Speed up line/column in OffsetPosition

Building the index again and again for every OffsetPosition instance makes
very little sense.  So this makes it build it only once for a given source.

I'm not sure if this is the best way to implement it and I'm afraid that a
global synchronized map may be a performance bottleneck once the number of
processors goes into the hundreds, but I know very little Scala/Java to do
this properly. :-(

Fixes http://stackoverflow.com/questions/14707127/accessing-position-information-in-a-scala-combinatorparser-kills-performance

Author: liskin

src/main/scala/scala/util/parsing/input/OffsetPosition.scala

@@ -23,6 +23,16 @@ case class OffsetPosition(source: java.lang.CharSequence, offset: Int) extends P
 
   /** An index that contains all line starts, including first line, and eof. */
   private lazy val index: Array[Int] = {
+    Option(OffsetPosition.indexCache.get(source)) match {
+      case Some(index) => index
+      case None =>
+        val index = genIndex
+        OffsetPosition.indexCache.put(source, index)
+        index
+    }
+  }
+
+  private def genIndex: Array[Int] = {
     val lineStarts = new ArrayBuffer[Int]
     lineStarts += 0
     for (i <- 0 until source.length)
@@ -71,3 +81,12 @@ case class OffsetPosition(source: java.lang.CharSequence, offset: Int) extends P
       this.line == that.line && this.column < that.column
   }
 }
+
+/** An object holding the index cache.
+ *
+ * @author Tomáš Janoušek
+ */
+object OffsetPosition {
+  private lazy val indexCache = java.util.Collections.synchronizedMap(
+    new java.util.WeakHashMap[java.lang.CharSequence, Array[Int]])
+}