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//! This crate provides an implementation of the [Recaman Sequence](http://oeis.org/A005132).
//!
//! # Examples
//!
//! ```
//! extern crate recaman;
//!
//! use recaman::Recaman;
//!
//! fn main() {
//!     let rec = Recaman::new();
//!     let sequence = rec.take(10).collect::<Vec<u64>>();
//! }
//! ```

#![deny(warnings, missing_docs)]

extern crate fxhash;

use fxhash::FxHashSet;

/// Instance of a Recaman sequence.
pub struct Recaman {
    current: u64,
    step: u64,
    hits: FxHashSet<u64>,
}

impl Recaman {
    /// Create a new Recaman sequence.
    pub fn new() -> Self {
        let mut hits = FxHashSet::default();
        hits.insert(0);

        Recaman {
            current: 0,
            step: 0,
            hits,
        }
    }

    #[inline]
    fn look_ahead(&self) -> u64 {
        if self.current < self.step {
            return 0;
        }
        self.current - self.step
    }
}

impl Iterator for Recaman {
    type Item = u64;

    fn next(&mut self) -> Option<Self::Item> {
        self.hits.insert(self.current);

        let next = self.look_ahead();

        self.current = match next == 0 || self.hits.contains(&next) {
            true => self.current + self.step,
            false => next,
        };
        self.step += 1;

        Some(self.current)
    }
}

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

    #[test]
    fn initial_zero() {
        let mut recaman = Recaman::new();

        assert_eq!(0, recaman.next().unwrap())
    }

    #[test]
    fn first_ten() {
        let recaman = Recaman::new();

        assert_eq!(
            vec![0, 1, 3, 6, 2, 7, 13, 20, 12, 21],
            recaman.take(10).collect::<Vec<u64>>()
        )
    }
}