/* Remove a single line from input.

   Usage:
      ./slash {line_number} < input.txt > output.txt

   Where {line_number} is the line to be removed.  Use positive integer
   to measure from start of file, or negative integer to measure from
   end of file.

   `./slash 123` is equivalent to `sed -e '123d'`.

   "cut" might have been a better name for this tool, since we are cutting
   one line from the input, but there is already a cut(1) tool that behaves
   very differently from this tool.  There isn't a standard "slash" or
   "slice" tool so either of these would work.  Of these, "slice" sounds
   like it's extracting a single slice, so "slash" seems like a better fit.
*/

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

int main(int argc, char **argv)
{
   char *buffer = NULL;
   int capacity = 0, size = 0;
   int line_number, current_line;
   char *r, *w, *e;

   if( argc != 2 )
      return printf("%s {line_number}\n", *argv);

   line_number = atoi(argv[1]);

   /* Load entire file to memory. */
   do
   {
      if( size == capacity )
      {
         capacity = (capacity != 0 ? capacity * 2 : 256);
         if( (buffer = (char*)realloc(buffer, capacity)) == NULL )
         {
            fprintf(stderr, "Out of memory (need %d)\n", capacity);
            return 1;
         }
      }
      size += fread(buffer + size, 1, capacity - size, stdin);
   } while( size == capacity );

   if( line_number >= 0 )
   {
      /* Scan forward and skip over requested line. */
      current_line = 1;
      for(r = w = buffer; r != buffer + size; r++)
      {
         if( current_line != line_number )
            *w++ = *r;
         if( *r == '\n' )
            current_line++;
      }
      fwrite(buffer, w - buffer, 1, stdout);
   }
   else
   {
      if( size > 0 )
      {
         /* Scan backward for the requested line. */
         r = e = buffer + size;
         if( r[-1] == '\n' )
            r--;
         current_line = -1;
         for(; r-- != buffer;)
         {
            if( *r == '\n' )
            {
               current_line--;
               if( current_line < line_number )
               {
                  current_line++;
                  break;
               }
               e = r + 1;
            }
         }
         r++;

         /* Overwrite the line that we want to skip over. */
         if( current_line == line_number )
         {
            memmove(r, e, size - (e - buffer));
            size -= e - r;
         }
         fwrite(buffer, size, 1, stdout);
      }
   }

   free(buffer);
   return 0;
}