// Package hcsr04 provides a driver for the HC-SR04 ultrasonic distance sensor
//
// Datasheet:
// https://cdn.sparkfun.com/datasheets/Sensors/Proximity/HCSR04.pdf
package hcsr04

import (
	"time"

	"tinygo.org/x/drivers"
)

const TIMEOUT = 23324 // max sensing distance (4m)

// Device holds the pins
type Device struct {
	trigger drivers.Pin
	echo    drivers.Pin
}

// New returns a new ultrasonic driver given 2 pins
func New(trigger, echo drivers.Pin) Device {
	return Device{
		trigger: trigger,
		echo:    echo,
	}
}

// Configure expects that the pins of the Device have already
// been configured by the user. Trigger pin should be an output
// and echo pin should be an input.
func (d *Device) Configure() {
}

// ReadDistance returns the distance of the object in mm
func (d *Device) ReadDistance() int32 {
	pulse := d.ReadPulse()

	// sound speed is 343000 mm/s
	// pulse is roundtrip measured in microseconds
	// distance = velocity * time
	// 2 * distance = 343000 * (pulse/1000000)
	return (pulse * 1715) / 10000 //mm
}

// ReadPulse returns the time of the pulse (roundtrip) in microseconds
func (d *Device) ReadPulse() int32 {
	t := time.Now()
	d.trigger.Low()
	time.Sleep(2 * time.Microsecond)
	d.trigger.High()
	time.Sleep(10 * time.Microsecond)
	d.trigger.Low()
	i := uint8(0)
	for {
		if d.echo.Get() {
			t = time.Now()
			break
		}
		i++
		if i > 10 {
			if time.Since(t).Microseconds() > TIMEOUT {
				return 0
			}
			i = 0
		}
	}
	i = 0
	for {
		if !d.echo.Get() {
			return int32(time.Since(t).Microseconds())
		}
		i++
		if i > 10 {
			if time.Since(t).Microseconds() > TIMEOUT {
				return 0
			}
			i = 0
		}
	}
	return 0
}