Implement RecursiveFullyUsedArrayProcessor

src/aud/exam/prep/array/FullyUsedArrayProcessor.java

@@ -1,5 +1,6 @@
 package aud.exam.prep.array;
 
+import aud.exam.prep.Pair;
 import aud.exam.prep.SequenceProcessor;
 
 abstract class FullyUsedArrayProcessor<T> implements SequenceProcessor<T, FullyUsedArray<T>> {
@@ -24,4 +25,34 @@ abstract class FullyUsedArrayProcessor<T> implements SequenceProcessor<T, FullyU
         return () ->
             new ArrayIterator<>(array.theArray, 0, array.theArray.length);
     }
+
+    protected Pair<FullyUsedArray<T>, FullyUsedArray<T>> createPairForDivideAlternating(FullyUsedArray<T> array) {
+        var p = new Pair<FullyUsedArray<T>, FullyUsedArray<T>>();
+        p.fst = new FullyUsedArray<>();
+        p.snd = new FullyUsedArray<>();
+
+        p.snd.theArray = Arrays.newArray(array.theArray.length / 2);
+        p.fst.theArray = Arrays.newArray(array.theArray.length - p.snd.theArray.length);
+        return p;
+    }
+
+    protected Pair<FullyUsedArray<T>, FullyUsedArray<T>> createPairForDivideByPivot(int less, int greater) {
+        var p = new Pair<FullyUsedArray<T>, FullyUsedArray<T>>();
+        p.fst = new FullyUsedArray<>();
+        p.snd = new FullyUsedArray<>();
+
+        p.fst.theArray = Arrays.newArray(less);
+        p.snd.theArray = Arrays.newArray(greater);
+        return p;
+    }
+
+    protected Pair<FullyUsedArray<T>, FullyUsedArray<T>> createPairForDivideAlternatingByRuns(FullyUsedArray<T> array, int elementsInFst) {
+        var p = new Pair<FullyUsedArray<T>, FullyUsedArray<T>>();
+        p.fst = new FullyUsedArray<>();
+        p.snd = new FullyUsedArray<>();
+
+        p.fst.theArray = Arrays.newArray(elementsInFst);
+        p.snd.theArray = Arrays.newArray(array.theArray.length - elementsInFst);
+        return p;
+    }
 }

src/aud/exam/prep/array/IterativeFullyUsedArrayProcessor.java

@@ -195,6 +195,7 @@ public class IterativeFullyUsedArrayProcessor<T> extends FullyUsedArrayProcessor
         if (b.theArray.length < a.theArray.length) {
             return false;
         }
+
         for (int i = 0; i < a.theArray.length; i++) {
             if (cmp.compare(a.theArray[i], b.theArray[i]) > 0) {
                 return false;
@@ -416,12 +417,7 @@ public class IterativeFullyUsedArrayProcessor<T> extends FullyUsedArrayProcessor
 
     @Override
     public Pair<FullyUsedArray<T>, FullyUsedArray<T>> divideAlternating(FullyUsedArray<T> array) {
-        var p = new Pair<FullyUsedArray<T>, FullyUsedArray<T>>();
+        var p = createPairForDivideAlternating(array);
-        p.fst = new FullyUsedArray<>();
-        p.snd = new FullyUsedArray<>();
-
-        p.snd.theArray = Arrays.newArray(array.theArray.length / 2);
-        p.fst.theArray = Arrays.newArray(array.theArray.length - p.snd.theArray.length);
 
         for (int i = 0; i < array.theArray.length; i++) {
             if (i % 2 == 0) {
@@ -436,10 +432,6 @@ public class IterativeFullyUsedArrayProcessor<T> extends FullyUsedArrayProcessor
 
     @Override
     public Pair<FullyUsedArray<T>, FullyUsedArray<T>> divideAlternatingByRuns(FullyUsedArray<T> array, Comparator<T> cmp) {
-        var p = new Pair<FullyUsedArray<T>, FullyUsedArray<T>>();
-        p.fst = new FullyUsedArray<>();
-        p.snd = new FullyUsedArray<>();
-
         var runInFst = true;
         var elementsInFst = 0;
 
@@ -453,8 +445,7 @@ public class IterativeFullyUsedArrayProcessor<T> extends FullyUsedArrayProcessor
             }
         }
 
-        p.fst.theArray = Arrays.newArray(elementsInFst);
+        var p = createPairForDivideAlternatingByRuns(array, elementsInFst);
-        p.snd.theArray = Arrays.newArray(array.theArray.length - elementsInFst);
 
         runInFst = true;
         var indexA = 0;
@@ -477,10 +468,6 @@ public class IterativeFullyUsedArrayProcessor<T> extends FullyUsedArrayProcessor
 
     @Override
     public Pair<FullyUsedArray<T>, FullyUsedArray<T>> divideByPivot(FullyUsedArray<T> array, T pivot, Comparator<T> cmp) {
-        var p = new Pair<FullyUsedArray<T>, FullyUsedArray<T>>();
-        p.fst = new FullyUsedArray<>();
-        p.snd = new FullyUsedArray<>();
-
         var less = 0;
         var greater = 0;
 
@@ -494,8 +481,7 @@ public class IterativeFullyUsedArrayProcessor<T> extends FullyUsedArrayProcessor
             }
         }
 
-        p.fst.theArray = Arrays.newArray(less);
+        var p = createPairForDivideByPivot(less, greater);
-        p.snd.theArray = Arrays.newArray(greater);
 
         var indexA = 0;
         var indexB = 0;

src/aud/exam/prep/array/RecursiveFullyUsedArrayProcessor.java

@@ -4,142 +4,698 @@ import aud.exam.prep.Pair;
 
 import java.util.Comparator;
 import java.util.Iterator;
+import java.util.Objects;
 
 public class RecursiveFullyUsedArrayProcessor<T> extends FullyUsedArrayProcessor<T> {
 
     @Override
     public boolean find(FullyUsedArray<T> array, T t) {
+        return findRec(array, t, 0);
+    }
+
+    private boolean findRec(FullyUsedArray<T> array, T t, int index) {
+        if (index >= array.theArray.length) {
             return false;
         }
+        if (Objects.equals(array.theArray[index], t)) {
+            return true;
+        }
+        return findRec(array, t, index+1);
+    }
 
     @Override
     public boolean findBinary(FullyUsedArray<T> array, T t, Comparator<T> cmp) {
+        return findBinaryRec(array, t, cmp, 0, array.theArray.length-1);
+    }
+
+    private boolean findBinaryRec(FullyUsedArray<T> array, T t, Comparator<T> cmp, int start, int end) {
+        if (start > end) {
             return false;
         }
 
+        var index = (start+end) / 2;
+        var c = cmp.compare(t, array.theArray[index]);
+
+        if (c == 0) {
+            return true;
+        }
+        if (c > 0) {
+            return findBinaryRec(array, t, cmp, index+1, end);
+        }
+        return findBinaryRec(array, t, cmp, start, index-1);
+    }
+
     @Override
     public boolean override(FullyUsedArray<T> array, T from, T to) {
+        return overrideRec(array, from, to, 0);
+    }
+
+    private boolean overrideRec(FullyUsedArray<T> array, T from, T to, int index) {
+        if (index >= array.theArray.length) {
             return false;
         }
+        if (Objects.equals(array.theArray[index], from)) {
+            array.theArray[index] = to;
+            return true;
+        }
+        return overrideRec(array, from, to, index+1);
+    }
 
     @Override
     public boolean overrideAll(FullyUsedArray<T> array, T from, T to) {
+        return overrideAllRec(array, from, to, 0);
+    }
+
+    private boolean overrideAllRec(FullyUsedArray<T> array, T from, T to, int index) {
+        if (index >= array.theArray.length) {
             return false;
         }
 
+        var modified = false;
+        if (Objects.equals(array.theArray[index], from)) {
+            array.theArray[index] = to;
+            modified = true;
+        }
+
+        return overrideAllRec(array, from, to, index+1) || modified;
+    }
+
     @Override
     public void insertInOrder(FullyUsedArray<T> array, T t, Comparator<T> cmp) {
+        T[] newArray = Arrays.newArray(array.theArray.length+1);
+        insertElementsInOrder(array.theArray, newArray, t, cmp, 0);
+        array.theArray = newArray;
+    }
 
+    private void insertElementsInOrder(T[] oldArray, T[] newArray, T t, Comparator<T> cmp, int index) {
+        if (index == oldArray.length) {
+            newArray[index] = t;
+        } else if (cmp.compare(t, oldArray[index]) < 0) {
+            newArray[index] = t;
+            insertRestOfElements(oldArray, newArray, index);
+        } else {
+            newArray[index] = oldArray[index];
+            insertElementsInOrder(oldArray, newArray, t, cmp, index+1);
+        }
+    }
+
+    private void insertRestOfElements(T[] oldArray, T[] newArray, int index) {
+        if (index >= oldArray.length) {
+            return;
+        }
+        newArray[index+1] = oldArray[index];
+        insertRestOfElements(oldArray, newArray, index+1);
     }
 
     @Override
     public boolean remove(FullyUsedArray<T> array, T t) {
+        if (!find(array, t)) {
             return false;
         }
 
+        T[] newArray = Arrays.newArray(array.theArray.length-1);
+        removeRec(array.theArray, newArray, t, 0);
+
+        array.theArray = newArray;
+        return true;
+    }
+
+    private void removeRec(T[] oldArray, T[] newArray, T t, int index) {
+        if (Objects.equals(oldArray[index], t)) {
+            copyRestOfElementsForRemove(oldArray, newArray, index+1);
+            return;
+        }
+
+        newArray[index] = oldArray[index];
+        removeRec(oldArray, newArray, t, index+1);
+    }
+
+    private void copyRestOfElementsForRemove(T[] oldArray, T[] newArray, int index) {
+        if (index >= oldArray.length) {
+            return;
+        }
+
+        newArray[index-1] = oldArray[index];
+        copyRestOfElementsForRemove(oldArray, newArray, index+1);
+    }
+
     @Override
     public boolean removeAll(FullyUsedArray<T> array, T t) {
+        var size = countRemainingElementsForRemoveAll(array.theArray, t, 0);
+        if (size == array.theArray.length) {
             return false;
         }
 
+        T[] newArray = Arrays.newArray(size);
+
+        removeAllRec(array.theArray, 0, newArray, 0, t);
+
+        array.theArray = newArray;
+        return true;
+    }
+
+    private void removeAllRec(T[] src, int indexSrc, T[] dst, int indexDst, T t) {
+        if (indexSrc >= src.length) {
+            return;
+        }
+
+        if (!Objects.equals(t, src[indexSrc])) {
+            dst[indexDst] = src[indexSrc];
+            indexDst++;
+        }
+
+        removeAllRec(src, indexSrc+1, dst, indexDst, t);
+    }
+
+    private int countRemainingElementsForRemoveAll(T[] array, T t, int index) {
+        if (index >= array.length) {
+            return 0;
+        }
+
+        var current = 0;
+        if (!Objects.equals(t, array[index])) {
+            current++;
+        }
+
+        return current + countRemainingElementsForRemoveAll(array, t, index+1);
+    }
+
     @Override
     public boolean isAscending(FullyUsedArray<T> array, Comparator<T> cmp) {
+        if (array.theArray.length < 2) {
+            return true;
+        }
+
+        return isAscendingRec(array.theArray, cmp, 0);
+    }
+
+    private boolean isAscendingRec(T[] array, Comparator<T> cmp, int index) {
+        if (index >= array.length-1) {
+            return true;
+        }
+
+        if (cmp.compare(array[index], array[index+1]) > 0) {
             return false;
         }
 
+        return isAscendingRec(array, cmp, index+1);
+    }
+
     @Override
     public T max(FullyUsedArray<T> array, Comparator<T> cmp) {
-        return null;
+        if (array.theArray.length == 0) {
+            throw new IllegalArgumentException("The sequence may not be empty");
+        }
+
+        return maxRec(array.theArray, cmp, 1, array.theArray[0]);
+    }
+
+    private T maxRec(T[] array, Comparator<T> cmp, int index, T max) {
+        if (index >= array.length) {
+            return max;
+        }
+
+        if (cmp.compare(max, array[index]) < 0) {
+            max = array[index];
+        }
+
+        return maxRec(array, cmp, index+1, max);
     }
 
     @Override
     public T secondMax(FullyUsedArray<T> array, Comparator<T> cmp) {
-        return null;
+        if (array.theArray.length < 2) {
+            throw new IllegalArgumentException("The sequence must contains at least 2 elements");
+        }
+
+        T secondMax, max;
+        if (cmp.compare(array.theArray[0], array.theArray[1]) < 0) {
+            secondMax = array.theArray[0];
+            max = array.theArray[1];
+        } else {
+            secondMax = array.theArray[1];
+            max = array.theArray[0];
+        }
+
+
+        return secondMaxRec(array.theArray, cmp, 2, max, secondMax);
+    }
+
+    private T secondMaxRec(T[] array, Comparator<T> cmp, int index, T max, T secondMax) {
+        if (index >= array.length) {
+            return secondMax;
+        }
+
+        if (cmp.compare(max, array[index]) < 0) {
+            secondMax = max;
+            max = array[index];
+        } else if (cmp.compare(secondMax, array[index]) < 0) {
+            secondMax = array[index];
+        }
+
+        return secondMaxRec(array, cmp, index+1, max, secondMax);
     }
 
     @Override
     public boolean isItemWiseLessOrEqual(FullyUsedArray<T> a, FullyUsedArray<T> b, Comparator<T> cmp) {
+        if (b.theArray.length < a.theArray.length) {
             return false;
         }
 
+        return isItemWiseLessOrEqualRec(a.theArray, b.theArray, 0, cmp);
+    }
+
+    private boolean isItemWiseLessOrEqualRec(T[] a, T[] b, int index, Comparator<T> cmp) {
+        if (index >= a.length) {
+            return true;
+        }
+
+        if (cmp.compare(a[index], b[index]) > 0) {
+            return false;
+        }
+
+        return isItemWiseLessOrEqualRec(a, b, index+1, cmp);
+    }
+
     @Override
     public boolean isItemWiseLess(FullyUsedArray<T> a, FullyUsedArray<T> b, Comparator<T> cmp) {
+        if (b.theArray.length < a.theArray.length) {
             return false;
         }
 
+        return isItemWiseLessRec(a.theArray, b.theArray, 0, cmp);
+    }
+
+    private boolean isItemWiseLessRec(T[] a, T[] b, int index, Comparator<T> cmp) {
+        if (index >= a.length) {
+            return index < b.length;
+        }
+
+        var c = cmp.compare(a[index], b[index]);
+
+        if (c > 0) {
+            return false;
+        }
+
+        if (c < 0) {
+            return isItemWiseLessOrEqualRec(a, b, index+1, cmp);
+        }
+
+        return isItemWiseLessRec(a, b, index+1, cmp);
+    }
+
     @Override
     public boolean isLexSmaller(FullyUsedArray<T> a, FullyUsedArray<T> b, Comparator<T> cmp) {
+        return isLexSmallerRec(a.theArray, 0, b.theArray, 0, cmp);
+    }
+
+    private boolean isLexSmallerRec(T[] a, int indexA, T[] b, int indexB, Comparator<T> cmp) {
+        if (indexA >= a.length) {
+            return indexB < b.length;
+        }
+
+        if (indexB >= b.length) {
             return false;
         }
 
+        var c = cmp.compare(a[indexA], b[indexB]);
+
+        if (c > 0) {
+            return false;
+        }
+
+        if (c < 0) {
+            return true;
+        }
+
+        return isLexSmallerRec(a, indexA+1, b, indexB+1, cmp);
+    }
+
     @Override
     public void exchangePairs(FullyUsedArray<T> array) {
+        exchangePairsRec(array.theArray, 1);
+    }
 
+    private void exchangePairsRec(T[] array, int index) {
+        if (index >= array.length-1) {
+            return;
+        }
+
+        T t = array[index];
+        array[index] = array[index +1];
+        array[index +1] = t;
+
+        exchangePairsRec(array, index+2);
     }
 
     @Override
     public void rotateTriples(FullyUsedArray<T> array) {
+        rotateTriplesRec(array.theArray, 1);
+    }
 
+    private void rotateTriplesRec(T[] array, int index) {
+        if (index >= array.length - 2) {
+            return;
+        }
+
+        T t = array[index+2];
+        array[index+2] = array[index+1];
+        array[index+1] = array[index];
+        array[index] = t;
+
+        rotateTriplesRec(array, index+3);
     }
 
     @Override
     public void removeEverySecond(FullyUsedArray<T> array) {
+        T[] newArray = Arrays.newArray(array.theArray.length - array.theArray.length / 2);
 
+        removeEverySecondRec(array.theArray, newArray, 0);
+
+        array.theArray = newArray;
+    }
+
+    private void removeEverySecondRec(T[] oldArray, T[] newArray, int index) {
+        if (index >= newArray.length) {
+            return;
+        }
+
+        newArray[index] = oldArray[index*2];
+        removeEverySecondRec(oldArray, newArray, index+1);
     }
 
     @Override
     public void doubleAllKeys(FullyUsedArray<T> array) {
+        T[] newArray = Arrays.newArray(array.theArray.length * 2);
 
+        doubleAllKeysRec(array.theArray, newArray, 0);
+
+        array.theArray = newArray;
+    }
+
+    private void doubleAllKeysRec(T[] array, T[] newArray, int index) {
+        if (index >= array.length) {
+            return;
+        }
+
+        newArray[index * 2] = array[index];
+        newArray[index * 2 + 1] = array[index];
+
+        doubleAllKeysRec(array, newArray, index+1);
     }
 
     @Override
     public FullyUsedArray<T> rotateRight(FullyUsedArray<T> array) {
-        return null;
+        if (array.theArray.length == 0) {
+            return array;
+        }
+
+        T t = array.theArray[array.theArray.length-1];
+
+        rotateRightRec(array.theArray, array.theArray.length-1);
+
+        array.theArray[0] = t;
+        return array;
+    }
+
+    private void rotateRightRec(T[] array, int index) {
+        if (index < 1) {
+            return;
+        }
+
+        array[index] = array[index-1];
+        rotateRightRec(array, index-1);
     }
 
     @Override
     public FullyUsedArray<T> rotateLeft(FullyUsedArray<T> array) {
-        return null;
+        if (array.theArray.length == 0) {
+            return array;
+        }
+
+        T t = array.theArray[0];
+
+        rotateLeftRec(array.theArray, 0);
+
+        array.theArray[array.theArray.length-1] = t;
+        return array;
+    }
+
+    private void rotateLeftRec(T[] array, int index) {
+        if (index >= array.length-1) {
+            return;
+        }
+
+        array[index] = array[index+1];
+        rotateLeftRec(array, index+1);
     }
 
     @Override
     public void removeDuplicates(FullyUsedArray<T> array) {
+        if (array.theArray.length < 2) {
+            return;
+        }
 
+        var count = 1 + countRemainingElementsForRemoveDuplicates(array.theArray, 1);
+
+        T[] newArray = Arrays.newArray(count);
+        newArray[0] = array.theArray[0];
+
+        removeDuplicatesRec(array.theArray, newArray, 1, 1);
+
+        array.theArray = newArray;
+    }
+
+    private void removeDuplicatesRec(T[] array, T[] newArray, int src, int dst) {
+        if (src >= array.length) {
+            return;
+        }
+
+        if (!Objects.equals(array[src], array[src-1])) {
+            newArray[dst] = array[src];
+            dst++;
+        }
+
+        removeDuplicatesRec(array, newArray, src+1, dst);
+    }
+
+    private int countRemainingElementsForRemoveDuplicates(T[] array, int index) {
+        if (index >= array.length) {
+            return 0;
+        }
+
+        var current = 0;
+        if (!Objects.equals(array[index], array[index-1])) {
+            current++;
+        }
+
+        return current + countRemainingElementsForRemoveDuplicates(array, index+1);
     }
 
     @Override
     public FullyUsedArray<T> invert(FullyUsedArray<T> array) {
-        return null;
+        invertRec(array.theArray, 0);
+        return array;
+    }
+
+    private void invertRec(T[] array, int index) {
+        if (index >= array.length / 2) {
+            return;
+        }
+
+        T t = array[index];
+        array[index] = array[array.length - 1 - index];
+        array[array.length - 1 - index] = t;
+
+        invertRec(array, index+1);
     }
 
     @Override
     public FullyUsedArray<T> clone(FullyUsedArray<T> array) {
-        return null;
+        var newArray = new FullyUsedArray<T>();
+        newArray.theArray = Arrays.newArray(array.theArray.length);
+
+        copyRestOfElements(array.theArray, 0, newArray.theArray, 0);
+
+        return newArray;
     }
 
     @Override
     public FullyUsedArray<T> alternate(FullyUsedArray<T> a, FullyUsedArray<T> b) {
-        return null;
+        var newArray = new FullyUsedArray<T>();
+        newArray.theArray = Arrays.newArray(a.theArray.length + b.theArray.length);
+
+        alternateRec(a.theArray, 0, b.theArray, 0, newArray.theArray, 0);
+
+        return newArray;
+    }
+
+    private void alternateRec(T[] a, int indexA, T[] b, int indexB, T[] array, int index) {
+        if (indexA >= a.length) {
+            copyRestOfElements(b, indexB, array, index);
+        } else if (indexB >= b.length) {
+            copyRestOfElements(a, indexA, array, index);
+        } else {
+            array[index] = a[indexA];
+            array[index+1] = b[indexB];
+            alternateRec(a, indexA+1, b, indexB+1, array, index+2);
+        }
     }
 
     @Override
     public FullyUsedArray<T> merge(FullyUsedArray<T> a, FullyUsedArray<T> b, Comparator<T> cmp) {
-        return null;
+        var newArray = new FullyUsedArray<T>();
+        newArray.theArray = Arrays.newArray(a.theArray.length + b.theArray.length);
+
+        mergeRec(a.theArray, 0, b.theArray, 0, newArray.theArray, 0, cmp);
+
+        return newArray;
+    }
+
+    private void mergeRec(T[] a, int indexA, T[] b, int indexB, T[] newArray, int index, Comparator<T> cmp) {
+        if (indexA >= a.length) {
+            copyRestOfElements(b, indexB, newArray, index);
+            return;
+        }
+        if (indexB >= b.length) {
+            copyRestOfElements(a, indexA, newArray, index);
+            return;
+        }
+
+        if (cmp.compare(a[indexA], b[indexB]) < 0) {
+            newArray[index] = a[indexA];
+            mergeRec(a, indexA+1, b, indexB, newArray, index+1, cmp);
+        } else {
+            newArray[index] = b[indexB];
+            mergeRec(a, indexA, b, indexB+1, newArray, index+1, cmp);
+        }
+    }
+
+    private void copyRestOfElements(T[] src, int srcIndex, T[] dst, int dstIndex) {
+        if (srcIndex >= src.length) {
+            return;
+        }
+
+        dst[dstIndex] = src[srcIndex];
+        copyRestOfElements(src, srcIndex+1, dst, dstIndex+1);
     }
 
     @Override
     public Pair<FullyUsedArray<T>, FullyUsedArray<T>> divideAlternating(FullyUsedArray<T> array) {
-        return null;
+        var p = createPairForDivideAlternating(array);
+        divideAlternatingRec(p, array.theArray, 0, 0, 0);
+        return p;
+    }
+
+    private void divideAlternatingRec(Pair<FullyUsedArray<T>, FullyUsedArray<T>> p, T[] array,
+                                      int index, int indexA, int indexB) {
+        if (index >= array.length) {
+            return;
+        }
+
+        if (index % 2 == 0) {
+            p.fst.theArray[indexA] = array[index];
+            divideAlternatingRec(p, array, index+1, indexA+1, indexB);
+        } else {
+            p.snd.theArray[indexB] = array[index];
+            divideAlternatingRec(p, array, index+1, indexA, indexB+1);
+        }
     }
 
     @Override
     public Pair<FullyUsedArray<T>, FullyUsedArray<T>> divideAlternatingByRuns(FullyUsedArray<T> array, Comparator<T> cmp) {
-        return null;
+        var elementsInFst = countElementsInFirst(array.theArray, cmp, true, 0);
+        var p = createPairForDivideAlternatingByRuns(array, elementsInFst);
+
+        divideAlternatingByRunsRec(array.theArray, p, cmp, true, 0, 0, 0);
+        return p;
+    }
+
+    private void divideAlternatingByRunsRec(T[] array, Pair<FullyUsedArray<T>, FullyUsedArray<T>> p, Comparator<T> cmp,
+                                            boolean runInFst, int index, int indexA, int indexB) {
+        if (index >= array.length) {
+            return;
+        }
+
+        if (runInFst) {
+            p.fst.theArray[indexA] = array[index];
+            indexA++;
+        } else {
+            p.snd.theArray[indexB] = array[index];
+            indexB++;
+        }
+
+        if (index < array.length-1 && cmp.compare(array[index], array[index + 1]) > 0) {
+            runInFst = !runInFst;
+        }
+
+        divideAlternatingByRunsRec(array, p, cmp, runInFst, index+1, indexA, indexB);
+    }
+
+    private int countElementsInFirst(T[] array, Comparator<T> cmp, boolean runInFst, int index) {
+        if (index >= array.length) {
+            return 0;
+        }
+
+        var countCurrentElement = 0;
+
+        if (runInFst) {
+            countCurrentElement++;
+        }
+
+        if (index < array.length-1 && cmp.compare(array[index], array[index + 1]) > 0) {
+            runInFst = !runInFst;
+        }
+
+        return countCurrentElement + countElementsInFirst(array, cmp, runInFst, index+1);
     }
 
     @Override
     public Pair<FullyUsedArray<T>, FullyUsedArray<T>> divideByPivot(FullyUsedArray<T> array, T pivot, Comparator<T> cmp) {
-        return null;
+        var counts = new Pair<Integer, Integer>();
+        counts.fst = 0;
+        counts.snd = 0;
+
+        countLessAndGreater(array.theArray, pivot, cmp, 0, counts);
+
+        var p = createPairForDivideByPivot(counts.fst, counts.snd);
+        divideByPivotRec(array.theArray, pivot, cmp, p, 0, 0, 0);
+
+        return p;
+    }
+
+    private void divideByPivotRec(T[] array, T pivot, Comparator<T> cmp, Pair<FullyUsedArray<T>, FullyUsedArray<T>> p,
+                                  int index, int indexA, int indexB) {
+        if (index >= array.length) {
+            return;
+        }
+
+        var c =  cmp.compare(array[index], pivot);
+        if (c < 0) {
+            p.fst.theArray[indexA] = array[index];
+            divideByPivotRec(array, pivot, cmp, p, index+1, indexA+1, indexB);
+        } else if (c > 0) {
+            p.snd.theArray[indexB] = array[index];
+            divideByPivotRec(array, pivot, cmp, p, index+1, indexA, indexB+1);
+        } else {
+            divideByPivotRec(array, pivot, cmp, p, index+1, indexA, indexB);
+        }
+    }
+
+    private void countLessAndGreater(T[] array, T pivot, Comparator<T> cmp, int index, Pair<Integer, Integer> counts) {
+        if (index >= array.length) {
+            return;
+        }
+
+        var c =  cmp.compare(array[index], pivot);
+        if (c < 0) {
+            counts.fst++;
+        }
+        if (c > 0) {
+            counts.snd++;
+        }
+
+        countLessAndGreater(array, pivot, cmp, index+1, counts);
     }
 
     @Override

test/aud/exam/prep/SequenceProcessorTest.java

@@ -313,10 +313,10 @@ public abstract class SequenceProcessorTest<S> {
     void testThat_isItemWiseLessWithAlmostSameIsTrue(List<Integer> list) {
         S a = processor.create(list);
 
-        var index = list.size()/2;
         if (list.isEmpty()) {
             list.add(1);
         } else {
+            var index = list.size()/2;
             list.set(index, list.get(index)+1);
         }
         S b = processor.create(list);
@@ -330,14 +330,15 @@ public abstract class SequenceProcessorTest<S> {
     void testThat_isItemWiseLessWithPlus5Works(List<Integer> list) {
         S a = processor.create(list);
 
+        if (list.isEmpty()) {
+            list.add(1);
+        }
+
         for (int i = 0; i < list.size(); i++) {
             if (i % 2 == 0) {
                 list.set(i, list.get(i)+5);
             }
         }
-        if (list.isEmpty()) {
-            list.add(1);
-        }
         S b = processor.create(list);
 
         assertTrue(processor.isItemWiseLess(a, b, cmp));
@@ -353,7 +354,7 @@ public abstract class SequenceProcessorTest<S> {
 
     @ParameterizedTest
     @ArgumentsSource(DoubleLatinProvider.class)
-    void testThat_isLexSmallerWithWorksLikeStrings(String stringA, String stringB) {
+    void testThat_isLexSmallerWorksLikeStringsDo(String stringA, String stringB) {
         S a = toSequenceByAscii(stringA);
         S b = toSequenceByAscii(stringB);