number guessing game

guess.c

#include<stdio.h>
#include<stdlib.h>
#include<assert.h>
#include<stdbool.h>

#define DUP (-1)
#define INVALID (-2)

static int comp(int a, int b, int c, int d) {
  if (a == c) {
    if (b == d) {
      return DUP;
    }
    return a;
  } else if (a == d) {
    if (b == c) {
      return DUP;
    }
    return b;
  } else if (b == c) {
    if (a == d) {
      return DUP;
    }
    return b;
  } else if (b == d) {
    if (a == d) {
      return DUP;
    }
    return b;
  }
  return INVALID;
}

static int do_guess(int size, int* guess, int* reply) {
  if (size < 2) {
    return DUP;
  }
  int prime = guess[0] + reply[0];
  int less = guess[0] - reply[0];
  bool only_one = false;
  for (int i=1; i<size; ++i) {
    int a = guess[i] + reply[i];
    int b = guess[i] - reply[i];
    if (only_one) {
      if (a != prime && b != prime) return INVALID;
    } else {
      int result = comp(prime, less, a, b);
      switch (result) {
        case DUP: break;
        case INVALID: return INVALID;
        default:
          prime = result;
          only_one = true;
          break;
      }
    }
  }
  return only_one ? prime : DUP;
}

int main(int argc, char* argv[]) {
  int size = atoi(argv[1]);
  assert(size * 2 == argc - 2);
  int* guess = (int*) malloc(sizeof(int) * size);
  int* reply = (int*) malloc(sizeof(int) * size);
  for (int i=0; i<size; ++i) {
    guess[i] = atoi(argv[i+2]);
    reply[i] = atoi(argv[i+2+size]);
  }
  int result = do_guess(size, guess, reply);
  printf("%d\n", result);
  free(guess);
  free(reply);
  return 0;
}