// Copyright 2012-2020 David Robillard <d@drobilla.net>
// SPDX-License-Identifier: ISC

#ifndef ZIX_SEM_H
#define ZIX_SEM_H

#include "zix/common.h"

#ifdef __APPLE__
#  include <mach/mach.h>
#elif defined(_WIN32)
#  include <limits.h>
#  include <windows.h>
#else
#  include <errno.h>
#  include <semaphore.h>
#endif

#ifdef __cplusplus
extern "C" {
#endif

#include <stdbool.h>

/**
   @addtogroup zix
   @{
   @name Semaphore
   @{
*/

struct ZixSemImpl;

/**
   A counting semaphore.

   This is an integer that is always positive, and has two main operations:
   increment (post) and decrement (wait).  If a decrement can not be performed
   (i.e. the value is 0) the caller will be blocked until another thread posts
   and the operation can succeed.

   Semaphores can be created with any starting value, but typically this will
   be 0 so the semaphore can be used as a simple signal where each post
   corresponds to one wait.

   Semaphores are very efficient (much moreso than a mutex/cond pair).  In
   particular, at least on Linux, post is async-signal-safe, which means it
   does not block and will not be interrupted.  If you need to signal from
   a realtime thread, this is the most appropriate primitive to use.
*/
typedef struct ZixSemImpl ZixSem;

/**
   Create and initialize `sem` to `initial`.
*/
static inline ZixStatus
zix_sem_init(ZixSem* sem, unsigned initial);

/**
   Destroy `sem`.
*/
static inline void
zix_sem_destroy(ZixSem* sem);

/**
   Increment (and signal any waiters).
   Realtime safe.
*/
static inline void
zix_sem_post(ZixSem* sem);

/**
   Wait until count is > 0, then decrement.
   Obviously not realtime safe.
*/
static inline ZixStatus
zix_sem_wait(ZixSem* sem);

/**
   Non-blocking version of wait().

   @return true if decrement was successful (lock was acquired).
*/
static inline bool
zix_sem_try_wait(ZixSem* sem);

/**
   @cond
*/

#ifdef __APPLE__

struct ZixSemImpl {
  semaphore_t sem;
};

static inline ZixStatus
zix_sem_init(ZixSem* sem, unsigned val)
{
  return semaphore_create(mach_task_self(), &sem->sem, SYNC_POLICY_FIFO, val)
           ? ZIX_STATUS_ERROR
           : ZIX_STATUS_SUCCESS;
}

static inline void
zix_sem_destroy(ZixSem* sem)
{
  semaphore_destroy(mach_task_self(), sem->sem);
}

static inline void
zix_sem_post(ZixSem* sem)
{
  semaphore_signal(sem->sem);
}

static inline ZixStatus
zix_sem_wait(ZixSem* sem)
{
  if (semaphore_wait(sem->sem) != KERN_SUCCESS) {
    return ZIX_STATUS_ERROR;
  }
  return ZIX_STATUS_SUCCESS;
}

static inline bool
zix_sem_try_wait(ZixSem* sem)
{
  const mach_timespec_t zero = {0, 0};
  return semaphore_timedwait(sem->sem, zero) == KERN_SUCCESS;
}

#elif defined(_WIN32)

struct ZixSemImpl {
  HANDLE sem;
};

static inline ZixStatus
zix_sem_init(ZixSem* sem, unsigned initial)
{
  sem->sem = CreateSemaphore(NULL, initial, LONG_MAX, NULL);
  return (sem->sem) ? ZIX_STATUS_SUCCESS : ZIX_STATUS_ERROR;
}

static inline void
zix_sem_destroy(ZixSem* sem)
{
  CloseHandle(sem->sem);
}

static inline void
zix_sem_post(ZixSem* sem)
{
  ReleaseSemaphore(sem->sem, 1, NULL);
}

static inline ZixStatus
zix_sem_wait(ZixSem* sem)
{
  if (WaitForSingleObject(sem->sem, INFINITE) != WAIT_OBJECT_0) {
    return ZIX_STATUS_ERROR;
  }
  return ZIX_STATUS_SUCCESS;
}

static inline bool
zix_sem_try_wait(ZixSem* sem)
{
  return WaitForSingleObject(sem->sem, 0) == WAIT_OBJECT_0;
}

#else /* !defined(__APPLE__) && !defined(_WIN32) */

struct ZixSemImpl {
  sem_t sem;
};

static inline ZixStatus
zix_sem_init(ZixSem* sem, unsigned initial)
{
  return sem_init(&sem->sem, 0, initial) ? ZIX_STATUS_ERROR
                                         : ZIX_STATUS_SUCCESS;
}

static inline void
zix_sem_destroy(ZixSem* sem)
{
  sem_destroy(&sem->sem);
}

static inline void
zix_sem_post(ZixSem* sem)
{
  sem_post(&sem->sem);
}

static inline ZixStatus
zix_sem_wait(ZixSem* sem)
{
  while (sem_wait(&sem->sem)) {
    if (errno != EINTR) {
      return ZIX_STATUS_ERROR;
    }
    /* Otherwise, interrupted, so try again. */
  }

  return ZIX_STATUS_SUCCESS;
}

static inline bool
zix_sem_try_wait(ZixSem* sem)
{
  return (sem_trywait(&sem->sem) == 0);
}

#endif

/**
   @endcond
   @}
   @}
*/

#ifdef __cplusplus
} /* extern "C" */
#endif

#endif /* ZIX_SEM_H */