--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/progs/crawler.scala	Tue Nov 19 06:38:20 2019 +0000
@@ -0,0 +1,65 @@
+// A Web-Crawler
+//================
+
+// call parallel version with
+//
+// scala -cp scala-parallel-collections_2.13-0.2.0.jar crawler.scala 
+
+
+import io.Source
+import scala.util._
+import scala.collection.parallel.CollectionConverters._
+
+// the idea is to look for links using the
+// regular expression "https?://[^"]*" and for
+// email addresses using yet another regex.
+
+
+// gets the first 10K of a web-page
+def get_page(url: String) : String = {
+  Try(Source.fromURL(url)("ISO-8859-1").take(10000).mkString).
+    getOrElse { println(s" Problem with: $url"); ""}
+}
+
+// regex for URLs and emails
+val http_pattern = """"https?://[^"]*"""".r
+val email_pattern = """([a-z0-9_\.-]+)@([\da-z\.-]+)\.([a-z\.]{2,6})""".r
+
+//  val s = "foo bla christian@kcl.ac.uk 1234567"
+//  email_pattern.findAllIn(s).toList
+
+// drops the first and last character from a string
+def unquote(s: String) = s.drop(1).dropRight(1)
+
+def get_all_URLs(page: String): Set[String] = 
+  http_pattern.findAllIn(page).map(unquote).toSet
+
+// a naive version of crawl - searches until a given depth,
+// visits pages potentially more than once
+def crawl(url: String, n: Int) : Unit = {
+  if (n == 0) ()
+  else {
+    println(s"  Visiting: $n $url")
+    val page = get_page(url)
+    for (u <- get_all_URLs(get_page(url)).par) crawl(u, n - 1)
+  }
+}
+
+// some starting URLs for the crawler
+val startURL = """https://nms.kcl.ac.uk/christian.urban/"""
+
+//crawl(startURL, 2)
+
+// a primitive email harvester
+def emails(url: String, n: Int) : Set[String] = {
+  if (n == 0) Set()
+  else {
+    println(s"  Visiting: $n $url")
+    val page = get_page(url)
+    val new_emails = email_pattern.findAllIn(page).toSet
+    new_emails ++ (for (u <- get_all_URLs(page).par) yield emails(u, n - 1)).flatten
+  }
+}
+
+println(emails(startURL, 3))
+