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const std = @import("std.zig");
const assert = std.debug.assert;
const testing = std.testing;
/// Thread-safe initialization of global data.
/// TODO use a mutex instead of a spinlock
pub fn lazyInit(comptime T: type) LazyInit(T) {
return LazyInit(T){
.data = undefined,
};
}
fn LazyInit(comptime T: type) type {
return struct {
state: State = .NotResolved,
data: Data,
const State = enum(u8) {
NotResolved,
Resolving,
Resolved,
};
const Self = @This();
// TODO this isn't working for void, investigate and then remove this special case
const Data = if (@sizeOf(T) == 0) u8 else T;
const Ptr = if (T == void) void else *T;
/// Returns a usable pointer to the initialized data,
/// or returns null, indicating that the caller should
/// perform the initialization and then call resolve().
pub fn get(self: *Self) ?Ptr {
while (true) {
var state = @cmpxchgWeak(State, &self.state, .NotResolved, .Resolving, .SeqCst, .SeqCst) orelse return null;
switch (state) {
.NotResolved => continue,
.Resolving => {
// TODO mutex instead of a spinlock
continue;
},
.Resolved => {
if (@sizeOf(T) == 0) {
return @as(T, undefined);
} else {
return &self.data;
}
},
else => unreachable,
}
}
}
pub fn resolve(self: *Self) void {
const prev = @atomicRmw(State, &self.state, .Xchg, .Resolved, .SeqCst);
assert(prev != .Resolved); // resolve() called twice
}
};
}
var global_number = lazyInit(i32);
test "std.lazyInit" {
if (global_number.get()) |_| @panic("bad") else {
global_number.data = 1234;
global_number.resolve();
}
if (global_number.get()) |x| {
testing.expect(x.* == 1234);
} else {
@panic("bad");
}
if (global_number.get()) |x| {
testing.expect(x.* == 1234);
} else {
@panic("bad");
}
}
var global_void = lazyInit(void);
test "std.lazyInit(void)" {
if (global_void.get()) |_| @panic("bad") else {
global_void.resolve();
}
testing.expect(global_void.get() != null);
testing.expect(global_void.get() != null);
}
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