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
//! Run closures in parallel.
//!
//! This is a simple primitive for spawning threads in bulk and waiting for them to complete.
//! Threads are allowed to borrow local variables from the main thread.
//!
//! # Examples
//!
//! Run two threads that increment a number:
//!
//! ```
//! use easy_parallel::Parallel;
//! use std::sync::Mutex;
//!
//! let mut m = Mutex::new(0);
//!
//! Parallel::new()
//!     .add(|| *m.lock().unwrap() += 1)
//!     .add(|| *m.lock().unwrap() += 1)
//!     .run();
//!
//! assert_eq!(*m.get_mut().unwrap(), 2);
//! ```
//!
//! Square each number of a vector on a different thread:
//!
//! ```
//! use easy_parallel::Parallel;
//!
//! let v = vec![10, 20, 30];
//!
//! let squares = Parallel::new()
//!     .each(0..v.len(), |i| v[i] * v[i])
//!     .run();
//!
//! assert_eq!(squares, [100, 400, 900]);
//! ```
//!
//! Compute the sum of numbers in an array:
//!
//! ```
//! use easy_parallel::Parallel;
//!
//! fn par_sum(v: &[i32]) -> i32 {
//!     const THRESHOLD: usize = 2;
//!
//!     if v.len() <= THRESHOLD {
//!         v.iter().copied().sum()
//!     } else {
//!         let half = (v.len() + 1) / 2;
//!         let sums = Parallel::new().each(v.chunks(half), par_sum).run();
//!         sums.into_iter().sum()
//!     }
//! }
//!
//! let v = [1, 25, -4, 10, 8];
//! assert_eq!(par_sum(&v), 40);
//! ```

#![warn(missing_docs, missing_debug_implementations, rust_2018_idioms)]

use std::fmt;
use std::mem;
use std::panic;
use std::process;
use std::sync::mpsc;
use std::thread;

/// A builder that runs closures in parallel.
#[must_use]
pub struct Parallel<'a, T> {
    /// Closures to run.
    closures: Vec<Box<dyn FnOnce() -> T + Send + 'a>>,
}

impl<'a, T> Parallel<'a, T> {
    /// Creates a builder for running closures in parallel.
    ///
    /// # Examples
    ///
    /// ```
    /// use easy_parallel::Parallel;
    ///
    /// let p = Parallel::<()>::new();
    /// ```
    pub fn new() -> Parallel<'a, T> {
        Parallel {
            closures: Vec::new(),
        }
    }

    /// Adds a closure to the list.
    ///
    /// # Examples
    ///
    /// ```
    /// use easy_parallel::Parallel;
    ///
    /// Parallel::new()
    ///     .add(|| println!("hello from a thread"))
    ///     .run();
    /// ```
    pub fn add<F>(mut self, f: F) -> Parallel<'a, T>
    where
        F: FnOnce() -> T + Send + 'a,
        T: Send + 'a,
    {
        self.closures.push(Box::new(f));
        self
    }

    /// Adds a cloned closure for each item in an iterator.
    ///
    /// Each clone of the closure takes an item as an argument.
    ///
    /// # Examples
    ///
    /// ```
    /// use easy_parallel::Parallel;
    ///
    /// Parallel::new()
    ///     .each(0..5, |i| println!("hello from thread #{}", i))
    ///     .run();
    /// ```
    pub fn each<A, I, F>(mut self, iter: I, f: F) -> Parallel<'a, T>
    where
        I: IntoIterator<Item = A>,
        F: FnOnce(A) -> T + Clone + Send + 'a,
        A: Send + 'a,
        T: Send + 'a,
    {
        for t in iter.into_iter() {
            let f = f.clone();
            self.closures.push(Box::new(|| f(t)));
        }
        self
    }

    /// Runs each closure on a separate thread and collects their results.
    ///
    /// Results are collected in the order in which closures were added. One of the closures always
    /// runs on the main thread because there is no point in spawning an extra thread for it.
    ///
    /// If a closure panics, panicking will resume in the main thread after all threads are joined.
    ///
    /// # Examples
    ///
    /// ```
    /// use easy_parallel::Parallel;
    /// use std::thread;
    /// use std::time::Duration;
    ///
    /// let res = Parallel::new()
    ///     .each(1..=3, |i| 10 * i)
    ///     .add(|| 100)
    ///     .run();
    ///
    /// assert_eq!(res, [10, 20, 30, 100]);
    /// ```
    pub fn run(mut self) -> Vec<T>
    where
        T: Send + 'a,
    {
        // Get the last closure.
        let f = match self.closures.pop() {
            None => return Vec::new(),
            Some(f) => f,
        };

        // Spawn threads, run the last closure on the current thread.
        let (mut results, r) = self.finish(f);
        results.push(r);
        results
    }

    /// Finishes with a closure to run on the main thread, starts threads, and collects results.
    ///
    /// Results are collected in the order in which closures were added.
    ///
    /// If a closure panics, panicking will resume in the main thread after all threads are joined.
    ///
    /// # Examples
    ///
    /// ```
    /// use easy_parallel::Parallel;
    /// use std::thread;
    /// use std::time::Duration;
    ///
    /// let (res, ()) = Parallel::new()
    ///     .each(1..=3, |i| 10 * i)
    ///     .finish(|| println!("Waiting for results"));
    ///
    /// assert_eq!(res, [10, 20, 30]);
    /// ```
    pub fn finish<F, R>(self, f: F) -> (Vec<T>, R)
    where
        F: FnOnce() -> R,
        T: Send + 'a,
    {
        // Set up a guard that aborts on panic.
        let guard = NoPanic;

        // Join handles for spawned threads.
        let mut handles = Vec::new();

        // Channels to collect results from spawned threads.
        let mut receivers = Vec::new();

        // Spawn a thread for each closure after the first one.
        for f in self.closures.into_iter() {
            // Wrap into a closure that sends the result back.
            let (sender, receiver) = mpsc::channel();
            let f = move || sender.send(f()).unwrap();

            // Erase the `'a` lifetime.
            let f: Box<dyn FnOnce() + Send + 'a> = Box::new(f);
            let f: Box<dyn FnOnce() + Send + 'static> = unsafe { mem::transmute(f) };

            // Spawn a thread for the closure.
            handles.push(thread::spawn(f));
            receivers.push(receiver);
        }

        let mut last_err = None;

        // Run the main closure on the main thread.
        let res = panic::catch_unwind(panic::AssertUnwindSafe(f));

        // Join threads and save the last panic if there was one.
        for h in handles {
            if let Err(err) = h.join() {
                last_err = Some(err);
            }
        }

        // Drop the guard because we may resume a panic now.
        drop(guard);

        // If a thread has panicked, resume the last collected panic.
        if let Some(err) = last_err {
            panic::resume_unwind(err);
        }

        // Collect the results from threads.
        let mut results = Vec::new();
        for receiver in receivers {
            results.push(receiver.recv().unwrap());
        }

        // If the main closure panicked, resume its panic.
        match res {
            Ok(r) => (results, r),
            Err(err) => panic::resume_unwind(err),
        }
    }
}

impl<T> fmt::Debug for Parallel<'_, T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("Parallel")
            .field("len", &self.closures.len())
            .finish()
    }
}

impl<T> Default for Parallel<'_, T> {
    fn default() -> Self {
        Self::new()
    }
}

/// Aborts the process if dropped while panicking.
struct NoPanic;

impl Drop for NoPanic {
    fn drop(&mut self) {
        if thread::panicking() {
            process::abort();
        }
    }
}