//
// libsemigroups - C++ library for semigroups and monoids
// Copyright (C) 2019 James D. Mitchell
//
// This program is free software: you can redistribute it and/or modify
// it under the terms of the GNU General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.
//
// This program is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU General Public License for more details.
//
// You should have received a copy of the GNU General Public License
// along with this program.  If not, see <http://www.gnu.org/licenses/>.
//

#ifndef LIBSEMIGROUPS_CONTAINERS_HPP_
#define LIBSEMIGROUPS_CONTAINERS_HPP_

#include <algorithm>         // for max
#include <array>             // for array
#include <cstddef>           // for size_t
#include <functional>        // for hash
#include <initializer_list>  // for initializer_list
#include <iterator>          // for reverse_iterator, distance
#include <sstream>           // for operator<<, ostream
#include <string>            // for to_string
#include <type_traits>       // for decay_t, false_type, is_def...
#include <utility>           // for forward
#include <vector>            // for vector, allocator

#include "debug.hpp"     // for LIBSEMIGROUPS_ASSERT
#include "iterator.hpp"  // for ConstIteratorStateful, ConstItera...

namespace libsemigroups {
  namespace detail {
    template <typename T, size_t N>
    class StaticTriVector2 {
      // So that StaticTriVector2<T, N> can access private data members
      // of StaticTriVector2<S, M> and vice versa.
      template <typename S, size_t M>
      friend class StaticTriVector2;

     public:
      // Aliases for iterators
      using const_iterator =
          typename std::array<T, N*(N + 1) / 2>::const_iterator;

      StaticTriVector2(StaticTriVector2 const&) = default;
      StaticTriVector2(StaticTriVector2&&)      = default;
      StaticTriVector2& operator=(StaticTriVector2 const&) = default;
      StaticTriVector2& operator=(StaticTriVector2&&) = default;
      ~StaticTriVector2()                            = default;

      StaticTriVector2() : _data(), _sizes() {
        clear();
      }

      // Not noexcept since std::array::fill is not.
      void clear() {
        _sizes.fill(0);
      }

      // Not noexcept because std::array::operator[] isn't
      void push_back(size_t depth, T x) {
        LIBSEMIGROUPS_ASSERT(depth < N);
        LIBSEMIGROUPS_ASSERT(_sizes[depth] < N - depth);
        _data[depth * (2 * N - depth + 1) / 2 + _sizes[depth]] = x;
        _sizes[depth]++;
      }

      // Not noexcept because std::array::operator[] isn't
      inline T back(size_t depth) const {
        LIBSEMIGROUPS_ASSERT(depth < N);
        return _data[depth * (2 * N - depth + 1) / 2 + _sizes[depth] - 1];
      }

      // Not noexcept because std::array::operator[] isn't
      inline T const& at(size_t depth, size_t index) const {
        LIBSEMIGROUPS_ASSERT(depth < N);
        LIBSEMIGROUPS_ASSERT(index < _sizes[depth]);
        return _data[depth * (2 * N - depth + 1) / 2 + index];
      }

      // Not noexcept because std::array::operator[] isn't
      inline size_t size(size_t depth) const {
        LIBSEMIGROUPS_ASSERT(depth < N);
        return _sizes[depth];
      }

      inline const_iterator cbegin(size_t depth) const noexcept {
        LIBSEMIGROUPS_ASSERT(depth < N);
        return _data.cbegin() + depth * (2 * N - depth + 1) / 2;
      }

      // Not noexcept because std::array::operator[] isn't
      inline const_iterator cend(size_t depth) const {
        LIBSEMIGROUPS_ASSERT(depth < N);
        return _data.cbegin() + depth * (2 * N - depth + 1) / 2 + _sizes[depth];
      }

      inline const_iterator begin(size_t depth) const noexcept {
        LIBSEMIGROUPS_ASSERT(depth < N);
        return _data.begin() + depth * (2 * N - depth + 1) / 2;
      }

      // Not noexcept because std::array::operator[] isn't
      inline const_iterator end(size_t depth) const {
        LIBSEMIGROUPS_ASSERT(depth < N);
        return _data.begin() + depth * (2 * N - depth + 1) / 2 + _sizes[depth];
      }

     private:
      std::array<T, N*(N + 1) / 2> _data;
      std::array<size_t, N>        _sizes;
    };

    // Template class for 2-dimensional dynamic arrays.
    template <typename T, typename A = std::allocator<T>>
    class DynamicArray2 final {
      // So that DynamicArray2<T> can access private data members of
      // DynamicArray2<S> and vice versa.
      template <typename S, typename B>
      friend class DynamicArray2;

     public:
      using allocator_type  = A;
      using value_type      = typename std::vector<T, A>::value_type;
      using reference       = typename std::vector<T, A>::reference;
      using const_reference = typename std::vector<T, A>::const_reference;
      using difference_type = typename std::vector<T, A>::difference_type;
      using size_type       = typename std::vector<T, A>::size_type;
      using const_pointer   = typename std::vector<T, A>::const_pointer;
      using pointer         = typename std::vector<T, A>::pointer;

      DynamicArray2(DynamicArray2 const&) = default;
      DynamicArray2(DynamicArray2&&)      = default;
      DynamicArray2& operator=(DynamicArray2 const&) = default;
      DynamicArray2& operator=(DynamicArray2&&) = default;

      ~DynamicArray2() = default;

      // Not noexcept because DynamicArray2::add_rows can throw.
      explicit DynamicArray2(size_type number_of_cols = 0,
                             size_type number_of_rows = 0,
                             T         default_val    = 0)
          : _vec(),
            _nr_used_cols(number_of_cols),
            _nr_unused_cols(0),
            _nr_rows(0),
            _default_val(default_val) {
        this->add_rows(number_of_rows);
      }

      // Not noexcept because DynamicArray2::DynamicArray2(size_type, size_type)
      // can throw.
      explicit DynamicArray2(std::initializer_list<std::initializer_list<T>> il)
          : DynamicArray2(il.begin()->size(), il.size()) {
        auto it = _vec.begin();
        for (auto const& row : il) {
          LIBSEMIGROUPS_ASSERT(row.size() == _nr_used_cols);
          std::copy(row.begin(), row.end(), it);
          it += _nr_used_cols + _nr_unused_cols;
        }
      }

      // Not noexcept
      template <typename S, typename B>
      DynamicArray2(DynamicArray2<S, B> const& copy,
                    size_type                  number_of_cols_to_add = 0)
          : _vec(),
            _nr_used_cols(copy._nr_used_cols),
            _nr_unused_cols(copy._nr_unused_cols),
            _nr_rows(copy.number_of_rows()),
            _default_val(copy._default_val) {
        if (number_of_cols_to_add <= _nr_unused_cols) {
          _vec.assign(copy._vec.cbegin(), copy._vec.cend());
          _nr_used_cols += number_of_cols_to_add;
          _nr_unused_cols -= number_of_cols_to_add;
          return;
        }

        size_type new_nr_cols = std::max(
            2 * number_of_cols(), number_of_cols_to_add + number_of_cols());
        _nr_used_cols += number_of_cols_to_add;
        _nr_unused_cols = new_nr_cols - _nr_used_cols;

        _vec.reserve(new_nr_cols * _nr_rows);
        // TODO(later) improve
        for (size_type i = 0; i < _nr_rows; i++) {
          size_type j;
          for (j = 0; j < copy._nr_used_cols; j++) {
            _vec.push_back(copy.get(i, j));
          }
          for (; j < new_nr_cols; j++) {
            _vec.push_back(_default_val);
          }
        }
      }

      // Not noexcept, since std::equal can throw
      bool operator==(DynamicArray2<T, A> const& that) const {
        if (_nr_used_cols != that._nr_used_cols || _nr_rows != that._nr_rows) {
          return false;
        }
        return std::equal(cbegin(), cend(), that.cbegin());
      }

      // Not noexcept, since operator== can throw
      bool operator!=(DynamicArray2<T, A> const& that) const {
        return !operator==(that);
      }

      bool empty() const noexcept {
        return _nr_rows == 0;
      }

      size_type size() const noexcept {
        return _nr_rows * _nr_used_cols;
      }

      size_type max_size() const noexcept {
        return _vec.max_size();
      }

      // Not noexcept, since std::fill can throw
      void fill(T const& val) {
        std::fill(_vec.begin(), _vec.end(), val);
      }

      void swap(DynamicArray2<T, A>& that) noexcept {
        _vec.swap(that._vec);
        std::swap(_nr_used_cols, that._nr_used_cols);
        std::swap(_nr_unused_cols, that._nr_unused_cols);
        std::swap(_nr_rows, that._nr_rows);
      }

      // Not noexcept because std::vector::operator[] isn't
      void swap(size_type i, size_type j, size_type k, size_type l) {
        LIBSEMIGROUPS_ASSERT(i < _nr_rows);
        LIBSEMIGROUPS_ASSERT(j < _nr_used_cols);
        LIBSEMIGROUPS_ASSERT(k < _nr_rows);
        LIBSEMIGROUPS_ASSERT(l < _nr_used_cols);
        std::swap(_vec[i * (_nr_used_cols + _nr_unused_cols) + j],
                  _vec[k * (_nr_used_cols + _nr_unused_cols) + l]);
      }

      // Not noexcept since std::swap_ranges can throw.
      void swap_rows(size_type i, size_type j) {
        LIBSEMIGROUPS_ASSERT(i < _nr_rows);
        LIBSEMIGROUPS_ASSERT(j < _nr_rows);
        size_type const number_of_cols = _nr_used_cols + _nr_unused_cols;
        std::swap_ranges(_vec.begin() + (i * number_of_cols),
                         _vec.begin() + ((i + 1) * number_of_cols),
                         _vec.begin() + (j * number_of_cols));
      }

      // The following is adapted from http://bit.ly/2X4xPlK
      // TODO(later) 1. make this a non-member function
      //             2. should perform checks that p actually permutes the
      //                given row
      // Not noexcept because std::vector::operator[] isn't
      void apply_row_permutation(std::vector<T> p) {
        for (size_type i = 0; i < p.size(); i++) {
          size_type current = i;
          while (i != p[current]) {
            size_type next = p[current];
            swap_rows(current, next);
            p[current] = current;
            current    = next;
          }
          p[current] = current;
        }
      }

      void clear() noexcept {
        _nr_unused_cols += _nr_used_cols;
        _nr_used_cols = 0;
        _nr_rows      = 0;
        _vec.clear();  // noexcept
      }

      // Throws if the assignment operator of T throws
      void shrink_rows_to(size_type n) {
        if (n < _nr_rows) {
          shrink_rows_to(0, n);
        }
      }

      void shrink_rows_to(size_type first, size_type last) {
        LIBSEMIGROUPS_ASSERT(first <= last);
        LIBSEMIGROUPS_ASSERT(first <= _nr_rows);
        LIBSEMIGROUPS_ASSERT(last <= _nr_rows);

        auto nr_cols = _nr_used_cols + _nr_unused_cols;
        _vec.erase(_vec.begin() + last * nr_cols, _vec.end());
        _vec.erase(_vec.begin(), _vec.begin() + first * nr_cols);
        _vec.shrink_to_fit();
        _nr_rows = last - first;
      }

      // Throws if the assignment operator of T throws
      void erase_column(size_type i) {
        LIBSEMIGROUPS_ASSERT(i < _nr_used_cols);
        size_type const n = _nr_used_cols + _nr_unused_cols;
        for (auto it = _vec.rbegin() + (n - i); it < _vec.rend(); it += n) {
          _vec.erase(it.base());
        }
        _nr_used_cols--;
      }

      // Not noexcept since std::vector::resize can throw.
      void inline add_rows(size_type nr) {
        _nr_rows += nr;
        if (nr != 0) {
          _vec.resize(_vec.size() + (_nr_used_cols + _nr_unused_cols) * nr,
                      _default_val);
        }
      }

      void inline set_default_value(T val) noexcept {
        _default_val = val;
      }

      // Not noexcept
      void add_cols(size_type nr) {
        if (nr <= _nr_unused_cols) {
          _nr_used_cols += nr;
          _nr_unused_cols -= nr;
          return;
        }

        size_type old_nr_cols = _nr_used_cols + _nr_unused_cols;
        size_type new_nr_cols = std::max(2 * old_nr_cols, nr + old_nr_cols);
        if (_nr_rows != 0) {
          _vec.resize(new_nr_cols * _nr_rows, _default_val);

          typename std::vector<T>::iterator old_it(
              _vec.begin() + (old_nr_cols * _nr_rows) - old_nr_cols);
          typename std::vector<T>::iterator new_it(
              _vec.begin() + (new_nr_cols * _nr_rows) - new_nr_cols);

          while (old_it != _vec.begin()) {
            std::move(old_it, old_it + _nr_used_cols, new_it);
            old_it -= old_nr_cols;
            new_it -= new_nr_cols;
          }
        }
        _nr_used_cols += nr;
        _nr_unused_cols = new_nr_cols - _nr_used_cols;
      }

      // Not noexcept because std::vector::operator[] isn't
      void inline set(size_type i, size_type j, T val) {
        LIBSEMIGROUPS_ASSERT(i < _nr_rows && j < _nr_used_cols);
        _vec[i * (_nr_used_cols + _nr_unused_cols) + j] = val;
      }

      // Not noexcept because std::vector::operator[] isn't
      inline T get(size_type i, size_type j) const {
        LIBSEMIGROUPS_ASSERT(i < _nr_rows && j < _nr_used_cols);
        return _vec[i * (_nr_used_cols + _nr_unused_cols) + j];
      }

      size_type number_of_rows() const noexcept {
        return _nr_rows;
      }

      size_type number_of_cols() const noexcept {
        return _nr_used_cols;
      }

      // Not noexcept
      template <typename S, typename B>
      void append(DynamicArray2<S, B> const& copy) {
        LIBSEMIGROUPS_ASSERT(copy._nr_used_cols == _nr_used_cols);

        size_type old_nr_rows = _nr_rows;
        add_rows(copy._nr_rows);

        if (copy._nr_unused_cols == _nr_unused_cols) {
          std::copy(copy._vec.begin(),
                    copy._vec.end(),
                    _vec.begin()
                        + (_nr_used_cols + _nr_unused_cols) * old_nr_rows);
        } else {  // TODO(later) improve this
          for (size_type i = old_nr_rows; i < _nr_rows; i++) {
            for (size_type j = 0; j < _nr_used_cols; j++) {
              set(i, j, copy.get(i - old_nr_rows, j));
            }
          }
        }
      }

      // Not noexcept
      void reserve(size_type number_of_rows) {
        _vec.reserve(number_of_rows * (_nr_unused_cols + _nr_used_cols));
      }

     private:
      std::vector<T, A> _vec;
      size_type         _nr_used_cols;
      size_type         _nr_unused_cols;
      size_type         _nr_rows;
      T                 _default_val;

      // Helper functions for iterators
      static inline size_type remainder(difference_type a,
                                        difference_type b) noexcept {
        if (a >= 0) {
          return a % b;
        } else {
          return (a % b) + b;
        }
      }

      static inline size_type quotient(difference_type a,
                                       difference_type b) noexcept {
        if (a % b >= 0) {
          return a / b;
        } else {
          return (a / b) - 1;
        }
      }

      // Types for ConstIteratorStateful
      template <typename TInternalIteratorType>
      friend struct AddAssign;

      template <typename TInternalIteratorType>
      struct AddAssign {
        TInternalIteratorType& operator()(DynamicArray2 const*   da,
                                          TInternalIteratorType& it,
                                          size_type val) const noexcept {
          if (da->_nr_unused_cols == 0 || val == 0) {
            return it += val;
          }
          size_type s = da->number_of_cols();
          size_type n = da->_nr_unused_cols;

          size_type q = val / s;
          size_type r = val % s;

          difference_type p = it - da->_vec.cbegin();
          it += q * (s + n) + r;

          if (r + remainder(p, s + n) >= s) {
            it += n;
          }
          return it;
        }
      };

      template <typename TInternalIteratorType>
      friend struct SubtractAssign;

      template <typename TInternalIteratorType>
      struct SubtractAssign {
        TInternalIteratorType& operator()(DynamicArray2 const*   da,
                                          TInternalIteratorType& it,
                                          size_type val) const noexcept {
          if (da->_nr_unused_cols == 0 || val == 0) {
            return it -= val;
          }
          size_type s = da->number_of_cols();
          size_type n = da->_nr_unused_cols;

          size_type q = val / s;
          size_type r = val % s;

          difference_type p = it - da->_vec.cbegin();
          it -= (q * (s + n) + r);
          if (remainder(p, s + n) < r) {
            it -= n;
          }
          return it;
        }
      };

      template <typename TOperator, typename TInternalIteratorType>
      struct PrefixIncrement {
        TInternalIteratorType&
        operator()(DynamicArray2 const*   da,
                   TInternalIteratorType& it) const noexcept {
          return TOperator()(da, it, 1);
        }
      };

      template <typename TOperator, typename TInternalIteratorType>
      struct PrefixDecrement {
        TInternalIteratorType&
        operator()(DynamicArray2 const*   da,
                   TInternalIteratorType& it) const noexcept {
          return TOperator()(da, it, 1);
        }
      };

      template <typename TInternalIteratorType>
      friend struct Difference;

      template <typename TInternalIteratorType>
      struct Difference {
        inline difference_type
        operator()(DynamicArray2 const*         da1,
                   TInternalIteratorType const& it1,
                   DynamicArray2 const*         da2,
                   TInternalIteratorType const& it2) const noexcept {
          LIBSEMIGROUPS_ASSERT(da1 == da2);
          (void) da2;
          difference_type s = da1->_nr_used_cols;
          difference_type n = da1->_nr_unused_cols;
          difference_type b = it1 - da1->_vec.begin();
          difference_type a = it2 - da1->_vec.begin();
          return (b - a) - n * (quotient(b, s + n) - quotient(a, s + n));
        }
      };

      template <typename TInternalIteratorType>
      friend struct ColumnAddAssign;

      template <typename TInternalIteratorType>
      struct ColumnAddAssign {
        TInternalIteratorType& operator()(DynamicArray2 const*   da,
                                          TInternalIteratorType& it,
                                          size_type val) const noexcept {
          return it += val * (da->_nr_used_cols + da->_nr_unused_cols);
        }
      };

      template <typename TInternalIteratorType>
      friend struct ColumnSubtractAssign;

      template <typename TInternalIteratorType>
      struct ColumnSubtractAssign {
        TInternalIteratorType& operator()(DynamicArray2 const*   da,
                                          TInternalIteratorType& it,
                                          size_type val) const noexcept {
          return it -= val * (da->_nr_used_cols + da->_nr_unused_cols);
        }
      };

      template <typename TInternalIteratorType>
      friend struct ColumnDifference;

      template <typename TInternalIteratorType>
      struct ColumnDifference {
        inline difference_type
        operator()(DynamicArray2 const*         da1,
                   TInternalIteratorType const& it1,
                   DynamicArray2 const*         da2,
                   TInternalIteratorType const& it2) const noexcept {
          LIBSEMIGROUPS_ASSERT(da1 == da2);
          (void) da2;
          return (it1 - it2)
                 / static_cast<difference_type>(da1->_nr_used_cols
                                                + da1->_nr_unused_cols);
        }
      };

      // Aliases for iterators
      using internal_const_iterator_type =
          typename std::vector<T, A>::const_iterator;

      using internal_iterator_type = typename std::vector<T, A>::iterator;

      struct IteratorTraits : public detail::IteratorTraits<std::vector<T, A>> {
        using state_type = DynamicArray2<T, A>*;
        using AddAssign  = DynamicArray2::AddAssign<internal_iterator_type>;
        using SubtractAssign
            = DynamicArray2::SubtractAssign<internal_iterator_type>;
        using PrefixIncrement = DynamicArray2::PrefixIncrement<
            DynamicArray2::AddAssign<internal_iterator_type>,
            internal_iterator_type>;
        using PrefixDecrement = DynamicArray2::PrefixDecrement<
            DynamicArray2::SubtractAssign<internal_iterator_type>,
            internal_iterator_type>;
        using Difference = DynamicArray2::Difference<internal_iterator_type>;
      };

      struct ConstIteratorTraits
          : public detail::ConstIteratorTraits<std::vector<T, A>> {
        using state_type = DynamicArray2<T, A> const*;
        using AddAssign
            = DynamicArray2::AddAssign<internal_const_iterator_type>;
        using SubtractAssign
            = DynamicArray2::SubtractAssign<internal_const_iterator_type>;
        using PrefixIncrement = DynamicArray2::PrefixIncrement<
            DynamicArray2::AddAssign<internal_const_iterator_type>,
            internal_const_iterator_type>;
        using PrefixDecrement = DynamicArray2::PrefixDecrement<
            DynamicArray2::SubtractAssign<internal_const_iterator_type>,
            internal_const_iterator_type>;
        using Difference
            = DynamicArray2::Difference<internal_const_iterator_type>;
      };

      struct ColumnIteratorTraits
          : public detail::IteratorTraits<std::vector<T, A>> {
        using state_type      = DynamicArray2<T, A>*;
        using AddAssign       = ColumnAddAssign<internal_iterator_type>;
        using SubtractAssign  = ColumnSubtractAssign<internal_iterator_type>;
        using PrefixIncrement = DynamicArray2::PrefixIncrement<
            ColumnAddAssign<internal_iterator_type>,
            internal_iterator_type>;
        using PrefixDecrement = DynamicArray2::PrefixDecrement<
            ColumnSubtractAssign<internal_iterator_type>,
            internal_iterator_type>;
        using Difference = ColumnDifference<internal_iterator_type>;
      };

      struct ConstColumnIteratorTraits
          : public detail::ConstIteratorTraits<std::vector<T, A>> {
        using state_type = DynamicArray2<T, A> const*;
        using AddAssign  = ColumnAddAssign<internal_const_iterator_type>;
        using SubtractAssign
            = ColumnSubtractAssign<internal_const_iterator_type>;
        using PrefixIncrement = DynamicArray2::PrefixIncrement<
            ColumnAddAssign<internal_const_iterator_type>,
            internal_const_iterator_type>;
        using PrefixDecrement = DynamicArray2::PrefixDecrement<
            ColumnSubtractAssign<internal_const_iterator_type>,
            internal_const_iterator_type>;
        using Difference = ColumnDifference<internal_const_iterator_type>;
      };

     public:
      using iterator       = IteratorStateful<IteratorTraits>;
      using const_iterator = ConstIteratorStateful<ConstIteratorTraits>;

      using column_iterator = IteratorStateful<ColumnIteratorTraits>;
      using const_column_iterator
          = ConstIteratorStateful<ConstColumnIteratorTraits>;

      iterator begin() noexcept {
        return iterator(this, _vec.begin());
      }

      iterator end() noexcept {
        return iterator(this, _vec.end());
      }

      const_iterator cbegin() const noexcept {
        return const_iterator(this, _vec.cbegin());
      }

      const_iterator cend() const noexcept {
        return const_iterator(this, _vec.cend());
      }

      typedef std::reverse_iterator<iterator>       reverse_iterator;
      typedef std::reverse_iterator<const_iterator> const_reverse_iterator;

      // Not noexcept
      reverse_iterator rbegin() {
        return reverse_iterator(end());
      }

      // Not noexcept
      reverse_iterator rend() {
        return reverse_iterator(begin());
      }

      // Not noexcept
      const_reverse_iterator rbegin() const {
        return const_reverse_iterator(end());
      }

      // Not noexcept
      const_reverse_iterator rend() const {
        return const_reverse_iterator(begin());
      }

      // Not noexcept
      const_reverse_iterator crbegin() const {
        return const_reverse_iterator(cend());
      }

      // Not noexcept
      const_reverse_iterator crend() const {
        return const_reverse_iterator(cbegin());
      }

      iterator begin_row(size_type row_index) noexcept {
        LIBSEMIGROUPS_ASSERT(row_index < _nr_rows);
        return iterator(
            this, _vec.begin() + (_nr_used_cols + _nr_unused_cols) * row_index);
      }

      iterator end_row(size_type row_index) noexcept {
        LIBSEMIGROUPS_ASSERT(row_index < _nr_rows);
        return begin_row(row_index) + _nr_used_cols;
      }

      column_iterator begin_column(size_type col_index) noexcept {
        LIBSEMIGROUPS_ASSERT(col_index < _nr_used_cols);
        return column_iterator(this, _vec.begin() + col_index);
      }

      column_iterator end_column(size_type col_index) noexcept {
        LIBSEMIGROUPS_ASSERT(col_index < _nr_used_cols);
        return column_iterator(this, _vec.end() + col_index);
      }

      const_iterator cbegin_row(size_type row_index) const noexcept {
        LIBSEMIGROUPS_ASSERT(row_index < _nr_rows);
        return const_iterator(
            this,
            _vec.cbegin() + (_nr_used_cols + _nr_unused_cols) * row_index);
      }

      const_iterator cend_row(size_type row_index) const noexcept {
        LIBSEMIGROUPS_ASSERT(row_index < _nr_rows);
        return cbegin_row(row_index) + _nr_used_cols;
      }

      const_column_iterator cbegin_column(size_type col_index) const noexcept {
        LIBSEMIGROUPS_ASSERT(col_index < _nr_used_cols);
        return const_column_iterator(this, _vec.begin() + col_index);
      }

      const_column_iterator cend_column(size_type col_index) const noexcept {
        LIBSEMIGROUPS_ASSERT(col_index < _nr_used_cols);
        return const_column_iterator(this, _vec.end() + col_index);
      }
    };

    // StaticVector1 wraps an array, providing it with some of the syntax of
    // std::vector.
    // TODO(later) add tests specifically targeting this class
    template <typename T, size_t N>
    class StaticVector1 final {
      static_assert(std::is_default_constructible<T>::value,
                    "StaticVector1<T> requires T to be default-constructible");

     public:
      using value_type      = typename std::array<T, N>::value_type;
      using reference       = typename std::array<T, N>::reference;
      using const_reference = typename std::array<T, N>::const_reference;
      using difference_type = typename std::array<T, N>::difference_type;
      using size_type       = typename std::array<T, N>::size_type;
      using const_pointer   = typename std::array<T, N>::const_pointer;
      using pointer         = typename std::array<T, N>::pointer;

      StaticVector1() : _array(), _size(0) {}

      StaticVector1(size_type n, value_type val) : StaticVector1() {
        LIBSEMIGROUPS_ASSERT(n <= N);
        for (size_type i = 0; i < n; ++i) {
          push_back(val);
        }
      }

      template <typename It>
      StaticVector1(It const& first, It const& last) : StaticVector1() {
        LIBSEMIGROUPS_ASSERT(static_cast<size_type>(std::distance(first, last))
                             <= N);
        for (auto it = first; it != last; ++it) {
          push_back(*it);
        }
      }

      StaticVector1(StaticVector1 const&) = default;
      StaticVector1(StaticVector1&&)      = default;
      StaticVector1& operator=(StaticVector1 const&) = default;
      StaticVector1& operator=(StaticVector1&&) = default;

      ~StaticVector1() = default;

      StaticVector1(std::initializer_list<T> il) : StaticVector1() {
        for (auto x : il) {
          push_back(x);
        }
      }

      // not noexcept because std::equal may not be
      bool operator==(StaticVector1 const& that) const {
        return size() == that.size()
               && std::equal(cbegin(), cend(), that.cbegin());
      }

      void clear() noexcept {
        _size = 0;
      }

      // Not noexcept because std::array::operator[] isn't
      void push_back(T x) {
        LIBSEMIGROUPS_ASSERT(_size < N);
        _array[_size] = x;
        _size++;
      }

      template <typename... Ss>
      void emplace_back(Ss&&... params) {
        LIBSEMIGROUPS_ASSERT(_size < N);
        _array[_size] = T(std::forward<Ss>(params)...);
        _size++;
      }

      size_t size() const noexcept {
        return _size;
      }

      size_t empty() const noexcept {
        return size() == 0;
      }

      void pop_back() {
        if (_size != 0) {
          _size--;
        }
      }

      // Not noexcept because std::array::operator[] isn't
      T const& back() const {
        LIBSEMIGROUPS_ASSERT(_size != 0);
        return _array[_size - 1];
      }

      // Not noexcept because std::array::operator[] isn't
      T& back() {
        LIBSEMIGROUPS_ASSERT(_size != 0);
        return _array[_size - 1];
      }

      using iterator       = typename std::array<T, N>::iterator;
      using const_iterator = typename std::array<T, N>::const_iterator;
      using const_reverse_iterator =
          typename std::array<T, N>::const_reverse_iterator;

      // noexcept because std::array::cbegin is noexcept
      inline const_iterator cbegin() const noexcept {
        return _array.cbegin();
      }

      inline const_reverse_iterator crbegin() const noexcept {
        return _array.crbegin() + (N - size());
      }

      // noexcept because std::array::cbegin is noexcept
      inline const_iterator cend() const noexcept {
        return _array.cbegin() + _size;
      }

      inline const_reverse_iterator crend() const noexcept {
        return _array.crend();
      }

      // noexcept because std::array::begin is noexcept
      inline iterator begin() noexcept {
        return _array.begin();
      }

      // noexcept because std::array::begin is noexcept
      inline iterator end() noexcept {
        return _array.begin() + _size;
      }

      // noexcept because std::array::cbegin is noexcept
      inline const_iterator begin() const noexcept {
        return _array.cbegin();
      }

      // noexcept because std::array::cbegin is noexcept
      inline const_iterator end() const noexcept {
        return _array.cbegin() + _size;
      }

      // not noexcept because std::copy can throw
      inline iterator erase(iterator first, iterator last) {
        iterator const old_end = end();
        std::copy(last, end(), first);
        _size -= std::distance(first, last);
        return (last == old_end ? end() : first + 1);
      }

      // not noexcept because iterator operations may throw
      inline void resize(size_t count) {
        LIBSEMIGROUPS_ASSERT(count <= N);
        if (count >= _size) {
          for (auto it = begin() + _size; it < begin() + (count - _size);
               ++it) {
            *it = T();
          }
        }
        _size = count;
      }

      // Not noexcept because std::array::operator[] isn't
      T& operator[](size_t pos) {
        return _array[pos];
      }

      // Not noexcept because std::array::operator[] isn't
      T const& operator[](size_t pos) const {
        return _array[pos];
      }

     private:
      std::array<T, N> _array;
      size_t           _size;
    };

    template <typename T, size_t N>
    std::ostream& operator<<(std::ostream& os, StaticVector1<T, N> const& vec) {
      if (vec.empty()) {
        os << "{}";
        return os;
      }
      os << "{{";  // {{ is an escaped single { for fmt
      for (auto it = vec.cbegin(); it < vec.cend() - 1; ++it) {
        os << std::to_string(*it) << ", ";
      }
      os << std::to_string(*(vec.cend() - 1)) << "}}";
      return os;
    }

    // Currently only supports N x N 2-dimensional static vectors
    template <typename T, size_t N>
    class StaticVector2 final {
      // So that StaticVector2<T, N> can access private data members of
      // StaticVector2<S, M> and vice versa.
      template <typename S, size_t M>
      friend class StaticVector2;

     public:
      StaticVector2() : _arrays(), _sizes() {
        clear();
      }

      StaticVector2(StaticVector2 const&) = default;
      StaticVector2(StaticVector2&&)      = default;
      StaticVector2& operator=(StaticVector2 const&) = default;
      StaticVector2& operator=(StaticVector2&&) = default;
      ~StaticVector2()                          = default;

      // Not noexcept since std::array::fill is not.
      void clear() {
        _sizes.fill(0);
      }

      // Not noexcept because std::array::operator[] isn't
      void push_back(size_t depth, T x) {
        LIBSEMIGROUPS_ASSERT(depth < N);
        LIBSEMIGROUPS_ASSERT(_sizes[depth] < N);
        _arrays[depth][_sizes[depth]] = x;
        _sizes[depth]++;
      }

      // Not noexcept because std::array::operator[] isn't
      inline T back(size_t depth) const {
        LIBSEMIGROUPS_ASSERT(depth < N);
        return _arrays[depth][_sizes[depth] - 1];
      }

      // Not noexcept because std::array::operator[] isn't
      inline T const& at(size_t depth, size_t index) const {
        LIBSEMIGROUPS_ASSERT(depth < N);
        LIBSEMIGROUPS_ASSERT(index < _sizes[depth]);
        return _arrays[depth][index];
      }

      // Not noexcept because std::array::operator[] isn't
      inline size_t size(size_t depth) const {
        LIBSEMIGROUPS_ASSERT(depth < N);
        return _sizes[depth];
      }

      // Not noexcept because std::array::operator[] isn't
      inline typename std::array<T, N>::const_iterator
      cbegin(size_t depth) const {
        LIBSEMIGROUPS_ASSERT(depth < N);
        return _arrays[depth].cbegin();
      }

      // Not noexcept because std::array::operator[] isn't
      inline typename std::array<T, N>::const_iterator
      cend(size_t depth) const {
        LIBSEMIGROUPS_ASSERT(depth < N);
        return _arrays[depth].cbegin() + _sizes[depth];
      }

      // Not noexcept because std::array::operator[] isn't
      inline typename std::array<T, N>::iterator begin(size_t depth) {
        LIBSEMIGROUPS_ASSERT(depth < N);
        return _arrays[depth].begin();
      }

      // Not noexcept because std::array::operator[] isn't
      inline typename std::array<T, N>::iterator end(size_t depth) {
        LIBSEMIGROUPS_ASSERT(depth < N);
        return _arrays[depth].begin() + _sizes[depth];
      }

     private:
      std::array<std::array<T, N>, N> _arrays;
      std::array<size_t, N>           _sizes;
    };

    // Currently only supports N x N 2-dimensional arrays
    template <typename T, size_t N>
    class Array2 final {
      // So that Array2<T, N> can access private data members of
      // Array2<S, M> and vice versa.
      template <typename S, size_t M>
      friend class Array2;

     public:
      Array2()              = default;
      Array2(Array2 const&) = default;
      Array2(Array2&&)      = default;
      Array2& operator=(Array2 const&) = default;
      Array2& operator=(Array2&&) = default;
      ~Array2()                   = default;

      // Not noexcept cos std::array::fill isn't
      inline void fill(T const& value) {
        for (auto& x : _arrays) {
          x.fill(value);
        }
      }

      // Not noexcept because std::array::operator[] isn't
      inline std::array<T, N>& operator[](size_t depth) {
        LIBSEMIGROUPS_ASSERT(depth < N);
        return _arrays[depth];
      }

      // Not noexcept because std::array::at can throw
      inline T const& at(size_t depth, size_t index) const {
        return _arrays.at(depth).at(index);
      }

      // Not noexcept because std::array::operator[] isn't
      inline typename std::array<T, N>::const_iterator
      cbegin(size_t depth) const {
        LIBSEMIGROUPS_ASSERT(depth < N);
        return _arrays[depth].cbegin();
      }

      // Not noexcept because std::array::operator[] isn't
      inline typename std::array<T, N>::const_iterator
      cend(size_t depth) const {
        LIBSEMIGROUPS_ASSERT(depth < N);
        return _arrays[depth].cend();
      }

      // Not noexcept because std::array::operator[] isn't
      inline typename std::array<T, N>::iterator begin(size_t depth) {
        LIBSEMIGROUPS_ASSERT(depth < N);
        return _arrays[depth].begin();
      }

      // Not noexcept because std::array::operator[] isn't
      inline typename std::array<T, N>::iterator end(size_t depth) {
        LIBSEMIGROUPS_ASSERT(depth < N);
        return _arrays[depth].end();
      }

     private:
      std::array<std::array<T, N>, N> _arrays;
    };

    template <typename T>
    struct IsDynamicArray2Helper : std::false_type {};

    template <typename T, typename A>
    struct IsDynamicArray2Helper<DynamicArray2<T, A>> : std::true_type {};

  }  // namespace detail

  template <typename T>
  static constexpr bool IsDynamicArray2
      = detail::IsDynamicArray2Helper<std::decay_t<T>>::value;

}  // namespace libsemigroups

namespace std {
  template <typename T, size_t N>
  struct hash<libsemigroups::detail::StaticVector1<T, N>> {
    size_t
    operator()(libsemigroups::detail::StaticVector1<T, N> const& sv) const {
      size_t seed = 0;
      for (T const& x : sv) {
        seed ^= std::hash<T>()(x) + 0x9e3779b97f4a7c16 + (seed << 6)
                + (seed >> 2);
      }
      return seed;
    }
  };
}  // namespace std
#endif  // LIBSEMIGROUPS_CONTAINERS_HPP_
