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Diffstat (limited to 'std/segmented_list.zig')
| -rw-r--r-- | std/segmented_list.zig | 379 |
1 files changed, 379 insertions, 0 deletions
diff --git a/std/segmented_list.zig b/std/segmented_list.zig new file mode 100644 index 0000000000..a89d332556 --- /dev/null +++ b/std/segmented_list.zig @@ -0,0 +1,379 @@ +const std = @import("index.zig"); +const assert = std.debug.assert; +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 prealloc_exp = blk: { + // we don't use the prealloc_exp constant when prealloc_item_count is 0. + assert(prealloc_item_count != 0); + + const value = std.math.log2_int(usize, prealloc_item_count); + assert((1 << value) == prealloc_item_count); // prealloc_item_count must be a power of 2 + break :blk @typeOf(1)(value); + }; + const ShelfIndex = std.math.Log2Int(usize); + + prealloc_segment: [prealloc_item_count]T, + dynamic_segments: []&T, + allocator: &Allocator, + len: usize, + + pub const prealloc_count = prealloc_item_count; + + /// 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(ShelfIndex(self.dynamic_segments.len), 0); + self.allocator.free(self.dynamic_segments); + *self = undefined; + } + + pub fn at(self: &Self, i: usize) &T { + assert(i < self.len); + return self.uncheckedAt(i); + } + + pub fn count(self: &const Self) usize { + return self.len; + } + + pub fn push(self: &Self, item: &const 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 = *self.uncheckedAt(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 = self.uncheckedAt(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 (new_capacity <= usize(1) << (prealloc_exp + self.dynamic_segments.len)) { + return self.shrinkCapacity(new_capacity); + } else { + 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 = ShelfIndex(self.dynamic_segments.len); + if (new_cap_shelf_count > old_shelf_count) { + self.dynamic_segments = try self.allocator.realloc(&T, 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(&T, 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 = 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 = 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(&T, self.dynamic_segments, new_cap_shelf_count); + } + + pub fn uncheckedAt(self: &Self, index: usize) &T { + 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 iterator(self: &Self, start_index: usize) Iterator { + var it = Iterator { + .list = self, + .index = start_index, + .shelf_index = undefined, + .box_index = undefined, + .shelf_size = undefined, + }; + if (start_index >= prealloc_item_count) { + it.shelf_index = shelfIndex(start_index); + it.box_index = boxIndex(start_index, it.shelf_index); + it.shelf_size = shelfSize(it.shelf_index); + } + return it; + } + }; +} + +test "std.SegmentedList" { + var da = std.heap.DirectAllocator.init(); + defer da.deinit(); + var a = &da.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(i32(i + 1)); + assert(list.len == i + 1); + }} + + {var i: usize = 0; while (i < 100) : (i += 1) { + assert(*list.at(i) == i32(i + 1)); + }} + + { + var it = list.iterator(0); + var x: i32 = 0; + while (it.next()) |item| { + x += 1; + assert(*item == x); + } + assert(x == 100); + while (it.prev()) |item| : (x -= 1) { + assert(*item == x); + } + assert(x == 0); + } + + assert(??list.pop() == 100); + assert(list.len == 99); + + try list.pushMany([]i32 { 1, 2, 3 }); + assert(list.len == 102); + assert(??list.pop() == 3); + assert(??list.pop() == 2); + assert(??list.pop() == 1); + assert(list.len == 99); + + try list.pushMany([]const i32 {}); + assert(list.len == 99); + + var i: i32 = 99; + while (list.pop()) |item| : (i -= 1) { + assert(item == i); + list.shrinkCapacity(list.len); + } +} |