//! USART1 transmit configuration for STM32F722.

use stm32f7::stm32f722;

/// UART baud-rate configuration error.
#[derive(Clone, Copy, Debug, Eq, PartialEq)]
pub enum BaudRateError {
    /// Zero is not a valid baud rate.
    Zero,
    /// The requested divisor cannot fit USART BRR.
    OutOfRange,
}

/// Owns configured transmit-only USART1 hardware.
///
/// Pin alternate-function setup remains board-owned because it depends on the
/// schematic. Configure pins before constructing this type.
pub struct Usart1Tx {
    peripheral: stm32f722::USART1,
}

impl Usart1Tx {
    /// Enables USART1 and configures 8N1 transmit-only operation.
    pub fn new(
        rcc: &stm32f722::RCC,
        peripheral: stm32f722::USART1,
        peripheral_clock_hz: u32,
        baud: u32,
    ) -> Result<Self, BaudRateError> {
        let brr = calculate_brr(peripheral_clock_hz, baud)?;

        rcc.apb2rstr()
            .modify(|_, write| write.usart1rst().set_bit());
        rcc.apb2rstr()
            .modify(|_, write| write.usart1rst().clear_bit());
        rcc.apb2enr().modify(|_, write| write.usart1en().set_bit());
        peripheral.brr().write(|write| {
            // SAFETY: `brr` is validated to fit the complete BRR register.
            unsafe { write.bits(u32::from(brr)) }
        });
        peripheral
            .cr1()
            .write(|write| write.te().set_bit().ue().set_bit());
        peripheral.cr2().write(|write| write.stop().stop1());

        Ok(Self { peripheral })
    }

    pub(crate) fn enable_dma_transmit(&self) {
        self.peripheral.cr3().write(|write| write.dmat().set_bit());
    }

    pub(crate) fn transmit_data_register_address(&self) -> u32 {
        self.peripheral.tdr().as_ptr() as u32
    }
}

const fn calculate_brr(peripheral_clock_hz: u32, baud: u32) -> Result<u16, BaudRateError> {
    if baud == 0 {
        return Err(BaudRateError::Zero);
    }

    let divisor = (peripheral_clock_hz + (baud / 2)) / baud;
    if divisor == 0 || divisor > u16::MAX as u32 {
        Err(BaudRateError::OutOfRange)
    } else {
        Ok(divisor as u16)
    }
}

#[cfg(test)]
mod tests {
    use super::{BaudRateError, calculate_brr};

    #[test]
    fn calculates_reset_clock_brr() {
        assert_eq!(calculate_brr(16_000_000, 115_200), Ok(139));
    }

    #[test]
    fn rejects_zero_baud() {
        assert_eq!(calculate_brr(16_000_000, 0), Err(BaudRateError::Zero));
    }
}