feat: update structure

cs2030s/labs/PE2/plab0124/Streaming.java

@@ -0,0 +1,43 @@
+import java.util.ArrayList;
+import java.util.List;
+import java.util.stream.Collectors;
+import java.util.stream.Stream;
+
+/**
+ * CS2030S PE2 
+ * AY2022/23 Semester 1.
+ * 
+ * @author A0253252M
+ **/
+
+public class Streaming {
+  public static Stream<Integer> evenTriangular(int n) {
+    return Stream.iterate(1, i -> i + 1)
+      .map(i -> i * (i + 1) / 2)
+      .filter(i -> i % 2 == 0)
+      .limit(n);
+  }
+  
+  // return a list of positive divisors for n
+  // go down from n, 
+  public static boolean isPerfect(int n) {
+    return Stream.iterate(n - 1, i -> i - 1)
+      .takeWhile(i -> i > 0)
+      .filter(i -> n % i == 0)
+      .reduce(0, (x, y) -> x + y) == n;
+  }
+
+  public static List<Integer> allPerfect(int start, int end) {
+    return Stream.iterate(start, i -> i + 1)
+      .takeWhile(i -> i <= end)
+      .filter(i -> isPerfect(i))
+      .collect(Collectors.toList());
+  }
+  
+  public static <T> List<T> decode(Stream<Pair<Integer, T>> stream) {
+    return stream.flatMap(p -> 
+        Stream.generate(() -> p.getSnd()).limit(p.getFst())
+        )
+      .collect(Collectors.toList());
+  }
+}