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+//
+// Copyright 2017 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+
+// This file declares INTERNAL parts of the Join API that are inlined/templated
+// or otherwise need to be available at compile time. The main abstractions
+// defined in this file are:
+//
+//   - A handful of default Formatters
+//   - JoinAlgorithm() overloads
+//   - JoinRange() overloads
+//   - JoinTuple()
+//
+// DO NOT INCLUDE THIS FILE DIRECTLY. Use this file by including
+// absl/strings/str_join.h
+//
+// IWYU pragma: private, include "absl/strings/str_join.h"
+
+#ifndef ABSL_STRINGS_INTERNAL_STR_JOIN_INTERNAL_H_
+#define ABSL_STRINGS_INTERNAL_STR_JOIN_INTERNAL_H_
+
+#include <cassert>
+#include <iterator>
+#include <memory>
+#include <string>
+#include <utility>
+
+#include "absl/strings/internal/ostringstream.h"
+#include "absl/strings/str_cat.h"
+
+namespace absl {
+namespace strings_internal {
+
+//
+// Formatter objects
+//
+// The following are implementation classes for standard Formatter objects. The
+// factory functions that users will call to create and use these formatters are
+// defined and documented in strings/join.h.
+//
+
+// The default formatter. Converts alpha-numeric types to strings.
+struct AlphaNumFormatterImpl {
+  // This template is needed in order to support passing in a dereferenced
+  // vector<bool>::iterator
+  template <typename T>
+  void operator()(std::string* out, const T& t) const {
+    StrAppend(out, AlphaNum(t));
+  }
+
+  void operator()(std::string* out, const AlphaNum& t) const {
+    StrAppend(out, t);
+  }
+};
+
+// A type that's used to overload the JoinAlgorithm() function (defined below)
+// for ranges that do not require additional formatting (e.g., a range of
+// strings).
+
+struct NoFormatter : public AlphaNumFormatterImpl {};
+
+// Formats types to strings using the << operator.
+class StreamFormatterImpl {
+ public:
+  // The method isn't const because it mutates state. Making it const will
+  // render StreamFormatterImpl thread-hostile.
+  template <typename T>
+  void operator()(std::string* out, const T& t) {
+    // The stream is created lazily to avoid paying the relatively high cost
+    // of its construction when joining an empty range.
+    if (strm_) {
+      strm_->clear();  // clear the bad, fail and eof bits in case they were set
+      strm_->str(out);
+    } else {
+      strm_.reset(new strings_internal::OStringStream(out));
+    }
+    *strm_ << t;
+  }
+
+ private:
+  std::unique_ptr<strings_internal::OStringStream> strm_;
+};
+
+// Formats a std::pair<>. The 'first' member is formatted using f1_ and the
+// 'second' member is formatted using f2_. sep_ is the separator.
+template <typename F1, typename F2>
+class PairFormatterImpl {
+ public:
+  PairFormatterImpl(F1 f1, absl::string_view sep, F2 f2)
+      : f1_(std::move(f1)), sep_(sep), f2_(std::move(f2)) {}
+
+  template <typename T>
+  void operator()(std::string* out, const T& p) {
+    f1_(out, p.first);
+    out->append(sep_);
+    f2_(out, p.second);
+  }
+
+  template <typename T>
+  void operator()(std::string* out, const T& p) const {
+    f1_(out, p.first);
+    out->append(sep_);
+    f2_(out, p.second);
+  }
+
+ private:
+  F1 f1_;
+  std::string sep_;
+  F2 f2_;
+};
+
+// Wraps another formatter and dereferences the argument to operator() then
+// passes the dereferenced argument to the wrapped formatter. This can be
+// useful, for example, to join a std::vector<int*>.
+template <typename Formatter>
+class DereferenceFormatterImpl {
+ public:
+  DereferenceFormatterImpl() : f_() {}
+  explicit DereferenceFormatterImpl(Formatter&& f)
+      : f_(std::forward<Formatter>(f)) {}
+
+  template <typename T>
+  void operator()(std::string* out, const T& t) {
+    f_(out, *t);
+  }
+
+  template <typename T>
+  void operator()(std::string* out, const T& t) const {
+    f_(out, *t);
+  }
+
+ private:
+  Formatter f_;
+};
+
+// DefaultFormatter<T> is a traits class that selects a default Formatter to use
+// for the given type T. The ::Type member names the Formatter to use. This is
+// used by the strings::Join() functions that do NOT take a Formatter argument,
+// in which case a default Formatter must be chosen.
+//
+// AlphaNumFormatterImpl is the default in the base template, followed by
+// specializations for other types.
+template <typename ValueType>
+struct DefaultFormatter {
+  typedef AlphaNumFormatterImpl Type;
+};
+template <>
+struct DefaultFormatter<const char*> {
+  typedef AlphaNumFormatterImpl Type;
+};
+template <>
+struct DefaultFormatter<char*> {
+  typedef AlphaNumFormatterImpl Type;
+};
+template <>
+struct DefaultFormatter<std::string> {
+  typedef NoFormatter Type;
+};
+template <>
+struct DefaultFormatter<absl::string_view> {
+  typedef NoFormatter Type;
+};
+template <typename ValueType>
+struct DefaultFormatter<ValueType*> {
+  typedef DereferenceFormatterImpl<typename DefaultFormatter<ValueType>::Type>
+      Type;
+};
+
+template <typename ValueType>
+struct DefaultFormatter<std::unique_ptr<ValueType>>
+    : public DefaultFormatter<ValueType*> {};
+
+//
+// JoinAlgorithm() functions
+//
+
+// The main joining algorithm. This simply joins the elements in the given
+// iterator range, each separated by the given separator, into an output std::string,
+// and formats each element using the provided Formatter object.
+template <typename Iterator, typename Formatter>
+std::string JoinAlgorithm(Iterator start, Iterator end, absl::string_view s,
+                     Formatter&& f) {
+  std::string result;
+  absl::string_view sep("");
+  for (Iterator it = start; it != end; ++it) {
+    result.append(sep.data(), sep.size());
+    f(&result, *it);
+    sep = s;
+  }
+  return result;
+}
+
+// No-op placeholder for input iterators which can not be iterated over.
+template <typename Iterator>
+size_t GetResultSize(Iterator, Iterator, size_t, std::input_iterator_tag) {
+  return 0;
+}
+
+// Calculates space to reserve, if the iterator supports multiple passes.
+template <typename Iterator>
+size_t GetResultSize(Iterator it, Iterator end, size_t separator_size,
+                     std::forward_iterator_tag) {
+  assert(it != end);
+  size_t length = it->size();
+  while (++it != end) {
+    length += separator_size;
+    length += it->size();
+  }
+  return length;
+}
+
+// A joining algorithm that's optimized for an iterator range of std::string-like
+// objects that do not need any additional formatting. This is to optimize the
+// common case of joining, say, a std::vector<std::string> or a
+// std::vector<absl::string_view>.
+//
+// This is an overload of the previous JoinAlgorithm() function. Here the
+// Formatter argument is of type NoFormatter. Since NoFormatter is an internal
+// type, this overload is only invoked when strings::Join() is called with a
+// range of std::string-like objects (e.g., std::string, absl::string_view), and an
+// explicit Formatter argument was NOT specified.
+//
+// The optimization is that the needed space will be reserved in the output
+// std::string to avoid the need to resize while appending. To do this, the iterator
+// range will be traversed twice: once to calculate the total needed size, and
+// then again to copy the elements and delimiters to the output std::string.
+template <typename Iterator>
+std::string JoinAlgorithm(Iterator start, Iterator end, absl::string_view s,
+                     NoFormatter) {
+  std::string result;
+  if (start != end) {
+    typename std::iterator_traits<Iterator>::iterator_category iterator_tag;
+    result.reserve(GetResultSize(start, end, s.size(), iterator_tag));
+
+    // Joins strings
+    absl::string_view sep("", 0);
+    for (Iterator it = start; it != end; ++it) {
+      result.append(sep.data(), sep.size());
+      result.append(it->data(), it->size());
+      sep = s;
+    }
+  }
+
+  return result;
+}
+
+// JoinTupleLoop implements a loop over the elements of a std::tuple, which
+// are heterogeneous. The primary template matches the tuple interior case. It
+// continues the iteration after appending a separator (for nonzero indices)
+// and formatting an element of the tuple. The specialization for the I=N case
+// matches the end-of-tuple, and terminates the iteration.
+template <size_t I, size_t N>
+struct JoinTupleLoop {
+  template <typename Tup, typename Formatter>
+  void operator()(std::string* out, const Tup& tup, absl::string_view sep,
+                  Formatter&& fmt) {
+    if (I > 0) out->append(sep.data(), sep.size());
+    fmt(out, std::get<I>(tup));
+    JoinTupleLoop<I + 1, N>()(out, tup, sep, fmt);
+  }
+};
+template <size_t N>
+struct JoinTupleLoop<N, N> {
+  template <typename Tup, typename Formatter>
+  void operator()(std::string*, const Tup&, absl::string_view, Formatter&&) {}
+};
+
+template <typename... T, typename Formatter>
+std::string JoinAlgorithm(const std::tuple<T...>& tup, absl::string_view sep,
+                     Formatter&& fmt) {
+  std::string result;
+  JoinTupleLoop<0, sizeof...(T)>()(&result, tup, sep, fmt);
+  return result;
+}
+
+template <typename Iterator>
+std::string JoinRange(Iterator first, Iterator last, absl::string_view separator) {
+  // No formatter was explicitly given, so a default must be chosen.
+  typedef typename std::iterator_traits<Iterator>::value_type ValueType;
+  typedef typename DefaultFormatter<ValueType>::Type Formatter;
+  return JoinAlgorithm(first, last, separator, Formatter());
+}
+
+template <typename Range, typename Formatter>
+std::string JoinRange(const Range& range, absl::string_view separator,
+                 Formatter&& fmt) {
+  using std::begin;
+  using std::end;
+  return JoinAlgorithm(begin(range), end(range), separator, fmt);
+}
+
+template <typename Range>
+std::string JoinRange(const Range& range, absl::string_view separator) {
+  using std::begin;
+  using std::end;
+  return JoinRange(begin(range), end(range), separator);
+}
+
+}  // namespace strings_internal
+}  // namespace absl
+
+#endif  // ABSL_STRINGS_INTERNAL_STR_JOIN_INTERNAL_H_