ona/source/coral/arena.zig

const debug = @import("./debug.zig");

const io = @import("./io.zig");

const list = @import("./list.zig");

const math = @import("./math.zig");

pub const Stacking = struct {
	allocator: io.Allocator,
	min_region_size: usize,
	head_region: ?*Region,
	tail_region: ?*Region,

	const Region = struct {
		next: ?*Region,
		count: usize,
		capacity: usize,

		fn allocate(self: *Region, region_size: usize) []usize {
			debug.assert(self.can_allocate(region_size));

			const offset = (@sizeOf(Region) / @alignOf(usize)) + self.count;
			const allocation = @as([*]usize, @ptrCast(self))[offset .. (offset + region_size)];

			self.count += region_size;

			return allocation;
		}

		fn as_raw(self: *Region) []usize {
			return @as([*]usize, @ptrCast(self))[0 .. ((@sizeOf(Region) / @alignOf(usize)) + self.capacity)];
		}

		fn can_allocate(self: Region, region_size: usize) bool {
			return (self.count + region_size) <= self.capacity;
		}
	};

	fn allocate_region(self: *Stacking, requested_region_size: usize) io.AllocationError!*Region {
		const region_size = @max(requested_region_size, self.min_region_size);
		const region = @as(*Region, @ptrCast(@alignCast(try self.allocator.reallocate(null, @alignOf(Region) + (@alignOf(usize) * region_size)))));

		region.* = .{
			.next = null,
			.count = 0,
			.capacity = region_size,
		};

		return region;
	}

	pub fn as_allocator(self: *Stacking) io.Allocator {
		return io.Allocator.bind(Stacking, self, .{
			.deallocate = deallocate,
			.reallocate = reallocate,
		});
	}

	pub fn deinit(self: *Stacking) void {
		while (self.head_region) |region| {
			const next_region = region.next;

			self.allocator.deallocate(region.as_raw());

			self.head_region = next_region;
		}

		self.head_region = null;
		self.tail_region = null;
	}

	fn deallocate(_: *Stacking, _: []io.Byte) void {
		// TODO: Decide how to implement.
		unreachable;
	}

	pub fn init(allocator: io.Allocator, min_region_size: usize) Stacking {
		return Stacking{
			.allocator = allocator,
			.min_region_size = min_region_size,
			.head_region = null,
			.tail_region = null,
		};
	}

	fn reallocate(self: *Stacking, _: usize, allocation: ?[]io.Byte, byte_size: usize) io.AllocationError![]io.Byte {
		if (allocation) |buffer| {
			if (byte_size < buffer.len) {
				return buffer[0 .. byte_size];
			}
		}

		const region_size = (byte_size + @sizeOf(usize) - 1) / @sizeOf(usize);

		const tail_region = self.tail_region orelse {
			const region = try self.allocate_region(region_size);

			self.tail_region = region;
			self.head_region = self.tail_region;

			return @as([*]io.Byte, @ptrCast(region.allocate(region_size)))[0 .. byte_size];
		};

		if (!tail_region.can_allocate(region_size)) {
			const region = try self.allocate_region(region_size);

			tail_region.next = region;
			self.tail_region = region;

			return @as([*]io.Byte, @ptrCast(region.allocate(region_size)))[0 .. byte_size];
		}

		return @as([*]io.Byte, @ptrCast(tail_region.allocate(region_size)))[0 .. byte_size];
	}
};