Combining two strings by removing duplicate substrings - c

Question

I have two strings that I would like to combine, removing duplicate substrings. Note that every two consecutive numbers constitute a substring. Consider string str1 and str2:
str1 = "#100#123#100#678"
str2 = "#100#678#100#56"
I would like to produce a combined string as:
comboStr = "#100#123#100#678#100#56" (i.e. I removed the duplicate #100#678)
What's the easiest way to do this? Is there a way I can achieve this using regular expressions?

Answer 1

I don't think regular expressions are a good way to solve this problem. Regexes might be useful in finding the #123 tokens, but the problem needs to backtrack on its own string in a way regex's back references are not desiged for.
I also don't think that there is an easy way (as in three lines of code) to solve this.
I assume that the strings always follow the pattern (#\d+)* and that the pair created at the seam when joining two strings is not trated as special, i.e. the resulting pair might be considered as duplicate. This means we can separate concatenation from pair removal.
Convert your string to a list of integers, operate on these lists and then join them back. That's some work, but it makes the actual code to strip duplicates easier - it's complicated enough - and might also come in handy when you need to operate on similar strings often.
#include <stdlib.h>
#include <stdio.h>
/*
* Convert a string to a list of at most max integers. The
* return value is the number of integers in the list (which
* max be greater than max!) or -1 if the string is invalid.
*/
int ilist_split(int *ilist, int max, const char *str)
{
const char *p = str;
int n = 0;
while (*p) {
int x;
int pos;
if (sscanf(p, "#%d %n", &x, &pos) < 1) return -1;
if (n < max) ilist[n] = x;
n++;
p += pos;
}
return n;
}
/*
* Convert a list of integers back to a string. The string
* is at most nbuf - 1 characters long and is assured to be
* zero-terminated if nbuf isn't 0. It is legal to pass NULL
* as char buffer if nbuf is 0. Returns the number of characters
* that would have been written ha dthe buffer been long enough,
* snprintf-style.
*/
int ilist_join(const int *ilist, int n, char *buf, int nbuf)
{
int len = 0;
int i;
for (i = 0; i < n; i++) {
len += snprintf(buf + len,
nbuf > len ? nbuf - len : 0, "#%d", ilist[i]);
}
return len;
}
/*
* Auxliary function to find a pair in an inteher list.
*/
int ilist_find_pair(int *ilist, int n, int a1, int a2)
{
int i;
for (i = 1; i < n; i++) {
if (ilist[i - 1] == a1 && ilist[i] == a2) return i - 1;
}
return -1;
}
/*
* Remove duplicate pairs from an integer list. The first
* pair is kept, subsequent pairs are deleted. Returns the
* new length of the array.
*/
int ilist_remove_dup_pairs(int *ilist, int n)
{
int i, j;
j = 1;
for (i = 1; i < n; i++) {
int a1 = ilist[i - 1];
int a2 = ilist[i];
if (ilist_find_pair(ilist, i - 1, a1, a2) < 0) {
ilist[j++] = ilist[i];
} else {
i++;
}
}
return j;
}
#define MAX 40
int main()
{
const char *str1 = "#100#123#100#678";
const char *str2 = "#100#678#100#56";
char res[80];
int ilist[MAX];
int nlist;
/* convert str1 */
nlist = ilist_split(ilist, MAX, str1);
if (nlist > MAX) nlist = MAX;
/* convert and concatenate str2 */
nlist += ilist_split(ilist + nlist, MAX - nlist, str2);
if (nlist > MAX) nlist = MAX;
/* remove duplicate pairs */
nlist = ilist_remove_dup_pairs(ilist, nlist);
/* convert back to string */
ilist_join(ilist, nlist, res, sizeof(res));
printf("%s\n", res);
return 0;
}