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@ityonemo
Created August 12, 2022 18:15
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Packed tagged union in zig
const E = enum(u12) {
Int = 0xFFF,
String = 0xFFE,
Pointer = 0xFFD,
}
const P = packed union(?e){
const this = @This();
Int: packed struct {
number: u32,
_padding: u20,
@tag(this) : e,
},
String: packed struct{
string: [6]u8,
@tag(this) : e,
},
Pointer: packed struct {
pointer: u48,
@tag(this) : e,
}
null: f64,
}
const std = @import("std");
const expect = std.testing.expect;
fn tag_of(p: P) u12 {return @intCast(u12, (@bitCast(u64, p) | 0xFFF0_0000_0000_0000) >> 52)}
test "NaN-boxed float" {
var with_int = P{.Int = .{number = 1234}};
var with_string = P{.String = .{string = "foo"}};
var as_float = P{.null = 1.234};
try expect(with_int.Int.number == 1234);
try expectEqual(with_string.String.string, "foo");
try expect(as_float.null == 1.234);
try expect(tag_of(with_int) == 0xFFF);
try expect(tag_of(with_string) == 0xFFE)
}
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ityonemo commented Aug 12, 2022

  • tag location is always comptime checked. It must be in the same place as all non-null tags.
  • might be possible to infer the location of the tag (first available high order locations - this would make "padding" unnecessary for "int")
  • it is considered safety-checked ub if the null enum writes an enumerated value into the respective bits of the tag location

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