// -*- c++ -*-
#pragma once

#include <algorithm>
#include <atomic>
#include <cstddef>
#include <cstring>
#include <initializer_list>

namespace std {
  class out_of_range;

  template<class charT>
  class basic_string
  {
    // The data backing the string.  This is shared and copy-on-write.
    // This is always allocated to allow one extra byte in buf beyond
    // cap, which provides space for a NUL charT.  Anything that
    // modifies len must ensure that buf[len] == 0.
    struct data
    {
      atomic<unsigned> refcnt;
      size_t len, cap;
      charT buf[];

      static data* alloc(data *cur, size_t cap)
      {
        // Allocate space for cap plus a NUL.
        struct data *res = reinterpret_cast<struct data*>(
          new char[sizeof(*res) + sizeof(charT) * (cap + 1)]);
        res->refcnt.store(1, memory_order_relaxed);
        res->cap = cap;
        if (cur) {
          res->len = cur->len;
          memmove(res->buf, cur->buf, sizeof(charT) * (cur->len + 1));
        } else {
          res->len = 0;
          res->buf[0] = 0;
        }
        return res;
      }

      void set_len(size_t nlen)
      {
        buf[nlen] = 0;
        len = nlen;
      }
    };

    struct data *data_;

    void release()
    {
      if (data_ && --data_->refcnt == 0)
        delete[] (char*)data_;
      data_ = nullptr;
    }

    void may_write()
    {
      if (data_ && data_->refcnt > 1) {
        struct data *ndata = data::alloc(data_, data_->cap);
        release();
        data_ = ndata;
      }
    }

  public:
    // XXX traits_type, allocator_type
    // XXX Traits and allocator-aware
    typedef charT value_type;
    typedef size_t size_type;
    typedef size_t difference_type;

    typedef value_type& reference;
    typedef const value_type& const_reference;
    typedef value_type* pointer;
    typedef const value_type* const_pointer;

    typedef value_type* iterator;
    typedef const value_type* const_iterator;

    static const size_type npos = -1;

    basic_string() : data_() { }

    basic_string(const basic_string& str) : data_(str.data_)
    {
      if (data_)
        ++data_->refcnt;
    }

    basic_string(basic_string&& str) noexcept : data_(str.data_)
    {
      str.data_ = nullptr;
    }

    basic_string(const basic_string& str, size_type pos, size_type n = npos)
      : basic_string()
    {
      append(str, pos, n);
    }

    basic_string(const charT* s, size_type n) : basic_string()
    {
      append(s, n);
    }

    basic_string(const charT* s) : basic_string()
    {
      append(s);
    }

    basic_string(size_type n, charT c) : basic_string()
    {
      append(n, c);
    }

    template<class InputIterator>
    basic_string(InputIterator begin, InputIterator end) : basic_string()
    {
      append(begin, end);
    }

    basic_string(initializer_list<charT> il) : basic_string()
    {
      append(il.begin(), il.size());
    }

    ~basic_string()
    {
      release();
    }

    basic_string& operator=(const basic_string& str)
    {
      if (&str != this) {
        release();
        if ((data_ = str.data_))
          ++data_->refcnt;
      }
      return *this;
    }

    basic_string& operator=(basic_string &&str) noexcept
    {
      release();
      data_ = str.data_;
      str.data_ = nullptr;
      return *this;
    }

    basic_string& operator=(const charT* s)
    {
      clear();
      return append(s);
    }

    basic_string& operator=(charT c)
    {
      clear();
      return append(1, c);
    }

    basic_string& operator=(initializer_list<charT> il)
    {
      clear();
      return append(il.begin(), il.size());
    }

    iterator begin() noexcept
    {
      may_write();
      if (data_)
        return &data_->buf[0];
      return nullptr;
    }

    const_iterator begin() const noexcept
    {
      return cbegin();
    }

    iterator end() noexcept
    {
      may_write();
      if (data_)
        return &data_->buf[data_->len];
      return nullptr;
    }

    const_iterator end() const noexcept
    {
      return cend();
    }

    // XXX rbegin, rend

    const_iterator cbegin() const noexcept
    {
      if (data_)
        return &data_->buf[0];
      return nullptr;
    }

    const_iterator cend() const noexcept
    {
      if (data_)
        return &data_->buf[data_->len];
      return nullptr;
    }

    // XXX crbegin, crend

    size_type size() const noexcept
    {
      if (data_)
        return data_->len;
      return 0;
    }

    size_type length() const noexcept
    {
      return size();
    }

    size_type max_size() const noexcept
    {
      return (size_type)-1;
    }

    // XXX resize

    size_type capacity() const noexcept
    {
      if (data_)
        return data_->cap;
      return 0;
    }

    void reserve(size_type n = 0)
    {
      // Make sure there's enough space besides the buffer itself for
      // the allocator to store a size plus alignment padding, for the
      // data header, and for a terminating NUL.  As a result, the
      // allocation will generally be power-of-two friendly and cap
      // will generally by a power of two minus this padding.
      const static auto PADDING =
        2 * sizeof(void*) + offsetof(struct data, buf) + 1;
      if (n < size())
        n = size();
      // Compute a the smallest power-of-two total allocation size
      // that will fit n.
      n += PADDING;
      size_type goal = 16;
      while (n > goal)
        goal *= 2;
      // The new buffer capacity of the total allocation goal minus
      // the padding we accounting for.
      size_type ncap = goal - PADDING;
      if (data_ && ncap == data_->cap)
        return;
      // Create new data
      struct data *ndata = data::alloc(data_, ncap);
      release();
      data_ = ndata;
    }

    void shrink_to_fit()
    {
      if (size() == capacity())
        return;
      struct data *ndata = data::alloc(data_, capacity());
      release();
      data_ = ndata;
    }

    void clear() noexcept
    {
      if (!data_)
        return;
      if (data_->refcnt > 1)
        release();
      else
        data_->set_len(0);
    }

    bool empty() const noexcept
    {
      return data_ == nullptr || data_->len == 0;
    }

    const_reference operator[](size_type pos) const
    {
      return *(begin() + pos);
    }

    reference operator[](size_type pos)
    {
      return *(begin() + pos);
    }

    const_reference at(size_type pos) const
    {
      if (pos >= size())
        throw out_of_range("index out of range");
      return *(begin() + pos);
    }

    reference at(size_type pos)
    {
      if (pos >= size())
        throw out_of_range("index out of range");
      return *(begin() + pos);
    }

    const charT& front() const
    {
      return *begin();
    }

    charT& front()
    {
      return *begin();
    }

    const charT& back() const
    {
      return *(end() - 1);
    }

    charT& back()
    {
      return *(end() - 1);
    }

    basic_string& operator+=(const basic_string& str)
    {
      return append(str);
    }

    basic_string& operator+=(const charT* s)
    {
      return append(s);
    }

    basic_string& operator+=(charT c)
    {
      return append(1, c);
    }

    basic_string& operator+=(initializer_list<charT> il)
    {
      return append(il.first(), il.size());
    }

    basic_string& append(const basic_string& str)
    {
      return append(str.data(), str.size());
    }

    basic_string& append(const basic_string& str, size_type pos, size_type n)
    {
      if (pos > str.size())
        throw out_of_range("pos out of range");
      if (n > str.size() - pos)
        n = str.size() - pos;
      return append(str.begin() + pos, n);
    }

    basic_string& append(const charT* s, size_type n)
    {
      reserve(size() + n);
      may_write();
      memmove(data_->buf + data_->len, s, sizeof(charT)*n);
      data_->set_len(data_->len + n);
      return *this;
    }

    basic_string& append(const charT* s)
    {
      // XXX traits::length
      return append(s, strlen(s));
    }

    basic_string& append(size_type n, charT c)
    {
      reserve(size() + n);
      may_write();
      fill(data_->buf + data_->len, data_->buf + data_->len + n, c);
      data_->set_len(data_->len + n);
      return *this;
    }

    template<class InputIterator>
    basic_string& append(InputIterator first, InputIterator last)
    {
      for (; first != last; ++first)
        push_back(*first);
      return *this;
    }

    basic_string& append(initializer_list<charT> il)
    {
      return append(il.begin(), il.size());
    }

    void push_back(charT c)
    {
      reserve(size() + 1);
      may_write();
      data_->buf[data_->len] = c;
      data_->set_len(data_->len + 1);
    }

    // XXX assign, insert, erase

    void pop_back()
    {
      if (!empty()) {
        may_write();
        data_->set_len(data_->len - 1);
      }
    }

    // XXX replace, copy, swap

    const charT* c_str() const noexcept
    {
      // We always maintain a NUL terminator
      return data();
    }

    const charT* data() const noexcept
    {
      return begin();
    }

    // XXX get_allocator, find, rfind, find_first_of, find_last_of,
    // find_first_not_of, find_last_not_of, substr

    int compare(const basic_string& str) const noexcept
    {
      return compare(0, size(), str.data(), str.size());
    }

    int compare(size_type pos1, size_type n1,
                const basic_string& str) const
    {
      return compare(pos1, n1, str.data(), str.size());
    }

    int compare(size_type pos1, size_type n1,
                const basic_string& str,
                size_type pos2, size_type n2) const
    {
      if (pos2 > str.size())
        throw out_of_range("compare pos out of range");
      if (n2 > str.size() - pos2)
        n2 = str.size() - pos2;
      return compare(pos1, n1, str.data() + pos2, n2);
    }

    int compare(const charT* s) const
    {
      return compare(0, size(), s);
    }

    int compare(size_type pos1, size_type n1,
                const charT* s) const
    {
      return compare(pos1, n1, s, strlen(s));
    }

    int compare(size_type pos1, size_type n1,
                const charT* s, size_type n2) const
    {
      if (pos1 > size())
        throw out_of_range("compare pos out of range");
      if (n1 > size() - pos1)
        n1 = size() - pos1;
      size_type rlen = n1 < n2 ? n1 : n2;
      int res = memcmp(data(), s, rlen * sizeof(charT));
      if (res == 0) {
        if (n1 < n2)
          res = -1;
        else if (n1 > n2)
          res = 1;
      }
      return res;
    }
  };

  template<class charT>
  bool operator==(const basic_string<charT>& a, const basic_string<charT>& b)
  {
    return a.compare(b) == 0;
  }

  template<class charT>
  bool operator==(const charT* a, const basic_string<charT>& b)
  {
    return b.compare(a) == 0;
  }

  template<class charT>
  bool operator==(const basic_string<charT>& a, const charT* b)
  {
    return a.compare(b) == 0;
  }

  template<class charT>
  bool operator!=(const basic_string<charT>& a, const basic_string<charT>& b)
  {
    return !(a == b);
  }

  template<class charT>
  bool operator!=(const charT* a, const basic_string<charT>& b)
  {
    return !(a == b);
  }

  template<class charT>
  bool operator!=(const basic_string<charT>& a, const charT* b)
  {
    return !(a == b);
  }

  template<class charT>
  bool operator<(const basic_string<charT>& a, const basic_string<charT>& b)
  {
    return a.compare(b) < 0;
  }

  template<class charT>
  bool operator<(const charT* a, const basic_string<charT>& b)
  {
    return b.compare(a) > 0;
  }

  template<class charT>
  bool operator<(const basic_string<charT>& a, const charT* b)
  {
    return a.compare(b) < 0;
  }

  template<class charT>
  bool operator>(const basic_string<charT>& a, const basic_string<charT>& b)
  {
    return a.compare(b) > 0;
  }

  template<class charT>
  bool operator>(const charT* a, const basic_string<charT>& b)
  {
    return b.compare(a) < 0;
  }

  template<class charT>
  bool operator>(const basic_string<charT>& a, const charT* b)
  {
    return a.compare(b) > 0;
  }

  template<class charT>
  bool operator<=(const basic_string<charT>& a, const basic_string<charT>& b)
  {
    return a.compare(b) <= 0;
  }

  template<class charT>
  bool operator<=(const charT* a, const basic_string<charT>& b)
  {
    return b.compare(a) >= 0;
  }

  template<class charT>
  bool operator<=(const basic_string<charT>& a, const charT* b)
  {
    return a.compare(b) <= 0;
  }

  template<class charT>
  bool operator>=(const basic_string<charT>& a, const basic_string<charT>& b)
  {
    return a.compare(b) >= 0;
  }

  template<class charT>
  bool operator>=(const charT* a, const basic_string<charT>& b)
  {
    return b.compare(a) <= 0;
  }

  template<class charT>
  bool operator>=(const basic_string<charT>& a, const charT* b)
  {
    return a.compare(b) >= 0;
  }


  template<class charT>
  basic_string<charT> operator+(const basic_string<charT>& lhs,
                                const basic_string<charT>& rhs)
  {
    basic_string<charT> o(lhs);
    return o.append(rhs);
  }

  template<class charT>
  basic_string<charT> operator+(basic_string<charT>&& lhs,
                                const basic_string<charT>& rhs)
  {
    return lhs.append(rhs);
  }

  template<class charT>
  basic_string<charT> operator+(const charT* lhs,
                                const basic_string<charT>& rhs)
  {
    basic_string<charT> o(lhs);
    return o.append(rhs);
  }

  template<class charT>
  basic_string<charT> operator+(charT lhs,
                                const basic_string<charT>& rhs)
  {
    basic_string<charT> o(1, lhs);
    return o.append(rhs);
  }
  
  template<class charT>
  basic_string<charT> operator+(const basic_string<charT>& lhs,
                                const charT* rhs)
  {
    basic_string<charT> o(lhs);
    return o.append(rhs);
  }

  template<class charT>
  basic_string<charT> operator+(basic_string<charT>&& lhs,
                                const charT* rhs)
  {
    return lhs.append(rhs);
  }

  template<class charT>
  basic_string<charT> operator+(const basic_string<charT>& lhs,
                                const charT rhs)
  {
    basic_string<charT> o(lhs);
    return o.append(1, rhs);
  }

  template<class charT>
  basic_string<charT> operator+(basic_string<charT>&& lhs,
                                const charT rhs)
  {
    return lhs.append(1, rhs);
  }

  typedef basic_string<char> string;
}