kindness 0.2.0

Methods for returning random elements from an iterator.
Documentation 
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use rand::{Rng, RngCore};

fn calculate_upper(min: u64) -> (u64, u8) {
    const fn inner_func(min: u64) -> (u64, u8) {
        let mut product = 1;
        let mut count: u8 = 0;

        loop {
            if let Some(p) = u64::checked_mul(product, min + (count as u64)) {
                product = p;
                count += 1;
            } else {
                return (product, count);
            }
        }
    }

    const RESULT2: (u64, u8) = inner_func(2);
    if min == 2 {
        return RESULT2;
    }

    inner_func(min)
}

/// ∀ n ∊ `1..U64::MAX` ∀ k ∊ `0..n`  
/// After n items have been returned, the probability that the highest index where true was returned is k is 1 / n
pub(crate) struct Chooser<R: RngCore> {
    pub rng: R,
    consumed: usize,
    chunk: u64,
    chunk_remaining: u8,
}

/* */
impl<R: RngCore> Chooser<R> {
    /// Create a new choice iterator
    pub fn new_zero(rng: R) -> Self {
        Self {
            rng,
            consumed: 0,
            //Set the chunk to one here because the first element is always true
            chunk: 1,
            chunk_remaining: 1,
        }
    }

    pub fn new_one(rng: R) -> Self {
        Self {
            rng,
            consumed: 1,
            //The chunk will be reset the first time we try to get an element
            chunk: 0,
            chunk_remaining: 0,
        }
    }

    /// Resets the chunk to one. Does not reset the chunk, but decrements the `chunk_remaining` by 1;
    /// This is okay because the product of any sequence of n consecutive integers is a multiple of the product of the first n integers
    pub fn set_consumed_to_one(&mut self) {
        self.consumed = 1;
        if self.chunk_remaining > 0 {
            self.chunk_remaining -= 1;
        }
    }

    pub fn get_consumed(&self) -> usize {
        self.consumed
    }
    pub fn set_consumed(&mut self, consumed: usize) {
        self.consumed = consumed;
        //no need to reset the chunk
        self.chunk_remaining = 0;
    }

    #[inline]
    pub fn next(&mut self) -> bool {
        self.consumed = self.consumed.saturating_add(1);

        if self.chunk_remaining == 0 {
            let (bound, remaining) = calculate_upper(self.consumed as u64);
            self.chunk_remaining = remaining;
            self.chunk = self.rng.gen_range(0..bound);
        }

        let result = self.chunk % (self.consumed as u64) == 0;
        self.chunk /= self.consumed as u64;
        self.chunk_remaining -= 1;
        result
    }
}