diff -r 9ed4a8502c54 -r 8a2f75e38eb2 spec/util_fsm_spec.lua
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/spec/util_fsm_spec.lua	Thu Mar 17 17:45:27 2022 +0000
@@ -0,0 +1,250 @@
+describe("util.fsm", function ()
+	local new_fsm = require "util.fsm".new;
+
+	do
+		local fsm = new_fsm({
+			transitions = {
+				{ name = "melt", from = "solid", to = "liquid" };
+				{ name = "freeze", from = "liquid", to = "solid" };
+			};
+		});
+
+		it("works", function ()
+			local water = fsm:init("liquid");
+			water:freeze();
+			assert.equal("solid", water.state);
+			water:melt();
+			assert.equal("liquid", water.state);
+		end);
+
+		it("does not allow invalid transitions", function ()
+			local water = fsm:init("liquid");
+			assert.has_errors(function ()
+				water:melt();
+			end, "Invalid state transition: liquid cannot melt");
+
+			water:freeze();
+			assert.equal("solid", water.state);
+
+			water:melt();
+			assert.equal("liquid", water.state);
+
+			assert.has_errors(function ()
+				water:melt();
+			end, "Invalid state transition: liquid cannot melt");
+		end);
+	end
+
+	it("notifies observers", function ()
+		local n = 0;
+		local has_become_solid = spy.new(function (transition)
+			assert.is_table(transition);
+			assert.equal("solid", transition.to);
+			assert.is_not_nil(transition.instance);
+			n = n + 1;
+			if n == 1 then
+				assert.is_nil(transition.from);
+				assert.is_nil(transition.from_attr);
+			elseif n == 2 then
+				assert.equal("liquid", transition.from);
+				assert.is_nil(transition.from_attr);
+				assert.equal("freeze", transition.name);
+			end
+		end);
+		local is_melting = spy.new(function (transition)
+			assert.is_table(transition);
+			assert.equal("melt", transition.name);
+			assert.is_not_nil(transition.instance);
+		end);
+		local fsm = new_fsm({
+			transitions = {
+				{ name = "melt", from = "solid", to = "liquid" };
+				{ name = "freeze", from = "liquid", to = "solid" };
+			};
+			state_handlers = {
+				solid = has_become_solid;
+			};
+
+			transition_handlers = {
+				melt = is_melting;
+			};
+		});
+
+		local water = fsm:init("liquid");
+		assert.spy(has_become_solid).was_not_called();
+
+		local ice = fsm:init("solid"); --luacheck: ignore 211/ice
+		assert.spy(has_become_solid).was_called(1);
+
+		water:freeze();
+
+		assert.spy(is_melting).was_not_called();
+		water:melt();
+		assert.spy(is_melting).was_called(1);
+	end);
+
+	local function test_machine(fsm_spec, expected_transitions, test_func)
+		fsm_spec.handlers = fsm_spec.handlers or {};
+		fsm_spec.handlers.transitioned = function (transition)
+			local expected_transition = table.remove(expected_transitions, 1);
+			assert.same(expected_transition, {
+				name = transition.name;
+				to = transition.to;
+				to_attr = transition.to_attr;
+				from = transition.from;
+				from_attr = transition.from_attr;
+			});
+		end;
+		local fsm = new_fsm(fsm_spec);
+		test_func(fsm);
+		assert.equal(0, #expected_transitions);
+	end
+
+
+	it("handles transitions with the same name", function ()
+		local expected_transitions = {
+			{ name = nil   , from = "none", to = "A" };
+			{ name = "step", from = "A", to = "B" };
+			{ name = "step", from = "B", to = "C" };
+			{ name = "step", from = "C", to = "D" };
+		};
+
+		test_machine({
+			default_state = "none";
+			transitions = {
+				{ name = "step", from = "A", to = "B" };
+				{ name = "step", from = "B", to = "C" };
+				{ name = "step", from = "C", to = "D" };
+			};
+		}, expected_transitions, function (fsm)
+			local i = fsm:init("A");
+			i:step(); -- B
+			i:step(); -- C
+			i:step(); -- D
+			assert.has_errors(function ()
+				i:step();
+			end, "Invalid state transition: D cannot step");
+		end);
+	end);
+
+	it("handles supports wildcard transitions", function ()
+		local expected_transitions = {
+			{ name = nil   , from = "none", to = "A" };
+			{ name = "step", from = "A", to = "B" };
+			{ name = "step", from = "B", to = "C" };
+			{ name = "reset", from = "C", to = "A" };
+			{ name = "step", from = "A", to = "B" };
+			{ name = "step", from = "B", to = "C" };
+			{ name = "step", from = "C", to = "D" };
+		};
+
+		test_machine({
+			default_state = "none";
+			transitions = {
+				{ name = "step", from = "A", to = "B" };
+				{ name = "step", from = "B", to = "C" };
+				{ name = "step", from = "C", to = "D" };
+				{ name = "reset", from = "*", to = "A" };
+			};
+		}, expected_transitions, function (fsm)
+			local i = fsm:init("A");
+			i:step(); -- B
+			i:step(); -- C
+			i:reset(); -- A
+			i:step(); -- B
+			i:step(); -- C
+			i:step(); -- D
+			assert.has_errors(function ()
+				i:step();
+			end, "Invalid state transition: D cannot step");
+		end);
+	end);
+
+	it("supports specifying multiple from states", function ()
+		local expected_transitions = {
+			{ name = nil   , from = "none", to = "A" };
+			{ name = "step", from = "A", to = "B" };
+			{ name = "step", from = "B", to = "C" };
+			{ name = "reset", from = "C", to = "A" };
+			{ name = "step", from = "A", to = "B" };
+			{ name = "step", from = "B", to = "C" };
+			{ name = "step", from = "C", to = "D" };
+		};
+
+		test_machine({
+			default_state = "none";
+			transitions = {
+				{ name = "step", from = "A", to = "B" };
+				{ name = "step", from = "B", to = "C" };
+				{ name = "step", from = "C", to = "D" };
+				{ name = "reset", from = {"B", "C", "D"}, to = "A" };
+			};
+		}, expected_transitions, function (fsm)
+			local i = fsm:init("A");
+			i:step(); -- B
+			i:step(); -- C
+			i:reset(); -- A
+			assert.has_errors(function ()
+				i:reset();
+			end, "Invalid state transition: A cannot reset");
+			i:step(); -- B
+			i:step(); -- C
+			i:step(); -- D
+			assert.has_errors(function ()
+				i:step();
+			end, "Invalid state transition: D cannot step");
+		end);
+	end);
+
+	it("handles transitions with the same start/end state", function ()
+		local expected_transitions = {
+			{ name = nil   , from = "none", to = "A" };
+			{ name = "step", from = "A", to = "B" };
+			{ name = "step", from = "B", to = "B" };
+			{ name = "step", from = "B", to = "B" };
+		};
+
+		test_machine({
+			default_state = "none";
+			transitions = {
+				{ name = "step", from = "A", to = "B" };
+				{ name = "step", from = "B", to = "B" };
+			};
+		}, expected_transitions, function (fsm)
+			local i = fsm:init("A");
+			i:step(); -- B
+			i:step(); -- B
+			i:step(); -- B
+		end);
+	end);
+
+	it("can identify instances of a specific fsm", function ()
+		local fsm1 = new_fsm({ default_state = "a" });
+		local fsm2 = new_fsm({ default_state = "a" });
+
+		local i1 = fsm1:init();
+		local i2 = fsm2:init();
+
+		assert.truthy(fsm1:is_instance(i1));
+		assert.truthy(fsm2:is_instance(i2));
+
+		assert.falsy(fsm1:is_instance(i2));
+		assert.falsy(fsm2:is_instance(i1));
+	end);
+
+	it("errors when an invalid initial state is passed", function ()
+		local fsm1 = new_fsm({
+			transitions = {
+				{ name = "", from = "A", to = "B" };
+			};
+		});
+
+		assert.has_no_errors(function ()
+			fsm1:init("A");
+		end);
+
+		assert.has_errors(function ()
+			fsm1:init("C");
+		end);
+	end);
+end);