Type Function MultiArrayList [src]

Alias for std.multi_array_list.MultiArrayList
A MultiArrayList stores a list of a struct or tagged union type. Instead of storing a single list of items, MultiArrayList stores separate lists for each field of the struct or lists of tags and bare unions. This allows for memory savings if the struct or union has padding, and also improves cache usage if only some fields or just tags are needed for a computation. The primary API for accessing fields is the slice() function, which computes the start pointers for the array of each field. From the slice you can call .items(.) to obtain a slice of field values. For unions you can call .items(.tags) or .items(.data).
Prototype

pub fn MultiArrayList(comptime T: type) type
Parameters

T: type
Source

  pub fn MultiArrayList(comptime T: type) type {
    return struct {
        bytes: [*]align(@alignOf(T)) u8 = undefined,
        len: usize = 0,
        capacity: usize = 0,

        pub const empty: Self = .{
            .bytes = undefined,
            .len = 0,
            .capacity = 0,
        };

        const Elem = switch (@typeInfo(T)) {
            .@"struct" => T,
            .@"union" => |u| struct {
                pub const Bare = @Type(.{ .@"union" = .{
                    .layout = u.layout,
                    .tag_type = null,
                    .fields = u.fields,
                    .decls = &.{},
                } });
                pub const Tag =
                    u.tag_type orelse @compileError("MultiArrayList does not support untagged unions");
                tags: Tag,
                data: Bare,

                pub fn fromT(outer: T) @This() {
                    const tag = meta.activeTag(outer);
                    return .{
                        .tags = tag,
                        .data = switch (tag) {
                            inline else => |t| @unionInit(Bare, @tagName(t), @field(outer, @tagName(t))),
                        },
                    };
                }
                pub fn toT(tag: Tag, bare: Bare) T {
                    return switch (tag) {
                        inline else => |t| @unionInit(T, @tagName(t), @field(bare, @tagName(t))),
                    };
                }
            },
            else => @compileError("MultiArrayList only supports structs and tagged unions"),
        };

        pub const Field = meta.FieldEnum(Elem);

        /// A MultiArrayList.Slice contains cached start pointers for each field in the list.
        /// These pointers are not normally stored to reduce the size of the list in memory.
        /// If you are accessing multiple fields, call slice() first to compute the pointers,
        /// and then get the field arrays from the slice.
        pub const Slice = struct {
            /// This array is indexed by the field index which can be obtained
            /// by using @intFromEnum() on the Field enum
            ptrs: [fields.len][*]u8,
            len: usize,
            capacity: usize,

            pub const empty: Slice = .{
                .ptrs = undefined,
                .len = 0,
                .capacity = 0,
            };

            pub fn items(self: Slice, comptime field: Field) []FieldType(field) {
                const F = FieldType(field);
                if (self.capacity == 0) {
                    return &[_]F{};
                }
                const byte_ptr = self.ptrs[@intFromEnum(field)];
                const casted_ptr: [*]F = if (@sizeOf(F) == 0)
                    undefined
                else
                    @ptrCast(@alignCast(byte_ptr));
                return casted_ptr[0..self.len];
            }

            pub fn set(self: *Slice, index: usize, elem: T) void {
                const e = switch (@typeInfo(T)) {
                    .@"struct" => elem,
                    .@"union" => Elem.fromT(elem),
                    else => unreachable,
                };
                inline for (fields, 0..) |field_info, i| {
                    self.items(@as(Field, @enumFromInt(i)))[index] = @field(e, field_info.name);
                }
            }

            pub fn get(self: Slice, index: usize) T {
                var result: Elem = undefined;
                inline for (fields, 0..) |field_info, i| {
                    @field(result, field_info.name) = self.items(@as(Field, @enumFromInt(i)))[index];
                }
                return switch (@typeInfo(T)) {
                    .@"struct" => result,
                    .@"union" => Elem.toT(result.tags, result.data),
                    else => unreachable,
                };
            }

            pub fn toMultiArrayList(self: Slice) Self {
                if (self.ptrs.len == 0 or self.capacity == 0) {
                    return .{};
                }
                const unaligned_ptr = self.ptrs[sizes.fields[0]];
                const aligned_ptr: [*]align(@alignOf(Elem)) u8 = @alignCast(unaligned_ptr);
                return .{
                    .bytes = aligned_ptr,
                    .len = self.len,
                    .capacity = self.capacity,
                };
            }

            pub fn deinit(self: *Slice, gpa: Allocator) void {
                var other = self.toMultiArrayList();
                other.deinit(gpa);
                self.* = undefined;
            }

            /// This function is used in the debugger pretty formatters in tools/ to fetch the
            /// child field order and entry type to facilitate fancy debug printing for this type.
            fn dbHelper(self: *Slice, child: *Elem, field: *Field, entry: *Entry) void {
                _ = self;
                _ = child;
                _ = field;
                _ = entry;
            }
        };

        const Self = @This();

        const fields = meta.fields(Elem);
        /// `sizes.bytes` is an array of @sizeOf each T field. Sorted by alignment, descending.
        /// `sizes.fields` is an array mapping from `sizes.bytes` array index to field index.
        const sizes = blk: {
            const Data = struct {
                size: usize,
                size_index: usize,
                alignment: usize,
            };
            var data: [fields.len]Data = undefined;
            for (fields, 0..) |field_info, i| {
                data[i] = .{
                    .size = @sizeOf(field_info.type),
                    .size_index = i,
                    .alignment = if (@sizeOf(field_info.type) == 0) 1 else field_info.alignment,
                };
            }
            const Sort = struct {
                fn lessThan(context: void, lhs: Data, rhs: Data) bool {
                    _ = context;
                    return lhs.alignment > rhs.alignment;
                }
            };
            mem.sort(Data, &data, {}, Sort.lessThan);
            var sizes_bytes: [fields.len]usize = undefined;
            var field_indexes: [fields.len]usize = undefined;
            for (data, 0..) |elem, i| {
                sizes_bytes[i] = elem.size;
                field_indexes[i] = elem.size_index;
            }
            break :blk .{
                .bytes = sizes_bytes,
                .fields = field_indexes,
            };
        };

        /// Release all allocated memory.
        pub fn deinit(self: *Self, gpa: Allocator) void {
            gpa.free(self.allocatedBytes());
            self.* = undefined;
        }

        /// The caller owns the returned memory. Empties this MultiArrayList.
        pub fn toOwnedSlice(self: *Self) Slice {
            const result = self.slice();
            self.* = .{};
            return result;
        }

        /// Compute pointers to the start of each field of the array.
        /// If you need to access multiple fields, calling this may
        /// be more efficient than calling `items()` multiple times.
        pub fn slice(self: Self) Slice {
            var result: Slice = .{
                .ptrs = undefined,
                .len = self.len,
                .capacity = self.capacity,
            };
            var ptr: [*]u8 = self.bytes;
            for (sizes.bytes, sizes.fields) |field_size, i| {
                result.ptrs[i] = ptr;
                ptr += field_size * self.capacity;
            }
            return result;
        }

        /// Get the slice of values for a specified field.
        /// If you need multiple fields, consider calling slice()
        /// instead.
        pub fn items(self: Self, comptime field: Field) []FieldType(field) {
            return self.slice().items(field);
        }

        /// Overwrite one array element with new data.
        pub fn set(self: *Self, index: usize, elem: T) void {
            var slices = self.slice();
            slices.set(index, elem);
        }

        /// Obtain all the data for one array element.
        pub fn get(self: Self, index: usize) T {
            return self.slice().get(index);
        }

        /// Extend the list by 1 element. Allocates more memory as necessary.
        pub fn append(self: *Self, gpa: Allocator, elem: T) !void {
            try self.ensureUnusedCapacity(gpa, 1);
            self.appendAssumeCapacity(elem);
        }

        /// Extend the list by 1 element, but asserting `self.capacity`
        /// is sufficient to hold an additional item.
        pub fn appendAssumeCapacity(self: *Self, elem: T) void {
            assert(self.len < self.capacity);
            self.len += 1;
            self.set(self.len - 1, elem);
        }

        /// Extend the list by 1 element, returning the newly reserved
        /// index with uninitialized data.
        /// Allocates more memory as necesasry.
        pub fn addOne(self: *Self, gpa: Allocator) Allocator.Error!usize {
            try self.ensureUnusedCapacity(gpa, 1);
            return self.addOneAssumeCapacity();
        }

        /// Extend the list by 1 element, asserting `self.capacity`
        /// is sufficient to hold an additional item.  Returns the
        /// newly reserved index with uninitialized data.
        pub fn addOneAssumeCapacity(self: *Self) usize {
            assert(self.len < self.capacity);
            const index = self.len;
            self.len += 1;
            return index;
        }

        /// Remove and return the last element from the list, or return `null` if list is empty.
        /// Invalidates pointers to fields of the removed element.
        pub fn pop(self: *Self) ?T {
            if (self.len == 0) return null;
            const val = self.get(self.len - 1);
            self.len -= 1;
            return val;
        }

        /// Inserts an item into an ordered list.  Shifts all elements
        /// after and including the specified index back by one and
        /// sets the given index to the specified element.  May reallocate
        /// and invalidate iterators.
        pub fn insert(self: *Self, gpa: Allocator, index: usize, elem: T) !void {
            try self.ensureUnusedCapacity(gpa, 1);
            self.insertAssumeCapacity(index, elem);
        }

        /// Inserts an item into an ordered list which has room for it.
        /// Shifts all elements after and including the specified index
        /// back by one and sets the given index to the specified element.
        /// Will not reallocate the array, does not invalidate iterators.
        pub fn insertAssumeCapacity(self: *Self, index: usize, elem: T) void {
            assert(self.len < self.capacity);
            assert(index <= self.len);
            self.len += 1;
            const entry = switch (@typeInfo(T)) {
                .@"struct" => elem,
                .@"union" => Elem.fromT(elem),
                else => unreachable,
            };
            const slices = self.slice();
            inline for (fields, 0..) |field_info, field_index| {
                const field_slice = slices.items(@as(Field, @enumFromInt(field_index)));
                var i: usize = self.len - 1;
                while (i > index) : (i -= 1) {
                    field_slice[i] = field_slice[i - 1];
                }
                field_slice[index] = @field(entry, field_info.name);
            }
        }

        /// Remove the specified item from the list, swapping the last
        /// item in the list into its position.  Fast, but does not
        /// retain list ordering.
        pub fn swapRemove(self: *Self, index: usize) void {
            const slices = self.slice();
            inline for (fields, 0..) |_, i| {
                const field_slice = slices.items(@as(Field, @enumFromInt(i)));
                field_slice[index] = field_slice[self.len - 1];
                field_slice[self.len - 1] = undefined;
            }
            self.len -= 1;
        }

        /// Remove the specified item from the list, shifting items
        /// after it to preserve order.
        pub fn orderedRemove(self: *Self, index: usize) void {
            const slices = self.slice();
            inline for (fields, 0..) |_, field_index| {
                const field_slice = slices.items(@as(Field, @enumFromInt(field_index)));
                var i = index;
                while (i < self.len - 1) : (i += 1) {
                    field_slice[i] = field_slice[i + 1];
                }
                field_slice[i] = undefined;
            }
            self.len -= 1;
        }

        /// Adjust the list's length to `new_len`.
        /// Does not initialize added items, if any.
        pub fn resize(self: *Self, gpa: Allocator, new_len: usize) !void {
            try self.ensureTotalCapacity(gpa, new_len);
            self.len = new_len;
        }

        /// Attempt to reduce allocated capacity to `new_len`.
        /// If `new_len` is greater than zero, this may fail to reduce the capacity,
        /// but the data remains intact and the length is updated to new_len.
        pub fn shrinkAndFree(self: *Self, gpa: Allocator, new_len: usize) void {
            if (new_len == 0) return clearAndFree(self, gpa);

            assert(new_len <= self.capacity);
            assert(new_len <= self.len);

            const other_bytes = gpa.alignedAlloc(
                u8,
                @alignOf(Elem),
                capacityInBytes(new_len),
            ) catch {
                const self_slice = self.slice();
                inline for (fields, 0..) |field_info, i| {
                    if (@sizeOf(field_info.type) != 0) {
                        const field = @as(Field, @enumFromInt(i));
                        const dest_slice = self_slice.items(field)[new_len..];
                        // We use memset here for more efficient codegen in safety-checked,
                        // valgrind-enabled builds. Otherwise the valgrind client request
                        // will be repeated for every element.
                        @memset(dest_slice, undefined);
                    }
                }
                self.len = new_len;
                return;
            };
            var other = Self{
                .bytes = other_bytes.ptr,
                .capacity = new_len,
                .len = new_len,
            };
            self.len = new_len;
            const self_slice = self.slice();
            const other_slice = other.slice();
            inline for (fields, 0..) |field_info, i| {
                if (@sizeOf(field_info.type) != 0) {
                    const field = @as(Field, @enumFromInt(i));
                    @memcpy(other_slice.items(field), self_slice.items(field));
                }
            }
            gpa.free(self.allocatedBytes());
            self.* = other;
        }

        pub fn clearAndFree(self: *Self, gpa: Allocator) void {
            gpa.free(self.allocatedBytes());
            self.* = .{};
        }

        /// Reduce length to `new_len`.
        /// Invalidates pointers to elements `items[new_len..]`.
        /// Keeps capacity the same.
        pub fn shrinkRetainingCapacity(self: *Self, new_len: usize) void {
            self.len = new_len;
        }

        /// Invalidates all element pointers.
        pub fn clearRetainingCapacity(self: *Self) void {
            self.len = 0;
        }

        /// Modify the array so that it can hold at least `new_capacity` items.
        /// Implements super-linear growth to achieve amortized O(1) append operations.
        /// Invalidates element pointers if additional memory is needed.
        pub fn ensureTotalCapacity(self: *Self, gpa: Allocator, new_capacity: usize) Allocator.Error!void {
            if (self.capacity >= new_capacity) return;
            return self.setCapacity(gpa, growCapacity(self.capacity, new_capacity));
        }

        const init_capacity = init: {
            var max = 1;
            for (fields) |field| max = @as(comptime_int, @max(max, @sizeOf(field.type)));
            break :init @as(comptime_int, @max(1, std.atomic.cache_line / max));
        };

        /// Called when memory growth is necessary. Returns a capacity larger than
        /// minimum that grows super-linearly.
        fn growCapacity(current: usize, minimum: usize) usize {
            var new = current;
            while (true) {
                new +|= new / 2 + init_capacity;
                if (new >= minimum)
                    return new;
            }
        }

        /// Modify the array so that it can hold at least `additional_count` **more** items.
        /// Invalidates pointers if additional memory is needed.
        pub fn ensureUnusedCapacity(self: *Self, gpa: Allocator, additional_count: usize) !void {
            return self.ensureTotalCapacity(gpa, self.len + additional_count);
        }

        /// Modify the array so that it can hold exactly `new_capacity` items.
        /// Invalidates pointers if additional memory is needed.
        /// `new_capacity` must be greater or equal to `len`.
        pub fn setCapacity(self: *Self, gpa: Allocator, new_capacity: usize) !void {
            assert(new_capacity >= self.len);
            const new_bytes = try gpa.alignedAlloc(
                u8,
                @alignOf(Elem),
                capacityInBytes(new_capacity),
            );
            if (self.len == 0) {
                gpa.free(self.allocatedBytes());
                self.bytes = new_bytes.ptr;
                self.capacity = new_capacity;
                return;
            }
            var other = Self{
                .bytes = new_bytes.ptr,
                .capacity = new_capacity,
                .len = self.len,
            };
            const self_slice = self.slice();
            const other_slice = other.slice();
            inline for (fields, 0..) |field_info, i| {
                if (@sizeOf(field_info.type) != 0) {
                    const field = @as(Field, @enumFromInt(i));
                    @memcpy(other_slice.items(field), self_slice.items(field));
                }
            }
            gpa.free(self.allocatedBytes());
            self.* = other;
        }

        /// Create a copy of this list with a new backing store,
        /// using the specified allocator.
        pub fn clone(self: Self, gpa: Allocator) !Self {
            var result = Self{};
            errdefer result.deinit(gpa);
            try result.ensureTotalCapacity(gpa, self.len);
            result.len = self.len;
            const self_slice = self.slice();
            const result_slice = result.slice();
            inline for (fields, 0..) |field_info, i| {
                if (@sizeOf(field_info.type) != 0) {
                    const field = @as(Field, @enumFromInt(i));
                    @memcpy(result_slice.items(field), self_slice.items(field));
                }
            }
            return result;
        }

        /// `ctx` has the following method:
        /// `fn lessThan(ctx: @TypeOf(ctx), a_index: usize, b_index: usize) bool`
        fn sortInternal(self: Self, a: usize, b: usize, ctx: anytype, comptime mode: std.sort.Mode) void {
            const sort_context: struct {
                sub_ctx: @TypeOf(ctx),
                slice: Slice,

                pub fn swap(sc: @This(), a_index: usize, b_index: usize) void {
                    inline for (fields, 0..) |field_info, i| {
                        if (@sizeOf(field_info.type) != 0) {
                            const field: Field = @enumFromInt(i);
                            const ptr = sc.slice.items(field);
                            mem.swap(field_info.type, &ptr[a_index], &ptr[b_index]);
                        }
                    }
                }

                pub fn lessThan(sc: @This(), a_index: usize, b_index: usize) bool {
                    return sc.sub_ctx.lessThan(a_index, b_index);
                }
            } = .{
                .sub_ctx = ctx,
                .slice = self.slice(),
            };

            switch (mode) {
                .stable => mem.sortContext(a, b, sort_context),
                .unstable => mem.sortUnstableContext(a, b, sort_context),
            }
        }

        /// This function guarantees a stable sort, i.e the relative order of equal elements is preserved during sorting.
        /// Read more about stable sorting here: https://en.wikipedia.org/wiki/Sorting_algorithm#Stability
        /// If this guarantee does not matter, `sortUnstable` might be a faster alternative.
        /// `ctx` has the following method:
        /// `fn lessThan(ctx: @TypeOf(ctx), a_index: usize, b_index: usize) bool`
        pub fn sort(self: Self, ctx: anytype) void {
            self.sortInternal(0, self.len, ctx, .stable);
        }

        /// Sorts only the subsection of items between indices `a` and `b` (excluding `b`)
        /// This function guarantees a stable sort, i.e the relative order of equal elements is preserved during sorting.
        /// Read more about stable sorting here: https://en.wikipedia.org/wiki/Sorting_algorithm#Stability
        /// If this guarantee does not matter, `sortSpanUnstable` might be a faster alternative.
        /// `ctx` has the following method:
        /// `fn lessThan(ctx: @TypeOf(ctx), a_index: usize, b_index: usize) bool`
        pub fn sortSpan(self: Self, a: usize, b: usize, ctx: anytype) void {
            self.sortInternal(a, b, ctx, .stable);
        }

        /// This function does NOT guarantee a stable sort, i.e the relative order of equal elements may change during sorting.
        /// Due to the weaker guarantees of this function, this may be faster than the stable `sort` method.
        /// Read more about stable sorting here: https://en.wikipedia.org/wiki/Sorting_algorithm#Stability
        /// `ctx` has the following method:
        /// `fn lessThan(ctx: @TypeOf(ctx), a_index: usize, b_index: usize) bool`
        pub fn sortUnstable(self: Self, ctx: anytype) void {
            self.sortInternal(0, self.len, ctx, .unstable);
        }

        /// Sorts only the subsection of items between indices `a` and `b` (excluding `b`)
        /// This function does NOT guarantee a stable sort, i.e the relative order of equal elements may change during sorting.
        /// Due to the weaker guarantees of this function, this may be faster than the stable `sortSpan` method.
        /// Read more about stable sorting here: https://en.wikipedia.org/wiki/Sorting_algorithm#Stability
        /// `ctx` has the following method:
        /// `fn lessThan(ctx: @TypeOf(ctx), a_index: usize, b_index: usize) bool`
        pub fn sortSpanUnstable(self: Self, a: usize, b: usize, ctx: anytype) void {
            self.sortInternal(a, b, ctx, .unstable);
        }

        pub fn capacityInBytes(capacity: usize) usize {
            comptime var elem_bytes: usize = 0;
            inline for (sizes.bytes) |size| elem_bytes += size;
            return elem_bytes * capacity;
        }

        fn allocatedBytes(self: Self) []align(@alignOf(Elem)) u8 {
            return self.bytes[0..capacityInBytes(self.capacity)];
        }

        fn FieldType(comptime field: Field) type {
            return meta.fieldInfo(Elem, field).type;
        }

        const Entry = entry: {
            var entry_fields: [fields.len]std.builtin.Type.StructField = undefined;
            for (&entry_fields, sizes.fields) |*entry_field, i| entry_field.* = .{
                .name = fields[i].name ++ "_ptr",
                .type = *fields[i].type,
                .default_value_ptr = null,
                .is_comptime = fields[i].is_comptime,
                .alignment = fields[i].alignment,
            };
            break :entry @Type(.{ .@"struct" = .{
                .layout = .@"extern",
                .fields = &entry_fields,
                .decls = &.{},
                .is_tuple = false,
            } });
        };
        /// This function is used in the debugger pretty formatters in tools/ to fetch the
        /// child field order and entry type to facilitate fancy debug printing for this type.
        fn dbHelper(self: *Self, child: *Elem, field: *Field, entry: *Entry) void {
            _ = self;
            _ = child;
            _ = field;
            _ = entry;
        }

        comptime {
            if (builtin.zig_backend == .stage2_llvm and !builtin.strip_debug_info) {
                _ = &dbHelper;
                _ = &Slice.dbHelper;
            }
        }
    };
}  