digitalroot.c

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <errno.h>
#include <limits.h>

int digitalroot(long num) {
  int sum = 0;

  // sum digits of num
  while (num > 0) {
    sum += num % 10;
    num = num / 10;
  }

  // reduce sum to digital root
  while (sum > 9) {
    sum = (sum / 10) + (sum % 10);
  }

  return sum;
}

int isdigitalroot(long num, int dr) {
  return num >= dr && (num - dr) % 9 == 0;
}

void print_combos_of_3(int dr) {
  for (int a = 1; a <= 8; a++) {
    for (int b = a + 1; b <= 8; b++) {
      for (int c = b + 1; c <= 8; c++) {
        if (isdigitalroot(a + b + c, dr)) {
          printf("%d + %d + %d -> %d\n", a, b, c, dr);
        }
      }
    }
  }
}

void print_combos_of_4(int dr) {
  for (int a = 1; a <= 8; a++) {
    for (int b = a + 1; b <= 8; b++) {
      for (int c = b + 1; c <= 8; c++) {
        for (int d = c + 1; d <= 8; d++) {
          if (isdigitalroot(a + b + c + d, dr)) {
            printf("%d + %d + %d + %d -> %d\n", a, b, c, d, dr);
          }
        }
      }
    }
  }
}

void print_combos_of_5(int dr) {
  for (int a = 1; a <= 8; a++) {
    for (int b = a + 1; b <= 8; b++) {
      for (int c = b + 1; c <= 8; c++) {
        for (int d = c + 1; d <= 8; d++) {
          for (int e = d + 1; e <= 8; e++) {
            if (isdigitalroot(a + b + c + d + e, dr)) {
              printf("%d + %d + %d + %d + %d -> %d\n", a, b, c, d, e, dr);
            }
          }
        }
      }
    }
  }
}

void print_usage() {
  fprintf(stderr, "usage: digitalroot [-c <count>] <num>\n");
}

int main(int argc, char *argv[]) {
  char *end;

  if (argc == 4 && strcmp(argv[1], "-c") == 0) {
    int count = strtol(argv[2], &end, 10);
    if (end == argv[3] || *end != '\0' || count < 3 || count > 5) {
      fprintf(stderr, "error: <count> must be a number between 3 and 5\n");
      return 2;
    }

    int dr = strtol(argv[3], &end, 10);
    if (end == argv[3] || *end != '\0' || dr < 1 || dr > 9) {
      fprintf(stderr, "error: <num> must be a number between 1 and 9\n");
      return 2;
    }

    if (count == 3) {
      print_combos_of_3(dr);
    } else if (count == 4) {
      print_combos_of_4(dr);
    } else if (count == 5) {
      print_combos_of_5(dr);
    }
  } else if (argc == 2) {
    errno = 0;
    long num = strtol(argv[1], &end, 10);
    if (end == argv[1] || *end != '\0' || num < 0 || num == LONG_MAX && errno == ERANGE) {
      fprintf(stderr, "error: <num> must be a positive number\n");
      return 2;
    }

    printf("%s -> %d\n", argv[1], digitalroot(num));
  } else {
    print_usage();
    return 1;
  }
}