From 41f81501498871045e6f0ce8d91b9d1dc9ec13a7 Mon Sep 17 00:00:00 2001
From: "Adam D. Ruppe" <destructionator@gmail.com>
Date: Mon, 13 Apr 2020 13:41:37 -0400
Subject: [PATCH] lang helper utility

---
 mvd.d | 93 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 93 insertions(+)
 create mode 100644 mvd.d

diff --git a/mvd.d b/mvd.d
new file mode 100644
index 0000000..b72d8f4
--- /dev/null
+++ b/mvd.d
@@ -0,0 +1,93 @@
+/++
+	mvd stands for Multiple Virtual Dispatch. It lets you
+	write functions that take any number of arguments of
+	objects and match based on the dynamic type of each
+	of them.
+
+	---
+	void foo(Object a, Object b) {} // 1
+	void foo(MyClass b, Object b) {} // 2
+	void foo(DerivedClass a, MyClass b) {} // 3
+
+	Object a = new MyClass();
+	Object b = new Object();
+
+	mvd!foo(a, b); // will call overload #2
+	---
+
+	The return values must be compatible; [mvd] will return
+	the least specialized static type of the return values
+	(most likely the shared base class type of all return types,
+	or `void` if there isn't one).
+
+	All non-class/interface types should be compatible among overloads.
+	Otherwise you are liable to get compile errors. (Or it might work,
+	that's up to the compiler's discretion.)
++/
+module arsd.mvd;
+
+import std.traits;
+
+/// This exists just to make the documentation of [mvd] nicer looking.
+alias CommonReturnOfOverloads(alias fn) = CommonType!(staticMap!(ReturnType, __traits(getOverloads, __traits(parent, fn), __traits(identifier, fn))));
+
+/// See details on the [arsd.mvd] page.
+CommonReturnOfOverloads!fn mvd(alias fn, T...)(T args) {
+	typeof(return) delegate() bestMatch;
+	int bestScore;
+
+	ov: foreach(overload; __traits(getOverloads, __traits(parent, fn), __traits(identifier, fn))) {
+		Parameters!overload pargs;
+		int score = 0;
+		foreach(idx, parg; pargs) {
+			alias t = typeof(parg);
+			static if(is(t == interface) || is(t == class)) {
+				pargs[idx] = cast(typeof(parg)) args[idx];
+				if(args[idx] !is null && pargs[idx] is null)
+					continue ov; // failed cast, forget it
+				else
+					score += BaseClassesTuple!t.length + 1;
+			} else
+				pargs[idx] = args[idx];
+		}
+		if(score == bestScore)
+			throw new Exception("ambiguous overload selection with args"); // FIXME: show the things
+		if(score > bestScore) {
+			bestMatch = () {
+				static if(is(typeof(return) == void))
+					overload(pargs);
+				else
+					return overload(pargs);
+			};
+			bestScore = score;
+		}
+	}
+
+	if(bestMatch is null)
+		throw new Exception("no match existed");
+
+	return bestMatch();
+}
+
+///
+unittest {
+
+	class MyClass {}
+	class DerivedClass : MyClass {}
+	class OtherClass {}
+
+	static struct Wrapper {
+		static: // this is just a namespace cuz D doesn't allow overloading inside unittest
+		int foo(Object a, Object b) { return 1; }
+		int foo(MyClass a, Object b) { return 2; }
+		int foo(DerivedClass a, MyClass b) { return 3; }
+	}
+
+	with(Wrapper) {
+		assert(mvd!foo(new Object, new Object) == 1);
+		assert(mvd!foo(new MyClass, new DerivedClass) == 2);
+		assert(mvd!foo(new DerivedClass, new DerivedClass) == 3);
+		assert(mvd!foo(new OtherClass, new OtherClass) == 1);
+		assert(mvd!foo(new OtherClass, new MyClass) == 1);
+	}
+}