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    // Copyright 2017 The Rust Project Developers. See the COPYRIGHT
    // file at the top-level directory of this distribution and at
    // http://rust-lang.org/COPYRIGHT.
    //
    // 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.

    //! Functions for randomly accessing and sampling sequences.

    use super::Rng;

    // This crate is only enabled when either std or alloc is available.
    // BTreeMap is not as fast in tests, but better than nothing.
    #[cfg(feature="std")] use std::collections::HashMap;
    #[cfg(not(feature="std"))] use alloc::btree_map::BTreeMap;

    #[cfg(not(feature="std"))] use alloc::Vec;

    /// Randomly sample `amount` elements from a finite iterator.
    ///
    /// The following can be returned:
    /// - `Ok`: `Vec` of `amount` non-repeating randomly sampled elements. The order is not random.
    /// - `Err`: `Vec` of all the elements from `iterable` in sequential order. This happens when the
    ///   length of `iterable` was less than `amount`. This is considered an error since exactly
    ///   `amount` elements is typically expected.
    ///
    /// This implementation uses `O(len(iterable))` time and `O(amount)` memory.
    ///
    /// # Example
    ///
    /// ```rust
    /// use rand::{thread_rng, seq};
    ///
    /// let mut rng = thread_rng();
    /// let sample = seq::sample_iter(&mut rng, 1..100, 5).unwrap();
    /// println!("{:?}", sample);
    /// ```
    pub fn sample_iter<T, I, R>(rng: &mut R, iterable: I, amount: usize) -> Result<Vec<T>, Vec<T>>
        where I: IntoIterator<Item=T>,
              R: Rng,
    {
        let mut iter = iterable.into_iter();
        let mut reservoir = Vec::with_capacity(amount);
        reservoir.extend(iter.by_ref().take(amount));

        // Continue unless the iterator was exhausted
        //
        // note: this prevents iterators that "restart" from causing problems.
        // If the iterator stops once, then so do we.
        if reservoir.len() == amount {
            for (i, elem) in iter.enumerate() {
                let k = rng.gen_range(0, i + 1 + amount);
                if let Some(spot) = reservoir.get_mut(k) {
                    *spot = elem;
                }
            }
            Ok(reservoir)
        } else {
            // Don't hang onto extra memory. There is a corner case where
            // `amount` was much less than `len(iterable)`.
            reservoir.shrink_to_fit();
            Err(reservoir)
        }
    }

    /// Randomly sample exactly `amount` values from `slice`.
    ///
    /// The values are non-repeating and in random order.
    ///
    /// This implementation uses `O(amount)` time and memory.
    ///
    /// Panics if `amount > slice.len()`
    ///
    /// # Example
    ///
    /// ```rust
    /// use rand::{thread_rng, seq};
    ///
    /// let mut rng = thread_rng();
    /// let values = vec![5, 6, 1, 3, 4, 6, 7];
    /// println!("{:?}", seq::sample_slice(&mut rng, &values, 3));
    /// ```
    pub fn sample_slice<R, T>(rng: &mut R, slice: &[T], amount: usize) -> Vec<T>
        where R: Rng,
              T: Clone
    {
        let indices = sample_indices(rng, slice.len(), amount);

        let mut out = Vec::with_capacity(amount);
        out.extend(indices.iter().map(|i| slice[*i].clone()));
        out
    }

    /// Randomly sample exactly `amount` references from `slice`.
    ///
    /// The references are non-repeating and in random order.
    ///
    /// This implementation uses `O(amount)` time and memory.
    ///
    /// Panics if `amount > slice.len()`
    ///
    /// # Example
    ///
    /// ```rust
    /// use rand::{thread_rng, seq};
    ///
    /// let mut rng = thread_rng();
    /// let values = vec![5, 6, 1, 3, 4, 6, 7];
    /// println!("{:?}", seq::sample_slice_ref(&mut rng, &values, 3));
    /// ```
    pub fn sample_slice_ref<'a, R, T>(rng: &mut R, slice: &'a [T], amount: usize) -> Vec<&'a T>
        where R: Rng
    {
        let indices = sample_indices(rng, slice.len(), amount);

        let mut out = Vec::with_capacity(amount);
        out.extend(indices.iter().map(|i| &slice[*i]));
        out
    }

    /// Randomly sample exactly `amount` indices from `0..length`.
    ///
    /// The values are non-repeating and in random order.
    ///
    /// This implementation uses `O(amount)` time and memory.
    ///
    /// This method is used internally by the slice sampling methods, but it can sometimes be useful to
    /// have the indices themselves so this is provided as an alternative.
    ///
    /// Panics if `amount > length`
    pub fn sample_indices<R>(rng: &mut R, length: usize, amount: usize) -> Vec<usize>
        where R: Rng,
    {
        if amount > length {
            panic!("`amount` must be less than or equal to `slice.len()`");
        }

        // We are going to have to allocate at least `amount` for the output no matter what. However,
        // if we use the `cached` version we will have to allocate `amount` as a HashMap as well since
        // it inserts an element for every loop.
        //
        // Therefore, if `amount >= length / 2` then inplace will be both faster and use less memory.
        // In fact, benchmarks show the inplace version is faster for length up to about 20 times
        // faster than amount.
        //
        // TODO: there is probably even more fine-tuning that can be done here since
        // `HashMap::with_capacity(amount)` probably allocates more than `amount` in practice,
        // and a trade off could probably be made between memory/cpu, since hashmap operations
        // are slower than array index swapping.
        if amount >= length / 20 {
            sample_indices_inplace(rng, length, amount)
        } else {
            sample_indices_cache(rng, length, amount)
        }
    }

    /// Sample an amount of indices using an inplace partial fisher yates method.
    ///
    /// This allocates the entire `length` of indices and randomizes only the first `amount`.
    /// It then truncates to `amount` and returns.
    ///
    /// This is better than using a HashMap "cache" when `amount >= length / 2` since it does not
    /// require allocating an extra cache and is much faster.
    fn sample_indices_inplace<R>(rng: &mut R, length: usize, amount: usize) -> Vec<usize>
        where R: Rng,
    {
        debug_assert!(amount <= length);
        let mut indices: Vec<usize> = Vec::with_capacity(length);
        indices.extend(0..length);
        for i in 0..amount {
            let j: usize = rng.gen_range(i, length);
            let tmp = indices[i];
            indices[i] = indices[j];
            indices[j] = tmp;
        }
        indices.truncate(amount);
        debug_assert_eq!(indices.len(), amount);
        indices
    }

    /// This method performs a partial fisher-yates on a range of indices using a HashMap
    /// as a cache to record potential collisions.
    ///
    /// The cache avoids allocating the entire `length` of values. This is especially useful when
    /// `amount <<< length`, i.e. select 3 non-repeating from 1_000_000
    fn sample_indices_cache<R>(
        rng: &mut R,
        length: usize,
        amount: usize,
    ) -> Vec<usize>
        where R: Rng,
    {
        debug_assert!(amount <= length);
        #[cfg(feature="std")] let mut cache = HashMap::with_capacity(amount);
        #[cfg(not(feature="std"))] let mut cache = BTreeMap::new();
        let mut out = Vec::with_capacity(amount);
        for i in 0..amount {
            let j: usize = rng.gen_range(i, length);

            // equiv: let tmp = slice[i];
            let tmp = match cache.get(&i) {
                Some(e) => *e,
                None => i,
            };

            // equiv: slice[i] = slice[j];
            let x = match cache.get(&j) {
                Some(x) => *x,
                None => j,
            };

            // equiv: slice[j] = tmp;
            cache.insert(j, tmp);

            // note that in the inplace version, slice[i] is automatically "returned" value
            out.push(x);
        }
        debug_assert_eq!(out.len(), amount);
        out
    }

    #[cfg(test)]
    mod test {
        use super::*;
        use {thread_rng, XorShiftRng, SeedableRng};

        #[test]
        fn test_sample_iter() {
            let min_val = 1;
            let max_val = 100;

            let mut r = thread_rng();
            let vals = (min_val..max_val).collect::<Vec<i32>>();
            let small_sample = sample_iter(&mut r, vals.iter(), 5).unwrap();
            let large_sample = sample_iter(&mut r, vals.iter(), vals.len() + 5).unwrap_err();

            assert_eq!(small_sample.len(), 5);
            assert_eq!(large_sample.len(), vals.len());
            // no randomization happens when amount >= len
            assert_eq!(large_sample, vals.iter().collect::<Vec<_>>());

            assert!(small_sample.iter().all(|e| {
                **e >= min_val && **e <= max_val
            }));
        }
        #[test]
        fn test_sample_slice_boundaries() {
            let empty: &[u8] = &[];

            let mut r = thread_rng();

            // sample 0 items
            assert_eq!(sample_slice(&mut r, empty, 0), vec![]);
            assert_eq!(sample_slice(&mut r, &[42, 2, 42], 0), vec![]);

            // sample 1 item
            assert_eq!(sample_slice(&mut r, &[42], 1), vec![42]);
            let v = sample_slice(&mut r, &[1, 42], 1)[0];
            assert!(v == 1 || v == 42);

            // sample "all" the items
            let v = sample_slice(&mut r, &[42, 133], 2);
            assert!(v == vec![42, 133] || v == vec![133, 42]);

            assert_eq!(sample_indices_inplace(&mut r, 0, 0), vec![]);
            assert_eq!(sample_indices_inplace(&mut r, 1, 0), vec![]);
            assert_eq!(sample_indices_inplace(&mut r, 1, 1), vec![0]);

            assert_eq!(sample_indices_cache(&mut r, 0, 0), vec![]);
            assert_eq!(sample_indices_cache(&mut r, 1, 0), vec![]);
            assert_eq!(sample_indices_cache(&mut r, 1, 1), vec![0]);

            // Make sure lucky 777's aren't lucky
            let slice = &[42, 777];
            let mut num_42 = 0;
            let total = 1000;
            for _ in 0..total {
                let v = sample_slice(&mut r, slice, 1);
                assert_eq!(v.len(), 1);
                let v = v[0];
                assert!(v == 42 || v == 777);
                if v == 42 {
                    num_42 += 1;
                }
            }
            let ratio_42 = num_42 as f64 / 1000 as f64;
            assert!(0.4 <= ratio_42 || ratio_42 <= 0.6, "{}", ratio_42);
        }

        #[test]
        fn test_sample_slice() {
            let xor_rng = XorShiftRng::from_seed;

            let max_range = 100;
            let mut r = thread_rng();

            for length in 1usize..max_range {
                let amount = r.gen_range(0, length);
                let seed: [u32; 4] = [
                    r.next_u32(), r.next_u32(), r.next_u32(), r.next_u32()
                ];

                println!("Selecting indices: len={}, amount={}, seed={:?}", length, amount, seed);

                // assert that the two index methods give exactly the same result
                let inplace = sample_indices_inplace(
                    &mut xor_rng(seed), length, amount);
                let cache = sample_indices_cache(
                    &mut xor_rng(seed), length, amount);
                assert_eq!(inplace, cache);

                // assert the basics work
                let regular = sample_indices(
                    &mut xor_rng(seed), length, amount);
                assert_eq!(regular.len(), amount);
                assert!(regular.iter().all(|e| *e < length));
                assert_eq!(regular, inplace);

                // also test that sampling the slice works
                let vec: Vec<usize> = (0..length).collect();
                {
                    let result = sample_slice(&mut xor_rng(seed), &vec, amount);
                    assert_eq!(result, regular);
                }

                {
                    let result = sample_slice_ref(&mut xor_rng(seed), &vec, amount);
                    let expected = regular.iter().map(|v| v).collect::<Vec<_>>();
                    assert_eq!(result, expected);
                }
            }
        }
    }

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