//! Arithmetic on `Iterator.size_hint()` values.
//!

use std::cmp;
use std::usize;

/// `SizeHint` is the return type of `Iterator::size_hint()`.
pub type SizeHint = (usize, Option<usize>);

/// Add `SizeHint` correctly.
#[inline]
pub fn add(a: SizeHint, b: SizeHint) -> SizeHint {
    let min = a.0.saturating_add(b.0);
    let max = match (a.1, b.1) {
        (Some(x), Some(y)) => x.checked_add(y),
        _ => None,
    };

    (min, max)
}

/// Add `x` correctly to a `SizeHint`.
#[inline]
pub fn add_scalar(sh: SizeHint, x: usize) -> SizeHint {
    let (mut low, mut hi) = sh;
    low = low.saturating_add(x);
    hi = hi.and_then(|elt| elt.checked_add(x));
    (low, hi)
}

/// Subtract `x` correctly from a `SizeHint`.
#[inline]
pub fn sub_scalar(sh: SizeHint, x: usize) -> SizeHint {
    let (mut low, mut hi) = sh;
    low = low.saturating_sub(x);
    hi = hi.map(|elt| elt.saturating_sub(x));
    (low, hi)
}

/// Multiply `SizeHint` correctly
#[inline]
pub fn mul(a: SizeHint, b: SizeHint) -> SizeHint {
    let low = a.0.saturating_mul(b.0);
    let hi = match (a.1, b.1) {
        (Some(x), Some(y)) => x.checked_mul(y),
        (Some(0), None) | (None, Some(0)) => Some(0),
        _ => None,
    };
    (low, hi)
}

/// Multiply `x` correctly with a `SizeHint`.
#[inline]
pub fn mul_scalar(sh: SizeHint, x: usize) -> SizeHint {
    let (mut low, mut hi) = sh;
    low = low.saturating_mul(x);
    hi = hi.and_then(|elt| elt.checked_mul(x));
    (low, hi)
}

/// Return the maximum
#[inline]
pub fn max(a: SizeHint, b: SizeHint) -> SizeHint {
    let (a_lower, a_upper) = a;
    let (b_lower, b_upper) = b;

    let lower = cmp::max(a_lower, b_lower);

    let upper = match (a_upper, b_upper) {
        (Some(x), Some(y)) => Some(cmp::max(x, y)),
        _ => None,
    };

    (lower, upper)
}

/// Return the minimum
#[inline]
pub fn min(a: SizeHint, b: SizeHint) -> SizeHint {
    let (a_lower, a_upper) = a;
    let (b_lower, b_upper) = b;
    let lower = cmp::min(a_lower, b_lower);
    let upper = match (a_upper, b_upper) {
        (Some(u1), Some(u2)) => Some(cmp::min(u1, u2)),
        _ => a_upper.or(b_upper),
    };
    (lower, upper)
}

#[test]
fn mul_size_hints() {
    assert_eq!(mul((3, Some(4)), (3, Some(4))), (9, Some(16)));
    assert_eq!(mul((3, Some(4)), (usize::MAX, None)), (usize::MAX, None));
    assert_eq!(mul((3, None), (0, Some(0))), (0, Some(0)));
}