#include "utils.h"

size_t str_replace(char *dest, size_t dest_len, char *orig, char *rep, char *with)
{
    char *ins;    // the next insert point
    char *tmp;    // varies
    size_t len_rep;  // length of rep (the string to remove)
    size_t len_with; // length of with (the string to replace rep with)
    size_t len_front; // distance between rep and end of last rep
    size_t count;    // number of replacements

    // sanity checks and initialization
    if (!orig || !rep) {
        return 0;
    }
    len_rep = strlen(rep);
    if (len_rep == 0) {
        return 0; // empty rep causes infinite loop during count
    }
    if (!with) {
        with = "";
    }
    len_with = strlen(with);

    // count the number of replacements needed
    ins = orig;
    for (count = 0; (tmp = strstr(ins, rep)); ++count) {
        ins = tmp + len_rep;
    }

    size_t required_len = strlen(orig) + (len_with - len_rep) * count + 1;
    if (dest_len < required_len) {
        return 0;
    }

    tmp = dest;

    // first time through the loop, all the variable are set correctly
    // from here on,
    //    tmp points to the end of the result string
    //    ins points to the next occurrence of rep in orig
    //    orig points to the remainder of orig after "end of rep"
    while (count--) {
        ins = strstr(orig, rep);
        len_front = ins - orig;
        tmp = strncpy(tmp, orig, len_front) + len_front;
        tmp = strcpy(tmp, with) + len_with;
        orig += len_front + len_rep; // move to next "end of rep"
    }

    strcpy(tmp, orig);

    return required_len - 1;
}