atoi_fast

atoi_fast.cpp

#include <cstdlib>

// Andrei Alexandrescu at the C++ and Beyond 2012 
class atoi_func
{
public:
    atoi_func(): value_() {}

    inline int value() const { return value_; }

    inline bool operator() (const char *str, size_t len)
    {
        value_ = 0;
        int sign = 1;
        if (str[0] == '-') { // handle negative
            sign = -1;
            ++str;
            --len;
        }

        switch (len) { // handle up to 10 digits, assume we're 32-bit
            case 10:    value_ += (str[len-10] - '0') * 1000000000;
            case  9:    value_ += (str[len- 9] - '0') * 100000000;
            case  8:    value_ += (str[len- 8] - '0') * 10000000;
            case  7:    value_ += (str[len- 7] - '0') * 1000000;
            case  6:    value_ += (str[len- 6] - '0') * 100000;
            case  5:    value_ += (str[len- 5] - '0') * 10000;
            case  4:    value_ += (str[len- 4] - '0') * 1000;
            case  3:    value_ += (str[len- 3] - '0') * 100;
            case  2:    value_ += (str[len- 2] - '0') * 10;
            case  1:    value_ += (str[len- 1] - '0');
                value_ *= sign;
                return value_ > 0;
            default:
                return false;
        }
    }
private:
    int value_;
};


static void atoiTest(benchmark::State& state) {
  std::string num("-981723944");
  for (auto _ : state) {
    auto i = atoi(num.c_str());
    benchmark::DoNotOptimize(i);
  }
}
// Register the function as a benchmark
BENCHMARK(atoiTest);

static void alexTest(benchmark::State& state) {
  std::string num("-981723944");
  atoi_func parse;
  for (auto _ : state) {
    auto i = parse(num.c_str(), num.length());
    benchmark::DoNotOptimize(i);
  }
}
BENCHMARK(alexTest);