190 lines
7.1 KiB
Zig
190 lines
7.1 KiB
Zig
// SPDX-License-Identifier: MIT
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// Copyright (c) 2015-2020 Zig Contributors
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// This file is part of [zig](https://ziglang.org/), which is MIT licensed.
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// The MIT license requires this copyright notice to be included in all copies
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// and substantial portions of the software.
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const std = @import("../std.zig");
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const meta = std.meta;
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const testing = std.testing;
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const mem = std.mem;
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const assert = std.debug.assert;
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const TypeInfo = std.builtin.TypeInfo;
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/// This is useful for saving memory when allocating an object that has many
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/// optional components. The optional objects are allocated sequentially in
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/// memory, and a single integer is used to represent each optional object
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/// and whether it is present based on each corresponding bit.
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pub fn TrailerFlags(comptime Fields: type) type {
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return struct {
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bits: Int,
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pub const Int = meta.Int(.unsigned, bit_count);
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pub const bit_count = @typeInfo(Fields).Struct.fields.len;
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pub const FieldEnum = blk: {
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comptime var fields: [bit_count]TypeInfo.EnumField = undefined;
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inline for (@typeInfo(Fields).Struct.fields) |struct_field, i|
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fields[i] = .{ .name = struct_field.name, .value = i };
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break :blk @Type(.{
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.Enum = .{
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.layout = .Auto,
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.tag_type = std.math.IntFittingRange(0, bit_count - 1),
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.fields = &fields,
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.decls = &[_]TypeInfo.Declaration{},
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.is_exhaustive = true,
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},
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});
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};
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pub const InitStruct = blk: {
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comptime var fields: [bit_count]TypeInfo.StructField = undefined;
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inline for (@typeInfo(Fields).Struct.fields) |struct_field, i| {
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fields[i] = TypeInfo.StructField{
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.name = struct_field.name,
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.field_type = ?struct_field.field_type,
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.default_value = @as(
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??struct_field.field_type,
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@as(?struct_field.field_type, null),
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),
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.is_comptime = false,
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.alignment = @alignOf(?struct_field.field_type),
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};
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}
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break :blk @Type(.{
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.Struct = .{
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.layout = .Auto,
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.fields = &fields,
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.decls = &[_]TypeInfo.Declaration{},
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.is_tuple = false,
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},
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});
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};
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pub const Self = @This();
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pub fn has(self: Self, comptime field: FieldEnum) bool {
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const field_index = @enumToInt(field);
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return (self.bits & (1 << field_index)) != 0;
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}
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pub fn get(self: Self, p: [*]align(@alignOf(Fields)) const u8, comptime field: FieldEnum) ?Field(field) {
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if (!self.has(field))
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return null;
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return self.ptrConst(p, field).*;
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}
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pub fn setFlag(self: *Self, comptime field: FieldEnum) void {
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const field_index = @enumToInt(field);
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self.bits |= 1 << field_index;
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}
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/// `fields` is a struct with each field set to an optional value.
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/// Only the non-null bits are observed and are used to set the flag bits.
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pub fn init(fields: InitStruct) Self {
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var self: Self = .{ .bits = 0 };
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inline for (@typeInfo(Fields).Struct.fields) |field, i| {
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if (@field(fields, field.name)) |_|
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self.bits |= 1 << i;
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}
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return self;
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}
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/// `fields` is a struct with each field set to an optional value (same as `init`).
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pub fn setMany(self: Self, p: [*]align(@alignOf(Fields)) u8, fields: InitStruct) void {
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inline for (@typeInfo(Fields).Struct.fields) |field, i| {
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if (@field(fields, field.name)) |value|
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self.set(p, @intToEnum(FieldEnum, i), value);
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}
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}
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pub fn set(
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self: Self,
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p: [*]align(@alignOf(Fields)) u8,
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comptime field: FieldEnum,
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value: Field(field),
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) void {
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self.ptr(p, field).* = value;
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}
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pub fn ptr(self: Self, p: [*]align(@alignOf(Fields)) u8, comptime field: FieldEnum) *Field(field) {
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if (@sizeOf(Field(field)) == 0)
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return undefined;
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const off = self.offset(p, field);
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return @ptrCast(*Field(field), @alignCast(@alignOf(Field(field)), p + off));
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}
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pub fn ptrConst(self: Self, p: [*]align(@alignOf(Fields)) const u8, comptime field: FieldEnum) *const Field(field) {
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if (@sizeOf(Field(field)) == 0)
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return undefined;
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const off = self.offset(p, field);
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return @ptrCast(*const Field(field), @alignCast(@alignOf(Field(field)), p + off));
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}
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pub fn offset(self: Self, p: [*]align(@alignOf(Fields)) const u8, comptime field: FieldEnum) usize {
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var off: usize = 0;
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inline for (@typeInfo(Fields).Struct.fields) |field_info, i| {
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const active = (self.bits & (1 << i)) != 0;
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if (i == @enumToInt(field)) {
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assert(active);
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return mem.alignForwardGeneric(usize, off, @alignOf(field_info.field_type));
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} else if (active) {
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off = mem.alignForwardGeneric(usize, off, @alignOf(field_info.field_type));
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off += @sizeOf(field_info.field_type);
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}
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}
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}
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pub fn Field(comptime field: FieldEnum) type {
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inline for (@typeInfo(Fields).Struct.fields) |field_info, i| {
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if (i == @enumToInt(field))
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return field_info.field_type;
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}
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}
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pub fn sizeInBytes(self: Self) usize {
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var off: usize = 0;
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inline for (@typeInfo(Fields).Struct.fields) |field, i| {
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if (@sizeOf(field.field_type) == 0)
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continue;
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if ((self.bits & (1 << i)) != 0) {
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off = mem.alignForwardGeneric(usize, off, @alignOf(field.field_type));
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off += @sizeOf(field.field_type);
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}
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}
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return off;
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}
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};
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}
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test "TrailerFlags" {
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const Flags = TrailerFlags(struct {
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a: i32,
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b: bool,
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c: u64,
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});
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testing.expectEqual(u2, @TagType(Flags.FieldEnum));
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var flags = Flags.init(.{
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.b = true,
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.c = 1234,
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});
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const slice = try testing.allocator.allocAdvanced(u8, 8, flags.sizeInBytes(), .exact);
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defer testing.allocator.free(slice);
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flags.set(slice.ptr, .b, false);
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flags.set(slice.ptr, .c, 12345678);
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testing.expect(flags.get(slice.ptr, .a) == null);
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testing.expect(!flags.get(slice.ptr, .b).?);
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testing.expect(flags.get(slice.ptr, .c).? == 12345678);
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flags.setMany(slice.ptr, .{
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.b = true,
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.c = 5678,
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});
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testing.expect(flags.get(slice.ptr, .a) == null);
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testing.expect(flags.get(slice.ptr, .b).?);
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testing.expect(flags.get(slice.ptr, .c).? == 5678);
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}
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