/* Words are read in and written out in reverse order. Command line syntax is:
 *
 *  ./a.out N [input-file [output-file]]
 *
 * where N is the number of words to read, input-file is the optional name of the input file (defaults to stdin).  If
 * output-file is specified, the input file must also be specified. The output file defaults to stdout if not specified.
 *
 * For example:
 *     ./a.out 3
 *     ./a.out 3 inputfile1
 *     ./a.out 3 inputfile1 outputfile1
 */

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

typedef struct node {
    char *string;
    struct node *next;
} Node;

/* Insert the new string into a new node. Treat the linked list like a stack. Note the address of the pointer
 * to the linked list has been passed in as a parameter. This allows its value to change as required -- it also
 *  means that it must be dereferenced when used.
 */
void insert( Node **head, char new_string[] ) {
    Node * ptr;

    ptr = (Node *) malloc( sizeof(Node) );
    if ( ptr == NULL ) {
	fprintf( stderr, "Error! Could not allocate node\n" );
	exit( -1 );
    } /* if */

    ptr->next = *head;
    ptr->string = (char *) calloc( strlen( new_string ) + 1, sizeof(char) );
    if ( ptr->string == NULL ) {
	fprintf( stderr, "Error! Could not allocate string\n" );
	exit( -1 );
    } /* if */
    ptr->string = strcpy( ptr->string, new_string );
    
    *head = ptr;
} /* insert */

int main( int argc, char (*argv[])[] ) {
    FILE *in  = stdin;		/* default to stdin */
    FILE *out = stdout;		/* default to stdout */
    char word[256];		/* handle large words */
    Node *root = NULL, *tmp;
    int NoOfWords = 0;
    int i;

    /* Note the "break" statement only occurs in the last case clause.
     */
    switch (argc) {
      case 4:
	out = fopen( argv[3], "w" );
	if ( out == NULL ) {
	    fprintf( stderr, "Error! Could not open output file %s\n", argv[3] );
	    exit( -1 );
	} /* if */
      case 3:
	in = fopen( argv[2], "r" );
	if ( in == NULL ) {
	    fprintf( stderr, "Error! Could not open input file %s\n", argv[2] );
	    exit( -1 );
	} /* if */
      case 2:
	NoOfWords = atoi( argv[1] );
	break;
      default:
	fprintf( stderr, "Usage:%s N [input-file [output-file]]\n", argv[0] );
	exit( -1 );
    } /* switch */

    /* interactive case
     */
    if ( in == stdin ) {
	printf( "Enter a list of %d words.\n", NoOfWords );
    } /* if */
    
    /* read and insert words into linked list
     */
    for ( i = 0; i < NoOfWords; i += 1 ) {
      if ( fscanf( in, "%s", word ) == EOF ) break;
	insert( &root, word );
    } /* for */

    fprintf( out, "The words in reverse order:\n" );
    for ( tmp = root; tmp != NULL; tmp = tmp->next ) {
	printf( "%s\n", tmp->string );
	free( root );
	root = tmp->next;
    } /* for */
} /* main */