gistfile1.c

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

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

struct List* makeList(int all, int inLoop) {
  struct List *head = malloc(sizeof(struct List));
  struct List *current = head;
  int marker = 1;
  for (int i = 0; i < all - inLoop; ++i) {
    current = current->next = malloc(sizeof(struct List));
    current->value = marker++;
  }
  if (inLoop > 0) {
    struct List *loopStart = current;
    for (int i = 0; i < inLoop; ++i) {
      current = current->next = malloc(sizeof(struct List));
      current->value = marker++;
    }
    current->next = loopStart->next;
  }
  return head;
}

typedef struct ReturnValue (* RecurFunction) (void *);

struct ReturnValue {
  RecurFunction function;
  void * value;
};

void * recur(RecurFunction function, void * input) {
  struct ReturnValue tmp;
  tmp.value = input;
  tmp.function = function;
  do {
    tmp = tmp.function(tmp.value);
  } while (tmp.function != NULL);
  return tmp.value;
}

// Тип входных данных.
struct RecurFactorial {
  int n, acc;
};
struct ReturnValue factorial(void * input) {
  struct RecurFactorial * args = input; // Приведение типа данных.
  struct ReturnValue result;
  result.value = input; // Входные и результируещие данный одни и теже.
  if (args->n == 1) {
    // Функция на следующий запуск не устанавливается для завершения вычислений.
    result.function = NULL;
  } else {
    // Формируем результаты итерации.
    args->acc *= args->n;
    args->n -= 1;
    // Устанвливаем функцию на следующий запуск.
    result.function = factorial;
  }
  return result;
}

struct RecurLoopInList {
  struct List * fastMarker, * slowMarker;
};

struct ReturnValue slowMarker(void * input);
struct ReturnValue fastMarker(void * input);

bool loopInList(struct List *head) {
  struct RecurLoopInList args;
  args.fastMarker = head->next;
  args.slowMarker = head;
  bool * resultP = recur(fastMarker, (void*)&args);
  bool result = *resultP;
  free(resultP);
  return result;
}

struct ReturnValue slowMarker(void * input) {
  struct RecurLoopInList * args = input;
  struct ReturnValue result;
  result.value = input;
  args->slowMarker = args->slowMarker->next;
  result.function = fastMarker;
  return result;
}

struct ReturnValue fastMarker(void * input) {
  struct RecurLoopInList * args = input;
  struct ReturnValue result;
  if (args->fastMarker->next == NULL ||
      args->fastMarker->next->next == NULL) {
    bool * ans = malloc(sizeof(bool));
    *ans = false;
    result.value = (void*)ans;
    result.function = NULL;
  } else if (args->fastMarker->next == args->slowMarker ||
             args->fastMarker->next->next == args->slowMarker) {
    bool * ans = malloc(sizeof(bool));
    *ans = true;
    result.value = (void*)ans;
    result.function = NULL;
  } else {
    args->fastMarker = args->fastMarker->next->next;
    result.value = (void*)input;
    result.function = slowMarker;
  }
  return result;
}

int main(int argc, char** argv) {
  struct RecurFactorial fac;
  fac.n = 5;
  fac.acc = 1;
  recur(factorial, (void *)&fac);
  printf("factorial result: %d\n", fac.acc);
  struct List * head = makeList(10, 5);
  printf("loopInList: %s\n", loopInList(head) ? "true" : "false");
  return 0;
}