src/base/strings/string_util.h - gn - Git at Google

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 //
 // This file defines utility functions for working with strings.

 #ifndef BASE_STRINGS_STRING_UTIL_H_
 #define BASE_STRINGS_STRING_UTIL_H_

 #include <ctype.h>
 #include <stdarg.h>  // va_list
 #include <stddef.h>
 #include <stdint.h>

 #include <initializer_list>
 #include <string>
 #include <string_view>
 #include <vector>

 #include "base/compiler_specific.h"
 #include "util/build_config.h"

 namespace base {

 // C standard-library functions that aren't cross-platform are provided as
 // "base::...", and their prototypes are listed below. These functions are
 // then implemented as inline calls to the platform-specific equivalents in the
 // platform-specific headers.

 // Wrapper for vsnprintf that always null-terminates and always returns the
 // number of characters that would be in an untruncated formatted
 // string, even when truncation occurs.
 int vsnprintf(char* buffer, size_t size, const char* format, va_list arguments)
     PRINTF_FORMAT(3, 0);

 // Some of these implementations need to be inlined.

 // We separate the declaration from the implementation of this inline
 // function just so the PRINTF_FORMAT works.
 inline int snprintf(char* buffer,
                     size_t size,
                     _Printf_format_string_ const char* format,
                     ...) PRINTF_FORMAT(3, 4);
 inline int snprintf(char* buffer,
                     size_t size,
                     _Printf_format_string_ const char* format,
                     ...) {
   va_list arguments;
   va_start(arguments, format);
   int result = vsnprintf(buffer, size, format, arguments);
   va_end(arguments);
   return result;
 }

 // ASCII-specific tolower.  The standard library's tolower is locale sensitive,
 // so we don't want to use it here.
 inline char ToLowerASCII(char c) {
   return (c >= 'A' && c <= 'Z') ? (c + ('a' - 'A')) : c;
 }
 inline char16_t ToLowerASCII(char16_t c) {
   return (c >= 'A' && c <= 'Z') ? (c + ('a' - 'A')) : c;
 }

 // ASCII-specific toupper.  The standard library's toupper is locale sensitive,
 // so we don't want to use it here.
 inline char ToUpperASCII(char c) {
   return (c >= 'a' && c <= 'z') ? (c + ('A' - 'a')) : c;
 }
 inline char16_t ToUpperASCII(char16_t c) {
   return (c >= 'a' && c <= 'z') ? (c + ('A' - 'a')) : c;
 }

 // Converts the given string to it's ASCII-lowercase equivalent.
 std::string ToLowerASCII(std::string_view str);
 std::u16string ToLowerASCII(std::u16string_view str);

 // Converts the given string to it's ASCII-uppercase equivalent.
 std::string ToUpperASCII(std::string_view str);
 std::u16string ToUpperASCII(std::u16string_view str);

 // Functor for case-insensitive ASCII comparisons for STL algorithms like
 // std::search.
 //
 // Note that a full Unicode version of this functor is not possible to write
 // because case mappings might change the number of characters, depend on
 // context (combining accents), and require handling UTF-16. If you need
 // proper Unicode support, use base::i18n::ToLower/FoldCase and then just
 // use a normal operator== on the result.
 template <typename Char>
 struct CaseInsensitiveCompareASCII {
  public:
   bool operator()(Char x, Char y) const {
     return ToLowerASCII(x) == ToLowerASCII(y);
   }
 };

 // Like strcasecmp for case-insensitive ASCII characters only. Returns:
 //   -1  (a < b)
 //    0  (a == b)
 //    1  (a > b)
 // (unlike strcasecmp which can return values greater or less than 1/-1). For
 // full Unicode support, use base::i18n::ToLower or base::i18h::FoldCase
 // and then just call the normal string operators on the result.
 int CompareCaseInsensitiveASCII(std::string_view a, std::string_view b);
 int CompareCaseInsensitiveASCII(std::u16string_view a, std::u16string_view b);

 // Equality for ASCII case-insensitive comparisons. For full Unicode support,
 // use base::i18n::ToLower or base::i18h::FoldCase and then compare with either
 // == or !=.
 bool EqualsCaseInsensitiveASCII(std::string_view a, std::string_view b);
 bool EqualsCaseInsensitiveASCII(std::u16string_view a, std::u16string_view b);

 // Contains the set of characters representing whitespace in the corresponding
 // encoding. Null-terminated. The ASCII versions are the whitespaces as defined
 // by HTML5, and don't include control characters.
 extern const char16_t kWhitespaceUTF16[];  // Includes Unicode.
 extern const char kWhitespaceASCII[];
 extern const char16_t kWhitespaceASCIIAs16[];  // No unicode.

 // Null-terminated string representing the UTF-8 byte order mark.
 extern const char kUtf8ByteOrderMark[];

 // Removes characters in |remove_chars| from anywhere in |input|.  Returns true
 // if any characters were removed.  |remove_chars| must be null-terminated.
 // NOTE: Safe to use the same variable for both |input| and |output|.
 bool RemoveChars(const std::u16string& input,
                  std::u16string_view remove_chars,
                  std::u16string* output);
 bool RemoveChars(const std::string& input,
                  std::string_view remove_chars,
                  std::string* output);

 // Replaces characters in |replace_chars| from anywhere in |input| with
 // |replace_with|.  Each character in |replace_chars| will be replaced with
 // the |replace_with| string.  Returns true if any characters were replaced.
 // |replace_chars| must be null-terminated.
 // NOTE: Safe to use the same variable for both |input| and |output|.
 bool ReplaceChars(const std::u16string& input,
                   std::u16string_view replace_chars,
                   const std::u16string& replace_with,
                   std::u16string* output);
 bool ReplaceChars(const std::string& input,
                   std::string_view replace_chars,
                   const std::string& replace_with,
                   std::string* output);

 enum TrimPositions {
   TRIM_NONE = 0,
   TRIM_LEADING = 1 << 0,
   TRIM_TRAILING = 1 << 1,
   TRIM_ALL = TRIM_LEADING | TRIM_TRAILING,
 };

 // Removes characters in |trim_chars| from the beginning and end of |input|.
 // The 8-bit version only works on 8-bit characters, not UTF-8. Returns true if
 // any characters were removed.
 //
 // It is safe to use the same variable for both |input| and |output| (this is
 // the normal usage to trim in-place).
 bool TrimString(const std::u16string& input,
                 std::u16string_view trim_chars,
                 std::u16string* output);
 bool TrimString(const std::string& input,
                 std::string_view trim_chars,
                 std::string* output);

 // std::string_view versions of the above. The returned pieces refer to the
 // original buffer.
 std::u16string_view TrimString(std::u16string_view input,
                                std::u16string_view trim_chars,
                                TrimPositions positions);
 std::string_view TrimString(std::string_view input,
                             std::string_view trim_chars,
                             TrimPositions positions);

 // Truncates a string to the nearest UTF-8 character that will leave
 // the string less than or equal to the specified byte size.
 void TruncateUTF8ToByteSize(const std::string& input,
                             const size_t byte_size,
                             std::string* output);

 // Trims any whitespace from either end of the input string.
 //
 // The std::string_view versions return a substring referencing the input
 // buffer. The ASCII versions look only for ASCII whitespace.
 //
 // The std::string versions return where whitespace was found.
 // NOTE: Safe to use the same variable for both input and output.
 TrimPositions TrimWhitespace(const std::u16string& input,
                              TrimPositions positions,
                              std::u16string* output);
 std::u16string_view TrimWhitespace(std::u16string_view input,
                                    TrimPositions positions);
 TrimPositions TrimWhitespaceASCII(const std::string& input,
                                   TrimPositions positions,
                                   std::string* output);
 std::string_view TrimWhitespaceASCII(std::string_view input,
                                      TrimPositions positions);

 // Searches for CR or LF characters.  Removes all contiguous whitespace
 // strings that contain them.  This is useful when trying to deal with text
 // copied from terminals.
 // Returns |text|, with the following three transformations:
 // (1) Leading and trailing whitespace is trimmed.
 // (2) If |trim_sequences_with_line_breaks| is true, any other whitespace
 //     sequences containing a CR or LF are trimmed.
 // (3) All other whitespace sequences are converted to single spaces.
 std::u16string CollapseWhitespace(const std::u16string& text,
                                   bool trim_sequences_with_line_breaks);
 std::string CollapseWhitespaceASCII(const std::string& text,
                                     bool trim_sequences_with_line_breaks);

 // Returns true if |input| is empty or contains only characters found in
 // |characters|.
 bool ContainsOnlyChars(std::string_view input, std::string_view characters);
 bool ContainsOnlyChars(std::u16string_view input,
                        std::u16string_view characters);

 // Returns true if the specified string matches the criteria. How can a wide
 // string be 8-bit or UTF8? It contains only characters that are < 256 (in the
 // first case) or characters that use only 8-bits and whose 8-bit
 // representation looks like a UTF-8 string (the second case).
 //
 // Note that IsStringUTF8 checks not only if the input is structurally
 // valid but also if it doesn't contain any non-character codepoint
 // (e.g. U+FFFE). It's done on purpose because all the existing callers want
 // to have the maximum 'discriminating' power from other encodings. If
 // there's a use case for just checking the structural validity, we have to
 // add a new function for that.
 //
 // IsStringASCII assumes the input is likely all ASCII, and does not leave early
 // if it is not the case.
 bool IsStringUTF8(std::string_view str);
 bool IsStringASCII(std::string_view str);
 bool IsStringASCII(std::u16string_view str);

 // Compare the lower-case form of the given string against the given
 // previously-lower-cased ASCII string (typically a constant).
 bool LowerCaseEqualsASCII(std::string_view str,
                           std::string_view lowecase_ascii);
 bool LowerCaseEqualsASCII(std::u16string_view str,
                           std::string_view lowecase_ascii);

 // Performs a case-sensitive string compare of the given 16-bit string against
 // the given 8-bit ASCII string (typically a constant). The behavior is
 // undefined if the |ascii| string is not ASCII.
 bool EqualsASCII(std::u16string_view str, std::string_view ascii);

 // starts_with/ends_with for ASCII case-insensitive comparisons.
 // If you need to do Unicode-aware case-insensitive StartsWith/EndsWith, it's
 // best to call base::i18n::ToLower() or base::i18n::FoldCase() (see
 // base/i18n/case_conversion.h for usage advice) on the arguments, and then use
 // the results to a case-sensitive comparison.
 bool StartsWithCaseInsensitiveASCII(std::string_view str,
                                     std::string_view search_for);
 bool StartsWithCaseInsensitiveASCII(std::u16string_view str,
                                     std::u16string_view search_for);
 bool EndsWithCaseInsensitiveASCII(std::string_view str,
                                   std::string_view search_for);
 bool EndsWithCaseInsensitiveASCII(std::u16string_view str,
                                   std::u16string_view search_for);

 // Determines the type of ASCII character, independent of locale (the C
 // library versions will change based on locale).
 template <typename Char>
 inline bool IsAsciiWhitespace(Char c) {
   return c == ' ' || c == '\r' || c == '\n' || c == '\t';
 }
 template <typename Char>
 inline bool IsAsciiAlpha(Char c) {
   return (c >= 'A' && c <= 'Z') || (c >= 'a' && c <= 'z');
 }
 template <typename Char>
 inline bool IsAsciiUpper(Char c) {
   return c >= 'A' && c <= 'Z';
 }
 template <typename Char>
 inline bool IsAsciiLower(Char c) {
   return c >= 'a' && c <= 'z';
 }
 template <typename Char>
 inline bool IsAsciiDigit(Char c) {
   return c >= '0' && c <= '9';
 }

 template <typename Char>
 inline bool IsHexDigit(Char c) {
   return (c >= '0' && c <= '9') || (c >= 'A' && c <= 'F') ||
          (c >= 'a' && c <= 'f');
 }

 // Returns the integer corresponding to the given hex character. For example:
 //    '4' -> 4
 //    'a' -> 10
 //    'B' -> 11
 // Assumes the input is a valid hex character. DCHECKs in debug builds if not.
 char HexDigitToInt(char16_t c);

 // Returns true if it's a Unicode whitespace character.
 bool IsUnicodeWhitespace(char16_t c);

 // Return a byte string in human-readable format with a unit suffix. Not
 // appropriate for use in any UI; use of FormatBytes and friends in ui/base is
 // highly recommended instead. TODO(avi): Figure out how to get callers to use
 // FormatBytes instead; remove this.
 std::u16string FormatBytesUnlocalized(int64_t bytes);

 // Starting at |start_offset| (usually 0), replace the first instance of
 // |find_this| with |replace_with|.
 void ReplaceFirstSubstringAfterOffset(std::u16string* str,
                                       size_t start_offset,
                                       std::u16string_view find_this,
                                       std::u16string_view replace_with);
 void ReplaceFirstSubstringAfterOffset(std::string* str,
                                       size_t start_offset,
                                       std::string_view find_this,
                                       std::string_view replace_with);

 // Starting at |start_offset| (usually 0), look through |str| and replace all
 // instances of |find_this| with |replace_with|.
 //
 // This does entire substrings; use std::replace in <algorithm> for single
 // characters, for example:
 //   std::replace(str.begin(), str.end(), 'a', 'b');
 void ReplaceSubstringsAfterOffset(std::u16string* str,
                                   size_t start_offset,
                                   std::u16string_view find_this,
                                   std::u16string_view replace_with);
 void ReplaceSubstringsAfterOffset(std::string* str,
                                   size_t start_offset,
                                   std::string_view find_this,
                                   std::string_view replace_with);

 // Reserves enough memory in |str| to accommodate |length_with_null| characters,
 // sets the size of |str| to |length_with_null - 1| characters, and returns a
 // pointer to the underlying contiguous array of characters.  This is typically
 // used when calling a function that writes results into a character array, but
 // the caller wants the data to be managed by a string-like object.  It is
 // convenient in that is can be used inline in the call, and fast in that it
 // avoids copying the results of the call from a char* into a string.
 //
 // |length_with_null| must be at least 2, since otherwise the underlying string
 // would have size 0, and trying to access &((*str)[0]) in that case can result
 // in a number of problems.
 //
 // Internally, this takes linear time because the resize() call 0-fills the
 // underlying array for potentially all
 // (|length_with_null - 1| * sizeof(string_type::value_type)) bytes.  Ideally we
 // could avoid this aspect of the resize() call, as we expect the caller to
 // immediately write over this memory, but there is no other way to set the size
 // of the string, and not doing that will mean people who access |str| rather
 // than str.c_str() will get back a string of whatever size |str| had on entry
 // to this function (probably 0).
 char* WriteInto(std::string* str, size_t length_with_null);
 char16_t* WriteInto(std::u16string* str, size_t length_with_null);

 // Does the opposite of SplitString()/SplitStringPiece(). Joins a vector or list
 // of strings into a single string, inserting |separator| (which may be empty)
 // in between all elements.
 //
 // If possible, callers should build a vector of std::string_views and use the
 // std::string_view variant, so that they do not create unnecessary copies of
 // strings. For example, instead of using SplitString, modifying the vector,
 // then using JoinString, use SplitStringPiece followed by JoinString so that no
 // copies of those strings are created until the final join operation.
 //
 // Use StrCat (in base/strings/strcat.h) if you don't need a separator.
 std::string JoinString(const std::vector<std::string>& parts,
                        std::string_view separator);
 std::u16string JoinString(const std::vector<std::u16string>& parts,
                           std::u16string_view separator);
 std::string JoinString(const std::vector<std::string_view>& parts,
                        std::string_view separator);
 std::u16string JoinString(const std::vector<std::u16string_view>& parts,
                           std::u16string_view separator);
 // Explicit initializer_list overloads are required to break ambiguity when used
 // with a literal initializer list (otherwise the compiler would not be able to
 // decide between the string and std::string_view overloads).
 std::string JoinString(std::initializer_list<std::string_view> parts,
                        std::string_view separator);
 std::u16string JoinString(std::initializer_list<std::u16string_view> parts,
                           std::u16string_view separator);

 // Replace $1-$2-$3..$9 in the format string with values from |subst|.
 // Additionally, any number of consecutive '$' characters is replaced by that
 // number less one. Eg $$->$, $$$->$$, etc. The offsets parameter here can be
 // NULL. This only allows you to use up to nine replacements.
 std::u16string ReplaceStringPlaceholders(
     const std::u16string& format_string,
     const std::vector<std::u16string>& subst,
     std::vector<size_t>* offsets);

 std::string ReplaceStringPlaceholders(std::string_view format_string,
                                       const std::vector<std::string>& subst,
                                       std::vector<size_t>* offsets);

 // Single-string shortcut for ReplaceStringHolders. |offset| may be NULL.
 std::u16string ReplaceStringPlaceholders(const std::u16string& format_string,
                                          const std::u16string& a,
                                          size_t* offset);

 }  // namespace base

 #if defined(OS_WIN)
 #include "base/strings/string_util_win.h"
 #elif defined(OS_POSIX) || defined(OS_FUCHSIA)
 #include "base/strings/string_util_posix.h"
 #else
 #error Define string operations appropriately for your platform
 #endif

 #endif  // BASE_STRINGS_STRING_UTIL_H_
	// Copyright 2013 The Chromium Authors. All rights reserved.
	// Use of this source code is governed by a BSD-style license that can be
	// found in the LICENSE file.
	//
	// This file defines utility functions for working with strings.

	#ifndef BASE_STRINGS_STRING_UTIL_H_
	#define BASE_STRINGS_STRING_UTIL_H_

	#include <ctype.h>
	#include <stdarg.h> // va_list
	#include <stddef.h>
	#include <stdint.h>

	#include <initializer_list>
	#include <string>
	#include <string_view>
	#include <vector>

	#include "base/compiler_specific.h"
	#include "util/build_config.h"

	namespace base {

	// C standard-library functions that aren't cross-platform are provided as
	// "base::...", and their prototypes are listed below. These functions are
	// then implemented as inline calls to the platform-specific equivalents in the
	// platform-specific headers.

	// Wrapper for vsnprintf that always null-terminates and always returns the
	// number of characters that would be in an untruncated formatted
	// string, even when truncation occurs.
	int vsnprintf(char* buffer, size_t size, const char* format, va_list arguments)
	PRINTF_FORMAT(3, 0);

	// Some of these implementations need to be inlined.

	// We separate the declaration from the implementation of this inline
	// function just so the PRINTF_FORMAT works.
	inline int snprintf(char* buffer,
	size_t size,
	_Printf_format_string_ const char* format,
	...) PRINTF_FORMAT(3, 4);
	inline int snprintf(char* buffer,
	size_t size,
	_Printf_format_string_ const char* format,
	...) {
	va_list arguments;
	va_start(arguments, format);
	int result = vsnprintf(buffer, size, format, arguments);
	va_end(arguments);
	return result;
	}

	// ASCII-specific tolower. The standard library's tolower is locale sensitive,
	// so we don't want to use it here.
	inline char ToLowerASCII(char c) {
	return (c >= 'A' && c <= 'Z') ? (c + ('a' - 'A')) : c;
	}
	inline char16_t ToLowerASCII(char16_t c) {
	return (c >= 'A' && c <= 'Z') ? (c + ('a' - 'A')) : c;
	}

	// ASCII-specific toupper. The standard library's toupper is locale sensitive,
	// so we don't want to use it here.
	inline char ToUpperASCII(char c) {
	return (c >= 'a' && c <= 'z') ? (c + ('A' - 'a')) : c;
	}
	inline char16_t ToUpperASCII(char16_t c) {
	return (c >= 'a' && c <= 'z') ? (c + ('A' - 'a')) : c;
	}

	// Converts the given string to it's ASCII-lowercase equivalent.
	std::string ToLowerASCII(std::string_view str);
	std::u16string ToLowerASCII(std::u16string_view str);

	// Converts the given string to it's ASCII-uppercase equivalent.
	std::string ToUpperASCII(std::string_view str);
	std::u16string ToUpperASCII(std::u16string_view str);

	// Functor for case-insensitive ASCII comparisons for STL algorithms like
	// std::search.
	//
	// Note that a full Unicode version of this functor is not possible to write
	// because case mappings might change the number of characters, depend on
	// context (combining accents), and require handling UTF-16. If you need
	// proper Unicode support, use base::i18n::ToLower/FoldCase and then just
	// use a normal operator== on the result.
	template <typename Char>
	struct CaseInsensitiveCompareASCII {
	public:
	bool operator()(Char x, Char y) const {
	return ToLowerASCII(x) == ToLowerASCII(y);
	}
	};

	// Like strcasecmp for case-insensitive ASCII characters only. Returns:
	// -1 (a < b)
	// 0 (a == b)
	// 1 (a > b)
	// (unlike strcasecmp which can return values greater or less than 1/-1). For
	// full Unicode support, use base::i18n::ToLower or base::i18h::FoldCase
	// and then just call the normal string operators on the result.
	int CompareCaseInsensitiveASCII(std::string_view a, std::string_view b);
	int CompareCaseInsensitiveASCII(std::u16string_view a, std::u16string_view b);

	// Equality for ASCII case-insensitive comparisons. For full Unicode support,
	// use base::i18n::ToLower or base::i18h::FoldCase and then compare with either
	// == or !=.
	bool EqualsCaseInsensitiveASCII(std::string_view a, std::string_view b);
	bool EqualsCaseInsensitiveASCII(std::u16string_view a, std::u16string_view b);

	// Contains the set of characters representing whitespace in the corresponding
	// encoding. Null-terminated. The ASCII versions are the whitespaces as defined
	// by HTML5, and don't include control characters.
	extern const char16_t kWhitespaceUTF16[]; // Includes Unicode.
	extern const char kWhitespaceASCII[];
	extern const char16_t kWhitespaceASCIIAs16[]; // No unicode.

	// Null-terminated string representing the UTF-8 byte order mark.
	extern const char kUtf8ByteOrderMark[];

	// Removes characters in \|remove_chars\| from anywhere in \|input\|. Returns true
	// if any characters were removed. \|remove_chars\| must be null-terminated.
	// NOTE: Safe to use the same variable for both \|input\| and \|output\|.
	bool RemoveChars(const std::u16string& input,
	std::u16string_view remove_chars,
	std::u16string* output);
	bool RemoveChars(const std::string& input,
	std::string_view remove_chars,
	std::string* output);

	// Replaces characters in \|replace_chars\| from anywhere in \|input\| with
	// \|replace_with\|. Each character in \|replace_chars\| will be replaced with
	// the \|replace_with\| string. Returns true if any characters were replaced.
	// \|replace_chars\| must be null-terminated.
	// NOTE: Safe to use the same variable for both \|input\| and \|output\|.
	bool ReplaceChars(const std::u16string& input,
	std::u16string_view replace_chars,
	const std::u16string& replace_with,
	std::u16string* output);
	bool ReplaceChars(const std::string& input,
	std::string_view replace_chars,
	const std::string& replace_with,
	std::string* output);

	enum TrimPositions {
	TRIM_NONE = 0,
	TRIM_LEADING = 1 << 0,
	TRIM_TRAILING = 1 << 1,
	TRIM_ALL = TRIM_LEADING \| TRIM_TRAILING,
	};

	// Removes characters in \|trim_chars\| from the beginning and end of \|input\|.
	// The 8-bit version only works on 8-bit characters, not UTF-8. Returns true if
	// any characters were removed.
	//
	// It is safe to use the same variable for both \|input\| and \|output\| (this is
	// the normal usage to trim in-place).
	bool TrimString(const std::u16string& input,
	std::u16string_view trim_chars,
	std::u16string* output);
	bool TrimString(const std::string& input,
	std::string_view trim_chars,
	std::string* output);

	// std::string_view versions of the above. The returned pieces refer to the
	// original buffer.
	std::u16string_view TrimString(std::u16string_view input,
	std::u16string_view trim_chars,
	TrimPositions positions);
	std::string_view TrimString(std::string_view input,
	std::string_view trim_chars,
	TrimPositions positions);

	// Truncates a string to the nearest UTF-8 character that will leave
	// the string less than or equal to the specified byte size.
	void TruncateUTF8ToByteSize(const std::string& input,
	const size_t byte_size,
	std::string* output);

	// Trims any whitespace from either end of the input string.
	//
	// The std::string_view versions return a substring referencing the input
	// buffer. The ASCII versions look only for ASCII whitespace.
	//
	// The std::string versions return where whitespace was found.
	// NOTE: Safe to use the same variable for both input and output.
	TrimPositions TrimWhitespace(const std::u16string& input,
	TrimPositions positions,
	std::u16string* output);
	std::u16string_view TrimWhitespace(std::u16string_view input,
	TrimPositions positions);
	TrimPositions TrimWhitespaceASCII(const std::string& input,
	TrimPositions positions,
	std::string* output);
	std::string_view TrimWhitespaceASCII(std::string_view input,
	TrimPositions positions);

	// Searches for CR or LF characters. Removes all contiguous whitespace
	// strings that contain them. This is useful when trying to deal with text
	// copied from terminals.
	// Returns \|text\|, with the following three transformations:
	// (1) Leading and trailing whitespace is trimmed.
	// (2) If \|trim_sequences_with_line_breaks\| is true, any other whitespace
	// sequences containing a CR or LF are trimmed.
	// (3) All other whitespace sequences are converted to single spaces.
	std::u16string CollapseWhitespace(const std::u16string& text,
	bool trim_sequences_with_line_breaks);
	std::string CollapseWhitespaceASCII(const std::string& text,
	bool trim_sequences_with_line_breaks);

	// Returns true if \|input\| is empty or contains only characters found in
	// \|characters\|.
	bool ContainsOnlyChars(std::string_view input, std::string_view characters);
	bool ContainsOnlyChars(std::u16string_view input,
	std::u16string_view characters);

	// Returns true if the specified string matches the criteria. How can a wide
	// string be 8-bit or UTF8? It contains only characters that are < 256 (in the
	// first case) or characters that use only 8-bits and whose 8-bit
	// representation looks like a UTF-8 string (the second case).
	//
	// Note that IsStringUTF8 checks not only if the input is structurally
	// valid but also if it doesn't contain any non-character codepoint
	// (e.g. U+FFFE). It's done on purpose because all the existing callers want
	// to have the maximum 'discriminating' power from other encodings. If
	// there's a use case for just checking the structural validity, we have to
	// add a new function for that.
	//
	// IsStringASCII assumes the input is likely all ASCII, and does not leave early
	// if it is not the case.
	bool IsStringUTF8(std::string_view str);
	bool IsStringASCII(std::string_view str);
	bool IsStringASCII(std::u16string_view str);

	// Compare the lower-case form of the given string against the given
	// previously-lower-cased ASCII string (typically a constant).
	bool LowerCaseEqualsASCII(std::string_view str,
	std::string_view lowecase_ascii);
	bool LowerCaseEqualsASCII(std::u16string_view str,
	std::string_view lowecase_ascii);

	// Performs a case-sensitive string compare of the given 16-bit string against
	// the given 8-bit ASCII string (typically a constant). The behavior is
	// undefined if the \|ascii\| string is not ASCII.
	bool EqualsASCII(std::u16string_view str, std::string_view ascii);

	// starts_with/ends_with for ASCII case-insensitive comparisons.
	// If you need to do Unicode-aware case-insensitive StartsWith/EndsWith, it's
	// best to call base::i18n::ToLower() or base::i18n::FoldCase() (see
	// base/i18n/case_conversion.h for usage advice) on the arguments, and then use
	// the results to a case-sensitive comparison.
	bool StartsWithCaseInsensitiveASCII(std::string_view str,
	std::string_view search_for);
	bool StartsWithCaseInsensitiveASCII(std::u16string_view str,
	std::u16string_view search_for);
	bool EndsWithCaseInsensitiveASCII(std::string_view str,
	std::string_view search_for);
	bool EndsWithCaseInsensitiveASCII(std::u16string_view str,
	std::u16string_view search_for);

	// Determines the type of ASCII character, independent of locale (the C
	// library versions will change based on locale).
	template <typename Char>
	inline bool IsAsciiWhitespace(Char c) {
	return c == ' ' \|\| c == '\r' \|\| c == '\n' \|\| c == '\t';
	}
	template <typename Char>
	inline bool IsAsciiAlpha(Char c) {
	return (c >= 'A' && c <= 'Z') \|\| (c >= 'a' && c <= 'z');
	}
	template <typename Char>
	inline bool IsAsciiUpper(Char c) {
	return c >= 'A' && c <= 'Z';
	}
	template <typename Char>
	inline bool IsAsciiLower(Char c) {
	return c >= 'a' && c <= 'z';
	}
	template <typename Char>
	inline bool IsAsciiDigit(Char c) {
	return c >= '0' && c <= '9';
	}

	template <typename Char>
	inline bool IsHexDigit(Char c) {
	return (c >= '0' && c <= '9') \|\| (c >= 'A' && c <= 'F') \|\|
	(c >= 'a' && c <= 'f');
	}

	// Returns the integer corresponding to the given hex character. For example:
	// '4' -> 4
	// 'a' -> 10
	// 'B' -> 11
	// Assumes the input is a valid hex character. DCHECKs in debug builds if not.
	char HexDigitToInt(char16_t c);

	// Returns true if it's a Unicode whitespace character.
	bool IsUnicodeWhitespace(char16_t c);

	// Return a byte string in human-readable format with a unit suffix. Not
	// appropriate for use in any UI; use of FormatBytes and friends in ui/base is
	// highly recommended instead. TODO(avi): Figure out how to get callers to use
	// FormatBytes instead; remove this.
	std::u16string FormatBytesUnlocalized(int64_t bytes);

	// Starting at \|start_offset\| (usually 0), replace the first instance of
	// \|find_this\| with \|replace_with\|.
	void ReplaceFirstSubstringAfterOffset(std::u16string* str,
	size_t start_offset,
	std::u16string_view find_this,
	std::u16string_view replace_with);
	void ReplaceFirstSubstringAfterOffset(std::string* str,
	size_t start_offset,
	std::string_view find_this,
	std::string_view replace_with);

	// Starting at \|start_offset\| (usually 0), look through \|str\| and replace all
	// instances of \|find_this\| with \|replace_with\|.
	//
	// This does entire substrings; use std::replace in <algorithm> for single
	// characters, for example:
	// std::replace(str.begin(), str.end(), 'a', 'b');
	void ReplaceSubstringsAfterOffset(std::u16string* str,
	size_t start_offset,
	std::u16string_view find_this,
	std::u16string_view replace_with);
	void ReplaceSubstringsAfterOffset(std::string* str,
	size_t start_offset,
	std::string_view find_this,
	std::string_view replace_with);

	// Reserves enough memory in \|str\| to accommodate \|length_with_null\| characters,
	// sets the size of \|str\| to \|length_with_null - 1\| characters, and returns a
	// pointer to the underlying contiguous array of characters. This is typically
	// used when calling a function that writes results into a character array, but
	// the caller wants the data to be managed by a string-like object. It is
	// convenient in that is can be used inline in the call, and fast in that it
	// avoids copying the results of the call from a char* into a string.
	//
	// \|length_with_null\| must be at least 2, since otherwise the underlying string
	// would have size 0, and trying to access &((*str)[0]) in that case can result
	// in a number of problems.
	//
	// Internally, this takes linear time because the resize() call 0-fills the
	// underlying array for potentially all
	// (\|length_with_null - 1\| * sizeof(string_type::value_type)) bytes. Ideally we
	// could avoid this aspect of the resize() call, as we expect the caller to
	// immediately write over this memory, but there is no other way to set the size
	// of the string, and not doing that will mean people who access \|str\| rather
	// than str.c_str() will get back a string of whatever size \|str\| had on entry
	// to this function (probably 0).
	char* WriteInto(std::string* str, size_t length_with_null);
	char16_t* WriteInto(std::u16string* str, size_t length_with_null);

	// Does the opposite of SplitString()/SplitStringPiece(). Joins a vector or list
	// of strings into a single string, inserting \|separator\| (which may be empty)
	// in between all elements.
	//
	// If possible, callers should build a vector of std::string_views and use the
	// std::string_view variant, so that they do not create unnecessary copies of
	// strings. For example, instead of using SplitString, modifying the vector,
	// then using JoinString, use SplitStringPiece followed by JoinString so that no
	// copies of those strings are created until the final join operation.
	//
	// Use StrCat (in base/strings/strcat.h) if you don't need a separator.
	std::string JoinString(const std::vector<std::string>& parts,
	std::string_view separator);
	std::u16string JoinString(const std::vector<std::u16string>& parts,
	std::u16string_view separator);
	std::string JoinString(const std::vector<std::string_view>& parts,
	std::string_view separator);
	std::u16string JoinString(const std::vector<std::u16string_view>& parts,
	std::u16string_view separator);
	// Explicit initializer_list overloads are required to break ambiguity when used
	// with a literal initializer list (otherwise the compiler would not be able to
	// decide between the string and std::string_view overloads).
	std::string JoinString(std::initializer_list<std::string_view> parts,
	std::string_view separator);
	std::u16string JoinString(std::initializer_list<std::u16string_view> parts,
	std::u16string_view separator);

	// Replace $1-$2-$3..$9 in the format string with values from \|subst\|.
	// Additionally, any number of consecutive '$' characters is replaced by that
	// number less one. Eg $$->$, $$$->$$, etc. The offsets parameter here can be
	// NULL. This only allows you to use up to nine replacements.
	std::u16string ReplaceStringPlaceholders(
	const std::u16string& format_string,
	const std::vector<std::u16string>& subst,
	std::vector<size_t>* offsets);

	std::string ReplaceStringPlaceholders(std::string_view format_string,
	const std::vector<std::string>& subst,
	std::vector<size_t>* offsets);

	// Single-string shortcut for ReplaceStringHolders. \|offset\| may be NULL.
	std::u16string ReplaceStringPlaceholders(const std::u16string& format_string,
	const std::u16string& a,
	size_t* offset);

	} // namespace base

	#if defined(OS_WIN)
	#include "base/strings/string_util_win.h"
	#elif defined(OS_POSIX) \|\| defined(OS_FUCHSIA)
	#include "base/strings/string_util_posix.h"
	#else
	#error Define string operations appropriately for your platform
	#endif

	#endif // BASE_STRINGS_STRING_UTIL_H_