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+;;  Copyright (c) Jeffrey Straszheim. All rights reserved.  The use and
+;;  distribution terms for this software are covered by the Eclipse Public
+;;  License 1.0 (http://opensource.org/licenses/eclipse-1.0.php) which can
+;;  be found in the file epl-v10.html at the root of this distribution.  By
+;;  using this software in any fashion, you are agreeing to be bound by the
+;;  terms of this license.  You must not remove this notice, or any other,
+;;  from this software.
+;;
+;;  util.clj
+;;
+;;  A Clojure implementation of Datalog -- Utilities
+;;
+;;  straszheimjeffrey (gmail)
+;;  Created 3 Feburary 2009
+
+
+(ns clojure.contrib.datalog.util
+  (:use [clojure.contrib.seq-utils :only (separate)]))
+
+
+
+;;; Bindings and logic vars.  A binding in a hash of logic vars to
+;;; bound values.  Logic vars are any symbol prefixed with a \?.
+
+(defn is-var?
+  "Is this a logic variable: e.g. a symbol prefixed with a ?"
+  [sym]
+  (when (symbol? sym)
+    (let [name (name sym)]
+      (and (= \? (first name))
+           (not= \? (fnext name))))))
+
+(defn is-query-var?
+  "Is this a query variable: e.g. a symbol prefixed with ??"
+  [sym]
+  (when (symbol? sym)
+    (let [name (name sym)]
+      (and (= \? (first name))
+           (= \? (fnext name))))))
+
+(defn map-values
+  "Like map, but works over the values of a hash map"
+  [f hash]
+  (let [key-vals (map (fn [[key val]] [key (f val)]) hash)]
+    (if (seq key-vals)
+      (apply conj (empty hash) key-vals)
+      hash)))
+
+(defn keys-to-vals
+  "Given a map and a collection of keys, return the collection of vals"
+  [m ks]
+  (vals (select-keys m ks)))
+
+(defn reverse-map
+  "Reverse the keys/values of a map"
+  [m]
+  (into {} (map (fn [[k v]] [v k]) m)))
+
+
+;;; Preduce -- A parallel reduce over hashes
+  
+(defn preduce
+  "Similar to merge-with, but the contents of each key are merged in
+   parallel using f.
+
+   f - a function of 2 arguments.
+   data - a collection of hashes."
+  [f data]
+  (let [data-1 (map (fn [h] (map-values #(list %) h)) data)
+        merged (doall (apply merge-with concat data-1))
+        ; Groups w/ multiple elements are identified for parallel processing
+        [complex simple] (separate (fn [[key vals]] (> (count vals) 1)) merged)
+        fold-group (fn [[key vals]] {key (reduce f vals)})
+        fix-single (fn [[key [val]]] [key val])]
+    (apply merge (concat (pmap fold-group merged) (map fix-single simple)))))
+  
+
+;;; Debuging and Tracing
+
+(def *trace-datalog* nil)
+
+(defmacro trace-datalog
+  "If *test-datalog* is set to true, run the enclosed commands"
+  [& body]
+  `(when *trace-datalog*
+     ~@body))
+
+ 	
+;; End of file