refactor: Enhance docs, code, add tests in SkylineProblem

Author: Hardvan

src/main/java/com/thealgorithms/others/SkylineProblem.java

@@ -1,17 +1,45 @@
+
 package com.thealgorithms.others;
 
 import java.util.ArrayList;
+import java.util.List;
+import java.util.Objects;
 
 /**
- * The {@code SkylineProblem} class is used to solve the skyline problem using a
- * divide-and-conquer approach.
- * It reads input for building data, processes it to find the skyline, and
- * prints the skyline.
+ * <h2>Skyline Problem</h2>
+ * <p>
+ * Solves the classic skyline problem using a divide-and-conquer approach. Given
+ * a list of buildings (each defined by left, height, right),
+ * computes the silhouette (skyline) formed by these buildings when viewed from
+ * a distance.
+ * </p>
+ * <p>
+ * Usage example:
+ *
+ * <pre>
+ * SkylineProblem sp = new SkylineProblem();
+ * sp.building = new SkylineProblem.Building[3];
+ * sp.add(1, 10, 5);
+ * sp.add(2, 15, 7);
+ * sp.add(3, 12, 9);
+ * List<SkylineProblem.Skyline> skyline = sp.findSkyline(0, 2);
+ * </pre>
+ * </p>
+ * <p>
+ * This class is not thread-safe.
+ * </p>
  */
 public class SkylineProblem {
 
-    Building[] building;
-    int count;
+    /**
+     * Array of buildings to process. Must be initialized before use.
+     */
+    public Building[] building;
+
+    /**
+     * Number of buildings added so far.
+     */
+    public int count;
 
     /**
      * Adds a building with the given left, height, and right values to the
@@ -20,101 +48,121 @@ public class SkylineProblem {
      * @param left   The left x-coordinate of the building.
      * @param height The height of the building.
      * @param right  The right x-coordinate of the building.
+     * @throws IllegalArgumentException if left >= right or height < 0
+     * @throws IllegalStateException    if building array is not initialized or is
+     *                                  full
      */
     public void add(int left, int height, int right) {
+        if (building == null) {
+            throw new IllegalStateException("Building array not initialized");
+        }
+        if (count >= building.length) {
+            throw new IllegalStateException("Building array is full");
+        }
+        if (left >= right) {
+            throw new IllegalArgumentException("Left coordinate must be less than right coordinate");
+        }
+        if (height < 0) {
+            throw new IllegalArgumentException("Height must be non-negative");
+        }
         building[count++] = new Building(left, height, right);
     }
 
     /**
      * Computes the skyline for a range of buildings using the divide-and-conquer
      * strategy.
      *
-     * @param start The starting index of the buildings to process.
-     * @param end   The ending index of the buildings to process.
+     * @param start The starting index of the buildings to process (inclusive).
+     * @param end   The ending index of the buildings to process (inclusive).
      * @return A list of {@link Skyline} objects representing the computed skyline.
+     * @throws IllegalArgumentException if indices are out of bounds or building
+     *                                  array is null
      */
-    public ArrayList<Skyline> findSkyline(int start, int end) {
+    public List<Skyline> findSkyline(int start, int end) {
+        if (building == null) {
+            throw new IllegalArgumentException("Building array is not initialized");
+        }
+        if (start < 0 || end >= count || start > end) {
+            throw new IllegalArgumentException("Invalid start or end index");
+        }
         // Base case: only one building, return its skyline.
         if (start == end) {
-            ArrayList<Skyline> list = new ArrayList<>();
+            List<Skyline> list = new ArrayList<>();
             list.add(new Skyline(building[start].left, building[start].height));
-            list.add(new Skyline(building[end].right, 0)); // Add the end of the building
+            list.add(new Skyline(building[start].right, 0)); // Add the end of the building
             return list;
         }
 
         int mid = (start + end) / 2;
-
-        ArrayList<Skyline> sky1 = this.findSkyline(start, mid); // Find the skyline of the left half
-        ArrayList<Skyline> sky2 = this.findSkyline(mid + 1, end); // Find the skyline of the right half
+        List<Skyline> sky1 = this.findSkyline(start, mid); // Find the skyline of the left half
+        List<Skyline> sky2 = this.findSkyline(mid + 1, end); // Find the skyline of the right half
         return this.mergeSkyline(sky1, sky2); // Merge the two skylines
     }
 
     /**
      * Merges two skylines (sky1 and sky2) into one combined skyline.
      *
-     * @param sky1 The first skyline list.
-     * @param sky2 The second skyline list.
+     * @param sky1 The first skyline list. Not modified.
+     * @param sky2 The second skyline list. Not modified.
      * @return A list of {@link Skyline} objects representing the merged skyline.
+     * @throws NullPointerException if either argument is null
      */
-    public ArrayList<Skyline> mergeSkyline(ArrayList<Skyline> sky1, ArrayList<Skyline> sky2) {
-        int currentH1 = 0;
-        int currentH2 = 0;
-        ArrayList<Skyline> skyline = new ArrayList<>();
-        int maxH = 0;
-
-        // Merge the two skylines
-        while (!sky1.isEmpty() && !sky2.isEmpty()) {
-            if (sky1.get(0).coordinates < sky2.get(0).coordinates) {
-                int currentX = sky1.get(0).coordinates;
-                currentH1 = sky1.get(0).height;
-
-                if (currentH1 < currentH2) {
-                    sky1.remove(0);
-                    if (maxH != currentH2) {
-                        skyline.add(new Skyline(currentX, currentH2));
-                    }
-                } else {
-                    maxH = currentH1;
-                    sky1.remove(0);
-                    skyline.add(new Skyline(currentX, currentH1));
-                }
-            } else {
-                int currentX = sky2.get(0).coordinates;
-                currentH2 = sky2.get(0).height;
-
-                if (currentH2 < currentH1) {
-                    sky2.remove(0);
-                    if (maxH != currentH1) {
-                        skyline.add(new Skyline(currentX, currentH1));
-                    }
-                } else {
-                    maxH = currentH2;
-                    sky2.remove(0);
-                    skyline.add(new Skyline(currentX, currentH2));
-                }
+    public List<Skyline> mergeSkyline(List<Skyline> sky1, List<Skyline> sky2) {
+        Objects.requireNonNull(sky1, "sky1 must not be null");
+        Objects.requireNonNull(sky2, "sky2 must not be null");
+        int i = 0, j = 0;
+        int h1 = 0, h2 = 0;
+        int prevHeight = 0;
+        List<Skyline> result = new ArrayList<>();
+        while (i < sky1.size() && j < sky2.size()) {
+            Skyline p1 = sky1.get(i);
+            Skyline p2 = sky2.get(j);
+            int x;
+            if (p1.coordinates < p2.coordinates) {
+                x = p1.coordinates;
+                h1 = p1.height;
+                i++;
+            } else if (p2.coordinates < p1.coordinates) {
+                x = p2.coordinates;
+                h2 = p2.height;
+                j++;
+            } else { // same x
+                x = p1.coordinates;
+                h1 = p1.height;
+                h2 = p2.height;
+                i++;
+                j++;
+            }
+            int maxH = Math.max(h1, h2);
+            if (result.isEmpty() || prevHeight != maxH) {
+                result.add(new Skyline(x, maxH));
+                prevHeight = maxH;
             }
         }
-
-        // Add any remaining points from sky1 or sky2
-        while (!sky1.isEmpty()) {
-            skyline.add(sky1.get(0));
-            sky1.remove(0);
+        // Append remaining points
+        while (i < sky1.size()) {
+            Skyline p = sky1.get(i++);
+            if (result.isEmpty() || result.get(result.size() - 1).height != p.height || result.get(result.size() - 1).coordinates != p.coordinates) {
+                result.add(new Skyline(p.coordinates, p.height));
+            }
         }
-
-        while (!sky2.isEmpty()) {
-            skyline.add(sky2.get(0));
-            sky2.remove(0);
+        while (j < sky2.size()) {
+            Skyline p = sky2.get(j++);
+            if (result.isEmpty() || result.get(result.size() - 1).height != p.height || result.get(result.size() - 1).coordinates != p.coordinates) {
+                result.add(new Skyline(p.coordinates, p.height));
+            }
         }
-
-        return skyline;
+        return result;
     }
 
     /**
-     * A class representing a point in the skyline with its x-coordinate and height.
+     * Represents a point in the skyline with its x-coordinate and height.
      */
-    public class Skyline {
-        public int coordinates;
-        public int height;
+    public static class Skyline {
+        /** The x-coordinate of the skyline point. */
+        public final int coordinates;
+        /** The height of the skyline at the given coordinate. */
+        public final int height;
 
         /**
          * Constructor for the {@code Skyline} class.
@@ -126,16 +174,36 @@ public Skyline(int coordinates, int height) {
             this.coordinates = coordinates;
             this.height = height;
         }
+
+        @Override
+        public boolean equals(Object o) {
+            if (this == o) return true;
+            if (o == null || getClass() != o.getClass()) return false;
+            Skyline skyline = (Skyline) o;
+            return coordinates == skyline.coordinates && height == skyline.height;
+        }
+
+        @Override
+        public int hashCode() {
+            return Objects.hash(coordinates, height);
+        }
+
+        @Override
+        public String toString() {
+            return "(" + coordinates + ", " + height + ")";
+        }
     }
 
     /**
-     * A class representing a building with its left, height, and right
-     * x-coordinates.
+     * Represents a building with its left, height, and right x-coordinates.
      */
-    public class Building {
-        public int left;
-        public int height;
-        public int right;
+    public static class Building {
+        /** The left x-coordinate of the building. */
+        public final int left;
+        /** The height of the building. */
+        public final int height;
+        /** The right x-coordinate of the building. */
+        public final int right;
 
         /**
          * Constructor for the {@code Building} class.
@@ -149,5 +217,11 @@ public Building(int left, int height, int right) {
             this.height = height;
             this.right = right;
         }
+
+        @Override
+        public String toString() {
+            return "Building{"
+                + "left=" + left + ", height=" + height + ", right=" + right + '}';
+        }
     }
 }