#include <stdio.h>
#include <errno.h>
#include <fcntl.h>
#include <unistd.h>
#include <stdlib.h>
#include <strings.h>

extern const char *__progname;

static const char *fromfile;
static off_t fromoff;
static const char *tofile;
static off_t tooff;
static off_t len;
static int reverse;

static int fromfd;
static int frompipe;
static int tofd;
static int topipe;

static int off_t_val(const char *s, off_t *otp, const char *tag)
{
 char *e;
 u_quad_t v;

 v = strtoq(s,&e,0);
 if (*e || (e == s))
  { fprintf(stderr,"%s: invalid %s value `%s'\n",__progname,tag,s);
    return(1);
  }
 *otp = v;
 return(0);
}

static void handleargs(int ac, char **av)
{
 int i;
 int j;
 int errs;

 errs = 0;
 reverse = 0;
 j = 1;
 for (i=1;i<ac;i++)
  { if (!strcmp(av[i],"-r"))
     { reverse = 1;
     }
    else if ((av[i][0] == '-') && av[i][1])
     { fprintf(stderr,"%s: unrecognized flag %s\n",__progname,av[i]);
       errs ++;
     }
    else
     { if (i != j) av[j] = av[i];
       j ++;
     }
  }
 if (j != 6)
  { fprintf(stderr,"Usage: %s [options] from-file from-offset to-file to-offset length\n",__progname);
    fprintf(stderr,"Options: -r      (reverse direction of copy)\n");
    errs ++;
  }
 else
  { fromfile = av[1];
    errs += off_t_val(av[2],&fromoff,"from-offset");
    tofile = av[3];
    errs += off_t_val(av[4],&tooff,"to-offset");
    errs += off_t_val(av[5],&len,"length");
  }
 if (errs) exit(1);
}

static void openfiles(void)
{
 if (strcmp(fromfile,"-"))
  { fromfd = open(fromfile,O_RDONLY,0);
    if (fromfd < 0)
     { fprintf(stderr,"%s: %s: %s\n",__progname,fromfile,strerror(errno));
       exit(1);
     }
    frompipe = 0;
  }
 else
  { if (fromoff != 0)
     { fprintf(stderr,"%s: reading from stdin requires a zero from-offset\n",__progname);
       exit(1);
     }
    if (reverse)
     { fprintf(stderr,"%s: -r is incompatible with reading from stdin\n",__progname);
       exit(1);
     }
    fromfd = 0;
    frompipe = 1;
  }
 if (strcmp(tofile,"-"))
  { tofd = open(tofile,O_WRONLY|O_CREAT,0666);
    if (tofd < 0)
     { fprintf(stderr,"%s: %s: %s\n",__progname,tofile,strerror(errno));
       exit(1);
     }
    topipe = 0;
  }
 else
  { if (tooff != 0)
     { fprintf(stderr,"%s: writing to stdout requires a zero to-offset\n",__progname);
       exit(1);
     }
    if (reverse)
     { fprintf(stderr,"%s: -r is incompatible with writing to stdout\n",__progname);
       exit(1);
     }
    tofd = 1;
    topipe = 1;
  }
}

static void docopy(void)
{
 static char buf[1048576];
 int n;
 off_t left;
 int r;
 int w;
 int wo;

 left = len;
 if (reverse)
  { fromoff += len;
    tooff += len;
  }
 while (left > 0)
  { n = sizeof(buf);
    if (n > left) n = left;
    if (reverse)
     { fromoff -= n;
       tooff -= n;
     }
    if (frompipe)
     { r = read(fromfd,&buf[0],n);
     }
    else
     { r = pread(fromfd,&buf[0],n,fromoff);
     }
    if (r < 0)
     { fprintf(stderr,"%s: read from %s at %llu (after copying %llu): %s\n",
		__progname, fromfile,
		(unsigned long long int)fromoff,
		(unsigned long long int)(len-left),
		strerror(errno) );
       exit(1);
     }
    if (r == 0)
     { fprintf(stderr,"%s: EOF on %s at %llu (after copying %llu)\n",
		__progname, fromfile,
		(unsigned long long int)fromoff,
		(unsigned long long int)(len-left) );
       exit(1);
     }
    if (reverse && (r != n))
     { fprintf(stderr,"%s: read from %s at %lld got %d (wanted %d)\n",__progname,fromfile,(long long int)fromoff,r,n);
       exit(1);
     }
    wo = 0;
    while (wo < r)
     { if (topipe)
	{ w = write(tofd,&buf[wo],r-wo);
	}
       else
	{ w = pwrite(tofd,&buf[wo],r-wo,tooff+wo);
	}
       if (w < 0)
	{ fprintf(stderr,"%s: write to %s at %llu (after copying %llu): %s\n",
		__progname, tofile,
		(unsigned long long int)tooff,
		(unsigned long long int)(wo+len-left),
		strerror(errno) );
	  exit(1);
	}
       if (w != r-wo)
	{ fprintf(stderr,"%s: short write to %s at %llu (after copying %llu): wanted %u, wrote %u (retrying remainder)\n",
		__progname, tofile,
		(unsigned long long int)tooff,
		(unsigned long long int)(wo+len-left),
		(unsigned int)(r-wo),
		(unsigned int)w );
	}
       wo += w;
     }
    left -= r;
    if (! reverse)
     { fromoff += r;
       tooff += r;
     }
  }
}

int main(int, char **);
int main(int ac, char **av)
{
 handleargs(ac,av);
 openfiles();
 docopy();
 exit(0);
}