#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <ctype.h>
#include <getopt.h>

/* Supported units and string conversion */
typedef enum TemperatureUnit { CELSIUS, DELISLE, FARENHEIT, KELVIN, RANKINE } TemperatureUnit;
const TemperatureUnit temperatureUnits[] = { CELSIUS, DELISLE, FARENHEIT, KELVIN, RANKINE };
const char* temperatureUnitNames[] = { "Celsius", "Delisle", "Farenheit", "Kelvin", "Rankine" };

/* Conversion table to kelvin */
const double kelvinMult[] = {1., -2. / 3., 5. / 9., 1, 5. / 9.};
const double kelvinAdd[] = {273.15, 373.15, 255.37, 0, 0};

static int const maxUnitLength = 31; // Maximum Length of members of temperatureUnitNames (not counting trailing \0)
static int const unknownUnit = -1; // Ugh.

void printUsage()
{
  printf("usage: temperaturUmrechner {-f|--from} <UNIT> {-t|--to} <UNIT> [<VALUE> [...]]\n"
         );
}

TemperatureUnit parseUnit(char str[])
{
  char lowerUnit[maxUnitLength + 1];
  
  char lowerStr[maxUnitLength + 1];
  memset(lowerStr, '\0', maxUnitLength + 1);
  strncpy(lowerStr, str, maxUnitLength);

  for (int i = 0; i < maxUnitLength; i++)
    {
      if (lowerStr[i] == '\0')
        break;

      lowerStr[i] = tolower(lowerStr[i]);
    }


  for (int unit = 0; unit < (int)( sizeof(temperatureUnits) / sizeof(TemperatureUnit) ); unit++)
    {
      memset(lowerUnit, '\0', maxUnitLength + 1);
      for (int i = 0; i < maxUnitLength; i++)
        {
          if (temperatureUnitNames[unit][i] == '\0')
            break;

          lowerUnit[i] = tolower(temperatureUnitNames[unit][i]);
        }

      if (strcmp(lowerUnit, lowerStr) == 0)
        return temperatureUnits[unit];
    }

  return unknownUnit;
}

double convert(double value, TemperatureUnit fromUnit, TemperatureUnit toUnit)
{
  int fromUnitIndex = -1;
  int toUnitIndex = -1;

  for (int unit = 0; unit < (int)( sizeof(temperatureUnits) / sizeof(TemperatureUnit) ); unit++)
    {
      if (temperatureUnits[unit] == fromUnit)
        fromUnitIndex = unit;

      if (temperatureUnits[unit] == toUnit)
        toUnitIndex = unit;

      if (fromUnitIndex != -1 && toUnitIndex != -1)
        break;
    }

  if (fromUnitIndex == -1 || toUnitIndex == -1)
    abort();

  return (value * kelvinMult[fromUnitIndex] + kelvinAdd[fromUnitIndex]) / kelvinMult[toUnitIndex] - kelvinAdd[toUnitIndex];
}

void main(int argc, char* argv[])
{
  TemperatureUnit fromUnit = unknownUnit;
  TemperatureUnit toUnit = unknownUnit;

  while (1) {
    int c;
    int optionIndex = 0;

    static struct option longOptions[] = {
      {"from", required_argument, 0, 'f'},
      {"to", required_argument, 0, 't'},
      {"help", no_argument, 0, 'h'},
      {0, 0, 0, 0}
    };

    c = getopt_long(argc, argv, "f:t:h", longOptions, &optionIndex);

    if (c == -1)
      break;

    switch (c)
      {
      case 'f':
        fromUnit = parseUnit(optarg);
        break;
      case 't':
        toUnit = parseUnit(optarg);
        break;
      case 'h':
        printUsage();
        exit(0);
        break;
      default:
        printUsage();
      }
  }

  if (fromUnit == unknownUnit)
    fprintf(stderr, "Unknown (or unset) unit to convert from\n");
  if (toUnit == unknownUnit)
    fprintf(stderr, "Unknown (or unset) unit to convert to\n");

  if (toUnit == unknownUnit || fromUnit == unknownUnit)
    {
      printUsage();
      exit(2);
    }

  for (double value;optind < argc;optind++)
    {
      sscanf(argv[optind], "%lf", &value);
      printf("%.3g\n", convert(value, fromUnit, toUnit));
    }

  exit(0);
}