cs50 pset2 substitution problem

substitution.c

#include<stdio.h>

int calculate_key_size(char argv[]); // returns size of key 
int is_alphabet_arr(char argv[]); // checks whether all elements of an array are alphabets
int is_alphabet_char(char a); // checks whether a character is an alphabet
int repeat_letter_check(char argv[]); // checks whether a letter repeats in an array
int check_key_validity(char argv[]); // checks whether the key given by the user is valid or not according to constraints
int case_check(char a); // returns 0 for lowercase and 1 for uppercase


char uppercase_alphabets[] = {'A','B','C','D','E','F','G','H','I','J','K','L','M','N','O','P','Q','R','S','T','U','V','W','X','Y','Z'};
char lowercase_aplhabets[] = {'a','b','c','d','e','f','g','h','i','j','k','l','m','n','o','p','q','r','s','t','u','v','w','x','y','z'};

int main(int argc, char* argv[]){
	if(argc != 2){
		printf("Usage : ./program_name key");
		return 1;
	}
	int key_validity = check_key_validity(argv[1]);
	if(key_validity != 0){
		printf("INVALID KEY\n");
		return 1;
	}
	char plaintext[1000];
	char ciphertext[1000];
	printf("plaintext: ");
	fgets(plaintext, sizeof(plaintext), stdin);
	//encrypting with user key
	int i = 0;
	while(plaintext[i] != '\0'){
		for(int y = 0; y < 26; y++){
			if(is_alphabet_char(plaintext[i]) == 1){
				ciphertext[i] = plaintext[i];
				break;
			}	
			else if(plaintext[i] == lowercase_aplhabets[y]){
				ciphertext[i] = argv[1][y];
				break;
			}
			else if(plaintext[i] == uppercase_alphabets[y]){
				ciphertext[i] = argv[1][y];
				break;
			}
		}
		//preserving case
		int plaintext_casecheck = case_check(plaintext[i]);
		int ciphertext_casecheck = case_check(ciphertext[i]);
		if (plaintext_casecheck != ciphertext_casecheck){
			if(plaintext_casecheck == 0){
				ciphertext[i] = (char)((int)(ciphertext[i] + 32));
			}
			else if(plaintext_casecheck == 1){
				ciphertext[i] = (char)((int)(ciphertext[i] - 32));
			}
		}
		i++;
	}
	printf("ciphertext: %s", ciphertext);
	return 0;
}

int calculate_key_size(char argv[]){
	int i = 0;
	int key_size = 0;
	while(argv[i] != '\0'){
		key_size += 1;
		i++;
	}
	if(key_size != 26)
		return 1;
	return 0;
}

int is_alphabet_arr(char argv[]){
	int a = 0;
	while(argv[a] != '\0'){
		if(!(((int)argv[a] >= 65 && (int)argv[a] <= 90) || ((int)argv[a] >= 97 && (int)argv[a] <= 122))){
			return 1;
		}
		a++;
	}
	return 0;
}

int is_alphabet_char(char a){
	for(int j = 65, k = 97; j < 91; j++, k++){
		if((int)a == j || (int)a == k)
			return 0;
	}
	return 1;
}
int repeat_letter_check(char argv[]){
	int c = 0;
	char alphabets[] = {'A','B','C','D','E','F','G','H','I','J','K','L','M','N','O','P','Q','R','S','T','U','V','W','X','Y','Z'}; 
	// above array's content does not matter;
	for(int b = 0; b < 27; b++){
		for(int j = 65, k = 97; j < 91; j++, k++){
			if((int)argv[b] == j || (int)argv[b] == k){
				alphabets[j-65] = '!';
				break;
			}
		}
	}
	while(argv[c] != '\0'){
		if(alphabets[c] != '!'){
			return 1;
		}
		c++;
	}
	return 0;
}

int check_key_validity(char argv[]){
	if(calculate_key_size(argv) == 1)
		return 1;
	if(is_alphabet_arr(argv) == 1)
		return 1;
	if(repeat_letter_check(argv) == 1)
		return 1;
	return 0;
}

int case_check(char a){
	for(int i = 0; i < 26; i++){
		if(a == lowercase_aplhabets[i]){
			return 0;
		}
		else if(a == uppercase_alphabets[i]){
			return 1;
		}
	}
	return -1; //can return any value as such because execution will never reach here
}