#include "fast_float/fast_float.h"

#include <chrono>
#include <cstdint>
#include <cstdlib>
#include <cstring>
#include <iomanip>
#include <iostream>
#include <string>
#include <system_error>
#include <vector>

namespace {

struct input_case {
  std::string input;
  std::string canonical;
};

struct target_case {
  fast_float::parsed_number_string number;
  fast_float::adjusted_mantissa am;

  target_case() : number(), am() {}
};

volatile uint64_t sink = 0;

std::string nonzero_tail(size_t length) {
  std::string result(length, '1');
  result.front() = '7';
  result.back() = '7';
  return result;
}

bool make_target_case(std::string const &input, target_case &target) {
  fast_float::parse_options options;
  fast_float::parsed_number_string const parsed =
      fast_float::parse_number_string<false, char>(
          input.data(), input.data() + input.size(), options, true);
  fast_float::adjusted_mantissa am =
      fast_float::compute_float<fast_float::binary_format<double>>(
          parsed.exponent, parsed.mantissa);
  if (!parsed.valid || !parsed.too_many_digits || am.power2 < 0 ||
      am == fast_float::compute_float<fast_float::binary_format<double>>(
                parsed.exponent, parsed.mantissa + 1)) {
    return false;
  }
  am = fast_float::compute_error<fast_float::binary_format<double>>(
      parsed.exponent, parsed.mantissa);
  if (am.power2 >= 0) {
    return false;
  }
  target.number = parsed;
  target.am = am;
  return true;
}

bool takes_digit_comp(std::string const &input) {
  target_case target;
  return make_target_case(input, target);
}

uint64_t bits(double value) {
  uint64_t result;
  ::memcpy(&result, &value, sizeof(result));
  return result;
}

void parse(std::string const &input, double &value, std::errc &error,
           size_t &parsed_length) {
  fast_float::from_chars_result const result = fast_float::from_chars(
      input.data(), input.data() + input.size(), value);
  error = result.ec;
  parsed_length = size_t(result.ptr - input.data());
}

bool verify_case(input_case const &test) {
  double input_value = 0;
  double canonical_value = 0;
  std::errc input_error;
  std::errc canonical_error;
  size_t input_length = 0;
  size_t canonical_length = 0;
  parse(test.input, input_value, input_error, input_length);
  parse(test.canonical, canonical_value, canonical_error, canonical_length);
  return takes_digit_comp(test.input) &&
         input_length + 1 == test.input.size() &&
         canonical_length + 1 == test.canonical.size() &&
         input_error == canonical_error &&
         bits(input_value) == bits(canonical_value);
}

std::vector<input_case> make_cases(
    std::vector<size_t> const &zero_counts,
    std::vector<size_t> const &core_lengths) {
  // This prefix makes compute_float(m) and compute_float(m + 1) differ at
  // exponent -18, so public from_chars reaches digit_comp after parsing a
  // coefficient longer than 19 digits.
  std::string const prefix = "6497987825129815764";
  std::vector<input_case> result;
  for (size_t core_length : core_lengths) {
    std::string const tail = nonzero_tail(core_length - prefix.size());
    for (size_t zero_count : zero_counts) {
      std::string const zeroes(zero_count, '0');
      std::string const integer_exponent =
          std::to_string(-18 - int(tail.size()) - int(zero_count));

      // Put a non-digit marker after each number so verification also checks
      // the public from_chars pointer result.
      result.push_back(input_case{prefix + tail + zeroes + "e" +
                                      integer_exponent + "x",
                                  prefix + tail + "e" +
                                      std::to_string(-18 - int(tail.size())) +
                                      "x"});
      result.push_back(input_case{"0." + prefix + tail + zeroes + "e1x",
                                  "0." + prefix + tail + "e1x"});
      result.push_back(input_case{prefix.substr(0, 1) + "." +
                                      prefix.substr(1) + tail + zeroes + "e0x",
                                  prefix.substr(0, 1) + "." +
                                      prefix.substr(1) + tail + "e0x"});
    }
  }
  return result;
}

std::vector<input_case> make_cases(std::vector<size_t> const &zero_counts) {
  return make_cases(zero_counts, {size_t(20), size_t(30), size_t(120)});
}

bool verify(std::vector<input_case> const &cases) {
  for (input_case const &test : cases) {
    if (!verify_case(test)) {
      return false;
    }
  }
  return true;
}

std::vector<input_case> make_ordinary_cases() {
  std::vector<input_case> result;
  result.reserve(1000);
  for (size_t index = 0; index < 1000; ++index) {
    int const integer = int(index % 97);
    int const fraction = int((index * 17) % 100000);
    int const exponent = int(index % 15) - 7;
    std::string input;
    switch (index % 4) {
    case 0:
      input = std::to_string(integer) + "." + std::to_string(fraction);
      break;
    case 1:
      input = std::to_string(integer) + "." + std::to_string(fraction) +
              "e" + std::to_string(exponent);
      break;
    case 2:
      input = "-" + std::to_string(integer) + "." +
              std::to_string(fraction) + "e" + std::to_string(exponent);
      break;
    default:
      input = "0." + std::to_string(fraction) + "e" +
              std::to_string(exponent);
      break;
    }
    result.push_back(input_case{input + "x", std::string()});
  }
  return result;
}

bool verify_ordinary(std::vector<input_case> const &cases) {
  for (input_case const &test : cases) {
    double value = 0;
    std::errc error;
    size_t parsed_length = 0;
    parse(test.input, value, error, parsed_length);
    if (error != std::errc() || parsed_length + 1 != test.input.size() ||
        takes_digit_comp(test.input)) {
      return false;
    }
  }
  return true;
}

void parse_all(std::vector<input_case> const &cases, size_t iterations) {
  uint64_t local_sink = 0;
  for (size_t iteration = 0; iteration < iterations; ++iteration) {
    for (input_case const &test : cases) {
      double value = 0;
      std::errc error;
      size_t parsed_length = 0;
      parse(test.input, value, error, parsed_length);
      local_sink += bits(value) + uint64_t(parsed_length) + uint64_t(error);
    }
  }
  sink += local_sink;
}

double benchmark(std::vector<input_case> const &cases, size_t iterations) {
  // Keep input construction and correctness validation out of the measured
  // parse operation, as callers normally own the input buffers already.
  parse_all(cases, 1);
  std::chrono::steady_clock::time_point const start =
      std::chrono::steady_clock::now();
  parse_all(cases, iterations);
  std::chrono::steady_clock::duration const elapsed =
      std::chrono::steady_clock::now() - start;
  double const operations = double(cases.size()) * double(iterations);
  return std::chrono::duration<double, std::nano>(elapsed).count() /
         operations;
}

double benchmark(std::vector<input_case> const &cases) {
  return benchmark(cases, 2000);
}

} // namespace

int main(int argc, char **argv) {
  if (argc != 2) {
    std::cerr << "usage: trailing_zero_benchmark "
                 "--verify|--boundary-benchmark|--pre-cutoff-benchmark|"
                 "--medium-suffix-benchmark|--ordinary-benchmark|--benchmark\n";
    return EXIT_FAILURE;
  }

  std::string const mode(argv[1]);
  if (mode == "--verify") {
    std::vector<input_case> const cases = make_cases(
        {size_t(0), size_t(1), size_t(8), size_t(15), size_t(16), size_t(17),
         size_t(32), size_t(63), size_t(64), size_t(65), size_t(700),
         size_t(769), size_t(1000), size_t(4096)},
        {size_t(20), size_t(30), size_t(120), size_t(720), size_t(769)});
    if (!verify(cases)) {
      std::cerr << "trailing-zero verification failed\n";
      return EXIT_FAILURE;
    }
    return EXIT_SUCCESS;
  }

  if (mode == "--boundary-benchmark") {
    std::vector<size_t> zero_counts;
    for (size_t zero_count = 0; zero_count <= 65; ++zero_count) {
      zero_counts.push_back(zero_count);
    }
    std::vector<input_case> const cases = make_cases(
        zero_counts, {size_t(20), size_t(120), size_t(720), size_t(769)});
    if (!verify(cases)) {
      std::cerr << "trailing-zero verification failed\n";
      return EXIT_FAILURE;
    }
    std::cout << "{\"metric\":\"ns_per_boundary_parse\",\"value\":"
              << std::fixed << std::setprecision(3) << benchmark(cases, 1000)
              << "}\n";
    return EXIT_SUCCESS;
  }

  if (mode == "--pre-cutoff-benchmark") {
    std::vector<size_t> zero_counts;
    for (size_t zero_count = 0; zero_count < 16; ++zero_count) {
      zero_counts.push_back(zero_count);
    }
    std::vector<input_case> const cases = make_cases(
        zero_counts,
        {size_t(20), size_t(30), size_t(120), size_t(720), size_t(769)});
    if (!verify(cases)) {
      std::cerr << "trailing-zero verification failed\n";
      return EXIT_FAILURE;
    }
    std::cout << "{\"metric\":\"ns_per_pre_cutoff_parse\",\"value\":"
              << std::fixed << std::setprecision(3) << benchmark(cases, 1000)
              << "}\n";
    return EXIT_SUCCESS;
  }

  if (mode == "--medium-suffix-benchmark") {
    std::vector<size_t> zero_counts;
    for (size_t zero_count = 16; zero_count < 64; ++zero_count) {
      zero_counts.push_back(zero_count);
    }
    std::vector<input_case> const cases = make_cases(
        zero_counts, {size_t(120), size_t(720), size_t(769)});
    if (!verify(cases)) {
      std::cerr << "trailing-zero verification failed\n";
      return EXIT_FAILURE;
    }
    std::cout << "{\"metric\":\"ns_per_medium_suffix_parse\",\"value\":"
              << std::fixed << std::setprecision(3) << benchmark(cases, 1000)
              << "}\n";
    return EXIT_SUCCESS;
  }

  if (mode == "--ordinary-benchmark") {
    std::vector<input_case> const cases = make_ordinary_cases();
    if (!verify_ordinary(cases)) {
      std::cerr << "ordinary verification failed\n";
      return EXIT_FAILURE;
    }
    std::cout << "{\"metric\":\"ns_per_ordinary_parse\",\"value\":"
              << std::fixed << std::setprecision(3) << benchmark(cases, 10000)
              << "}\n";
    return EXIT_SUCCESS;
  }

  if (mode == "--benchmark") {
    std::vector<input_case> const cases = make_cases(
        {size_t(64), size_t(700), size_t(769), size_t(4096)});
    if (!verify(cases)) {
      std::cerr << "trailing-zero verification failed\n";
      return EXIT_FAILURE;
    }
    std::cout << "{\"metric\":\"ns_per_parse\",\"value\":"
              << std::fixed << std::setprecision(3) << benchmark(cases)
              << "}\n";
    return EXIT_SUCCESS;
  }

  std::cerr << "unknown mode: " << mode << '\n';
  return EXIT_FAILURE;
}