add std.MultiArrayList

Also known as "Struct-Of-Arrays" or "SOA". The purpose of this data
structure is to provide a similar API to ArrayList but instead of
the element type being a struct, the fields of the struct are in N
different arrays, all with the same length and capacity.

Having this abstraction means we can put them in the same allocation,
avoiding overhead with the allocator. It also saves a tiny bit of
overhead from the redundant capacity and length fields, since each
struct element shares the same value.

This is an alternate implementation to #7854.

Author: andrewrk

lib/std/multi_array_list.zig

@@ -0,0 +1,353 @@
+// SPDX-License-Identifier: MIT
+// Copyright (c) 2015-2021 Zig Contributors
+// This file is part of [zig](https://ziglang.org/), which is MIT licensed.
+// The MIT license requires this copyright notice to be included in all copies
+// and substantial portions of the software.
+const std = @import("std.zig");
+const assert = std.debug.assert;
+const meta = std.meta;
+const mem = std.mem;
+const Allocator = mem.Allocator;
+
+pub fn MultiArrayList(comptime S: type) type {
+    return struct {
+        bytes: [*]align(@alignOf(S)) u8 = undefined,
+        len: usize = 0,
+        capacity: usize = 0,
+
+        pub const Elem = S;
+
+        pub const Field = meta.FieldEnum(S);
+
+        pub const Slice = struct {
+            /// The index corresponds to sizes.bytes, not in field order.
+            ptrs: [fields.len][*]u8,
+            len: usize,
+            capacity: usize,
+
+            pub fn items(self: Slice, comptime field: Field) []FieldType(field) {
+                const byte_ptr = self.ptrs[@enumToInt(field)];
+                const F = FieldType(field);
+                const casted_ptr = @ptrCast([*]F, @alignCast(@alignOf(F), byte_ptr));
+                return casted_ptr[0..self.len];
+            }
+
+            pub fn toMultiArrayList(self: Slice) Self {
+                if (self.ptrs.len == 0) {
+                    return .{};
+                }
+                const unaligned_ptr = self.ptrs[sizes.fields[0]];
+                const aligned_ptr = @alignCast(@alignOf(S), unaligned_ptr);
+                const casted_ptr = @ptrCast([*]align(@alignOf(S)) u8, aligned_ptr);
+                return .{
+                    .bytes = casted_ptr,
+                    .len = self.len,
+                    .capacity = self.capacity,
+                };
+            }
+
+            pub fn deinit(self: *Slice, gpa: *Allocator) void {
+                var other = self.toMultiArrayList();
+                other.deinit(gpa);
+                self.* = undefined;
+            }
+        };
+
+        const Self = @This();
+
+        const fields = meta.fields(S);
+        /// `sizes.bytes` is an array of @sizeOf each S field. Sorted by alignment, descending.
+        /// `sizes.indexes` is an array mapping from field to its index in the `sizes.bytes` array.
+        /// `sizes.fields` is an array with the field indexes of the `sizes.bytes` array.
+        const sizes = blk: {
+            const Data = struct {
+                size: usize,
+                size_index: usize,
+                alignment: usize,
+            };
+            var data: [fields.len]Data = undefined;
+            for (fields) |field_info, i| {
+                data[i] = .{
+                    .size = @sizeOf(field_info.field_type),
+                    .size_index = i,
+                    .alignment = field_info.alignment,
+                };
+            }
+            const Sort = struct {
+                fn lessThan(trash: *i32, lhs: Data, rhs: Data) bool {
+                    return lhs.alignment >= rhs.alignment;
+                }
+            };
+            var trash: i32 = undefined; // workaround for stage1 compiler bug
+            std.sort.sort(Data, &data, &trash, Sort.lessThan);
+            var sizes_bytes: [fields.len]usize = undefined;
+            var sizes_indexes: [fields.len]usize = undefined;
+            var field_indexes: [fields.len]usize = undefined;
+            for (data) |elem, i| {
+                sizes_bytes[i] = elem.size;
+                sizes_indexes[elem.size_index] = i;
+                field_indexes[i] = elem.size_index;
+            }
+            break :blk .{
+                .bytes = sizes_bytes,
+                .indexes = sizes_indexes,
+                .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;
+        }
+
+        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) |field_size, i| {
+                result.ptrs[sizes.fields[i]] = ptr;
+                ptr += field_size * self.capacity;
+            }
+            return result;
+        }
+
+        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: S) void {
+            const slices = self.slice();
+            inline for (fields) |field_info, i| {
+                slices.items(@intToEnum(Field, i))[index] = @field(elem, field_info.name);
+            }
+        }
+
+        /// Obtain all the data for one array element.
+        pub fn get(self: *Self, index: usize) S {
+            const slices = self.slice();
+            var result: S = undefined;
+            inline for (fields) |field_info, i| {
+                @field(elem, field_info.name) = slices.items(@intToEnum(Field, i))[index];
+            }
+            return result;
+        }
+
+        /// Extend the list by 1 element. Allocates more memory as necessary.
+        pub fn append(self: *Self, gpa: *Allocator, elem: S) !void {
+            try self.ensureCapacity(gpa, self.len + 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: S) void {
+            assert(self.len < self.capacity);
+            self.len += 1;
+            self.set(self.len - 1, elem);
+        }
+
+        /// 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.ensureCapacity(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) {
+                gpa.free(self.allocatedBytes());
+                self.* = .{};
+                return;
+            }
+            assert(new_len <= self.capacity);
+            assert(new_len <= self.len);
+
+            const other_bytes = gpa.allocAdvanced(
+                u8,
+                @alignOf(S),
+                capacityInBytes(new_len),
+                .exact,
+            ) catch {
+                self.len = new_len;
+                // TODO memset the invalidated items to undefined
+                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) |field_info, i| {
+                const field = @intToEnum(Field, i);
+                mem.copy(field_info.field_type, other_slice.items(field), self_slice.items(field));
+            }
+            gpa.free(self.allocatedBytes());
+            self.* = other;
+        }
+
+        /// 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;
+        }
+
+        /// 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 pointers if additional memory is needed.
+        pub fn ensureCapacity(self: *Self, gpa: *Allocator, new_capacity: usize) !void {
+            var better_capacity = self.capacity;
+            if (better_capacity >= new_capacity) return;
+
+            while (true) {
+                better_capacity += better_capacity / 2 + 8;
+                if (better_capacity >= new_capacity) break;
+            }
+
+            return self.setCapacity(gpa, better_capacity);
+        }
+
+        /// 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.allocAdvanced(
+                u8,
+                @alignOf(S),
+                capacityInBytes(new_capacity),
+                .exact,
+            );
+            if (self.len == 0) {
+                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) |field_info, i| {
+                const field = @intToEnum(Field, i);
+                mem.copy(field_info.field_type, other_slice.items(field), self_slice.items(field));
+            }
+            gpa.free(self.allocatedBytes());
+            self.* = other;
+        }
+
+        fn capacityInBytes(capacity: usize) usize {
+            const sizes_vector: std.meta.Vector(sizes.bytes.len, usize) = sizes.bytes;
+            const capacity_vector = @splat(sizes.bytes.len, capacity);
+            return @reduce(.Add, capacity_vector * sizes_vector);
+        }
+
+        fn allocatedBytes(self: Self) []align(@alignOf(S)) u8 {
+            return self.bytes[0..capacityInBytes(self.capacity)];
+        }
+
+        fn FieldType(field: Field) type {
+            return meta.fieldInfo(S, field).field_type;
+        }
+    };
+}
+
+test "basic usage" {
+    const testing = std.testing;
+    const ally = testing.allocator;
+
+    const Foo = struct {
+        a: u32,
+        b: []const u8,
+        c: u8,
+    };
+
+    var list = MultiArrayList(Foo){};
+    defer list.deinit(ally);
+
+    try list.ensureCapacity(ally, 2);
+
+    list.appendAssumeCapacity(.{
+        .a = 1,
+        .b = "foobar",
+        .c = 'a',
+    });
+
+    list.appendAssumeCapacity(.{
+        .a = 2,
+        .b = "zigzag",
+        .c = 'b',
+    });
+
+    testing.expectEqualSlices(u32, list.items(.a), &[_]u32{ 1, 2 });
+    testing.expectEqualSlices(u8, list.items(.c), &[_]u8{ 'a', 'b' });
+
+    testing.expectEqual(@as(usize, 2), list.items(.b).len);
+    testing.expectEqualStrings("foobar", list.items(.b)[0]);
+    testing.expectEqualStrings("zigzag", list.items(.b)[1]);
+
+    try list.append(ally, .{
+        .a = 3,
+        .b = "fizzbuzz",
+        .c = 'c',
+    });
+
+    testing.expectEqualSlices(u32, list.items(.a), &[_]u32{ 1, 2, 3 });
+    testing.expectEqualSlices(u8, list.items(.c), &[_]u8{ 'a', 'b', 'c' });
+
+    testing.expectEqual(@as(usize, 3), list.items(.b).len);
+    testing.expectEqualStrings("foobar", list.items(.b)[0]);
+    testing.expectEqualStrings("zigzag", list.items(.b)[1]);
+    testing.expectEqualStrings("fizzbuzz", list.items(.b)[2]);
+
+    // Add 6 more things to force a capacity increase.
+    var i: usize = 0;
+    while (i < 6) : (i += 1) {
+        try list.append(ally, .{
+            .a = @intCast(u32, 4 + i),
+            .b = "whatever",
+            .c = @intCast(u8, 'd' + i),
+        });
+    }
+
+    testing.expectEqualSlices(
+        u32,
+        &[_]u32{ 1, 2, 3, 4, 5, 6, 7, 8, 9 },
+        list.items(.a),
+    );
+    testing.expectEqualSlices(
+        u8,
+        &[_]u8{ 'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i' },
+        list.items(.c),
+    );
+
+    list.shrinkAndFree(ally, 3);
+
+    testing.expectEqualSlices(u32, list.items(.a), &[_]u32{ 1, 2, 3 });
+    testing.expectEqualSlices(u8, list.items(.c), &[_]u8{ 'a', 'b', 'c' });
+
+    testing.expectEqual(@as(usize, 3), list.items(.b).len);
+    testing.expectEqualStrings("foobar", list.items(.b)[0]);
+    testing.expectEqualStrings("zigzag", list.items(.b)[1]);
+    testing.expectEqualStrings("fizzbuzz", list.items(.b)[2]);
+}

lib/std/std.zig

@@ -20,6 +20,7 @@ pub const ComptimeStringMap = @import("comptime_string_map.zig").ComptimeStringM
 pub const DynLib = @import("dynamic_library.zig").DynLib;
 pub const HashMap = hash_map.HashMap;
 pub const HashMapUnmanaged = hash_map.HashMapUnmanaged;
+pub const MultiArrayList = @import("multi_array_list.zig").MultiArrayList;
 pub const PackedIntArray = @import("packed_int_array.zig").PackedIntArray;
 pub const PackedIntArrayEndian = @import("packed_int_array.zig").PackedIntArrayEndian;
 pub const PackedIntSlice = @import("packed_int_array.zig").PackedIntSlice;