72 lines
2.0 KiB
Zig
72 lines
2.0 KiB
Zig
// SPDX-License-Identifier: MIT
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// Copyright (c) 2015-2020 Zig Contributors
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// This file is part of [zig](https://ziglang.org/), which is MIT licensed.
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// The MIT license requires this copyright notice to be included in all copies
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// and substantial portions of the software.
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const std = @import("std.zig");
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const builtin = std.builtin;
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const testing = std.testing;
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pub fn once(comptime f: fn () void) Once(f) {
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return Once(f){};
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}
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/// An object that executes the function `f` just once.
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pub fn Once(comptime f: fn () void) type {
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return struct {
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done: bool = false,
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mutex: std.Mutex = std.Mutex{},
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/// Call the function `f`.
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/// If `call` is invoked multiple times `f` will be executed only the
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/// first time.
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/// The invocations are thread-safe.
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pub fn call(self: *@This()) void {
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if (@atomicLoad(bool, &self.done, .Acquire))
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return;
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return self.callSlow();
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}
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fn callSlow(self: *@This()) void {
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@setCold(true);
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const T = self.mutex.acquire();
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defer T.release();
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// The first thread to acquire the mutex gets to run the initializer
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if (!self.done) {
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f();
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@atomicStore(bool, &self.done, true, .Release);
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}
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}
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};
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}
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var global_number: i32 = 0;
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var global_once = once(incr);
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fn incr() void {
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global_number += 1;
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}
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test "Once executes its function just once" {
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if (builtin.single_threaded) {
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global_once.call();
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global_once.call();
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} else {
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var threads: [10]*std.Thread = undefined;
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defer for (threads) |handle| handle.wait();
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for (threads) |*handle| {
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handle.* = try std.Thread.spawn(@as(u8, 0), struct {
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fn thread_fn(x: u8) void {
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global_once.call();
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}
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}.thread_fn);
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}
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}
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testing.expectEqual(@as(i32, 1), global_number);
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}
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