CodeWars/4kyu/4_by_4_skyscrapers/solution.java

import java.util.ArrayList;
import java.util.Collections;
import java.util.Iterator;
import java.util.List;
import java.util.NoSuchElementException;
import java.util.stream.Stream;
import java.util.stream.StreamSupport;

public class SkyScrapers {
    private static class Permutations<T extends Comparable<T>> implements Iterable<List<T>> {
        public class PermutationsIterator implements Iterator<List<T>> {
            private List<T> elements;
            private boolean _hasNext;
            private boolean firstIteration = true;

            public PermutationsIterator(List<T> elements) {
                this.elements = elements;
                _hasNext = elements.size() > 0;
            }

            private void swap(int k, int l) {
                T tmp = elements.get(k);
                elements.set(k, elements.get(l));
                elements.set(l, tmp);
            }

            public boolean hasNext() {
                return _hasNext;
            }

            public List<T> next() {
                if (!_hasNext) {
                    throw new NoSuchElementException("No more permutations are left");
                }

                var lastIteration = new ArrayList<T>(elements);

                int k = 0, l = 0;
                _hasNext = false;
                for (int i = elements.size() - 1; i > 0; i--) {
                    if (elements.get(i).compareTo(elements.get(i - 1)) > 0) {
                        k = i - 1;
                        _hasNext = true;
                        break;
                    }
                }

                for (int i = elements.size() - 1; i > k; i--) {
                    if (elements.get(i).compareTo(elements.get(k)) > 0) {
                        l = i;
                        break;
                    }
                }

                swap(k, l);
                Collections.reverse(elements.subList(k + 1, elements.size()));

                return lastIteration;
            }
        }

        private List<T> elements;

        public Permutations(List<T> elements) {
            this.elements = elements;
            Collections.sort(this.elements);
        }

        public Iterator<List<T>> iterator() {
            return new PermutationsIterator(elements);
        }

        public Stream<List<T>> stream() {
            return StreamSupport.stream(this.spliterator(), false);
        }
    }

    private static class PossibleConfigurations implements Iterable<List<Integer>> {
        private List<Integer> initial;
        private int clue;

        private void generateInitial(int size) {
            initial = new ArrayList<Integer>();

            for (int i = 0; i < size; i++) {
                initial.add(i + 1);
            }
        }

        public PossibleConfigurations(int size, int clue) {
            generateInitial(size);
            this.clue = clue;
        }

        public Iterator<List<Integer>> iterator() {
            if (clue == 0) {
                return (new Permutations(initial)).iterator();
            }

            return (new Permutations(initial))
                .stream()
                .filter(heights -> isValid((List<Integer>) heights, clue))
                .iterator();
        }
    }

    private static boolean isValid(List<Integer> heights, int clue) {
        int canSee = 1;
        int lastHeight = heights.get(0);

        for (int i = 1; i < heights.size(); i++) {
            int currentHeight = heights.get(i);
            if (currentHeight > lastHeight) {
                lastHeight = currentHeight;
                canSee++;
            }
        }

        return canSee == clue;
    }

    private static int getDx(int clueIndex) {
        if (clueIndex >= 4 && clueIndex <= 7) {
            return -1;
        }

        if (clueIndex >= 12) {
            return 1;
        }

        return 0;
    }

    private static int getDy(int clueIndex) {
        if (clueIndex <= 3) {
            return 1;
        }

        if (clueIndex >= 8 && clueIndex <= 11) {
            return -1;
        }

        return 0;
    }

    private static int getStartX(int clueIndex) {
        if (clueIndex < 4) {
            return clueIndex;
        }

        if (clueIndex <= 7) {
            return 3;
        }

        if (clueIndex <= 11) {
            return 3 - clueIndex % 4;
        }

        return 0;
    }

    private static int getStartY(int clueIndex) {
        if (clueIndex < 4) {
            return 0;
        }

        if (clueIndex <= 7) {
            return clueIndex % 4;
        }

        if (clueIndex <= 11) {
            return 3;
        }

        return 3 - clueIndex % 4;
    }

    private static List<Integer> backupHeights(int[][] heights, int clueIndex) {
        List<Integer> backup = new ArrayList<Integer>();

        int dx = getDx(clueIndex);
        int dy = getDy(clueIndex);

        for (
            int x = getStartX(clueIndex), y = getStartY(clueIndex);
            x >= 0 && x < 4 && y >= 0 && y < 4;
            x += dx, y += dy
        ) {
            backup.add(heights[y][x]);
        }

        return backup;
    }

    private static boolean emplaceHeights(
        int[][] heights, int clueIndex, List<Integer> newHeights, boolean force
    ) {
        int i = 0;
        int dx = getDx(clueIndex);
        int dy = getDy(clueIndex);

        for (
            int x = getStartX(clueIndex), y = getStartY(clueIndex);
            x >= 0 && x < 4 && y >= 0 && y < 4;
            x += dx, y += dy
        ) {
            if (
                !force && heights[y][x] != 0 && heights[y][x] != newHeights.get(i)
            ) {
                return false;
            }

            heights[y][x] = newHeights.get(i++);
        }

        return true;
    }

    private static boolean solvePuzzle(
        int[][] heights, int[] clues, int clueIndex, boolean ignoreZeroes
    ) {
        while (clueIndex < 16 && ((ignoreZeroes && clues[clueIndex] == 0) || (!ignoreZeroes && clues[clueIndex] != 0))) {
            clueIndex++;
        }

        if (clueIndex >= 16) {
            return true;
        }

        // create copy of heights to ensure correct resetting
        List<Integer> currentHeights = backupHeights(heights, clueIndex);

        // iterate through the options
        for (List<Integer> possibleHeights : new PossibleConfigurations(4, clues[clueIndex])) {

            // emplace heights and if conflict occurs, reset and try next one
            if (!emplaceHeights(heights, clueIndex, possibleHeights, false)) {
                emplaceHeights(heights, clueIndex, currentHeights, true);
                continue;
            }

            // if no conflict present, try filling out other clues
            if (solvePuzzle(heights, clues, clueIndex + 1, ignoreZeroes)) {
                return true;
            }

            // otherwise reset heights and try again
            emplaceHeights(heights, clueIndex, currentHeights, true);
        }

        // if we got here, there is no feasible configuration of buildings
        return false;
    }

    static int[][] solvePuzzle(int[] clues) {
        var result = new int[4][4];
        solvePuzzle(result, clues, 0, true);

        // in case there are left zeroes
        solvePuzzle(result, clues, 0, false);

        return result;
    }
}