Compares strings composed of mixed digits and letters

smart_compare.cpp

#include <cctype>
#include <iostream>
#include <string>


int get_int(const std::string& str, std::size_t& i) {
  int val = 0;
  for (; i < str.size() && std::isdigit(str.at(i)); ++i) {
    val *= 10;
    val += (int)str.at(i) - (int)'0';
  }

  return val;
}

/**
 * @brief Compares composite strings made up of numbers and letters.
 *
 * Numeric sections of strings are compared as numbers while alpha sections are
 * compared as strings. For example, a normal string comparison would put
 * `"foo2"` after `"foo10"` (the character `'1'` vs `'2'`). This comparitor
 * would put `"foo2"` _before_ `"foo10"` (the number `10` vs `2`).
 *
 * @param a - A string possibly consisting of letters and numbers.
 * @param b - Another string possibly consisting of letters and numbers.
 *
 * @return Less than zero if `a` comes before `b`, greater than zero if `a`
 *  comes after `b`, or zero if they are equal.
 */
int smart_compare(const std::string& a, const std::string& b) {
  std::size_t aI = 0;
  std::size_t bI = 0;

  while (aI < a.size() && bI < b.size()) {
    const int aC = (int)a.at(aI);
    const int bC = (int)b.at(bI);

    if (std::isdigit(aC) && std::isdigit(bC)) {
      int aNum = get_int(a, aI);
      int bNum = get_int(b, bI);

      if (aNum != bNum) {
        return aNum - bNum;
      }
      else {
        continue;
      }
    }

    if (aC != bC) {
      return aC - bC;
    }

    ++aI;
    ++bI;
  }

  if (aI < a.size()) {
    return (int)a[aI] - 0;
  }
  else if (bI < b.size()) {
    return 0 - (int)b[bI];
  }
  else {
    return 0;
  }
}

int main() {
  std::cout << "a vs b: " << smart_compare("a", "b") << std::endl;
  std::cout << "b vs a: " << smart_compare("b", "a") << std::endl;
  std::cout << "foo vs bar: " << smart_compare("foo", "bar") << std::endl;
  std::cout << "4 vs 10: " << smart_compare("4", "10") << std::endl;
  std::cout << "10 vs 4: " << smart_compare("10", "4") << std::endl;
  std::cout << "foo4 vs foo10: " << smart_compare("foo4", "foo10") << std::endl;
  std::cout << "foo10 vs foo4: " << smart_compare("foo10", "foo4") << std::endl;
  std::cout << "foo10bar vs foo4bar: " << smart_compare("foo10bar", "foo4bar") << std::endl;
  std::cout << "foo10bar5 vs foo10bar30: " << smart_compare("foo10bar5", "foo10bar30") << std::endl;
  std::cout << "foo vs foo: " << smart_compare("foo", "foo") << std::endl;
  std::cout << "foo vs foo6: " << smart_compare("foo", "foo6") << std::endl;
  std::cout << "foo6 vs foo: " << smart_compare("foo6", "foo") << std::endl;
  std::cout << "foo6bar vs foobar: " << smart_compare("foo6bar", "foobar") << std::endl;
  std::cout << "foobar vs foo6bar: " << smart_compare("foobar", "foo6bar") << std::endl;

  return 0;
}