# Skyscraper Solver for NxN Size Version 3 (Using Bitmasks)

This is a follow up of Skyscraper Solver for NxN Size Version 2 (Using Backtracking)

I followed the advice in the last Codereview and did the following optimizations:

• Implementation of the class Field using a single Bitmask wiht std::uint32_t to represent Skyscrapers and Nopes. To give you an idea for n=4 the representation looks like this:

b0000 0 Nopes = {}
b0001 1 Nopes = 1
b0010 2 Nopes = 2
b0011 3 Nopes = 1, 2
b0100 4 Nopes = 3
b0101 5 Nopes = 1, 3
b0110 6 Nopes = 2, 3
b0111 7 Skyscraper = 4
b1000 8 Nopes = 4
b1001 9 Nopes = 1, 4
b1010 10 Nopes = 2, 4
b1011 11 Skyscraper = 3
b1100 12 Nopes = 3, 4
b1101 13 Skyscraper
b1110 14 Skyscraper = 1
b1111 15 Invalid all nope


Now the idea was that now that we represent Nopes and Skyscrapers with a bitmask the size for each field should go down a lot. Before this optimization I used an extra Nope class which is an std::unordered_set and an int for representing the Skyscraper.

So some of the old Nope class methods could get fused into the new Field class saving us the extra implementation and keeping track of all the Nope stuff.

• Storing the fields of the Board in an flat array. So instead of std::vector<std::vector<Field>> I know use std::vector<Field>. With the size of the puzzle we can still access each two dimensional position on the Board.

Now the bad news. With the optimization I was hoping the backtracking would solve the puzzles faster. Unfortunately the opposite happens. Now my solution is even more slow. I wonder if I just implemented the Field class with the Bitmask wrong.

So please let me know what stinks in terms of performance.

The full source code:

#include "codewarsbacktracking.h"

#include <algorithm>
#include <cassert>
#include <iomanip>
#include <iostream>
#include <numeric>
#include <string>
#include <unordered_set>

namespace codewarsbacktracking {

struct ClueHints {
ClueHints(std::size_t boardSize);
ClueHints();

void reverse();

void removeNopesOnSkyscrapers();

std::vector<int> skyscrapers{};
std::vector<std::vector<int>> nopes{};
};

ClueHints::ClueHints(std::size_t boardSize)
: skyscrapers{std::vector<int>(boardSize, 0)},
nopes{std::vector<std::vector<int>>(boardSize, std::vector<int>{})}
{
}

void ClueHints::reverse()
{
std::reverse(skyscrapers.begin(), skyscrapers.end());
std::reverse(nopes.begin(), nopes.end());
}

void ClueHints::removeNopesOnSkyscrapers()
{
for (std::size_t i = 0; i < skyscrapers.size(); ++i) {
if (skyscrapers[i] == 0) {
continue;
}
nopes[i].clear();
}
}

std::optional<ClueHints> getClueHints(int clue, std::size_t boardSize)
{
if (clue == 0) {
return {};
}

ClueHints clueHints{boardSize};

std::vector<std::unordered_set<int>> nopes(boardSize,
std::unordered_set<int>{});

if (clue == static_cast<int>(boardSize)) {
for (std::size_t i = 0; i < boardSize; ++i) {
clueHints.skyscrapers[i] = i + 1;
}
}
else if (clue == 1) {
clueHints.skyscrapers[0] = boardSize;
}
else if (clue == 2) {
nopes[0].insert(boardSize);
nopes[1].insert(boardSize - 1);
}
else {
for (std::size_t fieldIdx = 0;
fieldIdx < static_cast<std::size_t>(clue - 1); ++fieldIdx) {

for (std::size_t nopeValue = boardSize;
nopeValue >= (boardSize - (clue - 2) + fieldIdx);
--nopeValue) {
nopes[fieldIdx].insert(nopeValue);
}
}
}

assert(nopes.size() == clueHints.nopes.size());

for (std::size_t i = 0; i < nopes.size(); ++i) {
clueHints.nopes[i] = std::vector<int>(nopes[i].begin(), nopes[i].end());
}
return {clueHints};
}

std::optional<ClueHints> merge(std::optional<ClueHints> optFrontClueHints,
std::optional<ClueHints> optBackClueHints)
{
if (!optFrontClueHints && !optBackClueHints) {
return {};
}
if (!optFrontClueHints) {
optBackClueHints->reverse();
return optBackClueHints;
}
if (!optBackClueHints) {
return optFrontClueHints;
}

auto size = optFrontClueHints->skyscrapers.size();
ClueHints clueHints{size};

assert(optFrontClueHints->skyscrapers.size() ==
optFrontClueHints->nopes.size());
assert(optBackClueHints->skyscrapers.size() ==
optBackClueHints->nopes.size());
assert(optFrontClueHints->skyscrapers.size() ==
optBackClueHints->skyscrapers.size());

optBackClueHints->reverse();

for (std::size_t i = 0; i < optFrontClueHints->skyscrapers.size(); ++i) {

auto frontSkyscraper = optFrontClueHints->skyscrapers[i];
auto backSkyscraper = optBackClueHints->skyscrapers[i];

if (frontSkyscraper != 0 && backSkyscraper != 0) {
assert(frontSkyscraper == backSkyscraper);
clueHints.skyscrapers[i] = frontSkyscraper;
}
else if (frontSkyscraper != 0) {
clueHints.skyscrapers[i] = frontSkyscraper;
clueHints.nopes[i].clear();
}
else { // backSkyscraper != 0
clueHints.skyscrapers[i] = backSkyscraper;
clueHints.nopes[i].clear();
}

if (clueHints.skyscrapers[i] != 0) {
continue;
}

std::unordered_set<int> nopes(optFrontClueHints->nopes[i].begin(),
optFrontClueHints->nopes[i].end());
nopes.insert(optBackClueHints->nopes[i].begin(),
optBackClueHints->nopes[i].end());
clueHints.nopes[i] = std::vector<int>(nopes.begin(), nopes.end());
}
clueHints.removeNopesOnSkyscrapers();
return {clueHints};
}

void mergeClueHintsPerRow(std::vector<std::optional<ClueHints>> &clueHints)
{
std::size_t startOffset = clueHints.size() / 4 * 3 - 1;
std::size_t offset = startOffset;

for (std::size_t frontIdx = 0; frontIdx < clueHints.size() / 2;
++frontIdx, offset -= 2) {

if (frontIdx == clueHints.size() / 4) {
offset = startOffset;
}

int backIdx = frontIdx + offset;

clueHints[frontIdx] = merge(clueHints[frontIdx], clueHints[backIdx]);
}
clueHints.erase(clueHints.begin() + clueHints.size() / 2, clueHints.end());
}

std::vector<std::optional<ClueHints>>
getClueHints(const std::vector<int> &clues, std::size_t boardSize)
{
std::vector<std::optional<ClueHints>> clueHints;
clueHints.reserve(clues.size());

for (const auto &clue : clues) {
clueHints.emplace_back(getClueHints(clue, boardSize));
}
mergeClueHintsPerRow(clueHints);
return clueHints;
}

template <typename It> int missingNumberInSequence(It begin, It end)
{
int n = std::distance(begin, end) + 1;
double projectedSum = (n + 1) * (n / 2.0);
int actualSum = std::accumulate(begin, end, 0);
return projectedSum - actualSum;
}

/*
Example size = 4

b0000 0 Nopes = {}
b0001 1 Nopes = 1
b0010 2 Nopes = 2
b0011 3 Nopes = 1, 2
b0100 4 Nopes = 3
b0101 5 Nopes = 1, 3
b0110 6 Nopes = 2, 3
b0111 7 Skyscraper = 4
b1000 8 Nopes = 4
b1001 9 Nopes = 1, 4
b1010 10 Nopes = 2, 4
b1011 11 Skyscraper = 3
b1100 12 Nopes = 3, 4
b1101 13 Skyscraper
b1110 14 Skyscraper = 1
b1111 15 Invalid all nope
*/

class Field {
public:
Field(std::size_t size);

void insertSkyscraper(int skyscraper);
void insertNope(int nope);
void insertNopes(const std::vector<int> &nopes);

int skyscraper() const;
std::vector<int> nopes() const;

bool hasSkyscraper() const;

bool containsNope(int value) const;
bool containsNopes(const std::vector<int> &values);

private:

std::size_t mSize;

friend inline bool operator==(const Field &lhs, const Field &rhs);
friend inline bool operator!=(const Field &lhs, const Field &rhs);
};

inline bool operator==(const Field &lhs, const Field &rhs)
{
}
inline bool operator!=(const Field &lhs, const Field &rhs)
{
return !(lhs == rhs);
}

Field::Field(std::size_t size) : mSize{size}
{
}

void Field::insertSkyscraper(int skyscraper)
{
assert(skyscraper > 0 && skyscraper <= static_cast<int>(mSize));
for (int i = 0; i < static_cast<int>(mSize); ++i) {
if (i != skyscraper - 1) {
}
}
}

void Field::insertNope(int nope)
{
assert(nope > 0 && nope <= static_cast<int>(mSize));
mBitmask |= 1 << (nope - 1);
}

void Field::insertNopes(const std::vector<int> &nopes)
{
for (const auto nope : nopes) {
insertNope(nope);
}
}

int Field::skyscraper() const
{
if (!hasSkyscraper()) {
return 0;
}

for (std::size_t i = 0; i < mSize; ++i) {
return i + 1;
}
}
return 0;
}

std::vector<int> Field::nopes() const
{
std::vector<int> nopes;
nopes.reserve(mSize - 1);
for (std::size_t i = 0; i < mSize; ++i) {
nopes.emplace_back(i + 1);
}
}
return nopes;
}

bool Field::hasSkyscraper() const
{
bool foundZero = false;
for (std::size_t i = 0; i < mSize; ++i) {
if (!foundZero) {
foundZero = true;
}
else { // found more than one zero so no skyscraper present
return false;
}
}
}
return true;
}

bool Field::containsNope(int value) const
{
}

bool Field::containsNopes(const std::vector<int> &values)
{
for (const auto &value : values) {
if (!containsNope(value)) {
return false;
}
}
return true;
}

{
}

{
}

{
return bitmask & (1 << bit);
}

struct Point {
int x;
int y;
};

inline bool operator==(const Point &lhs, const Point &rhs)
{
return lhs.x == rhs.x && lhs.y == rhs.y;
}
inline bool operator!=(const Point &lhs, const Point &rhs)
{
return !(lhs == rhs);
}

enum class ReadDirection { topToBottom, rightToLeft };

int clueIdx);

{
++dir;
}

int clueIdx)
{
if (clueIdx == 0) {
return;
}
++point.x;
break;
++point.y;
break;
}
}

class Row {
public:
Row(std::vector<Field> &fields, std::size_t size, const Point &startPoint,

void insertSkyscraper(int pos, int skyscraper);

std::size_t size() const;

bool hasOnlyOneNopeField() const;

bool allFieldsContainSkyscraper() const;

int skyscraperCount() const;
int nopeCount(int nope) const;

void guessSkyscraperOutOfNeighbourNopes();

enum class Direction { front, back };

bool hasSkyscrapers(const std::vector<int> &skyscrapers,
Direction direction) const;
bool hasNopes(const std::vector<std::vector<int>> &nopes,
Direction direction) const;

Direction direction);
Direction direction);

std::vector<Field *> getFields() const;

private:
template <typename SkyIterator, typename FieldIterator>
bool hasSkyscrapers(SkyIterator skyItBegin, SkyIterator skyItEnd,
FieldIterator fieldItBegin,
FieldIterator fieldItEnd) const;

template <typename NopesIterator, typename FieldIterator>
bool hasNopes(NopesIterator nopesItBegin, NopesIterator nopesItEnd,
FieldIterator fieldItBegin, FieldIterator fieldItEnd) const;

template <typename SkyIterator, typename FieldIterator>
FieldIterator fieldItBegin, FieldIterator fieldItEnd);

template <typename NopesIterator, typename FieldIterator>
FieldIterator fieldItBegin, FieldIterator fieldItEnd);

template <typename IteratorType>
void insertSkyscraper(IteratorType it, int skyscraper);

template <typename IteratorType> void insertNope(IteratorType it, int nope);

template <typename IteratorType>
void insertNopes(IteratorType it, const std::vector<int> &nopes);

int getIdx(std::vector<Field *>::const_iterator cit) const;
int getIdx(std::vector<Field *>::const_reverse_iterator crit) const;

std::vector<Field> &boardFields,
std::size_t size,
const Point &startPoint);

bool onlyOneFieldWithoutNope(int nope) const;

bool nopeExistsAsSkyscraperInFields(const std::vector<Field *> &rowFields,
int nope) const;

std::optional<int> nopeValueInAllButOneField() const;

void insertSkyscraperToFirstFieldWithoutNope(int nope);

bool hasSkyscraper(int skyscraper) const;

std::vector<Row *> mCrossingRows;
std::vector<Field *> mRowFields;
};

Row::Row(std::vector<Field> &fields, std::size_t size, const Point &startPoint,
{
}

void Row::insertSkyscraper(int pos, int skyscraper)
{
assert(pos >= 0 && pos < static_cast<int>(mRowFields.size()));
assert(skyscraper > 0 && skyscraper <= static_cast<int>(mRowFields.size()));
auto it = mRowFields.begin() + pos;
insertSkyscraper(it, skyscraper);
}

std::size_t Row::size() const
{
return mRowFields.size();
}

{
assert(crossingRow != nullptr);
assert(mCrossingRows.size() < size());
mCrossingRows.push_back(crossingRow);
}

bool Row::hasOnlyOneNopeField() const
{
return skyscraperCount() == static_cast<int>(size() - 1);
}

{
assert(hasOnlyOneNopeField());

auto nopeFieldIt = mRowFields.end();
std::vector<int> sequence;
sequence.reserve(size() - 1);

for (auto it = mRowFields.begin(); it != mRowFields.end(); ++it) {
if ((*it)->hasSkyscraper()) {
sequence.emplace_back((*it)->skyscraper());
}
else {
nopeFieldIt = it;
}
}
assert(nopeFieldIt != mRowFields.end());
assert(skyscraperCount() == static_cast<int>(sequence.size()));

auto missingValue =
missingNumberInSequence(sequence.begin(), sequence.end());

assert(missingValue >= 0 && missingValue <= static_cast<int>(size()));
insertSkyscraper(nopeFieldIt, missingValue);
}

{
for (auto it = mRowFields.begin(); it != mRowFields.end(); ++it) {
if ((*it)->hasSkyscraper()) {
continue;
}
insertNope(it, nope);
}
}

bool Row::allFieldsContainSkyscraper() const
{
return skyscraperCount() == static_cast<int>(size());
}

int Row::skyscraperCount() const
{
int count = 0;
for (auto cit = mRowFields.cbegin(); cit != mRowFields.cend(); ++cit) {
if ((*cit)->hasSkyscraper()) {
++count;
}
}
return count;
}

int Row::nopeCount(int nope) const
{
int count = 0;
for (auto cit = mRowFields.cbegin(); cit != mRowFields.cend(); ++cit) {
if ((*cit)->hasSkyscraper()) {
continue;
}
if ((*cit)->containsNope(nope)) {
++count;
}
}
return count;
}

void Row::guessSkyscraperOutOfNeighbourNopes()
{
for (;;) {
auto optNope = nopeValueInAllButOneField();
if (!optNope) {
break;
}
insertSkyscraperToFirstFieldWithoutNope(*optNope);
}
}

bool Row::hasSkyscrapers(const std::vector<int> &skyscrapers,
Row::Direction direction) const
{
if (direction == Direction::front) {
return hasSkyscrapers(skyscrapers.cbegin(), skyscrapers.cend(),
mRowFields.cbegin(), mRowFields.cend());
}
return hasSkyscrapers(skyscrapers.cbegin(), skyscrapers.cend(),
mRowFields.crbegin(), mRowFields.crend());
}

bool Row::hasNopes(const std::vector<std::vector<int>> &nopes,
Direction direction) const
{
if (direction == Direction::front) {
return hasNopes(nopes.cbegin(), nopes.cend(), mRowFields.cbegin(),
mRowFields.cend());
}
return hasNopes(nopes.cbegin(), nopes.cend(), mRowFields.crbegin(),
mRowFields.crend());
}

Direction direction)
{
if (direction == Direction::front) {
mRowFields.begin(), mRowFields.end());
}
else {
mRowFields.rbegin(), mRowFields.rend());
}
}
Direction direction)
{
if (direction == Direction::front) {
mRowFields.end());
}
else {
mRowFields.rend());
}
}

std::vector<Field *> Row::getFields() const
{
return mRowFields;
}

template <typename SkyIterator, typename FieldIterator>
bool Row::hasSkyscrapers(SkyIterator skyItBegin, SkyIterator skyItEnd,
FieldIterator fieldItBegin,
FieldIterator fieldItEnd) const
{
auto skyIt = skyItBegin;
for (auto fieldIt = fieldItBegin;
fieldIt != fieldItEnd && skyIt != skyItEnd; ++fieldIt, ++skyIt) {
if (*skyIt == 0 && (*fieldIt)->hasSkyscraper()) {
continue;
}
if ((*fieldIt)->skyscraper() != *skyIt) {
return false;
}
}
return true;
}

template <typename NopesIterator, typename FieldIterator>
bool Row::hasNopes(NopesIterator nopesItBegin, NopesIterator nopesItEnd,
FieldIterator fieldItBegin, FieldIterator fieldItEnd) const
{
auto nopesIt = nopesItBegin;
for (auto fieldIt = fieldItBegin;
fieldIt != fieldItEnd && nopesIt != nopesItEnd; ++fieldIt, ++nopesIt) {

if (nopesIt->empty()) {
continue;
}
if ((*fieldIt)->hasSkyscraper()) {
return false;
}
if (!(*fieldIt)->containsNopes(*nopesIt)) {
return false;
}
}
return true;
}

template <typename SkyIterator, typename FieldIterator>
FieldIterator fieldItBegin, FieldIterator fieldItEnd)
{
auto skyIt = skyItBegin;
for (auto fieldIt = fieldItBegin;
fieldIt != fieldItEnd && skyIt != skyItEnd; ++fieldIt, ++skyIt) {
if (*skyIt == 0) {
continue;
}
insertSkyscraper(fieldIt, *skyIt);
}
}

template <typename NopesIterator, typename FieldIterator>
FieldIterator fieldItBegin, FieldIterator fieldItEnd)
{
auto nopesIt = nopesItBegin;
for (auto fieldIt = fieldItBegin;
fieldIt != fieldItEnd && nopesIt != nopesItEnd; ++fieldIt, ++nopesIt) {
if (nopesIt->empty()) {
continue;
}
insertNopes(fieldIt, *nopesIt);
}
}

template <typename FieldIterator>
void Row::insertSkyscraper(FieldIterator fieldIt, int skyscraper)
{
assert(mCrossingRows.size() == size());

if ((*fieldIt)->hasSkyscraper()) {
return;
}
(*fieldIt)->insertSkyscraper(skyscraper);

if (hasOnlyOneNopeField()) {
}

int idx = getIdx(fieldIt);

if (mCrossingRows[idx]->hasOnlyOneNopeField()) {
}

}

template <typename FieldIterator>
void Row::insertNope(FieldIterator fieldIt, int nope)
{
if ((*fieldIt)->hasSkyscraper()) {
return;
}
if ((*fieldIt)->containsNope(nope)) {
return;
}

bool hasSkyscraperBefore = (*fieldIt)->hasSkyscraper();
(*fieldIt)->insertNope(nope);

// skyscraper was added so we have to add nopes to the neighbours
// probaly could insert only nopes directly
if (!hasSkyscraperBefore && (*fieldIt)->hasSkyscraper()) {
insertSkyscraper(fieldIt, (*fieldIt)->skyscraper());
}

if (onlyOneFieldWithoutNope(nope)) {
insertSkyscraperToFirstFieldWithoutNope(nope);
}

int idx = getIdx(fieldIt);

if (mCrossingRows[idx]->onlyOneFieldWithoutNope(nope)) {
mCrossingRows[idx]->insertSkyscraperToFirstFieldWithoutNope(nope);
}
}

template <typename IteratorType>
void Row::insertNopes(IteratorType it, const std::vector<int> &nopes)
{
for (const auto &nope : nopes) {
insertNope(it, nope);
}
}

int Row::getIdx(std::vector<Field *>::const_iterator cit) const
{
return std::distance(mRowFields.cbegin(), cit);
}

int Row::getIdx(std::vector<Field *>::const_reverse_iterator crit) const
{
return size() - std::distance(mRowFields.crbegin(), crit) - 1;
}

std::vector<Field> &boardFields,
std::size_t size,
const Point &startPoint)
{
std::vector<Field *> fields;
fields.reserve(size);
std::size_t x = startPoint.x;
std::size_t y = startPoint.y;

for (std::size_t i = 0; i < size; ++i) {
fields.emplace_back(&boardFields[x + y * size]);
++y;
}
}
for (std::size_t i = 0; i < size; ++i) {
fields.emplace_back(&boardFields[x + y * size]);
--x;
}
}
return fields;
}

bool Row::onlyOneFieldWithoutNope(int nope) const
{
if (nopeExistsAsSkyscraperInFields(mRowFields, nope)) {
return false;
}
if (nopeCount(nope) < static_cast<int>(size()) - skyscraperCount() - 1) {
return false;
}
return true;
}

bool Row::nopeExistsAsSkyscraperInFields(const std::vector<Field *> &rowFields,
int nope) const
{
auto cit = std::find_if(
rowFields.cbegin(), rowFields.cend(),
[nope](const auto &field) { return field->skyscraper() == nope; });
return cit != rowFields.cend();
}

std::optional<int> Row::nopeValueInAllButOneField() const
{
std::unordered_map<int, int> nopeAndCount;

for (auto cit = mRowFields.cbegin(); cit != mRowFields.cend(); ++cit) {
if (!(*cit)->hasSkyscraper()) {
auto nopes = (*cit)->nopes();
for (const auto &nope : nopes) {
if (hasSkyscraper(nope)) {
continue;
}
++nopeAndCount[nope];
}
}
}
for (auto cit = nopeAndCount.cbegin(); cit != nopeAndCount.end(); ++cit) {
if (cit->second == static_cast<int>(size()) - skyscraperCount() - 1) {
return {cit->first};
}
}
return {};
}

void Row::insertSkyscraperToFirstFieldWithoutNope(int nope)
{
for (auto it = mRowFields.begin(); it != mRowFields.end(); ++it) {
if ((*it)->hasSkyscraper()) {
continue;
}
if (!(*it)->containsNope(nope)) {
insertSkyscraper(it, nope);
return; // there can be max one skyscraper per row;
}
}
}

bool Row::hasSkyscraper(int skyscraper) const
{
for (const auto &field : mRowFields) {
if (field->skyscraper() == skyscraper) {
return true;
}
}
return false;
}

class BorderIterator {
public:
BorderIterator(std::size_t boardSize);

Point point() const;

BorderIterator &operator++();

private:
int mIdx = 0;
std::size_t mBoardSize;
Point mPoint{0, 0};
};

BorderIterator::BorderIterator(std::size_t boardSize) : mBoardSize{boardSize}
{
}

Point BorderIterator::point() const
{
return mPoint;
}

{
}

BorderIterator &BorderIterator::operator++()
{
++mIdx;
if (mIdx == static_cast<int>(2 * mBoardSize)) {
return *this;
}
if (mIdx != 0 && mIdx % mBoardSize == 0) {
}

return *this;
}

struct Board {
Board(std::size_t size);

void insert(const std::vector<std::optional<ClueHints>> &clueHints);

void insert(const std::vector<std::vector<int>> &startingSkyscrapers);

bool isSolved() const;

std::vector<Field> fields;

std::vector<Row> mRows;

std::vector<std::vector<int>> skyscrapers2d() const;

std::size_t size() const;

private:
void makeRows();
void connnectRowsWithCrossingRows();

std::size_t mSize;
};

Board::Board(std::size_t size)
: fields{std::vector<Field>(size * size, Field{size})}, mSize{size}
{
makeRows();
}

void Board::insert(const std::vector<std::optional<ClueHints>> &clueHints)
{
assert(clueHints.size() == mRows.size());

for (std::size_t i = 0; i < clueHints.size(); ++i) {
if (!clueHints[i]) {
continue;
}
Row::Direction::front);
}
}

void Board::insert(const std::vector<std::vector<int>> &startingSkyscrapers)
{
if (startingSkyscrapers.empty()) {
return;
}
std::size_t boardSize = mRows.size() / 2;
assert(startingSkyscrapers.size() == boardSize);

for (std::size_t i = 0; i < startingSkyscrapers.size(); ++i) {
Row::Direction::back);
}
}

bool Board::isSolved() const
{
std::size_t endVerticalRows = mRows.size() / 2;
for (std::size_t i = 0; i < endVerticalRows; ++i) {
if (!mRows[i].allFieldsContainSkyscraper()) {
return false;
}
}
return true;
}

std::vector<std::vector<int>> Board::skyscrapers2d() const
{
std::vector<std::vector<int>> skyscrapers2d(mSize, std::vector<int>());

std::size_t j = 0;
skyscrapers2d[j].reserve(mSize);
for (std::size_t i = 0; i < fields.size(); ++i) {
if (i != 0 && i % mSize == 0) {
++j;
skyscrapers2d[j].reserve(mSize);
}
skyscrapers2d[j].emplace_back(fields[i].skyscraper());
}
return skyscrapers2d;
}

std::size_t Board::size() const
{
return mSize;
}

void Board::makeRows()
{
BorderIterator borderIterator{mSize};

std::size_t rowSize = mSize * 2;
mRows.reserve(rowSize);

for (std::size_t i = 0; i < rowSize; ++i, ++borderIterator) {
mRows.emplace_back(Row{fields, mSize, borderIterator.point(),
}
connnectRowsWithCrossingRows();
}

void Board::connnectRowsWithCrossingRows()
{
std::size_t boardSize = mRows.size() / 2;

std::vector<int> targetRowsIdx(boardSize);
std::iota(targetRowsIdx.begin(), targetRowsIdx.end(), boardSize);

for (std::size_t i = 0; i < mRows.size(); ++i) {
if (i == mRows.size() / 2) {
std::iota(targetRowsIdx.begin(), targetRowsIdx.end(), 0);
std::reverse(targetRowsIdx.begin(), targetRowsIdx.end());
}

for (const auto &targetRowIdx : targetRowsIdx) {
}
}
}

void debug_print(Board &board, const std::string &title)
{
std::cout << title << '\n';

for (std::size_t i = 0; i < board.fields.size(); ++i) {

if (i % board.size() == 0 && i != 0) {
std::cout << '\n';
}

if (board.fields[i].skyscraper() != 0) {
std::cout << std::setw(board.size() * 2);
std::cout << "V" + std::to_string(board.fields[i].skyscraper());
}
else if (board.fields[i].skyscraper() == 0 &&
!board.fields[i].nopes().empty()) {
auto nopes_set = board.fields[i].nopes();
std::vector<int> nopes(nopes_set.begin(), nopes_set.end());
std::sort(nopes.begin(), nopes.end());

std::string nopesStr;
for (std::size_t i = 0; i < nopes.size(); ++i) {
nopesStr.append(std::to_string(nopes[i]));
if (i != nopes.size() - 1) {
nopesStr.push_back(',');
}
}
std::cout << std::setw(board.size() * 2);
std::cout << nopesStr;
}
else {
std::cout << ' ';
}
}
std::cout << '\n';
}

template <typename FieldIterator>
int visibleBuildings(FieldIterator begin, FieldIterator end)
{
int visibleBuildingsCount = 0;
int highestSeen = 0;
for (auto it = begin; it != end; ++it) {
if (it->skyscraper() != 0 && it->skyscraper() > highestSeen) {
++visibleBuildingsCount;
highestSeen = it->skyscraper();
}
}
return visibleBuildingsCount;
}

bool rowsAreValid(const std::vector<Field> &fields, std::size_t index,
std::size_t rowSize)
{
std::size_t row = index / rowSize;
for (std::size_t currIndex = row * rowSize; currIndex < (row + 1) * rowSize;
++currIndex) {
if (currIndex == index) {
continue;
}
if (fields[currIndex].skyscraper() == fields[index].skyscraper()) {
return false;
}
}
return true;
}

bool columnsAreValid(const std::vector<Field> &fields, std::size_t index,
std::size_t rowSize)
{
std::size_t column = index % rowSize;

for (std::size_t i = 0; i < rowSize; ++i) {
std::size_t currIndex = column + i * rowSize;
if (currIndex == index) {
continue;
}
if (fields[currIndex].skyscraper() == fields[index].skyscraper()) {
return false;
}
}
return true;
}

std::tuple<int, int> getRowClues(const std::vector<int> &clues, std::size_t row,
std::size_t rowSize)
{
int frontClue = clues[clues.size() - 1 - row];
int backClue = clues[rowSize + row];
return {frontClue, backClue};
}

bool rowCluesAreValid(const std::vector<Field> &fields,
const std::vector<int> &clues, std::size_t index,
std::size_t rowSize)
{
std::size_t row = index / rowSize;

auto [frontClue, backClue] = getRowClues(clues, row, rowSize);

if (frontClue == 0 && backClue == 0) {
return true;
}

std::size_t rowIndexBegin = row * rowSize;
std::size_t rowIndexEnd = (row + 1) * rowSize;

auto citBegin = fields.cbegin() + rowIndexBegin;
auto citEnd = fields.cbegin() + rowIndexEnd;

bool rowIsFull = std::find_if(citBegin, citEnd, [](const Field &field) {
return !field.hasSkyscraper();
}) == citEnd;

if (!rowIsFull) {
return true;
}

if (frontClue != 0) {
auto frontVisible = visibleBuildings(citBegin, citEnd);

if (frontClue != frontVisible) {
return false;
}
}

auto critBegin = std::make_reverse_iterator(citEnd);
auto critEnd = std::make_reverse_iterator(citBegin);

if (backClue != 0) {
auto backVisible = visibleBuildings(critBegin, critEnd);

if (backClue != backVisible) {
return false;
}
}
return true;
}

std::tuple<int, int> getColumnClues(const std::vector<int> &clues,
std::size_t x, std::size_t size)
{
int frontClue = clues[x];
int backClue = clues[size * 3 - 1 - x];
return {frontClue, backClue};
}

bool columnCluesAreValid(const std::vector<Field> &fields,
const std::vector<int> &clues, std::size_t index,
std::size_t rowSize)
{
std::size_t column = index % rowSize;

auto [frontClue, backClue] = getColumnClues(clues, column, rowSize);

if (frontClue == 0 && backClue == 0) {
return true;
}

std::vector<Field> verticalFields;
verticalFields.reserve(rowSize);

for (std::size_t i = 0; i < rowSize; ++i) {
verticalFields.emplace_back(fields[column + i * rowSize]);
}

bool columnIsFull =
std::find_if(verticalFields.cbegin(), verticalFields.cend(),
[](const Field &field) {
return !field.hasSkyscraper();
}) == verticalFields.cend();

if (!columnIsFull) {
return true;
}

if (frontClue != 0) {
auto frontVisible =
visibleBuildings(verticalFields.cbegin(), verticalFields.cend());
if (frontClue != frontVisible) {
return false;
}
}
if (backClue != 0) {
auto backVisible =
visibleBuildings(verticalFields.crbegin(), verticalFields.crend());

if (backClue != backVisible) {
return false;
}
}
return true;
}

bool skyscrapersAreValidPositioned(const std::vector<Field> &fields,
const std::vector<int> &clues,
std::size_t index, std::size_t rowSize)
{
if (!rowsAreValid(fields, index, rowSize)) {
return false;
}
if (!columnsAreValid(fields, index, rowSize)) {
return false;
}
if (!rowCluesAreValid(fields, clues, index, rowSize)) {
return false;
}
if (!columnCluesAreValid(fields, clues, index, rowSize)) {
return false;
}
return true;
}

bool guessSkyscrapers(Board &board, const std::vector<int> &clues,
std::size_t index, std::size_t countOfElements,
std::size_t rowSize)
{
if (index == countOfElements) {
return true;
}

if (board.fields[index].skyscraper() != 0) {
if (!skyscrapersAreValidPositioned(board.fields, clues, index,
rowSize)) {
return false;
}
if (guessSkyscrapers(board, clues, index + 1, countOfElements,
rowSize)) {
return true;
}
return false;
}

for (int trySkyscraper = 1; trySkyscraper <= static_cast<int>(rowSize);
++trySkyscraper) {

if (board.fields[index].containsNope(trySkyscraper)) {
continue;
}
board.fields[index].insertSkyscraper(trySkyscraper);
if (!skyscrapersAreValidPositioned(board.fields, clues, index,
rowSize)) {
continue;
}
if (guessSkyscrapers(board, clues, index + 1, countOfElements,
rowSize)) {
return true;
}
}
return false;
}

std::vector<std::vector<int>>
SolvePuzzle(const std::vector<int> &clues,
std::vector<std::vector<int>> startingGrid, int)
{
assert(clues.size() % 4 == 0);

std::size_t boardSize = clues.size() / 4;

auto clueHints = getClueHints(clues, boardSize);

Board board{boardSize};

board.insert(clueHints);
board.insert(startingGrid);

if (board.isSolved()) {
return board.skyscrapers2d();
}
guessSkyscrapers(board, clues, 0, board.fields.size(), board.size());

return board.skyscrapers2d();
}

std::vector<std::vector<int>> SolvePuzzle(const std::vector<int> &clues)
{
return SolvePuzzle(clues, std::vector<std::vector<int>>{}, 0);
}

} // namespace codewarsbacktracking


Additional things like the unit test can be found here

My overall impression of this code is that you are trying too hard to add abstractions to the code, but are not paying attention to performance. Of course, the trick is to find the right balance between the two. It might be an interesting excercise to write a solution to the problem in C, where it is more obvious when you try to do something inefficient, and then try to convert it back to C++ by finding out which pieces of code can be replaced by use of standard containers and algorithms, and your own classes if there is nothing appropriate in the STL. Here is a list of some more specific problems that could be addressed:

# Avoid nested std::vectors

You are still nesting std::vectors, for eaxmple ClueHint::nopes. There's also a less obvious one: Board::mRows is a vector of Rows, but each Row contains a std::vector<Field *>. So this is effectively a nested vector again.

# Don't store redundant data

There is a big issue with class Row. It is storing a lot of redundant data. The only thing you need to know to be able to access a row on the board is (a reference to) the Board itself, and the number and direction of the row. If you really want to have a class Row to represent a row on the board, then you should make it work like a std::string_view or std::span.

Another issue is with class Field. It stores the bitmask of nopes, but also the size of the bitmask. The problem is that the size is the same for all the field in the board, so now you are storing a lot of redundant information. While the bitmask is 32 bits, mSize is a std::size_t so will most likely be 64 bits, and due to alignment restrictions, that means your Field will now be 128 bits large, 4 times more than necessary. I recommend you remove mSize from this class and just pass it to member functions that need to know the size.

The whole point of bitmasks is that bitwise operations on them are very cheap. In particular, you don't have to set or reset one bit at a time, you can apply a whole mask in one go. For example, Field::insertSkyscraper contains a for-loop to fill in mBitmask one bit at a time, but you can do that much more efficiently like so:

void Field::insertSkyscraper(int skyscraper)
{
mBitmask = 1 << (skyscraper - 1); // set just the bit corresponding to skyscraper
mBitmask ^= (1 << mSize) - 1;     // invert all bits up to the size of the board
}


The above still requires the size to be known, but you actually don't care about the higher bits until you want to check if a field has only one valid skyscraper height left. So you can replace this function with:

void Field::insertSkyscraper(int skyscraper)
{
mBitmask = ~(1 << (skyscraper - 1));
}


Looking at the code, it seems that it might actually be more efficient to store the bits inverted. The reason is that counting bits might not be as efficient, since not all CPUs doesn't support it in hardware (althought contemporary most CPUs do), and that you C++20 to get std::popcount(). If you invert the bits, then to check if only one possible skyscraper height is left on a field, you only have to check if exactly one bit is set, which is very simple to do yourself (C++20 gives us std::has_single_bit()).

Alternatively:

# Consider replacing Field with a std::bitset

Your class Field is basically reimplementing the features of std::bitset. Consider just using the latter directly:

std::vector<std::bitset<32>> fields;


# Be consistent naming things

Why are some member variable names prefixed with an m, but others are not? Why are some functions using camelCase, but others snake_case? Try to be more consistent. Especially if you use prefixes to distinguish member variables from local or global variables, it stops being useful the moment you don't do this consistently.

Also, ClueHint is redundant, either name it a Clue or a Hint.

# Avoid redundant std::optionals

A std::optional is sometimes necessary if the type you want to return doesn't have a way to represent invalid/empty/none. However, std::optional<ClueHints> seems redundant, since a ClueHint can just have empty skyscraper and nopes variables, and that seems to me to be equivalent to saying there is no clue. Note that std::optional<> won't make things more efficient; it always reserves space for the whole type.

• Member variables which are private are prefixed with an m. The public ones are not. The camelCase snake_case comes because the declaration of the method SolvePuzzle comes from codewars and is snake_case but I don't wanted to follow that style. Beside that many good advices. I will come back to it when I made all the changes. Mar 24, 2021 at 6:49
• Ah, I guess that makes sense then, I hadn't seen the convention to only use m for private member variables before. Mar 24, 2021 at 9:23
• beside replacing the bitmask with std::bitset I tried out all your suggestions but I don't see any speed improvements. At least the code got more readable. Are you sure there is not maybe also something wrong with the backtracking algorithm itself? Too me it looks like it just has to much insertions to do for certain puzzles. Mar 25, 2021 at 17:44