忘れるスタック実装

stack.c

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

typedef struct _stack {
  char **m;
  int index;
  int limit_index;
  int ttl;
  int size;
} STACK, *pSTACK;

pSTACK create_stack(int size, int ttl) {
  pSTACK p = NULL;
  if(size < ttl) { return p; }

  p = malloc(sizeof(STACK));
  p->m = (char**)malloc((sizeof(char*)) * size);
  memset(p->m, 0, (sizeof(char*)) * size);
  p->index = 0;
  p->limit_index = 0;
  p->size = size;
  p->ttl = ttl;
  return p;
}

int destroy_stack(pSTACK p) {
  free(p->m);
  p->m = NULL;
  free(p);
  return 0;
}

int slide_limit(pSTACK p) {
  int index = p->index;
  if(p->index < p->limit_index) {
    index += p->size;
  }
  if((index - p->limit_index) > p->ttl) {
    return 1; 
  }
  return 0;
}

int push(pSTACK p, char* data) {
  if(p->size > p->index) {
    p->m[p->index] = data;
    p->index = (p->index + 1) % p->size;
    p->limit_index = (p->limit_index + slide_limit(p)) % p->size;
    return 0;
  } else {
    return 1;
  }
}

char* pop(pSTACK p) {
  p->index--;
  if(p->index < 0) {
    p->index = p->size - 1;
  }
  return p->m[p->index];
}

int empty(pSTACK p) {
  return p->index == p->limit_index;
}

int main() {
  pSTACK stack = create_stack(16, 2);

  push(stack, "粘土を買う");
  push(stack, "おしごと");
  push(stack, "みすきー");
  while(!empty(stack)) {
    printf("%s\n", pop(stack));
  }
  destroy_stack(stack);
  stack = NULL;
  return 0;
}