Program for counting similarities between two strings, in C.

coincidence.c

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

void print_usage_on_error();
void print_coincidences(int[], int, size_t);
void count_coincidences(char[], char[], int *, int);
void remove_spaces(char *);

/*
Program for counting similarities between two strings. Main usage being finding the key
length of a polyalphabetic ciphertext (such as one created by the Vigenere cipher).
*/

int main(int argc, char *argv[]){
	
	//if the user hasn't supplied a significance level and a ciphertext,
	//prompt them to enter both
	if(argc < 3){
		print_usage_on_error();
		exit(EXIT_FAILURE);
	}
	
	char ciphertext[strlen(argv[1])];
	char ciphertext_copy[strlen(argv[1])];
	
	strcpy(ciphertext, argv[1]);
	remove_spaces(ciphertext);

	strcpy(ciphertext_copy, ciphertext);
	
	//get significance level from user
	char *endptr;
	int sig_level = strtol(argv[2], &endptr, 0);
	if(errno == ERANGE || *endptr != '\0'){
		print_usage_on_error();
		exit(EXIT_FAILURE);
	}
	
	int ciphertext_length = strlen(ciphertext);
	int counts[ciphertext_length];
	
	//initialise the array to all 0s
	memset(counts, 0, sizeof(counts));
	
	//count the number of coincidences and save those counts to an array
	count_coincidences(ciphertext, ciphertext_copy, counts, ciphertext_length);
	//we need to pass the size of the counts array because we're passing a 
	//pointer to an array so sizeof() won't work right - C can't pass arrays apparently.
	print_coincidences(counts, sig_level, sizeof(counts));

	return 0;
}

void print_usage_on_error(){
	fprintf(stderr, "Supply a ciphertext as a command line argument between quotes, followed by a significance level (how many coincidences a shift needs before it's displayed).\n");
}

//modifies the passed string in place
void remove_spaces(char *string){
	int str_length = strlen(string);
	int j;
	
	for(int i=0; i<str_length; i++){
		if(string[i] == ' '){
			for(j=i; j<str_length; j++){
				string[j] = string[j+1];
			}
		}
	}
	
	string[j] = '\0';
}

void print_coincidences(int counts[], int sig_level, size_t count_size){
	for(int i=0; i < count_size / sizeof(int); i++){
		if(counts[i] >= sig_level){
			printf("Shift: %d, Coincidences: %d\n",i, counts[i]);
		}
	}
}

void count_coincidences(char ciphertext[], char ciphertext_copy[], int *counts, int ciphertext_length){
	for(int shift=1; shift < ciphertext_length; shift++){
		for(int j=0; j < ciphertext_length - shift; j++){
			if(ciphertext[j + shift] == ciphertext_copy[j]){
				counts[shift] += 1;
			}
		}
	}
}