Add KeyFilter class

src/main/java/de/vanitasvitae/crypto/pgpainless/KeyFilter.java

@@ -0,0 +1,44 @@
+package de.vanitasvitae.crypto.pgpainless;
+
+import java.util.HashSet;
+import java.util.Iterator;
+import java.util.Set;
+
+import org.bouncycastle.openpgp.PGPPublicKey;
+import org.bouncycastle.openpgp.PGPPublicKeyRing;
+import org.bouncycastle.openpgp.PGPPublicKeyRingCollection;
+
+public abstract class KeyFilter {
+
+    public Set<Long> filter(Set<Long> initialIdSet, PGPPublicKeyRingCollection collection) {
+        Set<PGPPublicKeyRing> rings = new HashSet<>();
+        for (Iterator<PGPPublicKeyRing> i = collection.getKeyRings(); i.hasNext();) {
+            rings.add(i.next());
+        }
+        return filter(initialIdSet, rings, false);
+    }
+
+    public Set<Long> filter(Set<Long> initialIdSet, Set<PGPPublicKeyRing> rings, boolean boolToAvoidSameMethodErasure) {
+        Set<PGPPublicKey> keys = new HashSet<>();
+        for (PGPPublicKeyRing ring : rings) {
+            for (Iterator<PGPPublicKey> i = ring.getPublicKeys(); i.hasNext();) {
+                keys.add(i.next());
+            }
+        }
+        return filter(initialIdSet, keys);
+    }
+
+    public Set<Long> filter(Set<Long> initialIdSet, Set<PGPPublicKey> keys) {
+        Set<Long> filteredIds = new HashSet<>();
+        for (Long id : initialIdSet) {
+            for (PGPPublicKey key : keys) {
+                if (key.getKeyID() == id && filter(key)) {
+                    filteredIds.add(id);
+                }
+            }
+        }
+        return filteredIds;
+    }
+
+    public abstract boolean filter(PGPPublicKey key);
+}