add std.meta.TrailerFlags API

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.

lib/std/meta.zig

@@ -6,6 +6,7 @@ const math = std.math;
 const testing = std.testing;
 
 pub const trait = @import("meta/trait.zig");
+pub const TrailerFlags = @import("meta/trailer_flags.zig").TrailerFlags;
 
 const TypeInfo = builtin.TypeInfo;
 

lib/std/meta/trailer_flags.zig

@@ -0,0 +1,118 @@
+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;
+        }
+
+        pub fn init(comptime names: anytype) Self {
+            var self: Self = .{ .bits = 0 };
+            inline for (@typeInfo(@TypeOf(names)).Struct.fields) |field| {
+                if (@field(names, field.name)) {
+                    const field_index = meta.fieldIndex(Fields, field.name).?;
+                    self.bits |= 1 << field_index;
+                }
+            }
+            return self;
+        }
+
+        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) {
+            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) {
+            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 ((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 = true,
+    });
+    testing.expect(flags.sizeInBytes() == 16);
+    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);
+}