Add RotatingCalipers algorithm and tests

Author: ChavanSuhani

.github/RotatingCalipers.java

@@ -0,0 +1,238 @@
+package com.thealgorithms.geometry;
+
+import java.util.ArrayList;
+import java.util.Comparator;
+import java.util.List;
+
+/**
+ * RotatingCalipers - utility class for convex polygon computations:
+ * - diameter (farthest pair)
+ * - width (minimum distance between two parallel supporting lines)
+ * - minimum-area bounding rectangle (simple implementation)
+ *
+ * Note: This implementation computes the convex hull (monotonic chain).
+ * The min-area rectangle implementation below uses a simple edge-based projection
+ * approach (O(n^2) worst-case) for clarity and correctness; it can be optimized
+ * to the classic O(n) rotating-calipers minimum rectangle later.
+ *
+ * All methods are static. No instances.
+ */
+public final class RotatingCalipers {
+
+    private RotatingCalipers() {
+        throw new UnsupportedOperationException("Utility class");
+    }
+
+    /* ---------- Simple geometry primitives (replace with repo types if present) ---------- */
+
+    public static final class Point {
+        public final double x;
+        public final double y;
+
+        public Point(double x, double y) {
+            this.x = x;
+            this.y = y;
+        }
+    }
+
+    public static final class PointPair {
+        public final Point a;
+        public final Point b;
+
+        public PointPair(Point a, Point b) {
+            this.a = a;
+            this.b = b;
+        }
+
+        public double distance() {
+            double dx = a.x - b.x;
+            double dy = a.y - b.y;
+            return Math.hypot(dx, dy);
+        }
+    }
+
+    public static final class Rectangle {
+        // center point, width along angle, height perpendicular, rotation angle in radians
+        public final Point center;
+        public final double width;
+        public final double height;
+        public final double angle;
+
+        public Rectangle(Point center, double width, double height, double angle) {
+            this.center = center;
+            this.width = width;
+            this.height = height;
+            this.angle = angle;
+        }
+    }
+
+    /* ---------- Helpers ---------- */
+
+    private static double cross(Point o, Point a, Point b) {
+        return (a.x - o.x) * (b.y - o.y) - (a.y - o.y) * (b.x - o.x);
+    }
+
+    private static double dist2(Point a, Point b) {
+        double dx = a.x - b.x;
+        double dy = a.y - b.y;
+        return dx * dx + dy * dy;
+    }
+
+    /**
+     * Monotonic chain convex hull. Returns hull in CCW order (no duplicate final vertex).
+     * If input has <= 1 point, returns a copy.
+     */
+    public static List<Point> convexHull(List<Point> pts) {
+        List<Point> p = new ArrayList<>(pts);
+        p.sort(Comparator.comparingDouble((Point q) -> q.x).thenComparingDouble(q -> q.y));
+        int n = p.size();
+        if (n <= 1) {
+            return new ArrayList<>(p);
+        }
+        List<Point> lower = new ArrayList<>();
+        for (Point pt : p) {
+            while (lower.size() >= 2 && cross(lower.get(lower.size() - 2), lower.get(lower.size() - 1), pt) <= 0) {
+                lower.remove(lower.size() - 1);
+            }
+            lower.add(pt);
+        }
+        List<Point> upper = new ArrayList<>();
+        for (int i = n - 1; i >= 0; --i) {
+            Point pt = p.get(i);
+            while (upper.size() >= 2 && cross(upper.get(upper.size() - 2), upper.get(upper.size() - 1), pt) <= 0) {
+                upper.remove(upper.size() - 1);
+            }
+            upper.add(pt);
+        }
+        // Concatenate without duplicating end points
+        lower.remove(lower.size() - 1);
+        upper.remove(upper.size() - 1);
+        lower.addAll(upper);
+        return lower;
+    }
+
+    /**
+     * Diameter - farthest pair of points. If given arbitrary points, hull is computed.
+     *
+     * Complexity: O(n) on hull size after hull computation.
+     */
+    public static PointPair diameter(List<Point> points) {
+        List<Point> ch = convexHull(points);
+        int n = ch.size();
+        if (n == 0) return new PointPair(null, null);
+        if (n == 1) return new PointPair(ch.get(0), ch.get(0));
+        if (n == 2) return new PointPair(ch.get(0), ch.get(1));
+
+        int j = 1;
+        double best = 0;
+        Point bestA = ch.get(0);
+        Point bestB = ch.get(0);
+        for (int i = 0; i < n; ++i) {
+            int ni = (i + 1) % n;
+            while (Math.abs(cross(ch.get(i), ch.get(ni), ch.get((j + 1) % n)))
+                    > Math.abs(cross(ch.get(i), ch.get(ni), ch.get(j)))) {
+                j = (j + 1) % n;
+            }
+            double d2 = dist2(ch.get(i), ch.get(j));
+            if (d2 > best) {
+                best = d2;
+                bestA = ch.get(i);
+                bestB = ch.get(j);
+            }
+            d2 = dist2(ch.get(ni), ch.get(j));
+            if (d2 > best) {
+                best = d2;
+                bestA = ch.get(ni);
+                bestB = ch.get(j);
+            }
+        }
+        return new PointPair(bestA, bestB);
+    }
+
+    /**
+     * Width - minimal distance between two parallel supporting lines of the convex polygon.
+     *
+     * Complexity: O(n) on hull size after hull computation.
+     */
+    public static double width(List<Point> points) {
+        List<Point> ch = convexHull(points);
+        int n = ch.size();
+        if (n <= 1) return 0.0;
+        if (n == 2) {
+            return Math.hypot(ch.get(1).x - ch.get(0).x, ch.get(1).y - ch.get(0).y);
+        }
+
+        int j = 1;
+        double minWidth = Double.POSITIVE_INFINITY;
+        for (int i = 0; i < n; ++i) {
+            int ni = (i + 1) % n;
+            while (Math.abs(cross(ch.get(i), ch.get(ni), ch.get((j + 1) % n)))
+                    > Math.abs(cross(ch.get(i), ch.get(ni), ch.get(j)))) {
+                j = (j + 1) % n;
+            }
+            double distance = Math.abs(cross(ch.get(i), ch.get(ni), ch.get(j)))
+                    / Math.hypot(ch.get(ni).x - ch.get(i).x, ch.get(ni).y - ch.get(i).y);
+            minWidth = Math.min(minWidth, distance);
+        }
+        return minWidth;
+    }
+
+    /**
+     * Minimum-area bounding rectangle (simple, reliable approach).
+     * For each hull edge, rotate axes so edge is X-axis, project points, compute bounding rectangle.
+     * This implementation is easier to reason about and test. It is O(m^2) for hull size m.
+     *
+     * Returns null for empty input.
+     */
+    public static Rectangle minAreaBoundingRectangle(List<Point> points) {
+        List<Point> ch = convexHull(points);
+        int n = ch.size();
+        if (n == 0) return null;
+        if (n == 1) return new Rectangle(ch.get(0), 0.0, 0.0, 0.0);
+        if (n == 2) {
+            Point a = ch.get(0), b = ch.get(1);
+            double w = Math.hypot(b.x - a.x, b.y - a.y);
+            Point center = new Point((a.x + b.x) / 2.0, (a.y + b.y) / 2.0);
+            double angle = Math.atan2(b.y - a.y, b.x - a.x);
+            return new Rectangle(center, w, 0.0, angle);
+        }
+
+        double bestArea = Double.POSITIVE_INFINITY;
+        Rectangle bestRect = null;
+
+        for (int i = 0; i < n; ++i) {
+            Point a = ch.get(i);
+            Point b = ch.get((i + 1) % n);
+            double dx = b.x - a.x, dy = b.y - a.y;
+            double len = Math.hypot(dx, dy);
+            double ux = dx / len, uy = dy / len;  // axis along edge
+            double vx = -uy, vy = ux;             // perpendicular
+            double minU = Double.POSITIVE_INFINITY, maxU = -Double.POSITIVE_INFINITY;
+            double minV = Double.POSITIVE_INFINITY, maxV = -Double.POSITIVE_INFINITY;
+
+            for (Point p : ch) {
+                double u = (p.x - a.x) * ux + (p.y - a.y) * uy;
+                double v = (p.x - a.x) * vx + (p.y - a.y) * vy;
+                minU = Math.min(minU, u);
+                maxU = Math.max(maxU, u);
+                minV = Math.min(minV, v);
+                maxV = Math.max(maxV, v);
+            }
+
+            double width = maxU - minU;
+            double height = maxV - minV;
+            double area = width * height;
+            if (area < bestArea) {
+                bestArea = area;
+                double centerU = (minU + maxU) / 2.0;
+                double centerV = (minV + maxV) / 2.0;
+                Point center = new Point(a.x + centerU * ux + centerV * vx,
+                        a.y + centerU * uy + centerV * vy);
+                double angle = Math.atan2(uy, ux);
+                bestRect = new Rectangle(center, width, height, angle);
+            }
+        }
+        return bestRect;
+    }
+}
+