reverse-integer.cpp

/*
 * Copyright (c) 2018 Christopher Friedt
 *
 * SPDX-License-Identifier: MIT
 */

#include <algorithm>
#include <array>

using namespace std;

class Solution {
public:
  int reverse(int x) {

    // range: [ -(2^31), (2^31) )
    // range: -2147483648 : 2147483647

    if (0 == x) {
      return 0;
    }

    // uint8_t is the smallest supported type that can hold a decimal digit
    // we will have maximally 10 decimal digits
    array<uint8_t, 10> decimal_digits;

    int sign = (x >= 0) ? +1 : -1;
    x *= sign;

    size_t highest_pos = 0;

    for (size_t i = 0; i < decimal_digits.size() && x != 0; i++, x /= 10) {
      decimal_digits[i] = uint8_t(x % 10);
      highest_pos = i;
    }

    for (size_t i = 0; i <= highest_pos / 2; i++) {
      swap(decimal_digits[i], decimal_digits[highest_pos - i]);
    }

    int accumulator = 0;
    for (size_t i = 0, fac = 1; i <= highest_pos; i++, fac *= 10) {
      uint64_t diff = ((uint64_t(1) << 31) - 1) - accumulator;
      uint64_t xx = decimal_digits[i] * fac;
      if (xx > diff) {
        return 0;
      }
      accumulator += xx;
    }

    return accumulator * sign;
  }
};