Refactor in WriteAwareBruteSolver, add javadoc

Refactor WriteAwareBruteSolver constructor to guard against a situation
in which the class could have an inconsistent internal state

.gitignore

@@ -39,4 +39,5 @@ bin/
 .vscode/
 
 ### Mac OS ###
-.DS_Store
+.DS_Store
+performance.txt

.gitlab-ci.yml

@@ -2,8 +2,29 @@ image: azul/zulu-openjdk:8-latest
 
 stages:
   - test
+  - analyze
 
+# verify the padlock impl independently
 unit-test-padlock-impl:
   stage: test
   script:
     - ./gradlew :padlock-impl:test
+
+unit-test-all:
+  stage: test
+  script:
+    - ./gradlew test
+  artifacts:
+    when: always
+    reports:
+      junit: build/test-results/test/**/TEST-*.xml
+
+run-performance-analyze:
+  stage: analyze
+  needs:
+    - job: unit-test-all
+  script:
+    - ./gradlew runPerformanceAnalyze
+    - cat performance.txt
+  artifacts:
+    paths: ['performance.txt']

.idea/gradle.xml

@@ -5,7 +5,7 @@
     <option name="linkedExternalProjectsSettings">
       <GradleProjectSettings>
         <option name="externalProjectPath" value="$PROJECT_DIR$" />
-        <option name="gradleHome" value="" />
+        <option name="gradleJvm" value="azul-17" />
         <option name="modules">
           <set>
             <option value="$PROJECT_DIR$" />

README.md

@@ -48,6 +48,26 @@ You should design the proper architectures for your code.
 Last but not least, readability and maintainability are also important.
 Take this project as a show off to your designing and coding skills.
 
+## Build
+
+This repo uses Gradle to build and test.
+There is a unit test boilerplate and a gradle task configured to
+automatically test and evaluate the code when you push your commits.
+
+The `SolutionTestBase` is an abstract class for other tests.
+It tests the correctness of your solution and don't care about the run time.
+See `SolutionTest` for how to use that.
+
+The `PerformanceAnalyze` is not a unit test, but it do analyze roughly how
+fast your solution is. You need to fill in the `solve` method before you run it.
+
+Use `./gradlew test` to run all unit test configured in the project,
+and use `./gradlew runPerformanceTest` to get an analysis.
+
+> Note: You don't have to have a local gradle installation.
+> The `gradlew` script will download one for you.
+> Just install a valid jdk (version >= 8) and very thing should be fine.
+
 ## Still have unclear problems?
 
 Feel free to contact Jeffrey Freeman (jeffrey.freeman@cleverthis.com).

build.gradle.kts

@@ -1,3 +1,5 @@
+import java.io.FileOutputStream
+
 plugins {
     id("java")
 }
@@ -11,6 +13,7 @@ repositories {
 
 dependencies {
     implementation(project(":padlock-impl"))
+    implementation("org.apache.commons", "commons-text", "1.11.0")
     // if you ever need import more dependencies, following this format:
     // implementation("group-id:project-id:version")
     // just like the logback classic
@@ -27,4 +30,15 @@ java {
 
 tasks.test {
     useJUnitPlatform()
+    jvmArgs = listOf("-Dfast=true")
+}
+
+tasks.register<JavaExec>("runPerformanceAnalyze")
+tasks.named<JavaExec>("runPerformanceAnalyze") {
+    dependsOn("testClasses")
+    group = "verification"
+    classpath = sourceSets.test.get().runtimeClasspath
+    mainClass.set("com.cleverthis.interview.PerformanceAnalyze")
+    jvmArgs("-Dfast=true")
+    standardOutput = FileOutputStream("performance.txt")
 }

padlock-impl/build.gradle.kts

@@ -21,4 +21,5 @@ java {
 
 tasks.test {
     useJUnitPlatform()
+    jvmArgs = listOf("-Dfast=true")
 }

padlock-impl/src/main/java/com/cleverthis/interview/padlock/PadlockImpl.java

@@ -4,6 +4,7 @@ import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.Collections;
 import java.util.List;
+import java.util.concurrent.atomic.AtomicLong;
 
 import static com.cleverthis.interview.padlock.Utils.ensureSleep;
 
@@ -27,10 +28,12 @@ import static com.cleverthis.interview.padlock.Utils.ensureSleep;
  * After create, the input buffer is empty, you have to initialize.
  */
 public class PadlockImpl {
-    private final boolean debug;
     private final int numpadSize;
     private final Integer[] inputBuffer;
     private final Integer[] correctPasscode;
+    // performance counter
+    private final AtomicLong writeCounter = new AtomicLong(0);
+    private final AtomicLong checkCounter = new AtomicLong(0);
 
     /**
      * Create a padlock instance.
@@ -38,17 +41,6 @@ public class PadlockImpl {
      * @param numpadSize The number of buttons on the numpad of this lock.
      */
     public PadlockImpl(int numpadSize) {
-        this(numpadSize, false);
-    }
-
-    /**
-     * Create a padlock instance.
-     *
-     * @param numpadSize The number of buttons on the numpad of this lock.
-     * @param debug      Will skip sleep if is true
-     */
-    PadlockImpl(int numpadSize, boolean debug) {
-        this.debug = debug;
         if (numpadSize < 1) throw new IllegalArgumentException("numpadSize must be a positive number");
         this.numpadSize = numpadSize;
         this.inputBuffer = new Integer[numpadSize];
@@ -73,10 +65,11 @@ public class PadlockImpl {
      * @return The old value, null if not initialized.
      */
     public synchronized Integer writeInputBuffer(int address, int keyIndex) {
-        if (!debug) ensureSleep(1000);
+        ensureSleep(1000);
         if (keyIndex < 0 || keyIndex >= numpadSize)
             throw new IllegalArgumentException(
                     "keyIndex out of range. Keypad size: " + numpadSize + ", keyIndex: " + keyIndex);
+        writeCounter.incrementAndGet();
         Integer oldValue = inputBuffer[address];
         inputBuffer[address] = keyIndex;
         return oldValue;
@@ -98,10 +91,24 @@ public class PadlockImpl {
                     "Passcode invalid: contain duplicated value. " + Arrays.toString(inputBuffer));
             uniqueTestArr[i] = true;
         }
+        checkCounter.incrementAndGet();
         // if no exception, means:
         //     every digit is unique, and every digit is initialized
         // aka this is a valid code
         // now compare with our answer
         return Arrays.equals(correctPasscode, inputBuffer);
     }
-}
+
+    public long getWriteCounter() {
+        return writeCounter.get();
+    }
+
+    public long getCheckCounter() {
+        return checkCounter.get();
+    }
+
+    public void resetCounter() {
+        writeCounter.set(0);
+        checkCounter.set(0);
+    }
+}

padlock-impl/src/main/java/com/cleverthis/interview/padlock/Utils.java

@@ -1,12 +1,24 @@
 package com.cleverthis.interview.padlock;
 
 class Utils {
+    /**
+     * Check if the sleep is disabled.
+     * User can use `-Dfast=true` in the jvm args,
+     * or change it on the fly.
+     * Might waste sometime on checking this flag, but the effect should be minor.
+     * */
+    private static boolean shouldSkipSleep() {
+        return Boolean.parseBoolean(System.getProperty("fast"));
+    }
+
     /**
      * Ensure we will wait a given amount of time even if there are interruptions.
+     * Property `-Dfast=true` can disable the sleep.
      *
      * @param millis The time you want to sleep, measure in millisecond.
      */
     public static void ensureSleep(long millis) {
+        if (shouldSkipSleep()) return;
         long endTime = System.currentTimeMillis() + millis;
         while (endTime > System.currentTimeMillis()) {
             try {

padlock-impl/src/test/java/com/cleverthis/interview/padlock/PadlockTest.java

@@ -22,7 +22,7 @@ class PadlockTest {
 
     @Test
     void testInstantiationRest() {
-        PadlockImpl padlock = new PadlockImpl(5, true);
+        PadlockImpl padlock = new PadlockImpl(5);
         for (int i = 0; i < 5; i++) {
             // ensure input buffer is uninitialized
             // should return null when first set
@@ -34,7 +34,7 @@ class PadlockTest {
 
     @Test
     void testRejectInvalidInput() {
-        PadlockImpl padlock = new PadlockImpl(5, true);
+        PadlockImpl padlock = new PadlockImpl(5);
         for (int i = 0; i < 3; i++) {
             padlock.writeInputBuffer(i, i);
         }
@@ -54,7 +54,7 @@ class PadlockTest {
 
     @Test
     void testRejectInvalidInputBufferAddressAndValue() {
-        PadlockImpl padlock = new PadlockImpl(5, true);
+        PadlockImpl padlock = new PadlockImpl(5);
         // test address
         assertThrows(ArrayIndexOutOfBoundsException.class, () -> padlock.writeInputBuffer(-1, 1));
         assertThrows(ArrayIndexOutOfBoundsException.class, () -> padlock.writeInputBuffer(-10, 1));

src/main/java/com/cleverthis/interview/DumbBruteSolver.java

@@ -0,0 +1,106 @@
+package com.cleverthis.interview;
+
+import java.util.Arrays;
+import java.util.Collection;
+import java.util.Collections;
+import java.util.List;
+import java.util.stream.Stream;
+
+/**
+ * Brute-forces padlock using lexicographically ordered permutation generation
+ *
+ * Algorithm documented at: https://en.wikipedia.org/wiki/Permutation#Generation_in_lexicographic_order
+ */
+public class DumbBruteSolver implements SolverInterface {
+
+    /**
+     * Solves the padlock passed in to the method. The padlock's internal state should be correct after this method
+     * runs.
+     * @param padlockAdapter A padlock conforming to the PadlockAdapter contract
+     */
+    @Override
+    public void solve(PadlockAdapter padlockAdapter) {
+        int numpadSize = padlockAdapter.getNumpadSize();
+
+        Integer[] currentPermutation = new Integer[numpadSize];
+        for(int i = 0; i < numpadSize; i++) {
+            currentPermutation[i] = i;
+        }
+
+        while(true) {
+            boolean isCorrect = checkPermutation(currentPermutation, padlockAdapter);
+
+            if (!isCorrect) {
+                boolean nextPermutationExists = calculateNextPermutation(currentPermutation, numpadSize);
+                if(!nextPermutationExists) {
+                    return;
+                }
+            } else {
+                return;
+            }
+        }
+
+    }
+
+    /**
+     * Writes the permutation to the padlock's memory and checks whether this permutation is the correct passcode.
+     * This is a naive solution that makes no considerations for write-cost.
+     * @param permutation The permutation to write to the padlock
+     * @param padlockAdapter The padlock to write to
+     * @return True if the correct padlock passcode has been found, false otherwise
+     */
+    protected boolean checkPermutation(Integer[] permutation, PadlockAdapter padlockAdapter) {
+        for(int i = 0; i < padlockAdapter.getNumpadSize(); i++) {
+            padlockAdapter.writeInputBuffer(i, permutation[i]);
+        }
+
+        return padlockAdapter.isPasscodeCorrect();
+    }
+
+    /**
+     * Calculates the next permutation in lexicographic order, based on the algorithm linked on wikipedia
+     * @param currentPermutation The current permutation to run the algorithm on
+     * @param numpadSize The number of items in the set to be permuted
+     * @return true if next permutation successfully generated, false if permutations have been exhausted
+     */
+    protected boolean calculateNextPermutation(Integer[] currentPermutation, int numpadSize) {
+        if(numpadSize < 2) { return false; }
+
+        //Integer k, l;
+
+        // Find the k and l indices, such that they meet the criteria for the permutation algorithm.
+        // If such indice values are found, swap them, then reverse the array subset from k+1 to the end of the array
+        for(int k = (numpadSize - 2); k >= 0; k--) {
+            if(currentPermutation[k] < currentPermutation[k + 1]) {
+
+                for(int l = (numpadSize - 1); l > k; l--) {
+                    if(currentPermutation[k] < currentPermutation[l]) {
+                        // Swap index k value and index l value in permutations array
+                        // TODO: Could be a better swap algorithm
+                        int tempInt = currentPermutation[k];
+                        currentPermutation[k] = currentPermutation[l];
+                        currentPermutation[l] = tempInt;
+
+                        // Split the currentPermutation array into two slices. The slice happens at index k, with index k
+                        // inclusive to the first slice
+                        Integer[] firstSlice = Arrays.stream(currentPermutation, 0, k + 1).toArray(Integer[]::new);
+                        Integer[] secondSlice = Arrays.stream(currentPermutation, k + 1, numpadSize).toArray(Integer[]::new);
+
+                        // Reverse the subset of the permutation array from index k+1 to the end of the array
+                        Collections.reverse(Arrays.asList(secondSlice));
+
+                        // Concat the non-reversed and reversed subarrays into a new permutation
+                        Integer[] newPermutation = Stream.concat(Arrays.stream(firstSlice), Arrays.stream(secondSlice)).toArray(Integer[]::new);
+
+                        // Copy the new permutation into currentPermutation to return it
+                        System.arraycopy(newPermutation, 0, currentPermutation, 0, numpadSize);
+
+                        return true;
+                    }
+                }
+            }
+        }
+
+        return false;
+    }
+}

src/main/java/com/cleverthis/interview/IntegerLevenshtein.java

@@ -0,0 +1,62 @@
+package com.cleverthis.interview;
+
+import org.apache.commons.text.similarity.LevenshteinDistance;
+
+/**
+ * A wrapper class that holds possible padlock passcode permutations in an integer array. This class
+ * overloads the toString() method and is comparable, using its string value to calculate the levenshtein distance and
+ * use those distance values for sorting into a data structure.
+ */
+public class IntegerLevenshtein implements Comparable<IntegerLevenshtein> {
+    private Integer[] integerData;
+    private Integer size;
+
+    /**
+     * Creates a new IntegerLevenshtein
+     * @param size The number of integers (keypad size) of the passcode
+     */
+    public IntegerLevenshtein(int size) {
+        this.integerData = new Integer[size];
+        this.size = size;
+    }
+
+    /**
+     * Sets the integer data array
+     * @param integerData The new integer data
+     */
+    public void setIntegerData(Integer[] integerData) {
+        this.integerData = integerData.clone();
+    }
+
+    /**
+     * Gets the integer data array
+     * @return the integer data
+     */
+    public Integer[] getIntegerData() {
+        return this.integerData;
+    }
+
+    /**
+     * Casts each integer to a string and concatenates them together into a single string.
+     * For example, an internal integer array of [1,2,3,4] will be returned as the string "1234".
+     * @return A string representation of the integer array.
+     */
+    public String toString() {
+        String temp = new String();
+        for(int i = 0; i < size; i++) {
+            temp = temp.concat(integerData[i].toString());
+        }
+        return temp;
+    }
+
+    /**
+     * Overriden compareTo method that compares the calculated levenshtein distance between two IntegerLevenshtein objects
+     * @param otherInteger the object to be compared.
+     * @return The levenshtein distance between the two objects.
+     */
+    @Override
+    public int compareTo(IntegerLevenshtein otherInteger) {
+        LevenshteinDistance levenshtein = LevenshteinDistance.getDefaultInstance();
+        return levenshtein.apply(this.toString(), otherInteger.toString());
+    }
+}

src/main/java/com/cleverthis/interview/PadlockAdapter.java

@@ -0,0 +1,47 @@
+package com.cleverthis.interview;
+
+/**
+ * This defines the interface that padlocks must conform to.
+ * Concrete implementations will be adapted to this interface contract through concrete adapter classes.
+ */
+public interface PadlockAdapter {
+
+    /**
+     * Get the size of the padlock's physical number pad
+     *
+     * @return A count of the physical buttons on the padlock
+     */
+    int getNumpadSize();
+
+    /**
+     *  Write key presses to the input buffer of the padlock
+     * @param address The position / index of the button that is pressed. For example, address 0 is the first button pressed.
+     * @param keyIndex The value of the button that is pressed. Cannot be greater than the numpad size, as the buttons increment
+     *                 sequentially
+     * @return The old value of keyIndex at the inputted address
+     */
+    Integer writeInputBuffer(int address, int keyIndex);
+
+    /**
+     * Check whether the inputted password is correct
+     * @return True if password is correct, false otherwise
+     */
+    boolean isPasscodeCorrect();
+
+    /**
+     * Returns the write counter
+     * @return The number of times a write operation has occurred
+     */
+    long getWriteCounter();
+
+    /**
+     * Returns the check counter
+     * @return The number of times the password has been checked for correctness
+     */
+    long getCheckCounter();
+
+    /**
+     * Resets both the check and write counters
+     */
+    void resetCounter();
+}

src/main/java/com/cleverthis/interview/PadlockJavaAdapter.java

@@ -0,0 +1,49 @@
+package com.cleverthis.interview;
+
+import com.cleverthis.interview.padlock.PadlockImpl;
+
+/**
+ * The concrete implementation of PadlockAdapter that communicates with the padlock directly through a Java
+ * class. This implementation also contains a cache that it keeps in sync with the underlying API, as a front-line
+ * optimization against unnecessary write operations.
+ */
+public class PadlockJavaAdapter extends PadlockImpl implements PadlockAdapter {
+    /**
+     * Intermediate cache between the underlying padlock API and any code that interfaces with the PadlockAdapter
+     * API.
+     */
+    private final Integer[] inputBufferState;
+
+    /**
+     * Create a padlock instance.
+     *
+     * @param numpadSize The number of buttons on the numpad of this lock.
+     */
+    public PadlockJavaAdapter(int numpadSize) {
+        super(numpadSize);
+
+        inputBufferState = new Integer[numpadSize];
+
+        for(int i = 0; i < this.getNumpadSize(); i++) {
+            inputBufferState[i] = i;
+            super.writeInputBuffer(i, i);
+        }
+    }
+
+    /**
+     * Writes to the underlying input buffer, but only if the cache shows that the write is necessary
+     * @param address The position / index of the button that is pressed. For example, address 0 is the first button pressed.
+     * @param keyIndex The value of the button that is pressed. Cannot be greater than the numpad size, as the buttons increment
+     *                 sequentially
+     * @return The old value that was replaced, which is the same as keyIndex if a write operation doesn't actually occur.
+     */
+    @Override
+    public Integer writeInputBuffer(int address, int keyIndex) {
+        if(inputBufferState[address] != keyIndex) {
+            inputBufferState[address] = keyIndex;
+            return super.writeInputBuffer(address, keyIndex);
+        } else {
+            return keyIndex;
+        }
+    }
+}

src/main/java/com/cleverthis/interview/Solution.java

@@ -1,13 +0,0 @@
-package com.cleverthis.interview;
-
-import com.cleverthis.interview.padlock.PadlockImpl;
-
-/**
- * This is a placeholder class showing a simple boilerplate.
- * This class is not required, so you can replace with your own architecture.
- */
-public class Solution {
-    public void solve(PadlockImpl padlock) {
-        throw new RuntimeException("TODO");
-    }
-}

src/main/java/com/cleverthis/interview/SolverInterface.java

@@ -0,0 +1,13 @@
+package com.cleverthis.interview;
+
+/**
+ * Interface that defines the class signature for solver implementations
+ */
+public interface SolverInterface {
+
+    /**
+     * Solves the padlock passed in that conforms to the PadlockAdapter interface
+     * @param padlockAdapter the padlock object to solve
+     */
+    void solve(PadlockAdapter padlockAdapter);
+}

src/main/java/com/cleverthis/interview/WriteAwareBruteSolver.java

@@ -0,0 +1,90 @@
+package com.cleverthis.interview;
+
+import java.util.Iterator;
+import java.util.TreeSet;
+import org.apache.commons.text.similarity.LevenshteinDistance;
+
+/**
+ * This is a write-aware brute solver implementation that uses the levenshtein distance to sort passcode permutations
+ * into a tree. Walking the tree in-order (Likely a depth-first traversal internally) results in a naive nearest-neighbor
+ * optimization that generally performs well, but may perform poorly in some cases. Heuristic solutions exist that have
+ * acceptable performance, and this is a more naive heuristic implementation.
+ *
+ * This permutation optimization problem is NP-hard and can be represented as the traveling salesperson problem.
+ * Representing passcode permutations as vertices and the levenshtein distance as edges in an undirected, weighted graph,
+ * an optimal solution is the shortest path satisfying a tour of the entire graph.
+ *
+ * A more advanced solution (based on Christofides, or another TSP heuristic) may result in more performance gains, but
+ * the performance of this nearest-neighbor solution is adequate for the keypad sizes under test. A keypad size of 9
+ * results in a graph of vertex-count 9!. It's unlikely performance gains from a more intelligent TSP solver would
+ * offset the performance cost of building a graph and optimizing traversal.
+ */
+public class WriteAwareBruteSolver extends DumbBruteSolver {
+
+    private final TreeSet<IntegerLevenshtein> orderedTree;
+    private Integer numpadSize;
+
+    /**
+     * Creates an ordered tree on instantiation to be used as a cache for subsequent brute-force solves.
+     * @param numpadSize The size of the padlock's numpad, used in generation of the internal ordered tree
+     */
+    public WriteAwareBruteSolver(int numpadSize) {
+        orderedTree = new TreeSet<IntegerLevenshtein>();
+        this.numpadSize = numpadSize;
+
+        this.createOrderedTree(numpadSize);
+    }
+
+    /**
+     * Solves the padlock passed in to the method. The padlock's internal state should be correct after this method
+     * runs.
+     * @param padlockAdapter A padlock conforming to the PadlockAdapter contract
+     */
+    @Override
+    public void solve(PadlockAdapter padlockAdapter) {
+        int padlockNumpadSize = padlockAdapter.getNumpadSize();
+
+        if (this.numpadSize != padlockNumpadSize) {
+            this.createOrderedTree(padlockNumpadSize);
+        }
+
+        for (IntegerLevenshtein integerLevenshtein : orderedTree) {
+            if (this.checkPermutation(integerLevenshtein.getIntegerData(), padlockAdapter)) {
+                return;
+            }
+        }
+    }
+
+    /**
+     * Returns the size of the tree (the number of possible permutations)
+     * @return the size of the ordered tree
+     */
+    public Integer getTreeSize() {
+        return this.orderedTree.size();
+    }
+
+    /**
+     * Creates an ordered tree of possible passcode permutations
+     * @param size The number of keys on the numpad
+     */
+    protected void createOrderedTree(int size) {
+        if(this.numpadSize != size) {
+            this.numpadSize = size;
+            orderedTree.clear();
+        }
+
+        Integer[] currentPermutation = new Integer[numpadSize];
+        for(int i = 0; i < numpadSize; i++) {
+            currentPermutation[i] = i;
+        }
+
+        boolean morePermutationsExist = true;
+
+        do {
+            IntegerLevenshtein levenshteinPermutation = new IntegerLevenshtein(numpadSize);
+            levenshteinPermutation.setIntegerData(currentPermutation);
+            orderedTree.add(levenshteinPermutation);
+            morePermutationsExist = this.calculateNextPermutation(currentPermutation, numpadSize);
+        } while(morePermutationsExist);
+    }
+}

src/test/java/com/cleverthis/interview/DumbBruteSolverTest.java

@@ -0,0 +1,102 @@
+package com.cleverthis.interview;
+
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+/**
+ * Tests for DumbBruteSolver class
+ *
+ * Tests that lexicographically ordered permutations are properly calculated, and that the solver works
+ */
+public class DumbBruteSolverTest extends SolutionTestBase {
+
+    /**
+     * Overridden method that tests in SolutionTestBase.class expect defined and calls.
+     * Creates a DumbBruteSolver and calls the solve() method.
+     * @param padlock The padlock to solve
+     */
+    @Override
+    protected void solve(PadlockAdapter padlock) {
+        new DumbBruteSolver().solve(padlock);
+    }
+    /**
+     * Check whether a 7-button padlock can be solved
+     */
+    @Test
+    protected void testSolver() {
+        DumbBruteSolver dumbSolver = new DumbBruteSolver();
+        PadlockJavaAdapter padlock = new PadlockJavaAdapter(7);
+
+        dumbSolver.solve(padlock);
+
+        assertTrue(padlock.isPasscodeCorrect());
+    }
+
+    /**
+     * Check whether the next permutation is calculated correctly
+     */
+    @Test
+    protected void testOnePermutation() {
+        DumbBruteSolver dumbSolver = new DumbBruteSolver();
+
+        Integer[] permutation = new Integer[] {1, 2, 3, 4};
+        Integer[] correctPermutation = new Integer[] {1, 2, 4, 3};
+
+        dumbSolver.calculateNextPermutation(permutation, 4);
+
+        assertArrayEquals(permutation, correctPermutation);
+    }
+
+    /**
+     * Check whether two consecutive permutations are correct
+     */
+    @Test
+    protected void testTwoPermutations() {
+        DumbBruteSolver dumbSolver = new DumbBruteSolver();
+
+        Integer[] permutation = new Integer[] {1, 2, 3, 4};
+        Integer[] correctPermutation = new Integer[] {1, 3, 2, 4};
+
+        for(int i = 0; i < 2; i++) {
+            dumbSolver.calculateNextPermutation(permutation, 4);
+        }
+
+        assertArrayEquals(permutation, correctPermutation);
+    }
+
+    /**
+     * Check whether 23 consecutive permutations are correct
+     */
+    @Test
+    protected void test23Permutations() {
+        DumbBruteSolver dumbSolver = new DumbBruteSolver();
+
+        Integer[] permutation = new Integer[] {1, 2, 3, 4};
+        Integer[] correctPermutation = new Integer[] {4, 3, 2, 1};
+
+        for(int i = 0; i < 23; i++) {
+            dumbSolver.calculateNextPermutation(permutation, 4);
+        }
+
+        assertArrayEquals(permutation, correctPermutation);
+    }
+
+    /**
+     * Check whether the 24th permutation returns false, signifying we've exhausted all possible permutations
+     * for a list of size 4
+     */
+    @Test
+    protected void test24Permutations() {
+        DumbBruteSolver dumbSolver = new DumbBruteSolver();
+
+        Integer[] permutation = new Integer[] {1, 2, 3, 4};
+
+        for(int i = 0; i < 23; i++) {
+            assertTrue(dumbSolver.calculateNextPermutation(permutation, 4));
+        }
+
+        assertFalse(dumbSolver.calculateNextPermutation(permutation, 4));
+
+    }
+}

src/test/java/com/cleverthis/interview/PerformanceAnalyze.java

@@ -0,0 +1,51 @@
+package com.cleverthis.interview;
+
+import com.cleverthis.interview.padlock.PadlockImpl;
+
+/**
+ * Performance test but not mean to run in unit test.
+ */
+public class PerformanceAnalyze {
+
+    private static final int TOTAL_RUN = 500;
+    private static final int NUMPAD_SIZE = 9;
+    /**
+     * The solver object reference is held between tests, because the data structure used for optimization does not
+     * change between runs and keeping it in memory greatly increases performance.
+     */
+    static WriteAwareBruteSolver solver = new WriteAwareBruteSolver(NUMPAD_SIZE);
+
+    private static void solve(PadlockAdapter padlock) {
+        solver.solve(padlock);
+    }
+
+    static {
+        System.out.println("Total run: " + TOTAL_RUN);
+        System.out.println("Numpad size: " + NUMPAD_SIZE);
+    }
+
+    public static void main(String[] args) {
+        long timeSum = 0;
+        long writeSum = 0;
+        for (int i = 0; i < TOTAL_RUN; i++) {
+            PadlockAdapter padlock = new PadlockJavaAdapter(NUMPAD_SIZE);
+            padlock.resetCounter();
+            long start = System.currentTimeMillis();
+            solve((PadlockAdapter) padlock);
+            long end = System.currentTimeMillis();
+            if (!padlock.isPasscodeCorrect()) throw new IllegalStateException(
+                    "Invalid solution: passcode not correct after return");
+            long dT = end - start;
+            timeSum += dT;
+            writeSum += padlock.getWriteCounter();
+            System.out.println("Run #" + (i + 1) + ": time: " + dT + "ms; write: " + padlock.getWriteCounter());
+        }
+        System.out.println("Run time sum: " + timeSum + "ms");
+        System.out.println("Write sum: " + writeSum);
+        double avgTime = timeSum / (double) TOTAL_RUN;
+        double avgWrite = writeSum / (double) TOTAL_RUN;
+        System.out.println("Avg run time: " + avgTime + "ms");
+        System.out.println("Avg write: " + avgWrite);
+        System.out.println("Calculated estimate avg run time: " + (avgTime / 1000 + avgTime) + "s");
+    }
+}

src/test/java/com/cleverthis/interview/SolutionTest.java

@@ -1,30 +0,0 @@
-package com.cleverthis.interview;
-
-import com.cleverthis.interview.padlock.PadlockImpl;
-import org.junit.jupiter.api.Test;
-
-import java.util.Random;
-
-import static org.junit.jupiter.api.Assertions.*;
-
-/**
- * This is a simple placeholder to show how unit test works.
- * You can replace it with your own test.
- */
-class SolutionTest {
-    private void solve(PadlockImpl padlock) {
-        new Solution().solve(padlock);
-    }
-
-    @Test
-    void verify(){
-        Random random = new Random();
-        PadlockImpl padlock = new PadlockImpl(random.nextInt(1, 8));
-
-        long startTime = System.currentTimeMillis();
-        solve(padlock);
-        long endTime = System.currentTimeMillis();
-        assertTrue(padlock.isPasscodeCorrect());
-        System.out.println("Time usage: " + (endTime - startTime) + "ms");
-    }
-}

src/test/java/com/cleverthis/interview/SolutionTestBase.java

@@ -0,0 +1,34 @@
+package com.cleverthis.interview;
+
+import com.cleverthis.interview.padlock.PadlockImpl;
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+/**
+ * This is a base class for verifying the correctness of the solution.
+ */
+public abstract class SolutionTestBase {
+    
+    /**
+     * Implement your solution in this function.
+     * */
+    protected abstract void solve(PadlockAdapter padlock);
+
+    protected void verify(int numpadSize) {
+        PadlockAdapter padlock = new PadlockJavaAdapter(numpadSize);
+        solve(padlock);
+        assertTrue(padlock.isPasscodeCorrect());
+    }
+
+    /**
+     * Tests padlocks with numpad sizes of 1 to 7. This test runs for every class that extends this SolutionTestBase
+     * abstract class.
+     */
+    @Test
+    void verify1to7() {
+        for (int i = 1; i <= 7; i++) {
+            verify(i);
+        }
+    }
+}

src/test/java/com/cleverthis/interview/WriteAwareBruteSolverTest.java

@@ -0,0 +1,55 @@
+package com.cleverthis.interview;
+
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+/**
+ * Tests for WriteAwareBruteSolver class
+ *
+ * These tests do not keep a long-lived reference to WriteAwareBruteSolver, instead opting
+ * to reconstruct the permutation tree for every test. This is a trade-off between performance
+ * and avoiding any inadvertent behavior changes from state persisting between tests.
+ */
+public class WriteAwareBruteSolverTest extends SolutionTestBase {
+
+    /**
+     * Overridden method that tests in SolutionTestBase.class expect defined and calls.
+     * Creates a WriteAwareBruteSolver and calls the solve() method.
+     * @param padlock The padlock to solve
+     */
+    @Override
+    protected void solve(PadlockAdapter padlock) {
+        new WriteAwareBruteSolver(padlock.getNumpadSize()).solve(padlock);
+    }
+
+    /**
+     * Tests whether the solver can brute-force a 7-numpad padlock.
+     */
+    @Test
+    protected void testSolver() {
+        Integer numpadSize = 7;
+        WriteAwareBruteSolver writeAwareSolver = new WriteAwareBruteSolver(numpadSize);
+        PadlockJavaAdapter padlock = new PadlockJavaAdapter(numpadSize);
+
+        writeAwareSolver.solve(padlock);
+
+        assertTrue(padlock.isPasscodeCorrect());
+    }
+
+    /**
+     * Tests whether a numpad-size of 4 is correctly expanded to 24 possible permutations in the underlying
+     * tree
+     */
+    @Test
+    protected void testTreeSize() {
+        Integer numpadSize = 4;
+        WriteAwareBruteSolver writeAwareSolver = new WriteAwareBruteSolver(numpadSize);
+        PadlockJavaAdapter padlock = new PadlockJavaAdapter(4);
+
+        writeAwareSolver.solve(padlock);
+
+        assertEquals(writeAwareSolver.getTreeSize(), 24);
+    }
+}