// Copyright (c) 2020 Meng Huang (mhboy@outlook.com)
// This package is licensed under a MIT license that can be found in the LICENSE file.

// +build darwin linux dragonfly freebsd netbsd openbsd

package sem

import (
	"syscall"
	"unsafe"
)

const (
	// IPC_CREAT creates if key is nonexistent
	IPC_CREAT = 00001000

	// IPC_EXCL fails if key exists.
	IPC_EXCL = 00002000

	// IPC_NOWAIT returns error no wait.
	IPC_NOWAIT = 04000

	// SEM_UNDO sets up adjust on exit entry
	SEM_UNDO = 010000

	// IPC_RMID removes identifier
	IPC_RMID = 0
	// IPC_SET sets ipc_perm options.
	IPC_SET = 1
	// IPC_STAT gets ipc_perm options.
	IPC_STAT = 2
)

type sembuf struct {
	num uint16
	op  int16
	flg int16
}

// Get calls the semget system call.
func Get(key int) (uintptr, error) {
	semid, _, _ := syscall.Syscall(syscall.SYS_SEMGET, uintptr(key), 1, 0666)
	if int(semid) < 0 {
		semid, _, err := syscall.Syscall(syscall.SYS_SEMGET, uintptr(key), 1, IPC_CREAT|IPC_EXCL|0666)
		if int(semid) < 0 {
			return 0, err
		}
		var semun int = 1
		r1, _, err := syscall.Syscall6(syscall.SYS_SEMCTL, semid, 0, SETVAL, uintptr(semun), 0, 0)
		if int(r1) < 0 {
			return 0, err
		}
		return semid, nil
	}
	return semid, nil
}

// P calls the semop P system call.
func P(semid uintptr, flg int16) (bool, error) {
	if flg == 0 {
		flg = SEM_UNDO
	}
	buf := sembuf{num: 0, op: -1, flg: flg}
	r1, _, err := syscall.Syscall(syscall.SYS_SEMOP, semid, uintptr(unsafe.Pointer(&buf)), 1)
	var ok bool
	if r1 == 0 {
		ok = true
	}
	if err != 0 && err != syscall.EAGAIN {
		return ok, err
	}
	return ok, nil
}

// V calls the semop V system call.
func V(semid uintptr, flg int16) (bool, error) {
	if flg == 0 {
		flg = SEM_UNDO
	}
	buf := sembuf{num: 0, op: +1, flg: flg}
	r1, _, err := syscall.Syscall(syscall.SYS_SEMOP, semid, uintptr(unsafe.Pointer(&buf)), 1)
	var ok bool
	if r1 == 0 {
		ok = true
	}
	if err != 0 && err != syscall.EAGAIN {
		return ok, err
	}
	return ok, nil
}

// GetValue calls the semctl GETVAL system call.
func GetValue(semid uintptr) (int, error) {
	r1, _, err := syscall.Syscall(syscall.SYS_SEMCTL, semid, 0, GETVAL)
	if int(r1) < 0 {
		return 0, err
	}
	return int(r1), nil
}

// Remove removes the semaphore with the given id.
func Remove(semid uintptr) error {
	r1, _, errno := syscall.Syscall(syscall.SYS_SEMCTL, semid, IPC_RMID, 0)
	if int(r1) < 0 {
		return syscall.Errno(errno)
	}
	return nil
}