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use crate::{
    backend::riscv::simple_instruction::{param::Decided, Param},
    utility::parsing,
};
use bitvec::prelude::*;
use nom::{bytes::complete::tag, combinator::map, sequence::delimited, IResult};

use super::IsParamTransformer;

/// A transformer that extracts a bit at `Self.0`th bit of a parameter.
#[derive(Debug, Eq, PartialEq, Copy, Clone)]
pub struct BitAt(u8);

impl BitAt {
    pub const fn new(index: u8) -> Self {
        Self(index)
    }
}

pub fn parse(code: &str) -> IResult<&str, BitAt> {
    map(
        delimited(tag("bit_at("), parsing::integer, tag(")")),
        BitAt::new,
    )(code)
}

impl IsParamTransformer for BitAt {
    fn param_to_instruction_part(&self, _offset: u64, param: &Param) -> BitVec<u32> {
        // it is ok to use `as u32` here, see
        // https://doc.rust-lang.org/reference/expressions/operator-expr.html#type-cast-expressions
        let param = param.unwrap_immediate() as u32;
        let param_bits = &param.view_bits::<Lsb0>();
        let mut instruction_part = BitVec::new();
        instruction_part.push(param_bits[self.0 as usize]);
        instruction_part
    }

    fn update_param(&self, instruction_part: &BitSlice<u32>, param: &mut Param) {
        if let Param::Decided(Decided::Immediate(param_value))
        | Param::Resolved(_, Decided::Immediate(param_value)) = param
        {
            let mut param_bits_store = *param_value as u32;
            let param_bits = param_bits_store.view_bits_mut::<Lsb0>();
            param_bits.set(self.0 as usize, instruction_part[0]);
            *param_value = param_bits_store as i32;
        }
    }

    fn default_param(&self) -> Param {
        Param::Decided(Decided::Immediate(0))
    }

    fn bit_count(&self) -> usize {
        1
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    #[test]
    fn test_parse() {
        let result = parse("bit_at(0)").unwrap().1;
        assert_eq!(result, BitAt(0));
        let result = parse("bit_at(1)").unwrap().1;
        assert_eq!(result, BitAt(1));
        assert!(parse("bits_at(0, 7)").is_err());
    }

    #[test]
    fn param_to_instruction_part() {
        let transformer = BitAt(0);
        let param = Param::Decided(Decided::Immediate(0b1010));
        assert_eq!(
            transformer.param_to_instruction_part(0, &param),
            bits![u32, Lsb0; 0]
        );
        let transformer = BitAt(1);
        assert_eq!(
            transformer.param_to_instruction_part(0, &param),
            bits![u32, Lsb0; 1]
        );
        let transformer = BitAt(7);
        assert_eq!(
            transformer.param_to_instruction_part(0, &param),
            bits![u32, Lsb0; 0]
        );
    }

    #[test]
    fn update_param() {
        let transformer = BitAt(0);
        let mut param = Param::Decided(Decided::Immediate(0));
        transformer.update_param(bits![u32, Lsb0; 1], &mut param);
        assert_eq!(param, Param::Decided(Decided::Immediate(1)));

        let transformer = BitAt(1);
        let mut param = Param::Decided(Decided::Immediate(0));
        transformer.update_param(bits![u32, Lsb0; 0], &mut param);
        assert_eq!(param, Param::Decided(Decided::Immediate(0)));

        let transformer = BitAt(30);
        let mut param = Param::Decided(Decided::Immediate(0));
        transformer.update_param(bits![u32, Lsb0; 1], &mut param);
        assert_eq!(param, Param::Decided(Decided::Immediate(0x40000000)));

        let transformer = BitAt(31);
        let mut param = Param::Decided(Decided::Immediate(0));
        transformer.update_param(bits![u32, Lsb0; 1], &mut param);
        assert_eq!(param, Param::Decided(Decided::Immediate(-0x8000_0000)));
    }
}