use std::convert::TryInto;
pub struct Field {
    /// A bitmask we use to AND to the value, unshifted.
    /// E.g. for a width of `3` bits, this mask would be 0b111.
    mask: usize,

    /// Offset of the first bit in this field
    offset: usize,
}

impl Field {
    /// Define a new CSR field with the given width at a specified
    /// offset from the start of the register.
    pub const fn new(width: usize, offset: usize) -> Field {
        // assert!(width != 0, "field width cannot be 0");
        // assert!((width + offset) < 32, "field with and offset must fit within a 32-bit value");

        // It would be lovely if we could call `usize::pow()` in a const fn.
        let mask = match width {
            0 => 0,
            1 => 1,
            2 => 3,
            3 => 7,
            4 => 15,
            5 => 31,
            6 => 63,
            7 => 127,
            8 => 255,
            9 => 511,
            10 => 1023,
            11 => 2047,
            12 => 4095,
            13 => 8191,
            14 => 16383,
            15 => 32767,
            16 => 65535,
            17 => 131071,
            18 => 262143,
            19 => 524287,
            20 => 1048575,
            21 => 2097151,
            22 => 4194303,
            23 => 8388607,
            24 => 16777215,
            25 => 33554431,
            26 => 67108863,
            27 => 134217727,
            28 => 268435455,
            29 => 536870911,
            30 => 1073741823,
            31 => 2147483647,
            _ => 0,
        };
        Field { mask, offset }
    }
}

pub struct CSR<T> {
    base: *mut T,
}

impl<T> CSR<T>
where
    T: std::convert::TryFrom<usize> + std::convert::TryInto<usize> + std::default::Default,
{
    pub fn new(base: *mut T) -> Self {
        CSR { base }
    }
    pub fn r(&mut self, reg: usize, field: Field) -> T {
        let usize_base: *mut usize = unsafe { core::mem::transmute(&self.base) };
        ((unsafe { usize_base.add(reg).read_volatile() } >> field.offset) & field.mask)
            .try_into()
            .unwrap_or_default()
    }
    pub fn rw(&mut self, reg: usize, field: Field, value: T) {
        let usize_base: *mut usize = unsafe { core::mem::transmute(&self.base) };
        let value_as_usize: usize = value.try_into().unwrap_or_default() << field.offset;
        let previous = unsafe { usize_base.add(reg).read_volatile() } & ! field.mask;
        unsafe { usize_base.add(reg).write_volatile(previous | value_as_usize)};
    }
    pub fn ow(&mut self, reg: usize, field: Field, value: T) {
        let usize_base: *mut usize = unsafe { core::mem::transmute(&self.base) };
        let value_as_usize: usize = value.try_into().unwrap_or_default() << field.offset;
        unsafe { usize_base.add(reg).write_volatile(value_as_usize)};
    }
}

#[cfg(test)]
mod tests {
    pub mod pac {
        pub mod audio {
            pub const RX_CTL: usize = 0x0c;
            pub const RX_CTL_ENABLE: crate::Field = crate::Field::new(1, 0);
            pub const RX_CTL_RESET: crate::Field = crate::Field::new(1, 1);
        }
        pub mod uart {
            pub const RXTX: usize = 0x00;
            pub const RXTX_RXTX: crate::Field = crate::Field::new(8, 0);
            pub const TXFULL: usize = 0x04;
            pub const TXFULL_TXFULL: crate::Field = crate::Field::new(1, 0);
        }
    }
    #[test]
    fn compile_check() {
        use super::*;
        let mut audio = CSR::new(0x1000_0000 as *mut u32);
        audio.r(pac::audio::RX_CTL, pac::audio::RX_CTL_ENABLE);
        audio.rw(pac::audio::RX_CTL, pac::audio::RX_CTL_RESET, 1);

        let mut uart = CSR::new(0x1001_0000 as *mut u8);
        uart.ow(pac::uart::RXTX, pac::uart::RXTX_RXTX, b'a');
        assert_ne!(uart.r(pac::uart::TXFULL, pac::uart::TXFULL_TXFULL), 1);

        // Note: These also still compile.
        uart.ow(pac::uart::RXTX, pac::uart::TXFULL_TXFULL, b'a');

        // This compiles but requires a cast since `audio` is a pointer to
        // u32, whereas `uart` is a pointer to u8.
        audio.ow(pac::uart::RXTX, pac::uart::RXTX_RXTX, b'a' as _);
    }
}