use crate::modulus::barrett::{Barrett, BarrettPrecomp};
use crate::modulus::ReduceOnce;
use crate::modulus::{REDUCEMOD, NONE, ONCE, TWICE, FOURTIMES, BARRETT, BARRETTLAZY};

use num_bigint::BigUint;
use num_traits::cast::ToPrimitive;

impl BarrettPrecomp<u64>{
    
    pub fn new(q: u64) -> BarrettPrecomp<u64> {
        let big_r: BigUint = (BigUint::from(1 as usize)<<((u64::BITS<<1) as usize)) / BigUint::from(q);
        let lo: u64 = (&big_r & BigUint::from(u64::MAX)).to_u64().unwrap();
        let hi: u64 = (big_r >> u64::BITS).to_u64().unwrap();
        let mut precomp: BarrettPrecomp<u64> = Self{q:q, two_q:q<<1, four_q:q<<2, lo:lo, hi:hi, one:Barrett(0,0)};
        precomp.one = precomp.prepare(1);
        precomp
    }

    #[inline(always)]
    pub fn one(&self) -> Barrett<u64> {
        self.one
    }

    /// Applies a modular reduction on x based on REDUCE:
    /// - LAZY: no modular reduction.
    /// - ONCE: subtracts q if x >= q.
    /// - FULL: maps x to x mod q using Barrett reduction.
    #[inline(always)]
    pub fn reduce_assign<const REDUCE:REDUCEMOD>(&self, x: &mut u64){
        match REDUCE {
            NONE =>{},
            ONCE =>{x.reduce_once_assign(self.q)},
            TWICE=>{x.reduce_once_assign(self.two_q)},
            FOURTIMES =>{x.reduce_once_assign(self.four_q)},
            BARRETT =>{
                let (_, mhi) = x.widening_mul(self.hi);
                *x = *x - mhi.wrapping_mul(self.q);
                x.reduce_once_assign(self.q);
            },
            BARRETTLAZY =>{
                let (_, mhi) = x.widening_mul(self.hi);
                *x = *x - mhi.wrapping_mul(self.q)
            },
            _ => unreachable!("invalid REDUCE argument")
        }
    }

    #[inline(always)]
    pub fn prepare(&self, v: u64) -> Barrett<u64> {
        debug_assert!(v < self.q);
        let quotient: u64 = (((v as u128) << 64) / self.q as u128) as _;
        Barrett(v, quotient)
    }

    #[inline(always)]
    pub fn mul_external<const REDUCE:REDUCEMOD>(&self, lhs: Barrett<u64>, rhs: u64) -> u64 {
        let mut r: u64 = rhs;
        self.mul_external_assign::<REDUCE>(lhs, &mut r);
        r
    }

    #[inline(always)]
    pub fn mul_external_assign<const REDUCE:REDUCEMOD>(&self, lhs: Barrett<u64>, rhs: &mut u64){
        let t: u64 = ((*lhs.quotient() as u128 * *rhs as u128) >> 64) as _;
        *rhs = (rhs.wrapping_mul(*lhs.value())).wrapping_sub(self.q.wrapping_mul(t));
        self.reduce_assign::<REDUCE>(rhs);
    }
}