SteamKeyGen

functions.h

void generateKey (char* s, char type, char* chars) {
  switch (type) {
    case '1': {
      for (short i = 0; i < 17; i++)
        s[i] = chars[rand() % 36];
      
      s[5] = s[11] = '-';
      s[17] = '\0';
      break;
    }
    case '2': {
      for (short i = 0; i < 29; i++)
        s[i] = chars[rand() % 36];
      
      s[5] = s[11] = s[17] = s[23] = '-';
      s[29] = '\0';
      break;
    }
    case '3': {
      s[0] = chars[rand() % 10];
      s[1] = chars[rand() % 10];
      s[2] = chars[rand() % 10];
      
      for (short i = 3; i < 15; i++)
        s[i] = chars[rand() % 36];
      
      s[15] = ' ';
      s[16] = chars[rand() % 10];
      s[17] = chars[rand() % 10];
      s[18] = '\0';
      break;
    }
  }
}

void clearBuffer () {
  while (getchar() != '\n');
}

char safeTypeInput (char type) {
  while (type < '1' || type > '3') {
    printf("Type of a key: ");
    scanf("%c", &type);
    clearBuffer();
  }
  return type;
}

int safeNumberInput (char* number) {
  while (1) {
    printf("Number of keys: ");
    scanf("%s", number);
    
    for (short i = 0; number[i] != '\0'; i++) {
      if (!isdigit(number[i])) {
      	clearBuffer();
      	safeNumberInput(number);
      }

    return atoi(number);
    }
  }

main.c

#include "stdlib.h"
#include "stdio.h"
#include "ctype.h"
#include "time.h"
#include "functions.h"

char chars[36] = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ";

int main() {
  srand(time(NULL));

  char typeOfKeys = '0';
  char numberOfKeysString[21];
  int numberOfKeys;
  char stringForKey[29];
  
  printf("SteamKeyGen\n\n1. AAAAA-BBBBB-CCCCC\n2. AAAAA-BBBBB-CCCCC-DDDDD-EEEEE\n3. 237ABCDGHJLPRST 23\n\n");
  
  typeOfKeys = safeTypeInput(typeOfKeys);
  numberOfKeys = safeNumberInput(numberOfKeysString);

  FILE *f = fopen("Keys.txt", "w");
  while (numberOfKeys) {
    generateKey(stringForKey, typeOfKeys, chars);
    fprintf(f, "%s\n", stringForKey);
    numberOfKeys--;
  }
  fclose(f);

  printf("\nFinished!");
  clearBuffer();
  getchar();
  
  return 0;
}