1
  2
  3
  4
  5
  6
  7
  8
  9
 10
 11
 12
 13
 14
 15
 16
 17
 18
 19
 20
 21
 22
 23
 24
 25
 26
 27
 28
 29
 30
 31
 32
 33
 34
 35
 36
 37
 38
 39
 40
 41
 42
 43
 44
 45
 46
 47
 48
 49
 50
 51
 52
 53
 54
 55
 56
 57
 58
 59
 60
 61
 62
 63
 64
 65
 66
 67
 68
 69
 70
 71
 72
 73
 74
 75
 76
 77
 78
 79
 80
 81
 82
 83
 84
 85
 86
 87
 88
 89
 90
 91
 92
 93
 94
 95
 96
 97
 98
 99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
// file at the top-level directory of this distribution and at
// https://rust-lang.org/COPYRIGHT.
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// https://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or https://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.
#[cfg(all(feature="alloc", not(feature="std")))] use alloc::vec::Vec;
// BTreeMap is not as fast in tests, but better than nothing.
#[cfg(feature="std")] use std::collections::HashMap;
#[cfg(all(feature="alloc", not(feature="std")))] use alloc::collections::BTreeMap;

/// 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
///
/// ```
/// 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 + ?Sized,
{
    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
///
/// ```
/// 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 + ?Sized,
          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
///
/// ```
/// 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 + ?Sized
{
    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 + ?Sized,
{
    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 + ?Sized,
{
    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);
        indices.swap(i, j);
    }
    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 + ?Sized,
{
    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 {XorShiftRng, Rng, SeedableRng};
    #[cfg(not(feature="std"))]
    use alloc::vec::Vec;

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

        let mut r = ::test::rng(401);
        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 = ::test::rng(402);

        // sample 0 items
        assert_eq!(&sample_slice(&mut r, empty, 0)[..], [0u8; 0]);
        assert_eq!(&sample_slice(&mut r, &[42, 2, 42], 0)[..], [0u8; 0]);

        // sample 1 item
        assert_eq!(&sample_slice(&mut r, &[42], 1)[..], [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[..] == [42, 133] || v[..] == [133, 42]);

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

        assert_eq!(&sample_indices_cache(&mut r, 0, 0)[..], [0usize; 0]);
        assert_eq!(&sample_indices_cache(&mut r, 1, 0)[..], [0usize; 0]);
        assert_eq!(&sample_indices_cache(&mut r, 1, 1)[..], [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 = ::test::rng(403);

        for length in 1usize..max_range {
            let amount = r.gen_range(0, length);
            let mut seed = [0u8; 16];
            r.fill(&mut 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);
            }
        }
    }
}