Separated out remaining functions from unicode.h, documentation

10 files changed, 576 insertions, 5 deletions

diff --git a/include/unicode/conversion.h b/include/unicode/conversion.h
new file mode 100644
index 0000000..dc57084
--- /dev/null
+++ b/include/unicode/conversion.h
@@ -0,0 +1,113 @@
+//
+// Reichwein.IT Unicode Library
+//
+// Functions for conversion between UTF and ISO encodings
+//
+
+#pragma once
+
+#include "unicode/endian.h"
+#include "unicode/iso.h"
+#include "unicode/optimization.h"
+#include "unicode/predicate.h"
+#include "unicode/types.h"
+#include "unicode/type_traits.h"
+#include "unicode/utf.h"
+
+#include <algorithm>
+#include <array>
+#include <cstdint>
+#include <iterator>
+#include <memory>
+#include <stdexcept>
+#include <string>
+#include <type_traits>
+#include <utility>
+
+namespace unicode {
+
+ // First variant of convert(): Specification of encodings explicitly
+ //
+ // e.g.
+ // unicode::UTF_8
+ // unicode::UTF_16
+ // unicode::UTF_32
+ // unicode::ISO_8859_1
+ // unicode::ISO_8859_15
+ //
+ // see also utf.h and iso.h
+ //
+ // From and To are Encodings
+ //
+ // throws std::invalid_argument on conversion error
+ template<typename From, typename To, std::enable_if_t<is_encoding_v<From> && is_encoding_v<To>, bool> = true>
+ typename To::string_type convert(const typename From::string_type& s)
+ {
+  // At compile time, decide which optimization to use, with fallback to
+  // iterating with std::copy()
+
+  // if input type == output type, only validate and return input, if appropriate
+  if constexpr(sizeof(typename From::value_type) == sizeof(typename To::value_type) &&
+               is_utf_encoding_v<From> && is_utf_encoding_v<To>) {
+   if (validate_utf<typename From::value_type>(s)) {
+    return s;
+   } else {
+    throw std::invalid_argument("Invalid UTF input");
+   }
+  } else if constexpr(accu_size == 8 && is_little_endian() && is_utf_8_v<typename From::value_type> &&
+                      is_utf_encoding_v<From> && is_utf_encoding_v<To>) { // endian specific optimization
+   return convert_optimized_utf<From, To>(s);
+  } else if constexpr(accu_size == 4 || accu_size == 8) { // accu size specific optimization with speedup for 7bit input
+   return convert_optimized<From, To>(s);
+  } else {
+   typename To::string_type result;
+   std::copy(From::begin(s), From::end(s), To::back_inserter(result));
+   return result;
+  }
+ }
+
+ // Second variant of convert(): Specification of encodings via character type
+ //
+ // see also type_traits.h for is_char
+ //
+ // From and To are from: utf8_t (i.e. char or char8_t (C++20)), char16_t and char32_t, char, wchar_t, uint8_t, uint16_t, uint32_t
+ //
+ // throws std::invalid_argument on conversion error
+ template<typename From, typename To,
+  typename FromContainer=std::basic_string<From>,
+  typename ToContainer=std::basic_string<To>,
+  std::enable_if_t<is_char_v<From> && is_char_v<To>, bool> = true>
+ ToContainer convert(const FromContainer& s)
+ {
+  typedef UTF<utf_iterator<From>, utf_back_insert_iterator<To>> UTF_Trait;
+
+  ToContainer result;
+
+  std::copy(UTF_Trait::begin(s), UTF_Trait::end(s), UTF_Trait::back_inserter(result));
+
+  return result;
+ }
+
+ // Third variant of convert(): Specification of encodings via container type
+ //
+ // see also type_traits.h for is_container
+ //
+ // From and To are containers
+ //
+ // throws std::invalid_argument on conversion error
+ template<typename FromContainer, typename ToContainer,
+  std::enable_if_t<is_container_v<FromContainer> && is_container_v<ToContainer>, bool> = true
+ >
+ ToContainer convert(const FromContainer& s)
+ {
+  typedef UTF<utf_iterator<typename FromContainer::value_type, FromContainer>, utf_back_insert_iterator<typename ToContainer::value_type, ToContainer>> UTF_Trait;
+  
+  ToContainer result;
+
+  std::copy(UTF_Trait::begin(s), UTF_Trait::end(s), UTF_Trait::back_inserter(result));
+
+  return result;
+ }
+
+} // namespace unicode
+
diff --git a/include/unicode/endian.h b/include/unicode/endian.h
index 1230f06..d933a2b 100644
--- a/include/unicode/endian.h
+++ b/include/unicode/endian.h
@@ -1,3 +1,11 @@
+//
+// Reichwein.IT Unicode Library
+//
+// Endian handling functions
+//
+// In C++17, endian support is not yet available.
+//
+
 #pragma once
 
 #if __cplusplus >= 202002L
diff --git a/include/unicode/iso.h b/include/unicode/iso.h
index 1f5f007..24e3dd1 100644
--- a/include/unicode/iso.h
+++ b/include/unicode/iso.h
@@ -1,3 +1,16 @@
+//
+// Reichwein.IT Unicode Library
+//
+// ISO 8895 (-1 and -15) handling functions (i.e. Latin-1 and Latin-9)
+//
+// Implementation of iso_iterator for reading individual Unicode code points
+// from an string or container input, and a iso_back_insert_iterator for
+// writing them to the destination.
+//
+// The design is made to be compatible to the respective iterators in utf.h
+// to make it easy to combine them.
+//
+
 #pragma once
 
 #include "types.h"
diff --git a/include/unicode/optimization.h b/include/unicode/optimization.h
new file mode 100644
index 0000000..d7b054d
--- /dev/null
+++ b/include/unicode/optimization.h
@@ -0,0 +1,325 @@
+//
+// Reichwein.IT Unicode Library
+//
+// Optimized conversion functions for UTF input and output
+//
+
+#pragma once
+
+#include "unicode/endian.h"
+#include "unicode/iso.h"
+#include "unicode/predicate.h"
+#include "unicode/types.h"
+#include "unicode/type_traits.h"
+#include "unicode/utf.h"
+
+#include <algorithm>
+#include <array>
+#include <cstdint>
+#include <iterator>
+#include <memory>
+#include <stdexcept>
+#include <string>
+#include <type_traits>
+#include <utility>
+
+namespace unicode {
+
+ // Helper function: Item distance of specified iterators
+ // std::distance doesn't work here: it is based on "output" distance of iterators
+ template<class Iterator>
+ inline size_t input_distance(const Iterator& it1, const Iterator& it2)
+ {
+  return it2 - it1;
+ }
+ 
+ // Helper function: Distance of specified iterator content data in bytes
+ template<class Iterator>
+ inline size_t input_distance_bytes(const Iterator& it1, const Iterator& it2)
+ {
+  return input_distance(it1, it2) * sizeof(typename Iterator::value_type);
+ }
+
+ // Optimizations following:
+ static const size_t accu_size {sizeof(size_t)};
+
+ template<int value_size>
+ struct ConvertInputOptimizer {};
+
+ template<> struct ConvertInputOptimizer<1>
+ {
+  static const uint32_t ascii_mask { 0x80808080 };
+ };
+ 
+ template<> struct ConvertInputOptimizer<2>
+ {
+  static const uint32_t ascii_mask { 0xFF80FF80 };
+ };
+ 
+ template<> struct ConvertInputOptimizer<4>
+ {
+  static const uint32_t ascii_mask { 0xFFFFFF80 };
+ };
+
+ template<int AccuSize, class ConvertInputOptimizer>
+ struct ArchitectureOptimizer {};
+
+ // On 32 bit architecture, calculate with 32 bit accumulator value
+ // (hoping the compiler will put it into a 32 bit register)
+ template<class ConvertInputOptimizer>
+ struct ArchitectureOptimizer<4, ConvertInputOptimizer>
+ {
+  typedef ConvertInputOptimizer input_optimizer;
+  typedef uint32_t accu_type;
+  static const accu_type addr_mask {accu_size - 1};
+  static const accu_type ascii_mask { (accu_type)input_optimizer::ascii_mask };
+  static const accu_type ascii_value { 0ULL };
+  
+  template<typename input_value_type, class output_string_type>
+  inline static void append(const input_value_type* addr, output_string_type& s)
+  {
+   if constexpr(sizeof(input_value_type) == sizeof(typename output_string_type::value_type)) {
+    s.append(reinterpret_cast<const typename output_string_type::value_type*>(addr), accu_size / sizeof(input_value_type));
+   } else if constexpr(is_utf_8_v<input_value_type>) {
+    s.append({static_cast<typename output_string_type::value_type>(addr[0]),
+              static_cast<typename output_string_type::value_type>(addr[1]),
+              static_cast<typename output_string_type::value_type>(addr[2]),
+              static_cast<typename output_string_type::value_type>(addr[3])});
+   } else if constexpr(is_utf_16_v<input_value_type>) {
+    s.append({static_cast<typename output_string_type::value_type>(addr[0]),
+              static_cast<typename output_string_type::value_type>(addr[1])});
+   } else if constexpr(is_utf_32_v<input_value_type>) {
+    s.append({static_cast<typename output_string_type::value_type>(addr[0])});
+   }
+  }
+ };
+
+ // On 64 bit architecture, calculate with 64 bit accumulator value
+ // (hoping the compiler will put it into a 64 bit register)
+ template<class ConvertInputOptimizer>
+ struct ArchitectureOptimizer<8, ConvertInputOptimizer>
+ {
+  typedef ConvertInputOptimizer input_optimizer;
+  typedef uint64_t accu_type;
+  static const accu_type addr_mask {accu_size - 1};
+  static const accu_type ascii_mask { ((accu_type)input_optimizer::ascii_mask) << 32 | (accu_type)input_optimizer::ascii_mask };
+  static const accu_type ascii_value { 0ULL };
+  
+  template<typename input_value_type, class output_string_type>
+  inline static void append(const input_value_type* addr, output_string_type& s)
+  {
+   if constexpr(sizeof(input_value_type) == sizeof(typename output_string_type::value_type)) {
+    s.append(reinterpret_cast<const typename output_string_type::value_type*>(addr), accu_size / sizeof(input_value_type));
+   } else if constexpr(is_utf_8_v<input_value_type>) {
+    s.append({static_cast<typename output_string_type::value_type>(addr[0]),
+              static_cast<typename output_string_type::value_type>(addr[1]),
+              static_cast<typename output_string_type::value_type>(addr[2]),
+              static_cast<typename output_string_type::value_type>(addr[3]),
+              static_cast<typename output_string_type::value_type>(addr[4]),
+              static_cast<typename output_string_type::value_type>(addr[5]),
+              static_cast<typename output_string_type::value_type>(addr[6]),
+              static_cast<typename output_string_type::value_type>(addr[7])});
+   } else if constexpr(is_utf_16_v<input_value_type>) {
+    s.append({static_cast<typename output_string_type::value_type>(addr[0]),
+              static_cast<typename output_string_type::value_type>(addr[1]),
+              static_cast<typename output_string_type::value_type>(addr[2]),
+              static_cast<typename output_string_type::value_type>(addr[3])});
+   } else if constexpr(is_utf_32_v<input_value_type>) {
+    s.append({static_cast<typename output_string_type::value_type>(addr[0]),
+              static_cast<typename output_string_type::value_type>(addr[1])});
+   }
+  }
+
+ }; // class ArchitectureOptimizer
+
+ // Optimize for the case of all ASCII (7-bit) data in a accu size row
+ // From and To are Encodings
+ template<typename From, typename To, std::enable_if_t<is_encoding_v<From> && is_encoding_v<To>, bool> = true>
+ typename To::string_type convert_optimized(const typename From::string_type& s)
+ {
+  typename To::string_type result;
+  typedef ConvertInputOptimizer<sizeof(typename From::value_type)> input_optimizer;
+  typedef ArchitectureOptimizer<accu_size, input_optimizer> arch_optimizer;
+
+  auto begin{From::begin(s)};
+  auto end{From::end(s)};
+  auto back_inserter{To::back_inserter(result)};
+  auto addr{reinterpret_cast<const typename arch_optimizer::accu_type*>(&s.data()[s.size() - input_distance(begin, end)])};
+  while (input_distance_bytes(begin, end) >= accu_size) {
+   if (((uintptr_t)(void*)addr & arch_optimizer::addr_mask) == 0) {
+    while (input_distance_bytes(begin, end) >= accu_size) {
+     typename arch_optimizer::accu_type data{*addr};
+     if ((data & arch_optimizer::ascii_mask) == arch_optimizer::ascii_value)
+#if __cplusplus >= 202002L
+     [[likely]]
+#endif
+     {
+      arch_optimizer::template append(reinterpret_cast<const typename From::value_type*>(addr), result);
+      begin += accu_size / sizeof(typename From::value_type);
+      ++addr;
+     } else {
+      // just advance one code unit for now and break to trigger unoptimized
+      // version until next accu boundary
+      back_inserter = *begin;
+      ++begin;
+      break;
+     }
+    }
+   }
+
+   // keep up after unaligned Non-ASCII code points
+   while (begin != end && (uintptr_t)(void*)(addr = reinterpret_cast<const typename arch_optimizer::accu_type*>(&s.data()[s.size() - input_distance(begin, end)])) & arch_optimizer::addr_mask) {
+    back_inserter = *begin;
+    ++begin;
+   }
+  }
+
+  // remainder < 8 bytes   
+  while (begin != end) {
+   back_inserter = *begin;
+   ++begin;
+  }
+
+  return result;
+ }
+
+ template<size_t bits_to_compare = 32, typename To, typename std::enable_if_t<is_utf_8_v<To>, bool> = true>
+ inline void append_utf(std::basic_string<To>& result, const char32_t& value)
+ {
+  using From = char32_t;
+  if (bits_to_compare <= 7 || value < 0x80) { // 1 byte
+   result.push_back(static_cast<To>(value));
+  } else if (bits_to_compare <= 11 || value < 0x800) { // 2 bytes
+   result.append({utf8_byte_n_of_m<0,2,From,To>(value), utf8_byte_n_of_m<1,2,From,To>(value)});
+  } else if (bits_to_compare <= 16 || value < 0x10000) { // 3 bytes
+   result.append({utf8_byte_n_of_m<0,3,From,To>(value), utf8_byte_n_of_m<1,3,From,To>(value), utf8_byte_n_of_m<2,3,From,To>(value)});
+  } else { // 4 bytes
+   // expect value to be already valid Unicode values
+   result.append({utf8_byte_n_of_m<0,4,From,To>(value), utf8_byte_n_of_m<1,4,From,To>(value), utf8_byte_n_of_m<2,4,From,To>(value), utf8_byte_n_of_m<3,4,From,To>(value)});
+  }
+ }
+
+ template<size_t bits_to_compare = 32, typename To, typename std::enable_if_t<is_utf_16_v<To>, bool> = true>
+ inline void append_utf(std::basic_string<To>& result, const char32_t& value)
+ {
+  if (bits_to_compare <= 16 || value <= 0xFFFF) { // expect value to be already valid Unicode values
+   result.push_back(static_cast<To>(value));
+  } else {
+   char32_t value_reduced{value - 0x10000};
+   result.append({static_cast<To>((value_reduced >> 10) + 0xD800), static_cast<To>((value_reduced & 0x3FF) + 0xDC00)});
+  }
+ }
+
+ template<size_t bits_to_compare = 32, typename To, typename std::enable_if_t<is_utf_32_v<To>, bool> = true>
+ inline void append_utf(std::basic_string<To>& result, const char32_t& value)
+ {
+  // expect value to be already valid Unicode values (checked in input iterator)
+  result.push_back(static_cast<To>(value));
+ }
+
+ // Little Endian optimized version for UTF-8
+ // In block_mode, at least 4 bytes are in accu. On first call, even 8.
+ // otherwise, at least one code unit is in accu
+ template<typename From, typename To, bool block_mode = true, typename std::enable_if_t<is_utf_8_v<From>, bool> = true>
+ inline static void append_accu(std::basic_string<To>& result, uint64_t& accu, int& bytes_in_accu)
+ {
+  if (block_mode && bytes_in_accu == 8 && (accu & 0x8080808080808080) == 0)
+#if __cplusplus >= 202002L
+  [[likely]]
+#endif
+  {
+   result.append({
+                 static_cast<To>(accu & 0x7F),
+                 static_cast<To>((accu >> 8) & 0x7F),
+                 static_cast<To>((accu >> 16) & 0x7F),
+                 static_cast<To>((accu >> 24) & 0x7F),
+                 static_cast<To>((accu >> 32) & 0x7F),
+                 static_cast<To>((accu >> 40) & 0x7F),
+                 static_cast<To>((accu >> 48) & 0x7F),
+                 static_cast<To>((accu >> 56) & 0x7F),
+                 });
+   accu = 0;
+   bytes_in_accu = 0;
+  } else if ((accu & 0x80) == 0) { // 1 byte sequence
+   append_utf<7>(result, static_cast<char32_t>(accu & 0x7F));
+   accu >>= 8;
+   bytes_in_accu -= 1;
+  } else if ((block_mode || bytes_in_accu >= 2) && (accu & 0xC0E0) == 0x80C0) { // 2 byte sequence
+   char32_t value {static_cast<char32_t>(((accu & 0x1F) << 6) | ((accu >> 8) & 0x3f))};
+   accu >>= 16;
+   bytes_in_accu -= 2;
+   if (is_valid_unicode<11>(value))
+    append_utf<11>(result, value);
+   else
+#if __cplusplus >= 202002L
+    [[unlikely]]
+#endif
+    throw std::invalid_argument("Invalid Unicode character in 2 byte UTF-8 sequence");
+  } else if ((block_mode || bytes_in_accu >= 3) && (accu & 0xC0C0F0) == 0x8080E0) { // 3 byte sequence
+   char32_t value {static_cast<char32_t>(((accu & 0x0F) << 12) | ((accu >> 2) & 0x0FC0) | ((accu >> 16) & 0x3f))};
+   accu >>= 24;
+   bytes_in_accu -= 3;
+   if (is_valid_unicode<16>(value))
+    append_utf<16>(result, value);
+   else
+#if __cplusplus >= 202002L
+    [[unlikely]]
+#endif
+    throw std::invalid_argument("Invalid Unicode character in 3 byte UTF-8 sequence");
+  } else if ((block_mode || bytes_in_accu >= 4) && (accu & 0xC0C0C0F8) == 0x808080F0) { // 4 byte sequence
+   char32_t value {static_cast<char32_t>(((accu & 0x07) << 18) | ((accu << 4) & 0x3f000) | ((accu >> 10) & 0xFC0) | ((accu >> 24) & 0x3f))};
+   accu >>= 32;
+   bytes_in_accu -= 4;
+   if (is_valid_unicode<21>(value))
+    append_utf(result, value);
+   else
+#if __cplusplus >= 202002L
+    [[unlikely]]
+#endif
+    throw std::invalid_argument("Invalid Unicode character in 4 byte UTF-8 sequence");
+  } else
+#if __cplusplus >= 202002L
+   [[unlikely]]
+#endif
+   throw std::invalid_argument("Invalid UTF-8 byte sequence");
+ }
+
+ // Little Endian optimized version
+ template<typename From, typename To, std::enable_if_t<is_encoding_v<From> && is_encoding_v<To>, bool> = true>
+ typename To::string_type convert_optimized_utf(const typename From::string_type& s)
+ {
+  typename To::string_type result;
+  uint64_t accu{};
+  int bytes_in_accu{};
+
+  size_t s_index{};
+  size_t s_size{s.size()};
+  while (s_index + 8 / sizeof(typename From::value_type) <= s_size) {
+   // read input
+   // assume: bytes_in_accu < 8
+   accu |= (*reinterpret_cast<const uint64_t*>(&(s.data()[s_index]))) << (bytes_in_accu * 8);
+   s_index += (8 - bytes_in_accu) / sizeof(typename From::value_type);
+   bytes_in_accu = 8;
+
+   while (bytes_in_accu >= 4) {
+    append_accu<typename From::value_type, typename To::value_type, true>(result, accu, bytes_in_accu);
+   }
+  }
+
+  // 0..3 bytes left in accu
+  // 0..7 bytes left in s
+
+  while (s_index < s_size || bytes_in_accu > 0) {
+   while (s_index < s_size && bytes_in_accu < 8) {
+    accu |= static_cast<uint64_t>(*reinterpret_cast<const uint8_t*>(&(s.data()[s_index]))) << (bytes_in_accu * 8);
+    ++s_index;
+    bytes_in_accu += sizeof(typename From::value_type);
+   }
+
+   append_accu<typename From::value_type, typename To::value_type, false>(result, accu, bytes_in_accu);
+  }
+  return result;
+ }
+
+} // namespace unicode
+
diff --git a/include/unicode/predicate.h b/include/unicode/predicate.h
index 82031d1..f0a003d 100644
--- a/include/unicode/predicate.h
+++ b/include/unicode/predicate.h
@@ -1,5 +1,7 @@
 //
-// Unicode library - predicates for Unicode characters
+// Reichwein.IT Unicode Library
+//
+// Predicates for Unicode characters
 //
 
 #pragma once
diff --git a/include/unicode/type_traits.h b/include/unicode/type_traits.h
index 63c7d69..47789b9 100644
--- a/include/unicode/type_traits.h
+++ b/include/unicode/type_traits.h
@@ -1,3 +1,9 @@
+//
+// Reichwein.IT Unicode Library
+//
+// Type traits
+//
+
 #pragma once
 
 #include "utf_fwd.h"
@@ -32,7 +38,7 @@ namespace unicode {
  template<typename T>
  struct is_char
  {
-  static const bool value{std::is_trivial_v<T> && std::is_scalar_v<T> && !std::is_empty_v<T>};
+  static const bool value{std::is_trivial_v<T> && std::is_scalar_v<T>};
  };
  
  template<typename T>
diff --git a/include/unicode/types.h b/include/unicode/types.h
index a4461d7..6eac5f7 100644
--- a/include/unicode/types.h
+++ b/include/unicode/types.h
@@ -1,9 +1,20 @@
+//
+// Reichwein.IT Unicode Library
+//
+// Basic types
+//
+
 #pragma once
 
+// Definition of utf8_t as abstraction from char and char8_t, when available
+//
+// Be aware of char being signed on common architectures, while char8_t is
+// unsigned.
 #ifdef __cpp_char8_t
-// char8_t available
+ // char8_t available in C++20
  typedef char8_t utf8_t;
 #else
+ // fallback to char
  typedef char utf8_t;
 #endif
 typedef char iso_t;
diff --git a/include/unicode/utf.h b/include/unicode/utf.h
index 1d2f28e..5db9cac 100644
--- a/include/unicode/utf.h
+++ b/include/unicode/utf.h
@@ -1,3 +1,13 @@
+//
+// Reichwein.IT Unicode Library
+//
+// Functions for support of UTF encodings
+//
+// Implementation of utf_iterator and utf_back_insert_iterator templates for
+// validation and conversion via STL compatible iteration over standard
+// containers
+//
+
 #pragma once
 
 #include "utf_fwd.h"
diff --git a/include/unicode/utf_fwd.h b/include/unicode/utf_fwd.h
index c42dea1..7fd9329 100644
--- a/include/unicode/utf_fwd.h
+++ b/include/unicode/utf_fwd.h
@@ -1,6 +1,11 @@
-#pragma once
+//
+// Reichwein.IT Unicode Library
+//
+// Forward declarations for utf.h - Functions for reading and writing UTF
+// encodings
+//
 
-// Forward declarations
+#pragma once
 
 #include "types.h"
 
diff --git a/include/unicode/validation.h b/include/unicode/validation.h
new file mode 100644
index 0000000..b5060c4
--- /dev/null
+++ b/include/unicode/validation.h
@@ -0,0 +1,78 @@
+//
+// Reichwein.IT Unicode Library
+//
+// Functions for validation of UTF (Unicode Transformation Format) encodings
+//
+
+#pragma once
+
+#include "unicode/endian.h"
+#include "unicode/iso.h"
+#include "unicode/optimization.h"
+#include "unicode/predicate.h"
+#include "unicode/types.h"
+#include "unicode/type_traits.h"
+#include "unicode/utf.h"
+
+#include <algorithm>
+#include <array>
+#include <cstdint>
+#include <iterator>
+#include <memory>
+#include <stdexcept>
+#include <string>
+#include <type_traits>
+#include <utility>
+
+namespace unicode {
+
+ // First variant of is_valid_utf(): Specification of encoding explicitly
+ //
+ // e.g.
+ // unicode::UTF_8
+ // unicode::UTF_16
+ // unicode::UTF_32
+ //
+ // see also type_traits.h and utf.h
+ template<typename Encoding, std::enable_if_t<is_encoding_v<Encoding>, bool> = true>
+ bool is_valid_utf(const typename Encoding::string_type& s)
+ {
+  return validate_utf<typename Encoding::value_type>(s);
+ }
+
+ // Second variant of is_valid_utf(): Specification of encoding via character type
+ //
+ // see also type_traits.h for is_char
+ template<typename T,
+  typename Container=std::basic_string<T>,
+  std::enable_if_t<is_char_v<T>, bool> = true>
+ bool is_valid_utf(const Container& s)
+ {
+  typedef UTF<utf_iterator<T>, utf_back_insert_iterator<T>> UTF_Trait;
+  
+  try {
+   std::for_each(UTF_Trait::begin(s), UTF_Trait::end(s), [](const char32_t& c){});
+  } catch (const std::invalid_argument&) {
+   return false;
+  }
+  return true;
+ }
+
+ // Third variant of is_valid_utf(): Specification of encoding via container type
+ //
+ // see also type_traits.h for is_container
+ template<typename Container, std::enable_if_t<is_container_v<Container>, bool> = true>
+ bool is_valid_utf(const Container& s)
+ {
+  typedef UTF<utf_iterator<typename Container::value_type, Container>, utf_back_insert_iterator<typename Container::value_type, Container>> UTF_Trait;
+  
+  try {
+   std::for_each(UTF_Trait::begin(s), UTF_Trait::end(s), [](const char32_t& c){});
+  } catch (const std::invalid_argument&) {
+   return false;
+  }
+  return true;
+ }
+
+} // namespace unicode
+