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Projekte/common/src/main/java/pp/util/RandomPositionIterator.java

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+////////////////////////////////////////
+// Programming project code
+// UniBw M, 2022, 2023, 2024
+// www.unibw.de/inf2
+// (c) Mark Minas (mark.minas@unibw.de)
+////////////////////////////////////////
+
+package pp.util;
+
+import java.util.HashMap;
+import java.util.Iterator;
+import java.util.Map;
+import java.util.NoSuchElementException;
+import java.util.Random;
+
+/**
+ * An iterator that creates a random permutation of positions (x, y) where x and y are integer values
+ * in the range {0, ..., width-1} x {0, ..., height-1}. All permutations are uniformly distributed
+ * using the Fisher–Yates shuffle (also known as Knuth shuffle).
+ *
+ * @param <T> the type of elements returned by this iterator
+ */
+public class RandomPositionIterator<T> implements Iterator<T> {
+    private final Random random = new Random();
+    private final int height;
+    private final Map<Integer, T> movedMap = new HashMap<>();
+    private int remaining;
+    private final Creator<T> creator;
+
+    /**
+     * Functional interface to create instances of type T.
+     *
+     * @param <T> the type of elements created by this creator
+     */
+    public interface Creator<T> {
+        T create(int x, int y);
+    }
+
+    /**
+     * Creates a RandomPositionIterator for Position instances with float coordinates.
+     *
+     * @param width  the width of the rectangle
+     * @param height the height of the rectangle
+     * @return a RandomPositionIterator for Position instances
+     */
+    public static RandomPositionIterator<Position> floatPoints(int width, int height) {
+        return new RandomPositionIterator<>(FloatPoint::new, width, height);
+    }
+
+    /**
+     * Creates a new permutation iterator generating a random permutation of positions (x, y)
+     * where x and y are integer values in the range {0, ..., width-1} x {0, ..., height-1}.
+     *
+     * @param creator the creator to create instances of type T
+     * @param width   the width of the rectangle
+     * @param height  the height of the rectangle
+     */
+    public RandomPositionIterator(Creator<T> creator, int width, int height) {
+        this.height = height;
+        this.remaining = width * height;
+        this.creator = creator;
+    }
+
+    @Override
+    public boolean hasNext() {
+        return remaining > 0;
+    }
+
+    @Override
+    public T next() {
+        if (hasNext()) {
+            final int idx = random.nextInt(remaining--); // note that remaining is decremented
+            final T result = getWhere(idx);
+            if (idx < remaining)
+                movedMap.put(idx, getWhere(remaining));
+            movedMap.remove(remaining);
+            return result;
+        }
+        throw new NoSuchElementException();
+    }
+
+    private T getWhere(int idx) {
+        final T movedWhere = movedMap.get(idx);
+        if (movedWhere != null)
+            return movedWhere;
+        final int x = idx / height;
+        final int y = idx % height;
+        return creator.create(x, y);
+    }
+}