Address wagon aliasing issues

As it turns out, not fully testing new features is not necessarily a
good idea ...

This patch follows up 1F616EMO's patch by
* Making get_wagon_prototype return the resolved alias,
* Handling recursive wagon alises (in particular, loops), and
* Adding (partial) unittest for the wagon aliasing system. [v2]: The
testcases are complemented a bit more to cover situations where the
alias resolution system should return nil.

[v2]: This patch should hopefully also warn about not spawning wagons.
Note that this only warns about the missing wagon entity and does _not_
actually fix the issue.

How to test:
* In a world with both advtrains_train_subway and advtrains_train_japan
  enabled, place a subway wagon, a Japanese engine, and a regular
  Japanese wagon.
* Add the test mod to the world; do NOT remove advtrains_train_japan.
* Restart the world. Notice that the Japanese wagons still appear as
  Japanese wagons despite being aliased to subway wagons.
* Restart the world without the advtrains_train_japan mod. Notice that
  the engine appears as the subway wagon while the regular Japanese
  wagon appears as the wagon placeholder. [v2]: Also note that the
  warning message about the missing wagon prototype still mentions the
  regular Japanese wagon.
* Restart the world again with the advtrains_train_japan mod. Notice
  that both type of Japanese wagons reappear as Japanese wagons.
* Observe that unittests work.

Test mod:
advtrains.register_wagon_alias("advtrains:engine_japan", "advtrains:subway_wagon")
advtrains.register_wagon_alias("advtrains:wagon_japan", "advtrains:wagon_japan")

1 files changed, 40 insertions, 0 deletions

diff --git a/advtrains/spec/wagons_spec.lua b/advtrains/spec/wagons_spec.lua
new file mode 100644
index 0000000..df0687b
--- /dev/null
+++ b/advtrains/spec/wagons_spec.lua
@@ -0,0 +1,40 @@
+require "mineunit"
+mineunit "core"
+
+_G.advtrains = {
+	wagon_load_range = 32
+}
+sourcefile "wagons"
+
+local myproto = {_test = true}
+advtrains.register_wagon(":mywagon", myproto, "My wagon", "", false)
+advtrains.register_wagon_alias(":myalias", ":mywagon")
+advtrains.register_wagon_alias(":myotheralias", ":myalias")
+
+local myotherproto = {_other = true}
+advtrains.register_wagon(":noalias", myotherproto, "Not aliased wagon", "", false)
+advtrains.register_wagon_alias(":noalias", ":mywagon")
+
+advtrains.register_wagon_alias(":nilalias", ":nil")
+
+advtrains.register_wagon_alias(":R1", ":R2")
+advtrains.register_wagon_alias(":R2", ":R3")
+advtrains.register_wagon_alias(":R3", ":R1")
+
+describe("wagon alias system", function()
+	it("should work", function()
+		assert.same({":mywagon", myproto}, {advtrains.resolve_wagon_alias(":myalias")})
+		assert.equal(myproto, advtrains.wagon_prototypes[":myalias"])
+		assert.same({":mywagon", myproto}, {advtrains.resolve_wagon_alias(":myotheralias")})
+	end)
+	it("should respect wagon registration", function()
+		assert.same({":noalias", myotherproto}, {advtrains.resolve_wagon_alias(":noalias")})
+	end)
+	it("should handle recursive loops", function()
+		assert.same({}, {advtrains.resolve_wagon_alias(":R1")})
+	end)
+	it("should return nil for missing entries", function()
+		assert.same({}, {advtrains.resolve_wagon_alias(":what")})
+		assert.same({}, {advtrains.resolve_wagon_alias(":nilalias")})
+	end)
+end)