#include <stdio.h>
#include "pico/stdlib.h"
#include "hardware/gpio.h"
#include "hardware/timer.h"
#include "pico/sync.h" // Required for critical_section

// Define GPIO pins for Trigger and Echo
const uint TRIGGER_PIN = 17;
const uint ECHO_PIN = 16;
const uint LED_PIN = 2;

// Global variables to store timestamps, made atomic/thread-safe with a critical section
volatile uint64_t start_time_us = 0;
volatile uint64_t end_time_us = 0;
volatile bool new_measurement_ready = false;
critical_section_t time_section;

// Interrupt handler function
void echo_pin_irq_handler(uint gpio, uint32_t events) {
    critical_section_enter_blocking(&time_section);
    if (events & GPIO_IRQ_EDGE_RISE) {
        start_time_us = time_us_64();
        end_time_us = 0; // Reset end time on rising edge
    } else if (events & GPIO_IRQ_EDGE_FALL) {
        end_time_us = time_us_64();
        new_measurement_ready = true;
    }
    critical_section_exit(&time_section);
}

int main() {
    stdio_init_all();
    critical_section_init(&time_section);
    
    printf("HC-SR04 Ultrasonic Sensor on Raspberry Pi Pico (Interrupt Version)\n");

    // Initialize LED pin
    gpio_init(LED_PIN);
    gpio_set_dir(LED_PIN, GPIO_OUT);

    // Initialize HC-SR04 pins
    gpio_init(TRIGGER_PIN);
    gpio_init(ECHO_PIN);
    gpio_set_dir(TRIGGER_PIN, GPIO_OUT);
    gpio_set_dir(ECHO_PIN, GPIO_IN);

    // Ensure trigger is low initially
    gpio_put(TRIGGER_PIN, 0);
    sleep_ms(100);

    // Configure interrupt on the ECHO pin for both rising and falling edges
    gpio_set_irq_enabled_with_callback(ECHO_PIN, GPIO_IRQ_EDGE_RISE | GPIO_IRQ_EDGE_FALL, true, &echo_pin_irq_handler);

    // Main loop
    while (true) {
        // --- LED Blinking Logic (Runs without blocking) ---
        gpio_put(LED_PIN, 1);
        sleep_ms(1000); 
        gpio_put(LED_PIN, 0);
        sleep_ms(1000);

        // --- HC-SR04 Trigger (Non-blocking part 1) ---
        // Send 10us trigger pulse
        gpio_put(TRIGGER_PIN, 1);
        sleep_us(10);
        gpio_put(TRIGGER_PIN, 0);
        // The rest of the measurement happens in the IRQ handler

        // --- Distance Calculation (Check if data is ready) ---
        if (new_measurement_ready) {
            critical_section_enter_blocking(&time_section);
            uint64_t pulseLength_us = end_time_us - start_time_us;
            new_measurement_ready = false; // Reset flag after reading
            critical_section_exit(&time_section);

            // Calculate distance: Speed of sound = 0.0343 cm/us
            float distance_cm = (float)pulseLength_us * 0.0343 / 2.0;
            printf("Distance: %.2f cm\n", distance_cm);
        }

        // Wait a reasonable time before triggering the *next* measurement
        // This sleep replaces the original long blocking wait for the echo pulse to return.
        // It allows the LED blinking to dominate the inter-measurement time.
        sleep_ms(300); 
    }
}