/*

Package rpio provides GPIO access on the Raspberry PI without any need
for external c libraries (ex: WiringPI or BCM2835).

Supports simple operations such as:
- Pin mode/direction (input/output/clock)
- Pin write (high/low)
- Pin read (high/low)
- Pull up/down/off

Example of use:

	rpio.Open()
	defer rpio.Close()

	pin := rpio.Pin(4)
	pin.Output()

	for {
		pin.Toggle()
		time.Sleep(time.Second)
	}

The library use the raw BCM2835 pinouts, not the ports as they are mapped
on the output pins for the raspberry pi

   Rev 1 Raspberry Pi
+------+------+--------+
| GPIO | Phys | Name   |
+------+------+--------+
|   0  |   3  | SDA    |
|   1  |   5  | SCL    |
|   4  |   7  | GPIO 7 |
|   7  |  26  | CE1    |
|   8  |  24  | CE0    |
|   9  |  21  | MISO   |
|  10  |  19  | MOSI   |
|  11  |  23  | SCLK   |
|  14  |   8  | TxD    |
|  15  |  10  | RxD    |
|  17  |  11  | GPIO 0 |
|  18  |  12  | GPIO 1 |
|  21  |  13  | GPIO 2 |
|  22  |  15  | GPIO 3 |
|  23  |  16  | GPIO 4 |
|  24  |  18  | GPIO 5 |
|  25  |  22  | GPIO 6 |
+------+------+--------+

See the spec for full details of the BCM2835 controller:
http://www.raspberrypi.org/wp-content/uploads/2012/02/BCM2835-ARM-Peripherals.pdf

*/

package rpio

import (
	"bytes"
	"encoding/binary"
	"os"
	"reflect"
	"sync"
	"syscall"
	"time"
	"unsafe"
)

type Mode uint8
type Pin uint8
type State uint8
type Pull uint8

// Memory offsets for gpio, see the spec for more details
const (
	bcm2835Base = 0x20000000
	gpioOffset  = 0x200000
	clkOffset   = 0x101000

	memLength = 4096
)

var (
	base     int64
	gpioBase int64
	clkBase  int64
)

func init() {
	base = getBase()
	gpioBase = base + gpioOffset
	clkBase = base + clkOffset
}

// Pin mode, a pin can be set in Input or Output mode, or clock
const (
	Input Mode = iota
	Output
	Clock
)

// State of pin, High / Low
const (
	Low State = iota
	High
)

// Pull Up / Down / Off
const (
	PullOff Pull = iota
	PullDown
	PullUp
)

// Arrays for 8 / 32 bit access to memory and a semaphore for write locking
var (
	memlock  sync.Mutex
	gpioMem  []uint32
	clkMem   []uint32
	gpioMem8 []uint8
	clkMem8  []uint8
)

// Set pin as Input
func (pin Pin) Input() {
	PinMode(pin, Input)
}

// Set pin as Output
func (pin Pin) Output() {
	PinMode(pin, Output)
}

// Set pin as Clock
func (pin Pin) Clock() {
	PinMode(pin, Clock)
}

// Set pin High
func (pin Pin) High() {
	WritePin(pin, High)
}

// Set pin Low
func (pin Pin) Low() {
	WritePin(pin, Low)
}

// Toggle pin state
func (pin Pin) Toggle() {
	TogglePin(pin)
}

func (pin Pin) Freq(freq int) {
	SetFreq(pin, freq)
}

// Set pin Mode
func (pin Pin) Mode(mode Mode) {
	PinMode(pin, mode)
}

// Set pin state (high/low)
func (pin Pin) Write(state State) {
	WritePin(pin, state)
}

// Read pin state (high/low)
func (pin Pin) Read() State {
	return ReadPin(pin)
}

// Set a given pull up/down mode
func (pin Pin) Pull(pull Pull) {
	PullMode(pin, pull)
}

// Pull up pin
func (pin Pin) PullUp() {
	PullMode(pin, PullUp)
}

// Pull down pin
func (pin Pin) PullDown() {
	PullMode(pin, PullDown)
}

// Disable pullup/down on pin
func (pin Pin) PullOff() {
	PullMode(pin, PullOff)
}

// PinMode sets the mode (direction) of a given pin (Input, Output or Clock)
//
// Clock is possible only for some pins (bcm 4, 5, 6, 20, 21)
func PinMode(pin Pin, mode Mode) {

	// Pin fsel register, 0 or 1 depending on bank
	fselReg := uint8(pin) / 10
	shift := (uint8(pin) % 10) * 3
	f := uint32(0)

	switch mode {
	case Input:
		f = 0 // 000
	case Output:
		f = 1 // 001
	case Clock:
		switch pin {
		case 4, 5, 6, 32, 34, 42, 43, 44:
			f = 4 // 100 - alt0
		case 20, 21:
			f = 2 // 010 - alt5
		default:
			return
		}
	}

	memlock.Lock()
	defer memlock.Unlock()

	const pinMask = 7 // 0b111 - pinmode is 3 bits

	gpioMem[fselReg] = (gpioMem[fselReg] &^ (pinMask << shift)) | (f << shift)
}

// WritePin sets a given pin High or Low
// by setting the clear or set registers respectively
func WritePin(pin Pin, state State) {

	p := uint8(pin)

	// Clear register, 10 / 11 depending on bank
	// Set register, 7 / 8 depending on bank
	clearReg := p/32 + 10
	setReg := p/32 + 7

	memlock.Lock()
	defer memlock.Unlock()

	if state == Low {
		gpioMem[clearReg] = 1 << (p & 31)
	} else {
		gpioMem[setReg] = 1 << (p & 31)
	}

}

// Read the state of a pin
func ReadPin(pin Pin) State {
	// Input level register offset (13 / 14 depending on bank)
	levelReg := uint8(pin)/32 + 13

	if (gpioMem[levelReg] & (1 << uint8(pin&31))) != 0 {
		return High
	}

	return Low
}

// Toggle a pin state (high -> low -> high)
// TODO: probably possible to do this much faster without read
func TogglePin(pin Pin) {
	switch ReadPin(pin) {
	case Low:
		pin.High()
	case High:
		pin.Low()
	}
}

func PullMode(pin Pin, pull Pull) {
	// Pull up/down/off register has offset 38 / 39, pull is 37
	pullClkReg := uint8(pin)/32 + 38
	pullReg := 37
	shift := (uint8(pin) % 32)

	memlock.Lock()
	defer memlock.Unlock()

	switch pull {
	case PullDown, PullUp:
		gpioMem[pullReg] = gpioMem[pullReg]&^3 | uint32(pull)
	case PullOff:
		gpioMem[pullReg] = gpioMem[pullReg] &^ 3
	}

	// Wait for value to clock in, this is ugly, sorry :(
	time.Sleep(time.Microsecond)

	gpioMem[pullClkReg] = 1 << shift

	// Wait for value to clock in
	time.Sleep(time.Microsecond)

	gpioMem[pullReg] = gpioMem[pullReg] &^ 3
	gpioMem[pullClkReg] = 0

}

// Set clock speed for given pin
//
// freq should be in range 4688Hz - 19.2MHz to prevent unexpected behavior
// (for smaller frequencies implement custom software clock using output pin and sleep)
//
// Note that some pins share the same clock source, it means that
// changing frequency for one pin will change it also for all pins within a group
// The groups are: clk0 (4, 20, 32, 34), clk1 (5, 21, 42, 43) and clk2 (6 and 43)
func SetFreq(pin Pin, freq int) {
	const source = 19200000 // oscilator frequency
	const maxUint12 = 4095

	divi := uint32(source / freq)
	divf := uint32(((source % freq) << 12) / freq)

	divi &= maxUint12
	divf &= maxUint12

	clkCtlReg := 0x70
	clkDivReg := 0x74
	switch pin {
	case 4, 20, 32, 34: // clk0
		clkCtlReg += 0
		clkDivReg += 0
	case 5, 21, 42, 44: // clk1
		clkCtlReg += 8
		clkDivReg += 8
	case 6, 43: // clk2
		clkCtlReg += 16
		clkDivReg += 16
	default:
		return
	}

	memlock.Lock()
	defer memlock.Unlock()

	const PASSWORD = 0x5A000000
	const busy = 0x80
	const enab = 0x10
	const src = 0x01 // oscilator

	clkMem[clkCtlReg] = PASSWORD | src // stop gpio clock
	for clkMem[clkCtlReg]&busy != 0 {
	} // ... and wait

	clkMem[clkDivReg] = PASSWORD | (divi << 12) | divf // set dividers
	clkMem[clkCtlReg] = PASSWORD | enab | src          // start clock

}

// Open and memory map GPIO memory range from /dev/mem .
// Some reflection magic is used to convert it to a unsafe []uint32 pointer
func Open() (err error) {
	var file *os.File

	// Open fd for rw mem access; try gpiomem first
	file, err = os.OpenFile("/dev/gpiomem", os.O_RDWR|os.O_SYNC, 0)
	if os.IsNotExist(err) { // try mem (need root)
		file, err = os.OpenFile("/dev/mem", os.O_RDWR|os.O_SYNC, 0)
	}
	if err != nil {
		return
	}
	// FD can be closed after memory mapping
	defer file.Close()

	memlock.Lock()
	defer memlock.Unlock()

	// Memory map GPIO registers to slice
	gpioMem, gpioMem8, err = memMap(file.Fd(), gpioBase)
	if err != nil {
		return
	}

	// Memory map clock reisters to slice
	clkMem, clkMem8, err = memMap(file.Fd(), clkBase)
	if err != nil {
		return
	}

	return nil
}

func memMap(fd uintptr, offset int64) (mem []uint32, mem8 []byte, err error) {
	mem8, err = syscall.Mmap(
		int(fd),
		base+clkOffset,
		memLength,
		syscall.PROT_READ|syscall.PROT_WRITE,
		syscall.MAP_SHARED,
	)
	if err != nil {
		return
	}
	// Convert mapped byte memory to unsafe []uint32 pointer, adjust length as needed
	header := *(*reflect.SliceHeader)(unsafe.Pointer(&mem8))
	header.Len /= (32 / 8) // (32 bit = 4 bytes)
	header.Cap /= (32 / 8)
	mem = *(*[]uint32)(unsafe.Pointer(&header))
	return
}

// Close unmaps GPIO memory
func Close() error {
	memlock.Lock()
	defer memlock.Unlock()
	if err := syscall.Munmap(gpioMem8); err != nil {
		return err
	}
	if err := syscall.Munmap(clkMem8); err != nil {
		return err
	}
	return nil
}

// Read /proc/device-tree/soc/ranges and determine the base address.
// Use the default Raspberry Pi 1 base address if this fails.
func getBase() (base int64) {
	base = bcm2835Base
	ranges, err := os.Open("/proc/device-tree/soc/ranges")
	defer ranges.Close()
	if err != nil {
		return
	}
	b := make([]byte, 4)
	n, err := ranges.ReadAt(b, 4)
	if n != 4 || err != nil {
		return
	}
	buf := bytes.NewReader(b)
	var out uint32
	err = binary.Read(buf, binary.BigEndian, &out)
	if err != nil {
		return
	}
	return int64(out)
}