    const std = @import("std.zig");
    const assert = std.debug.assert;
    const testing = std.testing;
    const mem = std.mem;
    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 append() 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 append() 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) append 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 = undefined,
            dynamic_segments: [][*]T = &[_][*]T{},
            len: usize = 0,

            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;
                }
            }

            pub fn deinit(self: *Self, allocator: Allocator) void {
                self.freeShelves(allocator, @as(ShelfIndex, @intCast(self.dynamic_segments.len)), 0);
                allocator.free(self.dynamic_segments);
                self.* = undefined;
            }

            pub fn at(self: anytype, i: usize) AtType(@TypeOf(self)) {
                assert(i < self.len);
                return self.uncheckedAt(i);
            }

            pub fn count(self: Self) usize {
                return self.len;
            }

            pub fn append(self: *Self, allocator: Allocator, item: T) Allocator.Error!void {
                const new_item_ptr = try self.addOne(allocator);
                new_item_ptr.* = item;
            }

            pub fn appendSlice(self: *Self, allocator: Allocator, items: []const T) Allocator.Error!void {
                for (items) |item| {
                    try self.append(allocator, 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, allocator: Allocator) Allocator.Error!*T {
                const new_length = self.len + 1;
                try self.growCapacity(allocator, new_length);
                const result = uncheckedAt(self, self.len);
                self.len = new_length;
                return result;
            }

            /// Reduce length to `new_len`.
            /// Invalidates pointers for the elements at index new_len and beyond.
            pub fn shrinkRetainingCapacity(self: *Self, new_len: usize) void {
                assert(new_len <= self.len);
                self.len = new_len;
            }

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

            /// Invalidates all element pointers.
            pub fn clearAndFree(self: *Self, allocator: Allocator) void {
                self.setCapacity(allocator, 0) catch unreachable;
                self.len = 0;
            }

            /// Grows or shrinks capacity to match usage.
            /// TODO update this and related methods to match the conventions set by ArrayList
            pub fn setCapacity(self: *Self, allocator: Allocator, new_capacity: usize) Allocator.Error!void {
                if (prealloc_item_count != 0) {
                    if (new_capacity <= @as(usize, 1) << (prealloc_exp + @as(ShelfIndex, @intCast(self.dynamic_segments.len)))) {
                        return self.shrinkCapacity(allocator, new_capacity);
                    }
                }
                return self.growCapacity(allocator, new_capacity);
            }

            /// Only grows capacity, or retains current capacity.
            pub fn growCapacity(self: *Self, allocator: Allocator, new_capacity: usize) Allocator.Error!void {
                const new_cap_shelf_count = shelfCount(new_capacity);
                const old_shelf_count = @as(ShelfIndex, @intCast(self.dynamic_segments.len));
                if (new_cap_shelf_count <= old_shelf_count) return;

                const new_dynamic_segments = try allocator.alloc([*]T, new_cap_shelf_count);
                errdefer allocator.free(new_dynamic_segments);

                var i: ShelfIndex = 0;
                while (i < old_shelf_count) : (i += 1) {
                    new_dynamic_segments[i] = self.dynamic_segments[i];
                }
                errdefer while (i > old_shelf_count) : (i -= 1) {
                    allocator.free(new_dynamic_segments[i][0..shelfSize(i)]);
                };
                while (i < new_cap_shelf_count) : (i += 1) {
                    new_dynamic_segments[i] = (try allocator.alloc(T, shelfSize(i))).ptr;
                }

                allocator.free(self.dynamic_segments);
                self.dynamic_segments = new_dynamic_segments;
            }

            /// Only shrinks capacity or retains current capacity.
            /// It may fail to reduce the capacity in which case the capacity will remain unchanged.
            pub fn shrinkCapacity(self: *Self, allocator: Allocator, new_capacity: usize) void {
                if (new_capacity <= prealloc_item_count) {
                    const len = @as(ShelfIndex, @intCast(self.dynamic_segments.len));
                    self.freeShelves(allocator, len, 0);
                    allocator.free(self.dynamic_segments);
                    self.dynamic_segments = &[_][*]T{};
                    return;
                }

                const new_cap_shelf_count = shelfCount(new_capacity);
                const old_shelf_count = @as(ShelfIndex, @intCast(self.dynamic_segments.len));
                assert(new_cap_shelf_count <= old_shelf_count);
                if (new_cap_shelf_count == old_shelf_count) return;

                // freeShelves() must be called before resizing the dynamic

                // segments, but we don't know if resizing the dynamic segments

                // will work until we try it. So we must allocate a fresh memory

                // buffer in order to reduce capacity.

                const new_dynamic_segments = allocator.alloc([*]T, new_cap_shelf_count) catch return;
                self.freeShelves(allocator, old_shelf_count, new_cap_shelf_count);
                if (allocator.resize(self.dynamic_segments, new_cap_shelf_count)) {
                    // We didn't need the new memory allocation after all.

                    self.dynamic_segments = self.dynamic_segments[0..new_cap_shelf_count];
                    allocator.free(new_dynamic_segments);
                } else {
                    // Good thing we allocated that new memory slice.

                    @memcpy(new_dynamic_segments, self.dynamic_segments[0..new_cap_shelf_count]);
                    allocator.free(self.dynamic_segments);
                    self.dynamic_segments = new_dynamic_segments;
                }
            }

            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 writeToSlice(self: *Self, dest: []T, start: usize) void {
                const end = start + dest.len;
                assert(end <= self.len);

                var i = start;
                if (end <= prealloc_item_count) {
                    const src = self.prealloc_segment[i..end];
                    @memcpy(dest[i - start ..][0..src.len], src);
                    return;
                } else if (i < prealloc_item_count) {
                    const src = self.prealloc_segment[i..];
                    @memcpy(dest[i - start ..][0..src.len], src);
                    i = prealloc_item_count;
                }

                while (i < end) {
                    const shelf_index = shelfIndex(i);
                    const copy_start = boxIndex(i, shelf_index);
                    const copy_end = @min(shelfSize(shelf_index), copy_start + end - i);
                    const src = self.dynamic_segments[shelf_index][copy_start..copy_end];
                    @memcpy(dest[i - start ..][0..src.len], src);
                    i += (copy_end - copy_start);
                }
            }

            pub fn uncheckedAt(self: anytype, 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 log2_int_ceil(usize, box_count + 1);
                }
                return log2_int_ceil(usize, box_count + prealloc_item_count) - prealloc_exp - 1;
            }

            fn shelfSize(shelf_index: ShelfIndex) usize {
                if (prealloc_item_count == 0) {
                    return @as(usize, 1) << shelf_index;
                }
                return @as(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) - (@as(usize, 1) << shelf_index);
                }
                return list_index + prealloc_item_count - (@as(usize, 1) << ((prealloc_exp + 1) + shelf_index));
            }

            fn freeShelves(self: *Self, allocator: Allocator, from_count: ShelfIndex, to_count: ShelfIndex) void {
                var i = from_count;
                while (i != to_count) {
                    i -= 1;
                    allocator.free(self.dynamic_segments[i][0..shelfSize(i)]);
                }
            }

            pub const Iterator = BaseIterator(*Self, *T);
            pub const ConstIterator = BaseIterator(*const Self, *const T);
            fn BaseIterator(comptime SelfType: type, comptime ElementPtr: type) type {
                return struct {
                    list: SelfType,
                    index: usize,
                    box_index: usize,
                    shelf_index: ShelfIndex,
                    shelf_size: usize,

                    pub fn next(it: *@This()) ?ElementPtr {
                        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: *@This()) ?ElementPtr {
                        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: *@This()) ?ElementPtr {
                        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: *@This(), 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;
            }

            pub fn constIterator(self: *const Self, start_index: usize) ConstIterator {
                var it = ConstIterator{
                    .list = self,
                    .index = undefined,
                    .shelf_index = undefined,
                    .box_index = undefined,
                    .shelf_size = undefined,
                };
                it.set(start_index);
                return it;
            }
        };
    }

    test "SegmentedList basic usage" {
        try testSegmentedList(0);
        try testSegmentedList(1);
        try testSegmentedList(2);
        try testSegmentedList(4);
        try testSegmentedList(8);
        try testSegmentedList(16);
    }

    fn testSegmentedList(comptime prealloc: usize) !void {
        var list = SegmentedList(i32, prealloc){};
        defer list.deinit(testing.allocator);

        {
            var i: usize = 0;
            while (i < 100) : (i += 1) {
                try list.append(testing.allocator, @as(i32, @intCast(i + 1)));
                try testing.expect(list.len == i + 1);
            }
        }

        {
            var i: usize = 0;
            while (i < 100) : (i += 1) {
                try testing.expect(list.at(i).* == @as(i32, @intCast(i + 1)));
            }
        }

        {
            var it = list.iterator(0);
            var x: i32 = 0;
            while (it.next()) |item| {
                x += 1;
                try testing.expect(item.* == x);
            }
            try testing.expect(x == 100);
            while (it.prev()) |item| : (x -= 1) {
                try testing.expect(item.* == x);
            }
            try testing.expect(x == 0);
        }

        {
            var it = list.constIterator(0);
            var x: i32 = 0;
            while (it.next()) |item| {
                x += 1;
                try testing.expect(item.* == x);
            }
            try testing.expect(x == 100);
            while (it.prev()) |item| : (x -= 1) {
                try testing.expect(item.* == x);
            }
            try testing.expect(x == 0);
        }

        try testing.expect(list.pop().? == 100);
        try testing.expect(list.len == 99);

        try list.appendSlice(testing.allocator, &[_]i32{ 1, 2, 3 });
        try testing.expect(list.len == 102);
        try testing.expect(list.pop().? == 3);
        try testing.expect(list.pop().? == 2);
        try testing.expect(list.pop().? == 1);
        try testing.expect(list.len == 99);

        try list.appendSlice(testing.allocator, &[_]i32{});
        try testing.expect(list.len == 99);

        {
            var i: i32 = 99;
            while (list.pop()) |item| : (i -= 1) {
                try testing.expect(item == i);
                list.shrinkCapacity(testing.allocator, list.len);
            }
        }

        {
            var control: [100]i32 = undefined;
            var dest: [100]i32 = undefined;

            var i: i32 = 0;
            while (i < 100) : (i += 1) {
                try list.append(testing.allocator, i + 1);
                control[@as(usize, @intCast(i))] = i + 1;
            }

            @memset(dest[0..], 0);
            list.writeToSlice(dest[0..], 0);
            try testing.expect(mem.eql(i32, control[0..], dest[0..]));

            @memset(dest[0..], 0);
            list.writeToSlice(dest[50..], 50);
            try testing.expect(mem.eql(i32, control[50..], dest[50..]));
        }

        try list.setCapacity(testing.allocator, 0);
    }

    test "std.segmented_list clearRetainingCapacity" {
        var list = SegmentedList(i32, 1){};
        defer list.deinit(testing.allocator);

        try list.appendSlice(testing.allocator, &[_]i32{ 4, 5 });
        list.clearRetainingCapacity();
        try list.append(testing.allocator, 6);
        try testing.expect(list.at(0).* == 6);
        try testing.expect(list.len == 1);
        list.clearRetainingCapacity();
        try testing.expect(list.len == 0);
    }

    /// TODO look into why this std.math function was changed in
    /// fc9430f56798a53f9393a697f4ccd6bf9981b970.
    fn log2_int_ceil(comptime T: type, x: T) std.math.Log2Int(T) {
        assert(x != 0);
        const log2_val = std.math.log2_int(T, x);
        if (@as(T, 1) << log2_val == x)
            return log2_val;
        return log2_val + 1;
    }