Project Euler Problem1

C言語で関数型プログラミングの考え方でやりました

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

struct list {
  int value;
  struct list *next;
};

typedef int func_t(int);

struct list *cons(int val, struct list *lst) {
  struct list *p;
  if ((p = malloc(sizeof(struct list))) == NULL) exit(EXIT_FAILURE);
  p->value = val;
  p->next = lst;
  return p;
}

struct list *enumerate(int st, int end) {
  if (st > end) {
    return NULL;
  } else {
    return cons(st, enumerate(st+1, end));
  }
}

struct list *filter(func_t *f, struct list *lst) {
  if (lst == NULL) {
    return NULL;
  } else {
    if (f(lst->value) == 1) {
      return cons(lst->value, filter(f, lst->next));
    } else {
      return filter(f, lst->next);
    }
  }
}

int divisible_3or5(int n) {
  if (n%3==0 || n%5==0) {
    return 1;
  } else {
    return 0;
  }
}

int sum(struct list *lst) {
  if (lst == NULL) {
    return 0;
  } else {
    return lst->value + sum(lst->next);
  }
}

void free_memory(struct list *p) {
  struct list *temp;

  while (p != NULL) {
    temp = p->next;
    free(p);
    p = temp;
  }
}

int main(int argc, char const *argv[]) {
  struct list *p, *q;

  p = enumerate(1, 999);
  q = filter(divisible_3or5, p);

  printf("%d\n", sum(q));

  free_memory(p);
  free_memory(q);

  return 0;
}