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// Copyright 2014-2016 bluss and ndarray developers.
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.
use num_traits::Float;

/// An iterator of a sequence of evenly spaced floats.
///
/// Iterator element type is `F`.
pub struct Linspace<F> {
    start: F,
    step: F,
    index: usize,
    len: usize,
}

impl<F> Iterator for Linspace<F>
where
    F: Float,
{
    type Item = F;

    #[inline]
    fn next(&mut self) -> Option<F> {
        if self.index >= self.len {
            None
        } else {
            // Calculate the value just like numpy.linspace does
            let i = self.index;
            self.index += 1;
            Some(self.start + self.step * F::from(i).unwrap())
        }
    }

    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        let n = self.len - self.index;
        (n, Some(n))
    }
}

impl<F> DoubleEndedIterator for Linspace<F>
where
    F: Float,
{
    #[inline]
    fn next_back(&mut self) -> Option<F> {
        if self.index >= self.len {
            None
        } else {
            // Calculate the value just like numpy.linspace does
            self.len -= 1;
            let i = self.len;
            Some(self.start + self.step * F::from(i).unwrap())
        }
    }
}

impl<F> ExactSizeIterator for Linspace<F> where Linspace<F>: Iterator {}

/// Return an iterator of evenly spaced floats.
///
/// The `Linspace` has `n` elements from `a` to `b` (inclusive).
///
/// The iterator element type is `F`, where `F` must implement `Float`, e.g.
/// `f32` or `f64`.
///
/// **Panics** if converting `n - 1` to type `F` fails.
#[inline]
pub fn linspace<F>(a: F, b: F, n: usize) -> Linspace<F>
where
    F: Float,
{
    let step = if n > 1 {
        let num_steps = F::from(n - 1).expect("Converting number of steps to `A` must not fail.");
        (b - a) / num_steps
    } else {
        F::zero()
    };
    Linspace {
        start: a,
        step,
        index: 0,
        len: n,
    }
}

/// Return an iterator of floats from `start` to `end` (exclusive),
/// incrementing by `step`.
///
/// Numerical reasons can result in `b` being included in the result.
///
/// The iterator element type is `F`, where `F` must implement `Float`, e.g.
/// `f32` or `f64`.
///
/// **Panics** if converting `((b - a) / step).ceil()` to type `F` fails.
#[inline]
pub fn range<F>(a: F, b: F, step: F) -> Linspace<F>
where
    F: Float,
{
    let len = b - a;
    let steps = F::ceil(len / step);
    Linspace {
        start: a,
        step,
        len: steps.to_usize().expect(
            "Converting the length to `usize` must not fail. The most likely \
             cause of this failure is if the sign of `end - start` is \
             different from the sign of `step`.",
        ),
        index: 0,
    }
}