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// Copyright 2021 Benjamin Barenblat
//
// Licensed under the Apache License, Version 2.0 (the "License"); you may not
// use this file except in compliance with the License. You may obtain a copy of
// the License at
//
// https://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
// WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
// License for the specific language governing permissions and limitations under
// the License.
#include <getopt.h>
#include <locale.h>
#include <math.h>
#include <stdlib.h>
#include <iostream>
#include <optional>
#include <string_view>
namespace {
constexpr std::string_view kShortUsage =
"Usage: dewpoint TEMPERATURE HUMIDITY\n";
constexpr std::string_view kHelp =
R"(Compute the dew point from a given temperature and humidity. Temperatures are
interpreted by default according to the current locale; humidity is interpreted
as a percentage.
Options:
-c, --celsius, --centigrade
use the Celsius temperature scale
-f, --fahrenheit use the Fahrenheit temperature scale
--help display this help and exit
)";
constexpr std::string_view kAskForHelp =
"Try 'dewpoint --help' for more information\n";
enum {
kHelpLongOption = 128,
};
bool LocaleUsesFahrenheit(std::string_view locale) {
int underscore = locale.find('_');
int dot = locale.find('.');
if (underscore == std::string_view::npos || dot == std::string_view::npos ||
dot <= underscore) {
return false;
}
std::string_view territory =
locale.substr(underscore + 1, dot - underscore - 1);
return territory == "US" /* United States */ ||
territory == "LR" /* Liberia */ ||
territory == "FM" /* Micronesia */ ||
territory == "KY" /* Cayman Islands */ ||
territory == "MH" /* Marshall Islands */ ||
territory == "PW" /* Palau */;
}
std::optional<float> ReadFloat(const char* s) {
char* end;
float f = strtof(s, &end);
if (*end != '\0') {
return std::nullopt;
}
return f;
}
// Approximates the dew point of air at the specified temperature and relative
// humidity using the Magnus formula. For a complete derivation, see equation 8
// of Mark G. Lawrence, "The Relationship Between Relative Humidity and the
// Dewpoint Temperature in Moist Air," Bulletin of the American Meteorological
// Society 86(2) (February 2005), 225-234,
// https://doi.org/10.1175/BAMS-86-2-225.
float DewPoint(float celsius, float humidity) {
constexpr float kA = 17.625;
constexpr float kB = 243.04;
float x = log(humidity * 0.01f) + kA * celsius / (kB + celsius);
return kB * x / (kA - x);
}
float DewPointFahrenheit(float fahrenheit, float humidity) {
return 9.0f / 5.0f * DewPoint(5.0f / 9.0f * (fahrenheit - 32.0f), humidity) +
32.0f;
}
} // namespace
int main(int argc, char* argv[]) {
setlocale(LC_ALL, "");
std::string measurement_locale = "C";
if (const char* s = setlocale(LC_MEASUREMENT, ""); s != nullptr) {
measurement_locale = s;
}
const bool locale_uses_fahrenheit = LocaleUsesFahrenheit(measurement_locale);
bool use_fahrenheit = locale_uses_fahrenheit;
static option long_options[] = {
{"celsius", no_argument, nullptr, 'c'},
{"centigrade", no_argument, nullptr, 'c'},
{"fahrenheit", no_argument, nullptr, 'f'},
{"help", no_argument, nullptr, kHelpLongOption},
{nullptr, 0, nullptr, 0},
};
while (true) {
int c = getopt_long(argc, argv, "cf", long_options, nullptr);
if (c == -1) {
break;
}
switch (c) {
case 'c':
use_fahrenheit = false;
break;
case 'f':
use_fahrenheit = true;
break;
case kHelpLongOption:
std::cout << kShortUsage << kHelp
<< "\nYour current measurement locale is "
<< measurement_locale << ", which uses "
<< (locale_uses_fahrenheit ? "Fahrenheit" : "Celsius")
<< " by\ndefault.\n";
return 0;
case '?':
std::cerr << kAskForHelp;
return 1;
default:
std::cerr << "Internal error; please report.\n";
return 1;
}
}
if (optind != argc - 2) {
std::cerr << kShortUsage << kAskForHelp;
return 1;
}
std::optional<float> temperature = ReadFloat(argv[1]);
if (!temperature.has_value()) {
std::cerr << "dewpoint: invalid temperature \"" << argv[1] << "\"\n"
<< kAskForHelp;
return 1;
}
std::optional<float> humidity = ReadFloat(argv[2]);
if (!humidity.has_value() || *humidity <= 0.0f) {
std::cerr << "dewpoint: invalid humidity \"" << argv[1] << "\"\n"
<< kAskForHelp;
return 1;
}
float dew_point = use_fahrenheit ? DewPointFahrenheit(*temperature, *humidity)
: DewPoint(*temperature, *humidity);
std::cout << static_cast<int>(rintf(dew_point)) << '\n';
}
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