]);
	  fprintf (stderr," errno = %i\n",errno);
	  exit(1);
	}
     n = sdata.st_size;

     m = atoi(argv[2]);
     if ((m <= 0) || (m > n))
	{ fprintf (stderr,"Error: length must be >= 1 and <= file length"
			  " (%i)\n",n);
	  exit(1);
	}

     J = atoi(argv[3]);
     if (J < 1)
	{ fprintf (stderr,"Error: number of intervals must be >= 1\n");
	  exit(1);
	}

     ifile = fopen(argv[1],"r");
     if (ifile == NULL)
	{ fprintf (stderr,"Error: cannot open file %s for reading\n",argv[1]);
	  fprintf (stderr," errno = %i\n",errno);
	  exit(1);
	}

     buff = (unsigned char*) malloc (n);
     if (buff == NULL)
	{ fprintf (stderr,"Error: cannot allocate %i bytes\n",n);
	  fprintf (stderr," errno = %i\n",errno);
	  exit(1);
	}

     if (fread (buff,n,1,ifile) != 1)
	{ fprintf (stderr,"Error: cannot read file %s\n",argv[1]);
	  fprintf (stderr," errno = %i\n",errno);
	  exit(1);
	}
     fclose(ifile);

     ofile = fopen(argv[4],"w");
     if (ofile == NULL)
	{ fprintf (stderr,"Error: cannot open file %s for writing\n",argv[4]);
	  fprintf (stderr," errno = %i\n",errno);
	  exit(1);
	}

     if (fprintf (ofile,"# number=%i length=%i file=%s\n", J,m,argv[1]) <= 0)
	{ fprintf (stderr,"Error: cannot write file %s\n",argv[4]);
	  fprintf (stderr," errno = %i\n",errno);
	  exit(1);
	}

     for (t=0;t<J;t++)
	{ int j,l;
	  j = aleat(n-m+1);
          if (fprintf (ofile,"%i,%i\n",j,j+m-1) <= 0)
	     { fprintf (stderr,"Error: cannot write file %s\n",argv[4]);
	       fprintf (stderr," errno = %i\n",errno);
	       exit(1);
	     }
	}
   
     if (fclose(ofile) != 0)
	{ fprintf (stderr,"Error: cannot write file %s\n",argv[4]);
	  fprintf (stderr," errno = %i\n",errno);
	  exit(1);
	}

     fprintf (stderr,"File %s successfully generated\n",argv[4]);
     exit(1);
   }