diff -r e66bd5c563eb -r b5b5583a3a08 Attic/re3.scala
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/Attic/re3.scala	Thu Jul 30 13:50:54 2020 +0100
@@ -0,0 +1,127 @@
+// A version with simplification of derivatives;
+// this keeps the regular expressions small, which
+// is good for the run-time
+ 
+
+abstract class Rexp
+case object ZERO extends Rexp
+case object ONE extends Rexp
+case class CHAR(c: Char) extends Rexp
+case class ALT(r1: Rexp, r2: Rexp) extends Rexp 
+case class SEQ(r1: Rexp, r2: Rexp) extends Rexp 
+case class STAR(r: Rexp) extends Rexp 
+case class NTIMES(r: Rexp, n: Int) extends Rexp 
+
+
+
+// the nullable function: tests whether the regular 
+// expression can recognise the empty string
+def nullable (r: Rexp) : Boolean = r match {
+  case ZERO => false
+  case ONE => true
+  case CHAR(_) => false
+  case ALT(r1, r2) => nullable(r1) || nullable(r2)
+  case SEQ(r1, r2) => nullable(r1) && nullable(r2)
+  case STAR(_) => true
+  case NTIMES(r, i) => if (i == 0) true else nullable(r)
+}
+
+// the derivative of a regular expression w.r.t. a character
+def der (c: Char, r: Rexp) : Rexp = r match {
+  case ZERO => ZERO
+  case ONE => ZERO
+  case CHAR(d) => if (c == d) ONE else ZERO
+  case ALT(r1, r2) => ALT(der(c, r1), der(c, r2))
+  case SEQ(r1, r2) => 
+    if (nullable(r1)) ALT(SEQ(der(c, r1), r2), der(c, r2))
+    else SEQ(der(c, r1), r2)
+  case STAR(r1) => SEQ(der(c, r1), STAR(r1))
+  case NTIMES(r, i) => 
+    if (i == 0) ZERO else SEQ(der(c, r), NTIMES(r, i - 1))
+}
+
+def simp(r: Rexp) : Rexp = r match {
+  case ALT(r1, r2) => (simp(r1), simp(r2)) match {
+    case (ZERO, r2s) => r2s
+    case (r1s, ZERO) => r1s
+    case (r1s, r2s) => if (r1s == r2s) r1s else ALT (r1s, r2s)
+  }
+  case SEQ(r1, r2) =>  (simp(r1), simp(r2)) match {
+    case (ZERO, _) => ZERO
+    case (_, ZERO) => ZERO
+    case (ONE, r2s) => r2s
+    case (r1s, ONE) => r1s
+    case (r1s, r2s) => SEQ(r1s, r2s)
+  }
+  case r => r
+}
+
+
+// the derivative w.r.t. a string (iterates der)
+def ders(s: List[Char], r: Rexp) : Rexp = s match {
+  case Nil => r
+  case c::s => ders(s, simp(der(c, r)))
+}
+
+
+// the main matcher function
+def matcher(r: Rexp, s: String) : Boolean = 
+  nullable(ders(s.toList, r))
+
+
+// one or zero
+def OPT(r: Rexp) = ALT(r, ONE)
+
+
+// Test Cases
+
+// evil regular expressions: (a?){n} a{n}  and (a*)* b
+def EVIL1(n: Int) = SEQ(NTIMES(OPT(CHAR('a')), n), NTIMES(CHAR('a'), n))
+val EVIL2 = SEQ(STAR(STAR(CHAR('a'))), CHAR('b'))
+
+
+def time_needed[T](i: Int, code: => T) = {
+  val start = System.nanoTime()
+  for (j <- 1 to i) code
+  val end = System.nanoTime()
+  (end - start)/(i * 1.0e9)
+}
+
+
+//test: (a?{n}) (a{n})
+for (i <- 0 to 8000 by 1000) {
+  println(f"$i: ${time_needed(3, matcher(EVIL1(i), "a" * i))}%.5f")
+}
+
+//test: (a*)* b
+for (i <- 0 to 6000000 by 500000) {
+  println(f"$i: ${time_needed(3, matcher(EVIL2, "a" * i))}%.5f")
+}
+
+
+// size of a regular expressions - for testing purposes 
+def size(r: Rexp) : Int = r match {
+  case ZERO => 1
+  case ONE => 1
+  case CHAR(_) => 1
+  case ALT(r1, r2) => 1 + size(r1) + size(r2)
+  case SEQ(r1, r2) => 1 + size(r1) + size(r2)
+  case STAR(r) => 1 + size(r)
+  case NTIMES(r, _) => 1 + size(r)
+}
+
+
+// now the size of the derivatives grows 
+// much, much slower
+
+size(ders("".toList, EVIL2))      // 5
+size(ders("a".toList, EVIL2))     // 8
+size(ders("aa".toList, EVIL2))    // 8
+size(ders("aaa".toList, EVIL2))   // 8
+size(ders("aaaa".toList, EVIL2))  // 8
+size(ders("aaaaa".toList, EVIL2)) // 8
+
+
+
+
+