#include <stdint.h>

// Base addresses
#define RCC_BASE        0x40021000
#define GPIOA_BASE      0x50000000

// RCC register
#define RCC_IOPENR      (*(volatile uint32_t *)(RCC_BASE + 0x34))

// GPIOA registers
#define GPIOA_MODER     (*(volatile uint32_t *)(GPIOA_BASE + 0x00))
#define GPIOA_PUPDR     (*(volatile uint32_t *)(GPIOA_BASE + 0x0C))
#define GPIOA_IDR       (*(volatile uint32_t *)(GPIOA_BASE + 0x10))
#define GPIOA_ODR       (*(volatile uint32_t *)(GPIOA_BASE + 0x14))

// Pins
#define LED_PIN         1  // PA1
#define SWITCH_PIN      0  // PA0

// Optional delay
void delay(volatile uint32_t t) {
    while (t--) __asm__("nop");
}

int main(void)
{
    // 1. Enable GPIOA clock
    RCC_IOPENR |= (1 << 0); // GPIOAEN

    // 2. Configure PA1 as output (LED)
    GPIOA_MODER &= ~(3 << (LED_PIN * 2)); // Clear bits
    GPIOA_MODER |=  (1 << (LED_PIN * 2)); // 01 = output

    // 3. Configure PA0 as input (switch)
    GPIOA_MODER &= ~(3 << (SWITCH_PIN * 2)); // 00 = input

    // 4. Pull-up on PA0 (active-low switch)
    GPIOA_PUPDR &= ~(3 << (SWITCH_PIN * 2));
    GPIOA_PUPDR |=  (1 << (SWITCH_PIN * 2)); // 01 = pull-up

    while (1)
    {
        if ((GPIOA_IDR & (1 << SWITCH_PIN)) == 0) {
            // Switch pressed → turn LED ON
            GPIOA_ODR |= (1 << LED_PIN);
        } else {
            // Switch not pressed → turn LED OFF
            GPIOA_ODR &= ~(1 << LED_PIN);
        }

        delay(100000); // debounce-like delay
    }
}