#include <stdio.h>
#include <inttypes.h>
#include <math.h>
int16_t calculate_red(int16_t kelvin_temperature) {
if (kelvin_temperature <= 6600) {
return 255;
} else {
double res = pow(kelvin_temperature / 100 - 60, -0.1332047592) * 329.698727446;
if (res > 255) {
return 255;
} else if (res < 0) {
return 0;
}
return res;
}
}
int16_t calculate_green(int16_t kelvin_temperature) {
double res;
if (kelvin_temperature <= 6600) {
res = log(kelvin_temperature / 100) * 99.4708025861 - 161.1195681661;
} else {
res = pow(kelvin_temperature / 100 - 60, -0.0755148492) * 288.1221695283;
}
if (res > 255) {
return 255;
} else if (res < 0) {
return 0;
}
return res;
}
int16_t calculate_blue(int16_t kelvin_temperature) {
if (kelvin_temperature <= 1900) {
return 0;
} else if (kelvin_temperature >= 6600) {
return 255;
} else {
double res = log(kelvin_temperature / 100 - 10) * 138.5177312231 - 305.0447927307;
if (res > 255) {
return 255;
} else if (res < 0) {
return 0;
}
return res;
}
}
int main(int argc, char **argv) {
if (argc < 2) {
fputs("Specify the temperature in Kelvin whose color you want to calculate.\n", stderr);
return 1;
}
int16_t k;
if (argv[1][0] == '-' || sscanf(argv[1], "%" SCNd16, &k) != 1) {
fputs("The temperature in Kelvin must be a positive integer.\n", stderr);
return 1;
}
int r = calculate_red(k);
int g = calculate_green(k);
int b = calculate_blue(k);
printf("#%02X%02X%02X\n", r, g, b);
return 0;
}