pub fn TrailerFlags(comptime Fields: type) typebits: Intpub fn has(self: Self, comptime field: FieldEnum) boolpub fn get(self: Self, p: [*]align(@alignOf(Fields)) const u8, comptime field: FieldEnum) ?Field(field)pub fn setFlag(self: *Self, comptime field: FieldEnum) voidpub fn init(fields: ActiveFields) Selffields is a boolean struct where each active field is set to true
pub fn setMany(self: Self, p: [*]align(@alignOf(Fields)) u8, fields: FieldValues) voidfields is a struct with each field set to an optional value
pub fn set(self: Self, p: [*]align(@alignOf(Fields)) u8, comptime field: FieldEnum, value: Field(field)) voidpub fn ptr(self: Self, p: [*]align(@alignOf(Fields)) u8, comptime field: FieldEnum) *Field(field)pub fn ptrConst(self: Self, p: [*]align(@alignOf(Fields)) const u8, comptime field: FieldEnum) *const Field(field)pub fn offset(self: Self, comptime field: FieldEnum) usizepub fn sizeInBytes(self: Self) usize12
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
132pub fn TrailerFlags(comptime Fields: type) type {
return struct {
bits: Int,
pub const Int = @Int(.unsigned, bit_count);
pub const bit_count = @typeInfo(Fields).@"struct".field_names.len;
pub const FieldEnum = std.meta.FieldEnum(Fields);
pub const ActiveFields = std.enums.EnumFieldStruct(FieldEnum, bool, false);
pub const FieldValues = blk: {
var field_names: [bit_count][]const u8 = undefined;
var field_types: [bit_count]type = undefined;
var field_attrs: [bit_count]std.builtin.Type.Struct.FieldAttributes = undefined;
const fields_info = @typeInfo(Fields).@"struct";
for (
fields_info.field_names,
fields_info.field_types,
&field_names,
&field_types,
&field_attrs,
) |field_name, field_type, *new_name, *NewType, *new_attrs| {
new_name.* = field_name;
NewType.* = ?field_type;
const default: ?field_type = null;
new_attrs.* = .{ .default_value_ptr = &default };
}
break :blk @Struct(.auto, null, &field_names, &field_types, &field_attrs);
};
const Self = @This();
pub fn has(self: Self, comptime field: FieldEnum) bool {
const field_index = @backingInt(field);
return (self.bits & (1 << field_index)) != 0;
}
pub fn get(self: Self, p: [*]align(@alignOf(Fields)) const u8, comptime field: FieldEnum) ?Field(field) {
if (!self.has(field))
return null;
return self.ptrConst(p, field).*;
}
pub fn setFlag(self: *Self, comptime field: FieldEnum) void {
const field_index = @backingInt(field);
self.bits |= 1 << field_index;
}
/// `fields` is a boolean struct where each active field is set to `true`
pub fn init(fields: ActiveFields) Self {
var self: Self = .{ .bits = 0 };
inline for (@typeInfo(Fields).@"struct".field_names, 0..) |field_name, i| {
if (@field(fields, field_name))
self.bits |= 1 << i;
}
return self;
}
/// `fields` is a struct with each field set to an optional value
pub fn setMany(self: Self, p: [*]align(@alignOf(Fields)) u8, fields: FieldValues) void {
inline for (@typeInfo(Fields).@"struct".field_names, 0..) |field_name, i| {
if (@field(fields, field_name)) |value|
self.set(p, @as(FieldEnum, @fromBackingInt(@intCast(i))), value);
}
}
pub fn set(
self: Self,
p: [*]align(@alignOf(Fields)) u8,
comptime field: FieldEnum,
value: Field(field),
) void {
self.ptr(p, field).* = value;
}
pub fn ptr(self: Self, p: [*]align(@alignOf(Fields)) u8, comptime field: FieldEnum) *Field(field) {
if (@sizeOf(Field(field)) == 0)
return undefined;
const off = self.offset(field);
return @ptrCast(@alignCast(p + off));
}
pub fn ptrConst(self: Self, p: [*]align(@alignOf(Fields)) const u8, comptime field: FieldEnum) *const Field(field) {
if (@sizeOf(Field(field)) == 0)
return undefined;
const off = self.offset(field);
return @ptrCast(@alignCast(p + off));
}
pub fn offset(self: Self, comptime field: FieldEnum) usize {
var off: usize = 0;
inline for (@typeInfo(Fields).@"struct".field_types, 0..) |field_type, i| {
const active = (self.bits & (1 << i)) != 0;
if (i == @backingInt(field)) {
assert(active);
return mem.alignForward(usize, off, @alignOf(field_type));
} else if (active) {
off = mem.alignForward(usize, off, @alignOf(field_type));
off += @sizeOf(field_type);
}
}
}
pub fn Field(comptime field: FieldEnum) type {
return @typeInfo(Fields).@"struct".field_types[@backingInt(field)];
}
pub fn sizeInBytes(self: Self) usize {
var off: usize = 0;
inline for (@typeInfo(Fields).@"struct".field_types, 0..) |field_type, i| {
if (@sizeOf(field_type) == 0)
continue;
if ((self.bits & (1 << i)) != 0) {
off = mem.alignForward(usize, off, @alignOf(field_type));
off += @sizeOf(field_type);
}
}
return off;
}
};
}
This is useful for saving memory when allocating an object that has many optional components. The optional objects are allocated sequentially in memory, and a single integer is used to represent each optional object and whether it is present based on each corresponding bit.