const std = @import("std");
const testing = std.testing;
const mem = std.mem;
const Allocator = mem.Allocator;

pub const IDManager = struct {
    const Self = @This();

    allocator: *Allocator,
    nextID: usize = 0,
    availableIDs: std.ArrayList(usize),

    pub fn init(allocator: *Allocator) Self {
        return Self {
            .allocator = allocator,
            .availableIDs = std.ArrayList(usize).init(allocator),
        };
    }

    pub fn next(self: *Self) usize {
        if (self.availableIDs.items.len > 0)
        {
            return self.availableIDs.pop();
        }
        else
        {
            var result = self.nextID;
            self.nextID += 1;
            return result;
        }
    }

    pub fn free(self: *Self, id: usize) !void {
        self.availableIDs.append(id) catch |err| return err;
    }
};

test "next" {
    var arena = std.heap.ArenaAllocator.init(std.heap.page_allocator);
    defer arena.deinit();

    const allocator = &arena.allocator;

    var id_manager = IDManager.init(allocator);

    testing.expect(id_manager.next() == 0);
    testing.expect(id_manager.next() == 1);
}

test "free" {
    var arena = std.heap.ArenaAllocator.init(std.heap.page_allocator);
    defer arena.deinit();

    const allocator = &arena.allocator;

    var id_manager = IDManager.init(allocator);

    var id_zero = id_manager.next();
    var id_one = id_manager.next();
    var id_two = id_manager.next();

    try id_manager.free(id_one);

    testing.expect(id_manager.next() == id_one);
    testing.expect(id_manager.next() == 3);
}