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| 1 | const std = @import("std.zig"); |
| 2 | const builtin = std.builtin; |
| 3 | const testing = std.testing; |
| 4 | |
| 5 | pub fn once(comptime f: fn () void) Once(f) { |
| 6 | return Once(f){}; |
| 7 | } |
| 8 | |
| 9 | /// An object that executes the function `f` just once. |
| 10 | pub fn Once(comptime f: fn () void) type { |
| 11 | return struct { |
| 12 | done: bool = false, |
| 13 | mutex: std.Mutex = std.Mutex.init(), |
| 14 | |
| 15 | /// Call the function `f`. |
| 16 | /// If `call` is invoked multiple times `f` will be executed only the |
| 17 | /// first time. |
| 18 | /// The invocations are thread-safe. |
| 19 | pub fn call(self: *@This()) void { |
| 20 | if (@atomicLoad(bool, &self.done, .Acquire)) |
| 21 | return; |
| 22 | |
| 23 | return self.callSlow(); |
| 24 | } |
| 25 | |
| 26 | fn callSlow(self: *@This()) void { |
| 27 | @setCold(true); |
| 28 | |
| 29 | const T = self.mutex.acquire(); |
| 30 | defer T.release(); |
| 31 | |
| 32 | // The first thread to acquire the mutex gets to run the initializer |
| 33 | if (!self.done) { |
| 34 | f(); |
| 35 | @atomicStore(bool, &self.done, true, .Release); |
| 36 | } |
| 37 | } |
| 38 | }; |
| 39 | } |
| 40 | |
| 41 | var global_number: i32 = 0; |
| 42 | var global_once = once(incr); |
| 43 | |
| 44 | fn incr() void { |
| 45 | global_number += 1; |
| 46 | } |
| 47 | |
| 48 | test "Once executes its function just once" { |
| 49 | if (builtin.single_threaded) { |
| 50 | global_once.call(); |
| 51 | global_once.call(); |
| 52 | } else { |
| 53 | var threads: [10]*std.Thread = undefined; |
| 54 | defer for (threads) |handle| handle.wait(); |
| 55 | |
| 56 | for (threads) |*handle| { |
| 57 | handle.* = try std.Thread.spawn(@as(u8, 0), struct { |
| 58 | fn thread_fn(x: u8) void { |
| 59 | global_once.call(); |
| 60 | } |
| 61 | }.thread_fn); |
| 62 | } |
| 63 | } |
| 64 | |
| 65 | testing.expectEqual(@as(i32, 1), global_number); |
| 66 | } |
LemonBoy <thatlemon@gmail.com> 2020-04-18 13:26:12+02:00
Andrew Kelley <andrew@ziglang.org> 2020-04-18 15:48:32-04:00