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+const std = @import("std.zig");
+const assert = std.debug.assert;
+const testing = std.testing;
+const Allocator = std.mem.Allocator;
+
+// Imagine that `fn at(self: *Self, index: usize) &T` is a customer asking for a box
+// from a warehouse, based on a flat array, boxes ordered from 0 to N - 1.
+// But the warehouse actually stores boxes in shelves of increasing powers of 2 sizes.
+// So when the customer requests a box index, we have to translate it to shelf index
+// and box index within that shelf. Illustration:
+//
+// customer indexes:
+// shelf 0: 0
+// shelf 1: 1 2
+// shelf 2: 3 4 5 6
+// shelf 3: 7 8 9 10 11 12 13 14
+// shelf 4: 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
+// shelf 5: 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62
+// ...
+//
+// warehouse indexes:
+// shelf 0: 0
+// shelf 1: 0 1
+// shelf 2: 0 1 2 3
+// shelf 3: 0 1 2 3 4 5 6 7
+// shelf 4: 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
+// shelf 5: 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
+// ...
+//
+// With this arrangement, here are the equations to get the shelf index and
+// box index based on customer box index:
+//
+// shelf_index = floor(log2(customer_index + 1))
+// shelf_count = ceil(log2(box_count + 1))
+// box_index = customer_index + 1 - 2 ** shelf
+// shelf_size = 2 ** shelf_index
+//
+// Now we complicate it a little bit further by adding a preallocated shelf, which must be
+// a power of 2:
+// prealloc=4
+//
+// customer indexes:
+// prealloc: 0 1 2 3
+// shelf 0: 4 5 6 7 8 9 10 11
+// shelf 1: 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27
+// shelf 2: 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59
+// ...
+//
+// warehouse indexes:
+// prealloc: 0 1 2 3
+// shelf 0: 0 1 2 3 4 5 6 7
+// shelf 1: 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
+// shelf 2: 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
+// ...
+//
+// Now the equations are:
+//
+// shelf_index = floor(log2(customer_index + prealloc)) - log2(prealloc) - 1
+// shelf_count = ceil(log2(box_count + prealloc)) - log2(prealloc) - 1
+// box_index = customer_index + prealloc - 2 ** (log2(prealloc) + 1 + shelf)
+// shelf_size = prealloc * 2 ** (shelf_index + 1)
+
+/// This is a stack data structure where pointers to indexes have the same lifetime as the data structure
+/// itself, unlike ArrayList where push() invalidates all existing element pointers.
+/// The tradeoff is that elements are not guaranteed to be contiguous. For that, use ArrayList.
+/// Note however that most elements are contiguous, making this data structure cache-friendly.
+///
+/// Because it never has to copy elements from an old location to a new location, it does not require
+/// its elements to be copyable, and it avoids wasting memory when backed by an ArenaAllocator.
+/// Note that the push() and pop() convenience methods perform a copy, but you can instead use
+/// addOne(), at(), setCapacity(), and shrinkCapacity() to avoid copying items.
+///
+/// This data structure has O(1) push and O(1) pop.
+///
+/// It supports preallocated elements, making it especially well suited when the expected maximum
+/// size is small. `prealloc_item_count` must be 0, or a power of 2.
+pub fn SegmentedList(comptime T: type, comptime prealloc_item_count: usize) type {
+ return struct {
+ const Self = @This();
+ const ShelfIndex = std.math.Log2Int(usize);
+
+ const prealloc_exp: ShelfIndex = blk: {
+ // we don't use the prealloc_exp constant when prealloc_item_count is 0
+ // but lazy-init may still be triggered by other code so supply a value
+ if (prealloc_item_count == 0) {
+ break :blk 0;
+ } else {
+ assert(std.math.isPowerOfTwo(prealloc_item_count));
+ const value = std.math.log2_int(usize, prealloc_item_count);
+ break :blk value;
+ }
+ };
+
+ prealloc_segment: [prealloc_item_count]T,
+ dynamic_segments: [][*]T,
+ allocator: *Allocator,
+ len: usize,
+
+ pub const prealloc_count = prealloc_item_count;
+
+ fn AtType(comptime SelfType: type) type {
+ if (@typeInfo(SelfType).Pointer.is_const) {
+ return *const T;
+ } else {
+ return *T;
+ }
+ }
+
+ /// Deinitialize with `deinit`
+ pub fn init(allocator: *Allocator) Self {
+ return Self{
+ .allocator = allocator,
+ .len = 0,
+ .prealloc_segment = undefined,
+ .dynamic_segments = [_][*]T{},
+ };
+ }
+
+ pub fn deinit(self: *Self) void {
+ self.freeShelves(@intCast(ShelfIndex, self.dynamic_segments.len), 0);
+ self.allocator.free(self.dynamic_segments);
+ self.* = undefined;
+ }
+
+ pub fn at(self: var, i: usize) AtType(@typeOf(self)) {
+ assert(i < self.len);
+ return self.uncheckedAt(i);
+ }
+
+ pub fn count(self: Self) usize {
+ return self.len;
+ }
+
+ pub fn push(self: *Self, item: T) !void {
+ const new_item_ptr = try self.addOne();
+ new_item_ptr.* = item;
+ }
+
+ pub fn pushMany(self: *Self, items: []const T) !void {
+ for (items) |item| {
+ try self.push(item);
+ }
+ }
+
+ pub fn pop(self: *Self) ?T {
+ if (self.len == 0) return null;
+
+ const index = self.len - 1;
+ const result = uncheckedAt(self, index).*;
+ self.len = index;
+ return result;
+ }
+
+ pub fn addOne(self: *Self) !*T {
+ const new_length = self.len + 1;
+ try self.growCapacity(new_length);
+ const result = uncheckedAt(self, self.len);
+ self.len = new_length;
+ return result;
+ }
+
+ /// Grows or shrinks capacity to match usage.
+ pub fn setCapacity(self: *Self, new_capacity: usize) !void {
+ if (prealloc_item_count != 0) {
+ if (new_capacity <= usize(1) << (prealloc_exp + @intCast(ShelfIndex, self.dynamic_segments.len))) {
+ return self.shrinkCapacity(new_capacity);
+ }
+ }
+ return self.growCapacity(new_capacity);
+ }
+
+ /// Only grows capacity, or retains current capacity
+ pub fn growCapacity(self: *Self, new_capacity: usize) !void {
+ const new_cap_shelf_count = shelfCount(new_capacity);
+ const old_shelf_count = @intCast(ShelfIndex, self.dynamic_segments.len);
+ if (new_cap_shelf_count > old_shelf_count) {
+ self.dynamic_segments = try self.allocator.realloc(self.dynamic_segments, new_cap_shelf_count);
+ var i = old_shelf_count;
+ errdefer {
+ self.freeShelves(i, old_shelf_count);
+ self.dynamic_segments = self.allocator.shrink(self.dynamic_segments, old_shelf_count);
+ }
+ while (i < new_cap_shelf_count) : (i += 1) {
+ self.dynamic_segments[i] = (try self.allocator.alloc(T, shelfSize(i))).ptr;
+ }
+ }
+ }
+
+ /// Only shrinks capacity or retains current capacity
+ pub fn shrinkCapacity(self: *Self, new_capacity: usize) void {
+ if (new_capacity <= prealloc_item_count) {
+ const len = @intCast(ShelfIndex, self.dynamic_segments.len);
+ self.freeShelves(len, 0);
+ self.allocator.free(self.dynamic_segments);
+ self.dynamic_segments = [_][*]T{};
+ return;
+ }
+
+ const new_cap_shelf_count = shelfCount(new_capacity);
+ const old_shelf_count = @intCast(ShelfIndex, self.dynamic_segments.len);
+ assert(new_cap_shelf_count <= old_shelf_count);
+ if (new_cap_shelf_count == old_shelf_count) {
+ return;
+ }
+
+ self.freeShelves(old_shelf_count, new_cap_shelf_count);
+ self.dynamic_segments = self.allocator.shrink(self.dynamic_segments, new_cap_shelf_count);
+ }
+
+ pub fn shrink(self: *Self, new_len: usize) void {
+ assert(new_len <= self.len);
+ // TODO take advantage of the new realloc semantics
+ self.len = new_len;
+ }
+
+ pub fn uncheckedAt(self: var, index: usize) AtType(@typeOf(self)) {
+ if (index < prealloc_item_count) {
+ return &self.prealloc_segment[index];
+ }
+ const shelf_index = shelfIndex(index);
+ const box_index = boxIndex(index, shelf_index);
+ return &self.dynamic_segments[shelf_index][box_index];
+ }
+
+ fn shelfCount(box_count: usize) ShelfIndex {
+ if (prealloc_item_count == 0) {
+ return std.math.log2_int_ceil(usize, box_count + 1);
+ }
+ return std.math.log2_int_ceil(usize, box_count + prealloc_item_count) - prealloc_exp - 1;
+ }
+
+ fn shelfSize(shelf_index: ShelfIndex) usize {
+ if (prealloc_item_count == 0) {
+ return usize(1) << shelf_index;
+ }
+ return usize(1) << (shelf_index + (prealloc_exp + 1));
+ }
+
+ fn shelfIndex(list_index: usize) ShelfIndex {
+ if (prealloc_item_count == 0) {
+ return std.math.log2_int(usize, list_index + 1);
+ }
+ return std.math.log2_int(usize, list_index + prealloc_item_count) - prealloc_exp - 1;
+ }
+
+ fn boxIndex(list_index: usize, shelf_index: ShelfIndex) usize {
+ if (prealloc_item_count == 0) {
+ return (list_index + 1) - (usize(1) << shelf_index);
+ }
+ return list_index + prealloc_item_count - (usize(1) << ((prealloc_exp + 1) + shelf_index));
+ }
+
+ fn freeShelves(self: *Self, from_count: ShelfIndex, to_count: ShelfIndex) void {
+ var i = from_count;
+ while (i != to_count) {
+ i -= 1;
+ self.allocator.free(self.dynamic_segments[i][0..shelfSize(i)]);
+ }
+ }
+
+ pub const Iterator = struct {
+ list: *Self,
+ index: usize,
+ box_index: usize,
+ shelf_index: ShelfIndex,
+ shelf_size: usize,
+
+ pub fn next(it: *Iterator) ?*T {
+ if (it.index >= it.list.len) return null;
+ if (it.index < prealloc_item_count) {
+ const ptr = &it.list.prealloc_segment[it.index];
+ it.index += 1;
+ if (it.index == prealloc_item_count) {
+ it.box_index = 0;
+ it.shelf_index = 0;
+ it.shelf_size = prealloc_item_count * 2;
+ }
+ return ptr;
+ }
+
+ const ptr = &it.list.dynamic_segments[it.shelf_index][it.box_index];
+ it.index += 1;
+ it.box_index += 1;
+ if (it.box_index == it.shelf_size) {
+ it.shelf_index += 1;
+ it.box_index = 0;
+ it.shelf_size *= 2;
+ }
+ return ptr;
+ }
+
+ pub fn prev(it: *Iterator) ?*T {
+ if (it.index == 0) return null;
+
+ it.index -= 1;
+ if (it.index < prealloc_item_count) return &it.list.prealloc_segment[it.index];
+
+ if (it.box_index == 0) {
+ it.shelf_index -= 1;
+ it.shelf_size /= 2;
+ it.box_index = it.shelf_size - 1;
+ } else {
+ it.box_index -= 1;
+ }
+
+ return &it.list.dynamic_segments[it.shelf_index][it.box_index];
+ }
+
+ pub fn peek(it: *Iterator) ?*T {
+ if (it.index >= it.list.len)
+ return null;
+ if (it.index < prealloc_item_count)
+ return &it.list.prealloc_segment[it.index];
+
+ return &it.list.dynamic_segments[it.shelf_index][it.box_index];
+ }
+
+ pub fn set(it: *Iterator, index: usize) void {
+ it.index = index;
+ if (index < prealloc_item_count) return;
+ it.shelf_index = shelfIndex(index);
+ it.box_index = boxIndex(index, it.shelf_index);
+ it.shelf_size = shelfSize(it.shelf_index);
+ }
+ };
+
+ pub fn iterator(self: *Self, start_index: usize) Iterator {
+ var it = Iterator{
+ .list = self,
+ .index = undefined,
+ .shelf_index = undefined,
+ .box_index = undefined,
+ .shelf_size = undefined,
+ };
+ it.set(start_index);
+ return it;
+ }
+ };
+}
+
+test "std.SegmentedList" {
+ var a = std.heap.direct_allocator;
+
+ try testSegmentedList(0, a);
+ try testSegmentedList(1, a);
+ try testSegmentedList(2, a);
+ try testSegmentedList(4, a);
+ try testSegmentedList(8, a);
+ try testSegmentedList(16, a);
+}
+
+fn testSegmentedList(comptime prealloc: usize, allocator: *Allocator) !void {
+ var list = SegmentedList(i32, prealloc).init(allocator);
+ defer list.deinit();
+
+ {
+ var i: usize = 0;
+ while (i < 100) : (i += 1) {
+ try list.push(@intCast(i32, i + 1));
+ testing.expect(list.len == i + 1);
+ }
+ }
+
+ {
+ var i: usize = 0;
+ while (i < 100) : (i += 1) {
+ testing.expect(list.at(i).* == @intCast(i32, i + 1));
+ }
+ }
+
+ {
+ var it = list.iterator(0);
+ var x: i32 = 0;
+ while (it.next()) |item| {
+ x += 1;
+ testing.expect(item.* == x);
+ }
+ testing.expect(x == 100);
+ while (it.prev()) |item| : (x -= 1) {
+ testing.expect(item.* == x);
+ }
+ testing.expect(x == 0);
+ }
+
+ testing.expect(list.pop().? == 100);
+ testing.expect(list.len == 99);
+
+ try list.pushMany([_]i32{
+ 1,
+ 2,
+ 3,
+ });
+ testing.expect(list.len == 102);
+ testing.expect(list.pop().? == 3);
+ testing.expect(list.pop().? == 2);
+ testing.expect(list.pop().? == 1);
+ testing.expect(list.len == 99);
+
+ try list.pushMany([_]i32{});
+ testing.expect(list.len == 99);
+
+ var i: i32 = 99;
+ while (list.pop()) |item| : (i -= 1) {
+ testing.expect(item == i);
+ list.shrinkCapacity(list.len);
+ }
+
+ try list.setCapacity(0);
+}
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