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const std = @import("../std.zig");
const meta = std.meta;
const testing = std.testing;
const mem = std.mem;
const assert = std.debug.assert;
/// 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.
pub fn TrailerFlags(comptime Fields: type) type {
return struct {
bits: Int,
pub const Int = @Type(.{ .Int = .{ .bits = bit_count, .is_signed = false } });
pub const bit_count = @typeInfo(Fields).Struct.fields.len;
pub const Self = @This();
pub fn has(self: Self, comptime name: []const u8) bool {
const field_index = meta.fieldIndex(Fields, name).?;
return (self.bits & (1 << field_index)) != 0;
}
pub fn get(self: Self, p: [*]align(@alignOf(Fields)) const u8, comptime name: []const u8) ?Field(name) {
if (!self.has(name))
return null;
return self.ptrConst(p, name).*;
}
pub fn setFlag(self: *Self, comptime name: []const u8) void {
const field_index = meta.fieldIndex(Fields, name).?;
self.bits |= 1 << field_index;
}
/// `fields` is a struct with each field set to an optional value.
/// Missing fields are assumed to be `null`.
/// Only the non-null bits are observed and are used to set the flag bits.
pub fn init(fields: anytype) Self {
var self: Self = .{ .bits = 0 };
inline for (@typeInfo(@TypeOf(fields)).Struct.fields) |field| {
const opt: ?Field(field.name) = @field(fields, field.name);
const field_index = meta.fieldIndex(Fields, field.name).?;
self.bits |= @as(Int, @boolToInt(opt != null)) << field_index;
}
return self;
}
/// `fields` is a struct with each field set to an optional value (same as `init`).
/// Missing fields are assumed to be `null`.
pub fn setMany(self: Self, p: [*]align(@alignOf(Fields)) u8, fields: anytype) void {
inline for (@typeInfo(@TypeOf(fields)).Struct.fields) |field| {
const opt: ?Field(field.name) = @field(fields, field.name);
if (opt) |value| {
self.set(p, field.name, value);
}
}
}
pub fn set(
self: Self,
p: [*]align(@alignOf(Fields)) u8,
comptime name: []const u8,
value: Field(name),
) void {
self.ptr(p, name).* = value;
}
pub fn ptr(self: Self, p: [*]align(@alignOf(Fields)) u8, comptime name: []const u8) *Field(name) {
if (@sizeOf(Field(name)) == 0)
return undefined;
const off = self.offset(p, name);
return @ptrCast(*Field(name), @alignCast(@alignOf(Field(name)), p + off));
}
pub fn ptrConst(self: Self, p: [*]align(@alignOf(Fields)) const u8, comptime name: []const u8) *const Field(name) {
if (@sizeOf(Field(name)) == 0)
return undefined;
const off = self.offset(p, name);
return @ptrCast(*const Field(name), @alignCast(@alignOf(Field(name)), p + off));
}
pub fn offset(self: Self, p: [*]align(@alignOf(Fields)) const u8, comptime name: []const u8) usize {
var off: usize = 0;
inline for (@typeInfo(Fields).Struct.fields) |field, i| {
const active = (self.bits & (1 << i)) != 0;
if (comptime mem.eql(u8, field.name, name)) {
assert(active);
return mem.alignForwardGeneric(usize, off, @alignOf(field.field_type));
} else if (active) {
off = mem.alignForwardGeneric(usize, off, @alignOf(field.field_type));
off += @sizeOf(field.field_type);
}
}
@compileError("no field named " ++ name ++ " in type " ++ @typeName(Fields));
}
pub fn Field(comptime name: []const u8) type {
return meta.fieldInfo(Fields, name).field_type;
}
pub fn sizeInBytes(self: Self) usize {
var off: usize = 0;
inline for (@typeInfo(Fields).Struct.fields) |field, i| {
if (@sizeOf(field.field_type) == 0)
continue;
if ((self.bits & (1 << i)) != 0) {
off = mem.alignForwardGeneric(usize, off, @alignOf(field.field_type));
off += @sizeOf(field.field_type);
}
}
return off;
}
};
}
test "TrailerFlags" {
const Flags = TrailerFlags(struct {
a: i32,
b: bool,
c: u64,
});
var flags = Flags.init(.{
.b = true,
.c = 1234,
});
const slice = try testing.allocator.allocAdvanced(u8, 8, flags.sizeInBytes(), .exact);
defer testing.allocator.free(slice);
flags.set(slice.ptr, "b", false);
flags.set(slice.ptr, "c", 12345678);
testing.expect(flags.get(slice.ptr, "a") == null);
testing.expect(!flags.get(slice.ptr, "b").?);
testing.expect(flags.get(slice.ptr, "c").? == 12345678);
flags.setMany(slice.ptr, .{
.b = true,
.c = 5678,
});
testing.expect(flags.get(slice.ptr, "a") == null);
testing.expect(flags.get(slice.ptr, "b").?);
testing.expect(flags.get(slice.ptr, "c").? == 5678);
}
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