c-exercises/circular-buffer.c

/* Circular buffer, using a counter to distinguish full from empty buffer. */

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

#include <assert.h>

#define DEBUG 0
#define debug_printf(fmt, ...) \
          do { if (DEBUG) fprintf(stderr, fmt, __VA_ARGS__); } while (0)

#define CB_SIZE 42

/* Types */

struct elem {
  int foo;
};
typedef struct elem elem_t;

struct cb {
  int count;
  int size;
  int start;
  int end;

  elem_t *elems;
};
typedef struct cb cb_t;


/* Helpers */

void dienomem() {
  fprintf(stderr, "ERROR: No memory?!\n");
  exit(1);
}

/* Operations */

void cb_init(cb_t * cb, int size) {
  cb->count = 0;
  cb->size = size;

  cb->start = 0;

  cb->elems = malloc(sizeof(elem_t) * size);
  if (!cb->elems)
    dienomem();
}

void cb_free(cb_t * cb) {
  free(cb->elems);
}

bool cb_is_full(cb_t * cb) {
  return (cb->count == cb->size);
}

bool cb_is_empty(cb_t * cb) {
  return (cb->count == 0);
}

int cb_count(cb_t * cb) {
  return cb->count;
}

void cb_write(cb_t * cb, elem_t * elem) {
  int end = (cb->start + cb->count) % cb->size;
  cb->elems[end] = *elem;

  if (cb_is_full(cb))
    /* We have just overwritten the first element of the buffer,
     * therefore move the start. */
    cb->start = (cb->start + 1) % cb->size;
  else
    cb->count++;
}

bool cb_read(cb_t * cb, elem_t * elem) {
  if (!cb_is_empty(cb)) {
    *elem = cb->elems[cb->start];
    cb->start = (cb->start + 1) % cb->size;
    cb->count--;
    return true;
  } else {
    return false;
  }
}

/* Test it. */

int main(void) {
  /* Initialize empty CB_SIZE-element circular buffer. */
  cb_t *test_cb = malloc(sizeof(cb_t));
  if (!test_cb)
    dienomem();
  cb_init(test_cb, CB_SIZE);

  /* Should be empty. */
  assert(cb_is_empty(test_cb) == true);
  assert(cb_is_full(test_cb) == false);

  /* Add one element. */
  elem_t *one = malloc(sizeof(elem_t));
  if (!one)
    dienomem();
  one->foo = 23;
  cb_write(test_cb, one);
  free(one);
  assert(cb_is_empty(test_cb) == false);

  /* Read the one element. */
  elem_t *oneback = malloc(sizeof(elem_t));
  if (!oneback)
    dienomem();
  cb_read(test_cb, oneback);
  assert(oneback->foo == 23);
  free(oneback);
  assert(cb_is_empty(test_cb) == true);

  /* Fill the buffer. */
  elem_t *e = malloc(sizeof(elem_t));
  if (!e)
    dienomem();
  for (int i = 1; i <= CB_SIZE; i++) {
    e->foo = i;
    cb_write(test_cb, e);
  }
  assert(cb_is_empty(test_cb) == false);
  assert(cb_is_full(test_cb) == true);
  assert(cb_count(test_cb) == CB_SIZE);

  /* Add another element. */
  e->foo = CB_SIZE + 1;
  cb_write(test_cb, e);
  assert(cb_is_empty(test_cb) == false);
  assert(cb_is_full(test_cb) == true);
  assert(cb_count(test_cb) == CB_SIZE);

  /* Read all elements and check their values. */
  for (int i = 2; i <= CB_SIZE + 1; i++) {
    bool rc = cb_read(test_cb, e);
    assert(rc == true);
    debug_printf("e->foo == %d\n", e->foo);
    assert(e->foo == i);
  }
  assert(cb_is_empty(test_cb) == true);

  /* Should not be possible to read a value now. */
  bool rc = cb_read(test_cb, e);
  assert(rc == false);
  assert(cb_is_empty(test_cb) == true);
  assert(cb_count(test_cb) == 0);

  /* Fill the buffer - twice. */
  for (int i = 1; i <= CB_SIZE * 2; i++) {
    e->foo = i;
    cb_write(test_cb, e);
  }
  assert(cb_is_empty(test_cb) == false);
  assert(cb_is_full(test_cb) == true);
  assert(cb_count(test_cb) == CB_SIZE);

  /* Read all elements and check their values. */
  for (int i = CB_SIZE + 1; i <= CB_SIZE * 2; i++) {
    bool rc = cb_read(test_cb, e);
    assert(rc == true);
    assert(e->foo == i);
  }
  assert(cb_is_empty(test_cb) == true);

  /* Free buffer etc. */
  free(e);
  cb_free(test_cb);
  free(test_cb);
}