[MISC] Update writable_alphabet concept

CHANGELOG.md

@@ -63,6 +63,11 @@ If possible, provide tooling that performs the changes, e.g. a shell-script.
 
 ## API changes
 
+#### Alphabet
+
+* Removed seqan3::char_is_valid_for requirement from seqan3::writable_alphabet and seqan3::writable_constexpr_alphabet
+  ([\#2337](https://github.com/seqan/seqan3/pull/2337)).
+
 #### Argument Parser
 
 * The enum names of `seqan3::option_spec` were changed to lower case

doc/howto/write_an_alphabet/index.md

@@ -136,7 +136,7 @@ your type also model these concepts.
 \endsolution
 
 At this point the seqan3::alphabet concept should be modelled successfully and even seqan3::writable_alphabet
-is fine because we implemented `assign_char` and `char_is_valid`.
+is fine because we implemented `assign_char`.
 \snippet dna2_alphabet.cpp writable_alphabet_concept
 
 ## Shortcut: alphabet base template
@@ -183,4 +183,3 @@ from '1', 't' and 'T' for value 1 as well as from '0', 'f' and 'F' for value 0.
 \note
 You should really make your alphabet types [no-throw-default-constructible](\ref std::is_nothrow_default_constructible)
 if you can!
-

include/seqan3/alphabet/composite/alphabet_variant.hpp

@@ -567,21 +567,30 @@ class alphabet_variant : public alphabet_base<alphabet_variant<alternative_types
         for (size_t i = 0u; i < table_size; ++i)
         {
             char_type chr = static_cast<char_type>(i);
-            bool there_was_no_valid_representation{true};
 
-            meta::for_each(alternatives{}, [&] (auto && alt)
+            std::array<bool, sizeof...(alternative_types)> is_char_valid{char_is_valid_for<alternative_types>(chr)...};
+            std::array<rank_type, sizeof...(alternative_types)> ranks
             {
-                using alt_type = std::remove_cvref_t<decltype(alt)>;
-
-                if (there_was_no_valid_representation && char_is_valid_for<alt_type>(chr))
+                rank_by_type_(assign_char_to(chr, alternative_types{}))...
+            };
+
+            // if no char_is_valid_for any alternative use the rank of the first alternative
+#if defined(__cpp_lib_constexpr_algorithms) && __cpp_lib_constexpr_algorithms >= 201806L
+            // the following lines only works beginning from c++20
+            auto found_it = std::find(is_char_valid.begin(), is_char_valid.end(), true);
+            auto found_index = found_it == is_char_valid.end() ? 0 : found_it - is_char_valid.begin();
+#else
+            size_t found_index = 0u;
+            for (size_t k = 0u; k < is_char_valid.size(); ++k)
+            {
+                if (is_char_valid[k])
                 {
-                    there_was_no_valid_representation = false;
-                    char_to_rank[i] = rank_by_type_(assign_char_to(chr, alt_type{}));
+                    found_index = k;
+                    break;
                 }
-            });
-
-            if (there_was_no_valid_representation)
-                char_to_rank[i] = rank_by_type_(assign_char_to(chr, meta::front<alternatives>{}));
+            }
+#endif // defined(__cpp_lib_constexpr_algorithms) && __cpp_lib_constexpr_algorithms >= 201806L
+            char_to_rank[i] = ranks[found_index];
         }
 
         return char_to_rank;

include/seqan3/alphabet/concept.hpp

@@ -872,7 +872,6 @@ SEQAN3_CONCEPT alphabet = semialphabet<t> && requires (t v)
  *   1. `t` shall model seqan3::alphabet
  *   2. `t` shall model seqan3::writable_semialphabet
  *   3. seqan3::assign_char_to needs to be defined for objects of type `t`
- *   4. seqan3::char_is_valid_for needs to be defined for type `t` and an argument of the character representation
  *
  * See the documentation pages for the respective requirements.
  *
@@ -895,8 +894,6 @@ template <typename t>
 SEQAN3_CONCEPT writable_alphabet = alphabet<t> && writable_semialphabet<t> && requires (t v, alphabet_char_t<t> c)
 {
     { seqan3::assign_char_to(c, v) };
-
-    { seqan3::char_is_valid_for<t>(c) };
 };
 //!\endcond
 
@@ -1035,8 +1032,7 @@ SEQAN3_CONCEPT constexpr_alphabet = constexpr_semialphabet<t> && alphabet<t> &&
  * \ingroup alphabet
  *
  * Refines seqan3::detail::constexpr_alphabet, seqan3::detail::writable_constexpr_semialphabet and
- * seqan3::writable_alphabet and requires that the calls to seqan3::assign_char_to and seqan3::char_is_valid_for
- * can happen in a `constexpr`-context.
+ * seqan3::writable_alphabet and requires that the call to seqan3::assign_char_to can happen in a `constexpr`-context.
  */
 //!\cond
 template <typename t>
@@ -1045,7 +1041,6 @@ SEQAN3_CONCEPT writable_constexpr_alphabet =
 {
     // currently only tests rvalue interfaces, because we have no constexpr values in this scope to get references to
     requires SEQAN3_IS_CONSTEXPR(seqan3::assign_char_to(alphabet_char_t<t>{}, std::remove_reference_t<t>{}));
-    requires SEQAN3_IS_CONSTEXPR(seqan3::char_is_valid_for<t>(alphabet_char_t<t>{}));
 };
 //!\endcond