lord.c

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

// prints light if color (hex) passed is light, else dark
// usage: lord hexcolorcode

/*
 * print error message and exit
 * */
void error(char *msg) {
  printf("%s\n", msg);
  exit(1);
}

/*
 * checks if passed hexcolorcode is valid or not (0 = invalid, 1 = valid)
 * */
int validatehex(char *hex) {
  if (strlen(hex) != 6) // hex color code should be 6 digits
    return 0;
  for (int i = 0; i < 6; i++) { // check if 0-9 or a-f or A-F or not
    if (hex[i] >= '0' && hex[i] <= '9')
      continue;
    if (hex[i] >= 'a' && hex[i] <= 'f')
      continue;
    if (hex[i] >= 'A' && hex[i] <= 'F')
      continue;
    return 0;
  }
  return 1;
}

/*
 * convert arbitrary length string to lowercase
 * */
void lower(char *s) {
  int l = strlen(s);
  for (int i = 0; i < l; i++) {
    s[i] = tolower(s[i]);
  }
}

/*
 * convert a single hex character to its integer decimal representation
 * */
int hexchartoint(char c) {
  c = tolower(c);
  if (c >= '0' && c <= '9')
    return c - 48;
  if (c >= 'a' && c <= 'f')
    return c - 'a' + 10;
  return -1;
}

/*
 * efficient integer exponentiation using binary representation
 * */
int intpow(int base, int exp) {
  int rv = 1;
  while (exp > 0) {
    if (exp & 1)
      rv *= base;
    exp >>= 1;
    base *= base;
  }
  return rv;
}

/*
 * convert arbitrary length hexadecimal string to integer decimal representation
 * */
int hextoint(char *hex) {
  int len = strlen(hex);
  int rv = 0;
  for (int i = len - 1; i >= 0; i--) {
    int val = hexchartoint(hex[i]);      // get value of current character
    rv += val * intpow(16, len - 1 - i); // base conversion
  }
  return rv;
}
int main(int argc, char **argv) {
  char *color = calloc(8, sizeof(char)); // allocate memory for 6/7 digit hex
  if (color == NULL)
    error("memory allocation error");
  if (argc > 1) { // if color passed as args, copy it from args to color
    if (strlen(argv[1]) > 7)
      error("Invalid hex code"); // if > 6 digits + #
    strcpy(color, argv[1]);      // copy hex to color
  } else {
    printf("enter hex color code: "); // if not passed as arg, input
    scanf("%s", color);
  }

  /*
   * Remove the # from begining if it exists
   * */
  if (color[0] == '#') {
    char *c2 = calloc(7, sizeof(char));
    if (c2 == NULL)
      error("memory allocation error");
    strncpy(c2, color + 1, 6);
    free(color);
    color = c2;
  }

  lower(color); // convert hex to lowercase
  if (!validatehex(color))
    error("Invalid hex code");   // if hexcode not valid, checks length = 6 too
  int icolor = hextoint(color);  // get int representation of 16bit hex
  free(color);                   // the string is no longer needed
  int r = (icolor >> 16) & 0xff; // get red value (0-255)
  int g = (icolor >> 8) & 0xff;  // get green value (0-255)
  int b = (icolor >> 0) & 0xff;  // get blue value (0-255)
  float luma = 0.2126 * r + 0.7152 * g + 0.0722 * b; // per ITU-R BT.709
  printf("%s\n", (luma < 128 ? "dark" : "light"));   // luma from 0 to 255
  return 0;
}