ClojureBindings: implement all accessor methods

Previously, they threw UnsupportedOperationException due to the
fact that it is tricky to access the list of available user keys.

But it _can_ be done, so let's do it!

This makes Clojure work much better in the SciJava REPL, which needs
the ability to iterate through the variables of the engine bindings.

Author: ctrueden

src/main/java/org/scijava/plugins/scripting/clojure/ClojureBindings.java

@@ -32,11 +32,14 @@
 package org.scijava.plugins.scripting.clojure;
 
 import java.util.Collection;
+import java.util.HashMap;
 import java.util.Map;
 import java.util.Set;
 
 import javax.script.Bindings;
 
+import clojure.lang.MapEntry;
+import clojure.lang.Namespace;
 import clojure.lang.RT;
 import clojure.lang.Symbol;
 import clojure.lang.Var;
@@ -75,7 +78,7 @@ public boolean containsKey(final Object key) {
 
 	@Override
 	public boolean containsValue(final Object value) {
-		throw new UnsupportedOperationException();
+		return map().containsValue(value);
 	}
 
 	@Override
@@ -147,17 +150,33 @@ public void clear() {
 
 	@Override
 	public Set<String> keySet() {
-		throw new UnsupportedOperationException();
+		return map().keySet();
 	}
 
 	@Override
 	public Collection<Object> values() {
-		throw new UnsupportedOperationException();
+		return map().values();
 	}
 
 	@Override
 	public Set<Entry<String, Object>> entrySet() {
-		throw new UnsupportedOperationException();
+		return map().entrySet();
 	}
 
+	// -- Helper methods --
+
+	private static Map<String, Object> map() {
+		final Map<String, Object> map = new HashMap<String, Object>();
+
+		final Namespace ns = Namespace.find(Symbol.intern(null, USER_NS));
+		for (final Object el : ns.getMappings()) {
+			final MapEntry entry = (MapEntry) el;
+			final Symbol key = (Symbol) entry.key();
+			final Object value = Var.intern(ns, key).get();
+			if (value instanceof Var.Unbound) continue; // skip weird variables
+			map.put(key.getName(), value);
+		}
+
+		return map;
+	}
 }