/**
 *
 * Reverse the order of words in a string using minimal memory.
 *
 * This is an old an ancient technique so it's not exactly new or clever.
 *
 * Compile: cc -o reverse reverse.c
 *
 * Usage:   ./reverse 
 *             or
 *          ./reverse "Your phrase goes here"
 *
 * Copyright (c) 2001, Simon Wistow <simon@thegestalt.org>
 *
 * Released under the terms of the Gnu Public Licence
 * http://www.gnu.org/licenses/old-licenses/gpl-2.0.html
 *
 **/
#include <stdio.h>
#include <string.h>
#include <stdlib.h>

void reverse(char* start, char* end);

/* Reverse the order of words in a string */
int main (int argc, char ** argv)
{
    /* The input */
    char * string;
    /* Walk through the string */
    char *p1, *p2;

   /* If we've been given an argument, then use that. Otherwise use the default.
    * we have to strdup it otherwise newer compilers automatically make it const
    * this could be fixed by using the -fwriteable-strings flag 
    */
    if (argc>1) {
        string = strdup(argv[1]);
    } else { 
        string = strdup("The cow jumped over the moon"); 
    }
    /* If there are no spaces then we don't need to do anything */
    if(!strchr(string, ' ')) {
        goto PRINT; /* So, I like gotos ... sue me */
    }
    /* First reverse the String in place using the old xor trick*/
    reverse(string, string+strlen(string)-1);

    /* Now walk through the string and reverse strings in place */
    for(p1=string; p1<string+strlen(string); ++p1) {
        /* Find the next space character */
        for (p2=p1; ' ' != *p2 && p2<string+strlen(string); ++p2);

        /* Now reverse this string */
        reverse(p1, p2-1);
        /* And update p1 */
        /* (which will get incremented to the start of the next string) */
        p1=p2; 
    }
    PRINT: printf("%s\n", string);
}

void reverse(char *string, char* end) {
    char *start;
    for (start=string; end>start; ++start, --end) {
        *start ^= *end;        
        *end   ^= *start;
        *start ^= *end;  
        /* Or start ^= *end ^= *start ^= *end; */
    }
}