Move referrer mapping to a dedicated class

src/org/adblockplus/android/ReferrerMapping.java

Index: src/org/adblockplus/android/ReferrerMapping.java
===================================================================
new file mode 100644
--- /dev/null
+++ b/src/org/adblockplus/android/ReferrerMapping.java
@@ -0,0 +1,47 @@
+package org.adblockplus.android;
+
+import java.util.ArrayList;
+import java.util.LinkedHashMap;
+import java.util.List;
+import java.util.Map;
+
+public class ReferrerMapping
+{
+  private static class MappingCache extends LinkedHashMap<String, String>
+  {
+    private static final long serialVersionUID = 1L;
+    private static final int MAX_SIZE = 5000;
+
+    public MappingCache()
+    {
+      super(MAX_SIZE + 1, 0.75f, true);
+    }
+
+    @Override
+    protected boolean removeEldestEntry(final Map.Entry<String, String> eldest)
+    {
+      return size() > MAX_SIZE;
+    }
+  };
+
+  private final MappingCache mappingCache = new MappingCache();
+
+  public void add(String fullUrl, String referrer)
+  {
+    mappingCache.put(fullUrl, referrer);
+  }
+
+  public List<String> buildReferrerChain(String url)
+  {
+    final List<String> referrerChain = new ArrayList<String>();
+    // We need to limit the chain length to ensure we don't block indefinitely
+    // if there's a referrer loop.
+    final int maxChainLength = 10;
+    for (int i = 0; i < maxChainLength && url != null; i++)
+    {
+      referrerChain.add(0, url);
+      url = mappingCache.get(url);
+    }
+    return referrerChain;
+  }
+}