rusty-numbers/src/bigint.rs

//! \[WIP\] Arbitrarily large integers
//!
//! Traits to implement:
//! * Add
//! * AddAssign
//! * Div
//! * DivAssign
//! * Mul
//! * MulAssign
//! * Neg
//! * Rem
//! * RemAssign
//! * Sub
//! * SubAssign
use crate::num::*;

#[derive(Clone, Debug)]
pub struct BigInt {
    inner: Vec<usize>,
    sign: Sign,
}

macro_rules! impl_from_smaller {
    ($($Type: ty),* ) => {
        $(
            impl From<$Type> for BigInt {
                fn from(n: $Type) -> Self {
                    let mut new = Self::default();
                    new.inner[0] = n as usize;

                    new
                }
            }
        )*
    }
}

#[cfg(target_pointer_width = "16")]
impl_from_smaller!(u8, u16);

#[cfg(target_pointer_width = "16")]
static BITS:usize = 16;

#[cfg(target_pointer_width = "32")]
impl_from_smaller!(u8, u16, u32);

#[cfg(target_pointer_width = "32")]
static BITS:usize = 32;

#[cfg(target_pointer_width = "64")]
impl_from_smaller!(u8, u16, u32, u64);

#[cfg(target_pointer_width = "64")]
static BITS:usize = 64;

fn add(u: BigInt, v: BigInt) -> BigInt {
    let mut out = BigInt::default();

    let u_digits = u.inner.len();
    let v_digits = v.inner.len();

    let digits = if v_digits > u_digits {
        v_digits
    } else {
        u_digits
    };

    let mut carry = 0usize;
    for i in 0..=digits {
        let a = *u.inner.get(i).unwrap_or(&0usize);
        let b = *v.inner.get(i).unwrap_or(&0usize);

        let (res, overflowed) = a.overflowing_add(b);
        // assert!(!overflowed, "Expected to overflow: {:#?}", (res, overflowed));
        if overflowed {
            if i == 0 {
               out.inner[i] = res;
            } else {
                out.inner.push(res);
            }
            carry = core::usize::MAX - res;
        } else {
            if res < core::usize::MAX {
                out.inner.push(res + carry);
                carry = 0;
            } else {
                out.inner.push(0usize);
                carry = 1;
            }

            carry = 0;
        }
    }

    out
}

impl Default for BigInt {
    fn default() -> Self {
        Self {
            inner: vec![0],
            sign: Sign::Positive,
        }
    }
}

impl From<usize> for BigInt {
    fn from(n: usize) -> Self {
        Self {
            inner: vec![n],
            sign: Sign::Positive,
        }
    }
}

impl From<&str> for BigInt {
    fn from(_: &str) -> Self {
        unimplemented!()
    }
}

impl From<String> for BigInt {
    fn from(_: String) -> Self {
        unimplemented!()
    }
}

impl BigInt {
    pub fn new() -> Self {
        Self::default()
    }

    pub fn shrink_to_fit(&mut self) {
        todo!();
    }

    pub fn from_str_radix<T: ToString>(s: T, radix: usize) {
        todo!();
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn sanity_checks() {
        let int = BigInt::from(45u8);
        assert_eq!(int.inner[0], 45usize)
    }

    #[test]
    fn test_add() {
        // MAX is 2^Bitsize - 1,
        // so the sum should be
        // [MAX -1, 1]
        // Compare base 10: 9 + 9 = 18
        let a = BigInt::from(core::usize::MAX);
        let b = BigInt::from(core::usize::MAX);

        let sum = add(a, b);

        assert_eq!(sum.inner[0], core::usize::MAX - 1, "least significant place should be MAX - 1");
        assert_eq!(sum.inner[1], 1usize, "most significant place should be 1");

        // assert!(false, "{:#?} should have inner equal to {:?}", sum, core::u64::MAX as u128 + core::u64::MAX as u128);
    }
}