c10::string_view (#26616)

Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/26616

Implement C++17 std::string_view for C++11.

This is useful for compile time type name retrievaly which I'm going to stack on top of this.
It is also useful to replace `const std::string&` with throughout our codebase.
ghstack-source-id: 92100314

Test Plan: unit tests

Differential Revision: D17518992

fbshipit-source-id: 48e31c677d51b0041f4b37e89a92bd176d4a0b08

c10/macros/Macros.h

@@ -263,4 +263,13 @@ constexpr uint32_t CUDA_THREADS_PER_BLOCK_FALLBACK = 256;
 #define C10_CPP14_HOST_CONSTEXPR AT_CPP14_CONSTEXPR
 #endif
 
+#if !defined(__clang__) && !defined(_MSC_VER) && defined(__GNUC__) && \
+    __GNUC__ < 6
+#define CONSTEXPR_EXCEPT_GCC5
+#define IS_NOT_GCC5_CONSTEXPR 0
+#else
+#define CONSTEXPR_EXCEPT_GCC5 AT_CPP14_CONSTEXPR
+#define IS_NOT_GCC5_CONSTEXPR AT_IS_CPP14_CONSTEXPR
+#endif
+
 #endif // C10_MACROS_MACROS_H_

c10/test/util/C++17_test.cpp

@@ -0,0 +1,14 @@
+#include <c10/util/C++17.h>
+
+using c10::guts::max;
+using c10::guts::min;
+
+static_assert(min(3, 5) == 3, "");
+static_assert(min(5, 3) == 3, "");
+static_assert(min(3, 3) == 3, "");
+static_assert(min(3.0, 3.1) == 3.0, "");
+
+static_assert(max(3, 5) == 5, "");
+static_assert(max(5, 3) == 5, "");
+static_assert(max(3, 3) == 3, "");
+static_assert(max(3.0, 3.1) == 3.1, "");

c10/util/C++17.h

@@ -294,6 +294,15 @@ template<class T> inline std::string to_string(T value) {
     return detail::to_string_<T>::call(value);
 }
 
+template <class T>
+constexpr const T& min(const T& a, const T& b) {
+  return (b < a) ? b : a;
+}
+
+template <class T>
+constexpr const T& max(const T& a, const T& b) {
+  return (a < b) ? b : a;
+}
 }}
 
 #endif // C10_UTIL_CPP17_H_

c10/util/reverse_iterator.h

@@ -0,0 +1,295 @@
+#pragma once
+
+/**
+ * A constexpr std::reverse_iterator for C++11.
+ * Implementation taken from libstdc++,
+ * https://raw.githubusercontent.com/gcc-mirror/gcc/gcc-9_2_0-release/libstdc%2B%2B-v3/include/bits/stl_iterator.h
+ * adapted to our code base and constexpr'ified.
+ */
+
+// Copyright (C) 2001-2019 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library.  This library 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, or (at your option)
+// any later version.
+
+// This library 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.
+
+// Under Section 7 of GPL version 3, you are granted additional
+// permissions described in the GCC Runtime Library Exception, version
+// 3.1, as published by the Free Software Foundation.
+
+// You should have received a copy of the GNU General Public License and
+// a copy of the GCC Runtime Library Exception along with this program;
+// see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
+// <http://www.gnu.org/licenses/>.
+
+/*
+ *
+ * Copyright (c) 1994
+ * Hewlett-Packard Company
+ *
+ * Permission to use, copy, modify, distribute and sell this software
+ * and its documentation for any purpose is hereby granted without fee,
+ * provided that the above copyright notice appear in all copies and
+ * that both that copyright notice and this permission notice appear
+ * in supporting documentation.  Hewlett-Packard Company makes no
+ * representations about the suitability of this software for any
+ * purpose.  It is provided "as is" without express or implied warranty.
+ *
+ *
+ * Copyright (c) 1996-1998
+ * Silicon Graphics Computer Systems, Inc.
+ *
+ * Permission to use, copy, modify, distribute and sell this software
+ * and its documentation for any purpose is hereby granted without fee,
+ * provided that the above copyright notice appear in all copies and
+ * that both that copyright notice and this permission notice appear
+ * in supporting documentation.  Silicon Graphics makes no
+ * representations about the suitability of this software for any
+ * purpose.  It is provided "as is" without express or implied warranty.
+ */
+
+#include <c10/util/C++17.h>
+#include <iterator>
+
+namespace c10 {
+
+template <typename _Iterator>
+class reverse_iterator
+    : public std::iterator<
+          typename std::iterator_traits<_Iterator>::iterator_category,
+          typename std::iterator_traits<_Iterator>::value_type,
+          typename std::iterator_traits<_Iterator>::difference_type,
+          typename std::iterator_traits<_Iterator>::pointer,
+          typename std::iterator_traits<_Iterator>::reference> {
+ protected:
+  _Iterator current;
+
+  using __traits_type = std::iterator_traits<_Iterator>;
+
+ public:
+  using iterator_type = _Iterator;
+  using difference_type = typename __traits_type::difference_type;
+  using pointer = typename __traits_type::pointer;
+  using reference = typename __traits_type::reference;
+
+  constexpr reverse_iterator() : current() {}
+
+  explicit constexpr reverse_iterator(iterator_type __x) : current(__x) {}
+
+  constexpr reverse_iterator(const reverse_iterator& __x)
+      : current(__x.current) {}
+
+  AT_CPP14_CONSTEXPR reverse_iterator& operator=(
+      const reverse_iterator& rhs) noexcept {
+    current = rhs.current;
+  }
+
+  template <typename _Iter>
+  constexpr reverse_iterator(const reverse_iterator<_Iter>& __x)
+      : current(__x.base()) {}
+
+  constexpr iterator_type base() const {
+    return current;
+  }
+
+  constexpr reference operator*() const {
+#if defined(__cpp_constexpr) && __cpp_constexpr >= 201304
+    _Iterator iter = current;
+    return *--iter;
+#else
+    // Only works for random access iterators if we're not C++14 :(
+    return *(current - 1);
+#endif
+  }
+
+  constexpr pointer operator->() const {
+#if defined(__cpp_constexpr) && __cpp_constexpr >= 201304
+    _Iterator iter = current;
+    return _S_to_pointer(--iter);
+#else
+    // Only works for random access iterators if we're not C++14 :(
+    return _S_to_pointer(current - 1);
+#endif
+  }
+
+  AT_CPP14_CONSTEXPR reverse_iterator& operator++() {
+    --current;
+    return *this;
+  }
+
+  AT_CPP14_CONSTEXPR reverse_iterator operator++(int) {
+    reverse_iterator __tmp = *this;
+    --current;
+    return __tmp;
+  }
+
+  AT_CPP14_CONSTEXPR reverse_iterator& operator--() {
+    ++current;
+    return *this;
+  }
+
+  AT_CPP14_CONSTEXPR reverse_iterator operator--(int) {
+    reverse_iterator __tmp = *this;
+    ++current;
+    return __tmp;
+  }
+
+  constexpr reverse_iterator operator+(difference_type __n) const {
+    return reverse_iterator(current - __n);
+  }
+
+  AT_CPP14_CONSTEXPR reverse_iterator& operator+=(difference_type __n) {
+    current -= __n;
+    return *this;
+  }
+
+  constexpr reverse_iterator operator-(difference_type __n) const {
+    return reverse_iterator(current + __n);
+  }
+
+  AT_CPP14_CONSTEXPR reverse_iterator& operator-=(difference_type __n) {
+    current += __n;
+    return *this;
+  }
+
+  constexpr reference operator[](difference_type __n) const {
+    return *(*this + __n);
+  }
+
+ private:
+  template <typename _Tp>
+  static constexpr _Tp* _S_to_pointer(_Tp* __p) {
+    return __p;
+  }
+
+  template <typename _Tp>
+  static constexpr pointer _S_to_pointer(_Tp __t) {
+    return __t.operator->();
+  }
+};
+
+template <typename _Iterator>
+inline constexpr bool operator==(
+    const reverse_iterator<_Iterator>& __x,
+    const reverse_iterator<_Iterator>& __y) {
+  return __x.base() == __y.base();
+}
+
+template <typename _Iterator>
+inline constexpr bool operator<(
+    const reverse_iterator<_Iterator>& __x,
+    const reverse_iterator<_Iterator>& __y) {
+  return __y.base() < __x.base();
+}
+
+template <typename _Iterator>
+inline constexpr bool operator!=(
+    const reverse_iterator<_Iterator>& __x,
+    const reverse_iterator<_Iterator>& __y) {
+  return !(__x == __y);
+}
+
+template <typename _Iterator>
+inline constexpr bool operator>(
+    const reverse_iterator<_Iterator>& __x,
+    const reverse_iterator<_Iterator>& __y) {
+  return __y < __x;
+}
+
+template <typename _Iterator>
+inline constexpr bool operator<=(
+    const reverse_iterator<_Iterator>& __x,
+    const reverse_iterator<_Iterator>& __y) {
+  return !(__y < __x);
+}
+
+template <typename _Iterator>
+inline constexpr bool operator>=(
+    const reverse_iterator<_Iterator>& __x,
+    const reverse_iterator<_Iterator>& __y) {
+  return !(__x < __y);
+}
+
+template <typename _IteratorL, typename _IteratorR>
+inline constexpr bool operator==(
+    const reverse_iterator<_IteratorL>& __x,
+    const reverse_iterator<_IteratorR>& __y) {
+  return __x.base() == __y.base();
+}
+
+template <typename _IteratorL, typename _IteratorR>
+inline constexpr bool operator<(
+    const reverse_iterator<_IteratorL>& __x,
+    const reverse_iterator<_IteratorR>& __y) {
+  return __y.base() < __x.base();
+}
+
+template <typename _IteratorL, typename _IteratorR>
+inline constexpr bool operator!=(
+    const reverse_iterator<_IteratorL>& __x,
+    const reverse_iterator<_IteratorR>& __y) {
+  return !(__x == __y);
+}
+
+template <typename _IteratorL, typename _IteratorR>
+inline constexpr bool operator>(
+    const reverse_iterator<_IteratorL>& __x,
+    const reverse_iterator<_IteratorR>& __y) {
+  return __y < __x;
+}
+
+template <typename _IteratorL, typename _IteratorR>
+inline constexpr bool operator<=(
+    const reverse_iterator<_IteratorL>& __x,
+    const reverse_iterator<_IteratorR>& __y) {
+  return !(__y < __x);
+}
+
+template <typename _IteratorL, typename _IteratorR>
+inline constexpr bool operator>=(
+    const reverse_iterator<_IteratorL>& __x,
+    const reverse_iterator<_IteratorR>& __y) {
+  return !(__x < __y);
+}
+
+template <typename _IteratorL, typename _IteratorR>
+inline constexpr auto operator-(
+    const reverse_iterator<_IteratorL>& __x,
+    const reverse_iterator<_IteratorR>& __y)
+    -> decltype(__y.base() - __x.base()) {
+  return __y.base() - __x.base();
+}
+
+template <typename _Iterator>
+inline constexpr reverse_iterator<_Iterator> operator+(
+    typename reverse_iterator<_Iterator>::difference_type __n,
+    const reverse_iterator<_Iterator>& __x) {
+  return reverse_iterator<_Iterator>(__x.base() - __n);
+}
+
+template <typename _Iterator>
+inline constexpr reverse_iterator<_Iterator> __make_reverse_iterator(
+    _Iterator __i) {
+  return reverse_iterator<_Iterator>(__i);
+}
+
+template <typename _Iterator>
+inline constexpr reverse_iterator<_Iterator> make_reverse_iterator(
+    _Iterator __i) {
+  return reverse_iterator<_Iterator>(__i);
+}
+
+template <typename _Iterator>
+auto __niter_base(reverse_iterator<_Iterator> __it)
+    -> decltype(__make_reverse_iterator(__niter_base(__it.base()))) {
+  return __make_reverse_iterator(__niter_base(__it.base()));
+}
+
+} // namespace c10

c10/util/string_view.h

@@ -0,0 +1,677 @@
+#pragma once
+
+#include <c10/macros/Macros.h>
+#include <c10/util/C++17.h>
+#include <c10/util/reverse_iterator.h>
+#include <cstring>
+#include <limits>
+#include <stdexcept>
+#include <string>
+
+namespace c10 {
+
+/**
+ * Reimplementation of std::string_view for C++11.
+ * Implemented following the interface definition in
+ * https://en.cppreference.com/w/cpp/string/basic_string_view
+ * See there for the API documentation.
+ *
+ * Difference: We don't have a Traits template parameter because
+ * std::char_traits isn't constexpr and we'd have to reimplement
+ * std::char_traits if we wanted to use it with our constexpr basic_string_view.
+ */
+template <class CharT>
+class basic_string_view final {
+ public:
+  using value_type = CharT;
+  using pointer = CharT*;
+  using const_pointer = const CharT*;
+  using reference = CharT&;
+  using const_reference = const CharT&;
+  using const_iterator = const CharT*;
+  using iterator = const_iterator;
+  using const_reverse_iterator = c10::reverse_iterator<const_iterator>;
+  using reverse_iterator = const_reverse_iterator;
+  using size_type = std::size_t;
+  using difference_type = std::ptrdiff_t;
+
+  static constexpr size_type npos = size_type(-1);
+
+  constexpr basic_string_view() noexcept : begin_(nullptr), size_(0) {}
+
+  explicit constexpr basic_string_view(const_pointer str, size_type count)
+      : begin_(str), size_(count) {}
+
+  /* implicit */ constexpr basic_string_view(const_pointer str)
+      : basic_string_view(str, strlen_(str)) {}
+
+  /* implicit */ basic_string_view(const ::std::basic_string<CharT>& str)
+      : basic_string_view(str.data(), str.size()) {}
+
+  constexpr basic_string_view(const basic_string_view&) noexcept = default;
+
+  AT_CPP14_CONSTEXPR basic_string_view& operator=(
+      const basic_string_view& rhs) noexcept {
+    begin_ = rhs.begin_;
+    size_ = rhs.size_;
+    return *this;
+  }
+
+  explicit operator ::std::basic_string<CharT>() const {
+    return ::std::basic_string<CharT>(data(), size());
+  }
+
+  constexpr const_iterator begin() const noexcept {
+    return cbegin();
+  }
+
+  constexpr const_iterator cbegin() const noexcept {
+    return begin_;
+  }
+
+  constexpr const_iterator end() const noexcept {
+    return cend();
+  }
+
+  constexpr const_iterator cend() const noexcept {
+    return begin_ + size_;
+  }
+
+  constexpr const_reverse_iterator rbegin() const noexcept {
+    return crbegin();
+  }
+
+  constexpr const_reverse_iterator crbegin() const noexcept {
+    return const_reverse_iterator(this->end());
+  }
+
+  constexpr const_reverse_iterator rend() const noexcept {
+    return crend();
+  }
+
+  constexpr const_reverse_iterator crend() const noexcept {
+    return const_reverse_iterator(this->begin());
+  }
+
+  friend constexpr const_iterator begin(basic_string_view sv) noexcept {
+    return sv.begin();
+  }
+
+  friend constexpr const_iterator end(basic_string_view sv) noexcept {
+    return sv.end();
+  }
+
+  constexpr const_reference operator[](size_type pos) const {
+    return at(pos);
+  }
+
+  constexpr const_reference at(size_type pos) const {
+#if !defined( \
+    __CUDA_ARCH__) // CUDA doesn't like std::out_of_range in device code
+    return C10_UNLIKELY(pos >= size_)
+        ? (throw std::out_of_range(
+               "string_view::operator[] or string_view::at() out of range. Index: " +
+               std::to_string(pos) + ", size: " + std::to_string(size())),
+           at_(0))
+        : at_(pos);
+#else
+    return at_(pos);
+#endif
+  }
+
+  constexpr const_reference front() const {
+    return *begin_;
+  }
+
+  constexpr const_reference back() const {
+    return *(begin_ + size_ - 1);
+  }
+
+  constexpr const_pointer data() const noexcept {
+    return begin_;
+  }
+
+  constexpr size_type size() const noexcept {
+    return size_;
+  }
+
+  constexpr size_type length() const noexcept {
+    return size();
+  }
+
+  constexpr size_type max_size() const noexcept {
+    return std::numeric_limits<difference_type>::max();
+  }
+
+  C10_NODISCARD constexpr bool empty() const noexcept {
+    return size() == 0;
+  }
+
+  CONSTEXPR_EXCEPT_GCC5 void remove_prefix(size_type n) {
+    if (n > size()) {
+      throw std::out_of_range(
+          "basic_string_view::remove_prefix: out of range. PrefixLength: " +
+          c10::guts::to_string(n) + ", size: " + c10::guts::to_string(size()));
+    }
+    begin_ += n;
+    size_ -= n;
+  }
+
+  CONSTEXPR_EXCEPT_GCC5 void remove_suffix(size_type n) {
+    if (n > size()) {
+      throw std::out_of_range(
+          "basic_string_view::remove_suffix: out of range. SuffixLength: " +
+          c10::guts::to_string(n) + ", size: " + c10::guts::to_string(size()));
+    }
+    size_ -= n;
+  }
+
+  CONSTEXPR_EXCEPT_GCC5 void swap(basic_string_view& sv) noexcept {
+    auto tmp = *this;
+    *this = sv;
+    sv = tmp;
+  }
+
+  size_type copy(pointer dest, size_type count, size_type pos = 0) const {
+    if (pos > size_) {
+      throw std::out_of_range(
+          "basic_string_view::copy: out of range. Index: " +
+          c10::guts::to_string(pos) +
+          ", size: " + c10::guts::to_string(size()));
+    }
+    size_type copy_length = c10::guts::min(count, size_ - pos);
+    for (auto iter = begin() + pos, end = iter + copy_length; iter != end;) {
+      *(dest++) = *(iter++);
+    }
+    return copy_length;
+  }
+
+  constexpr basic_string_view substr(size_type pos = 0, size_type count = npos)
+      const {
+#if !defined( \
+    __CUDA_ARCH__) // CUDA doesn't like std::out_of_range in device code
+    return (pos > size_)
+        ? (throw std::out_of_range(
+               "basic_string_view::substr parameter out of bounds. Index: " +
+               std::to_string(pos) + ", size: " + std::to_string(size())),
+           substr_())
+        : substr_(pos, count);
+#else
+    return substr_(pos, count);
+#endif
+  }
+
+  constexpr int compare(basic_string_view rhs) const noexcept {
+#if __cpp_constexpr >= 201304
+    // if we are in C++14, write it iteratively. This is faster.
+    for (size_t i = 0, end = c10::guts::min(size(), rhs.size()); i < end; ++i) {
+      if (at_(i) < rhs.at_(i)) {
+        return -1;
+      } else if (at_(i) > rhs.at_(i)) {
+        return 1;
+      }
+    }
+    if (size() < rhs.size()) {
+      return -1;
+    } else if (size() > rhs.size()) {
+      return 1;
+    }
+    return 0;
+#else
+    // if we are in C++11, we need to do it recursively because of constexpr
+    // restrictions.
+    return (size() == 0 && rhs.size() == 0) ? 0
+                                            : (size() == 0) ? -1
+                                                            : (rhs.size() == 0)
+                ? 1
+                : (front() < rhs.front()) ? -1
+                                          : (front() > rhs.front())
+                        ? 1
+                        : substr_(1).compare(rhs.substr_(1));
+#endif
+  }
+
+  constexpr int compare(size_type pos1, size_type count1, basic_string_view v)
+      const {
+    return substr(pos1, count1).compare(v);
+  }
+
+  constexpr int compare(
+      size_type pos1,
+      size_type count1,
+      basic_string_view v,
+      size_type pos2,
+      size_type count2) const {
+    return substr(pos1, count1).compare(v.substr(pos2, count2));
+  }
+
+  constexpr int compare(const_pointer s) const {
+    return compare(basic_string_view(s));
+  }
+
+  constexpr int compare(size_type pos1, size_type count1, const_pointer s)
+      const {
+    return substr(pos1, count1).compare(basic_string_view(s));
+  }
+
+  constexpr int compare(
+      size_type pos1,
+      size_type count1,
+      const_pointer s,
+      size_type count2) const {
+    return substr(pos1, count1).compare(basic_string_view(s, count2));
+  }
+
+  friend constexpr bool operator==(
+      basic_string_view lhs,
+      basic_string_view rhs) noexcept {
+    return lhs.equals_(rhs);
+  }
+
+  friend constexpr bool operator!=(
+      basic_string_view lhs,
+      basic_string_view rhs) noexcept {
+    return !(lhs == rhs);
+  }
+
+  friend constexpr bool operator<(
+      basic_string_view lhs,
+      basic_string_view rhs) noexcept {
+    return lhs.compare(rhs) < 0;
+  }
+
+  friend constexpr bool operator>=(
+      basic_string_view lhs,
+      basic_string_view rhs) noexcept {
+    return !(lhs < rhs);
+  }
+
+  friend constexpr bool operator>(
+      basic_string_view lhs,
+      basic_string_view rhs) noexcept {
+    return rhs < lhs;
+  }
+
+  friend constexpr bool operator<=(
+      basic_string_view lhs,
+      basic_string_view rhs) noexcept {
+    return !(lhs > rhs);
+  }
+
+  constexpr bool starts_with(basic_string_view prefix) const noexcept {
+    return (prefix.size() > size()) ? false
+                                    : prefix.equals_(substr_(0, prefix.size()));
+  }
+
+  constexpr bool starts_with(CharT prefix) const noexcept {
+    return !empty() && prefix == front();
+  }
+
+  constexpr bool starts_with(const_pointer prefix) const {
+    return starts_with(basic_string_view(prefix));
+  }
+
+  constexpr bool ends_with(basic_string_view suffix) const noexcept {
+    return (suffix.size() > size())
+        ? false
+        : suffix.equals_(substr_(size() - suffix.size(), suffix.size()));
+  }
+
+  constexpr bool ends_with(CharT suffix) const noexcept {
+    return !empty() && suffix == back();
+  }
+
+  constexpr bool ends_with(const_pointer suffix) const {
+    return ends_with(basic_string_view(suffix));
+  }
+
+  constexpr size_type find(basic_string_view v, size_type pos = 0) const
+      noexcept {
+#if __cpp_constexpr >= 201304
+    // if we are in C++14, write it iteratively. This is faster.
+    if (v.size() == 0) {
+      return pos <= size() ? pos : npos;
+    }
+
+    if (pos + v.size() <= size()) {
+      for (size_type cur = pos, end = size() - v.size(); cur <= end; ++cur) {
+        if (v.at_(0) == at_(cur) &&
+            v.substr_(1).equals_(substr_(cur + 1, v.size() - 1))) {
+          return cur;
+        }
+      }
+    }
+    return npos;
+#else
+    // if we are in C++11, we need to do it recursively because of constexpr
+    // restrictions.
+    return (v.size() == 0) ? (pos <= size() ? pos : npos)
+                           : (pos + v.size() > size())
+            ? npos
+            : (v.at_(0) == at_(pos) &&
+               v.substr_(1).equals_(substr_(pos + 1, v.size() - 1)))
+                ? pos
+                : find(v, pos + 1);
+#endif
+  }
+
+  constexpr size_type find(CharT ch, size_type pos = 0) const noexcept {
+    return find_first_if_(pos, charIsEqual_{ch});
+  }
+
+  constexpr size_type find(const_pointer s, size_type pos, size_type count)
+      const {
+    return find(basic_string_view(s, count), pos);
+  }
+
+  constexpr size_type find(const_pointer s, size_type pos = 0) const {
+    return find(basic_string_view(s), pos);
+  }
+
+  constexpr size_type rfind(basic_string_view v, size_type pos = npos) const
+      noexcept {
+#if __cpp_constexpr >= 201304
+    // if we are in C++14, write it iteratively. This is faster.
+    if (v.size() == 0) {
+      return pos <= size() ? pos : size();
+    }
+
+    if (v.size() <= size()) {
+      pos = c10::guts::min(size() - v.size(), pos);
+      do {
+        if (v.at_(0) == at_(pos) &&
+            v.substr_(1).equals_(substr_(pos + 1, v.size() - 1))) {
+          return pos;
+        }
+      } while (pos-- > 0);
+    }
+    return npos;
+#else
+    // if we are in C++11, we need to do it recursively because of constexpr
+    // restrictions.
+    return (v.size() == 0) ? (pos <= size() ? pos : size())
+                           : (v.size() > size()) ? npos
+                                                 : (size() - v.size() < pos)
+                ? rfind(v, size() - v.size())
+                : (v.at_(0) == at_(pos) &&
+                   v.substr_(1).equals_(substr_(pos + 1, v.size() - 1)))
+                    ? pos
+                    : (pos == 0) ? npos : rfind(v, pos - 1);
+#endif
+  }
+
+  constexpr size_type rfind(CharT ch, size_type pos = npos) const noexcept {
+    return find_last_if_(pos, charIsEqual_{ch});
+  }
+
+  constexpr size_type rfind(const_pointer s, size_type pos, size_type count)
+      const {
+    return rfind(basic_string_view(s, count), pos);
+  }
+
+  constexpr size_type rfind(const_pointer s, size_type pos = npos) const {
+    return rfind(basic_string_view(s), pos);
+  }
+
+  constexpr size_type find_first_of(basic_string_view v, size_type pos = 0)
+      const noexcept {
+    return find_first_if_(pos, stringViewContainsChar_{v});
+  }
+
+  constexpr size_type find_first_of(CharT ch, size_type pos = 0) const
+      noexcept {
+    return find_first_if_(pos, charIsEqual_{ch});
+  }
+
+  constexpr size_type find_first_of(
+      const_pointer s,
+      size_type pos,
+      size_type count) const {
+    return find_first_of(basic_string_view(s, count), pos);
+  }
+
+  constexpr size_type find_first_of(const_pointer s, size_type pos = 0) const {
+    return find_first_of(basic_string_view(s), pos);
+  }
+
+  constexpr size_type find_last_of(basic_string_view v, size_type pos = npos)
+      const noexcept {
+    return find_last_if_(pos, stringViewContainsChar_{v});
+  }
+
+  constexpr size_type find_last_of(CharT ch, size_type pos = npos) const
+      noexcept {
+    return find_last_if_(pos, charIsEqual_{ch});
+  }
+
+  constexpr size_type find_last_of(
+      const_pointer s,
+      size_type pos,
+      size_type count) const {
+    return find_last_of(basic_string_view(s, count), pos);
+  }
+
+  constexpr size_type find_last_of(const_pointer s, size_type pos = npos)
+      const {
+    return find_last_of(basic_string_view(s), pos);
+  }
+
+  constexpr size_type find_first_not_of(basic_string_view v, size_type pos = 0)
+      const noexcept {
+    return find_first_if_(pos, stringViewDoesNotContainChar_{v});
+  }
+
+  constexpr size_type find_first_not_of(CharT ch, size_type pos = 0) const
+      noexcept {
+    return find_first_if_(pos, charIsNotEqual_{ch});
+  }
+
+  constexpr size_type find_first_not_of(
+      const_pointer s,
+      size_type pos,
+      size_type count) const {
+    return find_first_not_of(basic_string_view(s, count), pos);
+  }
+
+  constexpr size_type find_first_not_of(const_pointer s, size_type pos = 0)
+      const {
+    return find_first_not_of(basic_string_view(s), pos);
+  }
+
+  constexpr size_type find_last_not_of(
+      basic_string_view v,
+      size_type pos = npos) const noexcept {
+    return find_last_if_(pos, stringViewDoesNotContainChar_{v});
+  }
+
+  constexpr size_type find_last_not_of(CharT ch, size_type pos = npos) const
+      noexcept {
+    return find_last_if_(pos, charIsNotEqual_{ch});
+  }
+
+  constexpr size_type find_last_not_of(
+      const_pointer s,
+      size_type pos,
+      size_type count) const {
+    return find_last_not_of(basic_string_view(s, count), pos);
+  }
+
+  constexpr size_type find_last_not_of(const_pointer s, size_type pos = npos)
+      const {
+    return find_last_not_of(basic_string_view(s), pos);
+  }
+
+ private:
+  static constexpr size_type strlen_(const_pointer str) noexcept {
+#if __cpp_constexpr >= 201304
+    // if we are in C++14, write it iteratively. This is faster.
+    const_pointer current = str;
+    while (*current != '\0') {
+      ++current;
+    }
+    return current - str;
+#else
+    // if we are in C++11, we need to do it recursively because of constexpr
+    // restrictions.
+    return (*str == '\0') ? 0 : 1 + strlen_(str + 1);
+#endif
+  }
+
+  constexpr const_reference at_(size_type pos) const noexcept {
+    return *(begin_ + pos);
+  }
+
+  constexpr basic_string_view substr_(size_type pos = 0, size_type count = npos)
+      const {
+    return basic_string_view{begin_ + pos, c10::guts::min(count, size() - pos)};
+  }
+
+  template <class Condition>
+  constexpr size_type find_first_if_(size_type pos, Condition&& condition) const
+      noexcept {
+#if __cpp_constexpr >= 201304
+    // if we are in C++14, write it iteratively. This is faster.
+    if (pos + 1 <= size()) {
+      for (size_type cur = pos; cur < size(); ++cur) {
+        if (condition(at_(cur))) {
+          return cur;
+        }
+      }
+    }
+    return npos;
+#else
+    // if we are in C++11, we need to do it recursively because of constexpr
+    // restrictions.
+    return (pos + 1 > size()) ? npos
+                              : condition(at_(pos))
+            ? pos
+            : find_first_if_(pos + 1, std::forward<Condition>(condition));
+#endif
+  }
+
+  template <class Condition>
+  constexpr size_type find_last_if_(size_type pos, Condition&& condition) const
+      noexcept {
+#if __cpp_constexpr >= 201304
+    // if we are in C++14, write it iteratively. This is faster.
+    if (size() > 0) {
+      pos = c10::guts::min(size() - 1, pos);
+      do {
+        if (condition(at_(pos))) {
+          return pos;
+        }
+      } while (pos-- > 0);
+    }
+    return npos;
+#else
+    // if we are in C++11, we need to do it recursively because of constexpr
+    // restrictions.
+    return (size() == 0) ? npos
+                         : (pos >= size())
+            ? find_last_if_(size() - 1, std::forward<Condition>(condition))
+            : condition(at_(pos))
+                ? pos
+                : (pos == 0) ? npos
+                             : find_last_if_(
+                                   pos - 1, std::forward<Condition>(condition));
+#endif
+  }
+
+  constexpr bool equals_(basic_string_view rhs) const {
+// We don't use string_view::compare() here but implement it manually because
+// only looking at equality allows for more optimized code.
+#if __cpp_constexpr >= 201304
+    // if we are in C++14, write it iteratively. This is faster.
+    if (size() != rhs.size()) {
+      return false;
+    }
+    // Yes, memcmp would be laster than this loop, but memcmp isn't constexpr
+    // and I didn't feel like implementing a constexpr memcmp variant.
+    // TODO At some point this should probably be done, including tricks
+    // like comparing one machine word instead of a byte per iteration.
+    for (typename basic_string_view<CharT>::size_type pos = 0; pos < size();
+         ++pos) {
+      if (at_(pos) != rhs.at_(pos)) {
+        return false;
+      }
+    }
+    return true;
+#else
+    // if we are in C++11, we need to do it recursively because of constexpr
+    // restrictions.
+    return (size() != rhs.size()) ? false
+                                  : (size() == 0)
+            ? true
+            : (front() != rhs.front()) ? false
+                                       : (substr_(1).equals_(rhs.substr_(1)));
+#endif
+  }
+
+  struct charIsEqual_ final {
+    CharT expected;
+    constexpr bool operator()(CharT actual) const noexcept {
+      return expected == actual;
+    }
+  };
+
+  struct charIsNotEqual_ final {
+    CharT expected;
+    constexpr bool operator()(CharT actual) const noexcept {
+      return expected != actual;
+    }
+  };
+
+  struct stringViewContainsChar_ final {
+    basic_string_view expected;
+    constexpr bool operator()(CharT ch) const noexcept {
+      return npos != expected.find(ch);
+    }
+  };
+
+  struct stringViewDoesNotContainChar_ final {
+    basic_string_view expected;
+    constexpr bool operator()(CharT ch) const noexcept {
+      return npos == expected.find(ch);
+    }
+  };
+
+  const_pointer begin_;
+  size_type size_;
+};
+
+template <class CharT>
+inline std::basic_ostream<CharT>& operator<<(
+    std::basic_ostream<CharT>& stream,
+    basic_string_view<CharT> sv) {
+  // The rules for operator<< are quite complex, but std::string has the same.
+  // Let's just rely on the std::string implementation. This might be a bit
+  // slower, but I don't think performance matters here.
+  return stream << ::std::basic_string<CharT>(sv);
+}
+
+template <class CharT>
+CONSTEXPR_EXCEPT_GCC5 inline void swap(
+    basic_string_view<CharT>& lhs,
+    basic_string_view<CharT>& rhs) {
+  lhs.swap(rhs);
+}
+
+using string_view = basic_string_view<char>;
+
+} // namespace c10
+
+namespace std {
+template <class CharT>
+struct hash<::c10::basic_string_view<CharT>> {
+  size_t operator()(::c10::basic_string_view<CharT> x) const {
+    // The standard says that std""string_view hashing must do the same as
+    // std::string hashing but leaves the details of std::string hashing
+    // up to the implementer. So, to be conformant, we need to have the same
+    // behavior as the implementer-defined std::string hasher of the STL
+    // we're built against. Let's just call it. This is probably slow
+    // but the only way to be conformant.
+    return std::hash<::std::basic_string<CharT>>()(
+        ::std::basic_string<CharT>(x));
+  }
+};
+} // namespace std