docs: add example of custom alphabet for seqan2::Dna

Author: eseiler

doc/cookbook/index.md

@@ -202,6 +202,13 @@ Here is an example for a dna4-like alphabet:
 
 \snippet test/performance/simd_dna4.hpp cookbook
 
+# Adaptation of seqan2::Dna
+
+In case you want to use `seqan2::Dna` in your SeqAn3 application, you can adapt it to the SeqAn3 alphabet
+concepts by specialising the `seqan3::custom::alphabet` class template.
+
+\include test/unit/alphabet/nucleotide/custom_alphabet_seqan2_dna.hpp
+
 # All SeqAn documentation snippets
 
 The following lists all snippets that appear in our documentation.

doc/howto/write_an_alphabet/index.md

@@ -188,3 +188,10 @@ 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!
+
+## Adaptation of seqan2::Dna
+
+In case you want to use `seqan2::Dna` in your SeqAn3 application, you can adapt it to the SeqAn3 alphabet
+concepts by specialising the `seqan3::custom::alphabet` class template.
+
+\include test/unit/alphabet/nucleotide/custom_alphabet_seqan2_dna.hpp

test/unit/alphabet/nucleotide/CMakeLists.txt

@@ -2,6 +2,7 @@
 # SPDX-FileCopyrightText: 2016-2026 Knut Reinert & MPI für molekulare Genetik
 # SPDX-License-Identifier: CC0-1.0
 
+seqan3_test (custom_alphabet_seqan2_dna_test.cpp)
 seqan3_test (dna4_test.cpp)
 seqan3_test (dna5_test.cpp)
 seqan3_test (dna15_test.cpp)

test/unit/alphabet/nucleotide/custom_alphabet_seqan2_dna.hpp

@@ -0,0 +1,72 @@
+// SPDX-FileCopyrightText: 2006-2026 Knut Reinert & Freie Universität Berlin
+// SPDX-FileCopyrightText: 2016-2026 Knut Reinert & MPI für molekulare Genetik
+// SPDX-License-Identifier: CC0-1.0
+
+#pragma once
+
+#include <seqan3/alphabet/concept.hpp>
+
+#include <seqan/basic.h>
+#include <seqan/modifier/modifier_functors.h>
+
+// Ranks and letters of `seqan2::Dna` and `seqan3::dna4` are the same:
+// 'A' = 0, 'C' = 1, 'G' = 2, 'T' = 3.
+template <>
+struct seqan3::custom::alphabet<seqan2::Dna>
+{
+    using alphabet_t = seqan2::Dna;
+
+    static constexpr size_t alphabet_size = 4u;
+
+    static uint8_t to_rank(alphabet_t const a) noexcept
+    {
+        return seqan2::ordValue(a);
+    }
+
+    static alphabet_t & assign_rank_to(uint8_t const r, alphabet_t & a) noexcept
+    {
+        seqan2::assign(a, r);
+        return a;
+    }
+
+    static char to_char(alphabet_t const a) noexcept
+    {
+        char c;
+        seqan2::assign(c, a);
+        return c;
+    }
+
+    static alphabet_t & assign_char_to(char const c, alphabet_t & a) noexcept
+    {
+        seqan2::assign(a, c);
+        return a;
+    }
+
+    static alphabet_t complement(alphabet_t const a) noexcept
+    {
+        static seqan2::FunctorComplement<alphabet_t> func;
+        return func(a);
+    }
+};
+
+// Disable `seqan2::begin` overload for `seqan::Dna`.
+// Otherwise, `seqan2::Dna` would be considered a range.
+// `seqan2::Dna` is indeed not a range, and assuming otherwise breaks many views.
+namespace seqan2
+{
+
+void begin(seqan2::Dna) = delete;
+
+} // namespace seqan2
+
+// This is only needed if you want to use `seqan3::debug_stream` with `seqan2::Dna`.
+namespace seqan3
+{
+
+inline debug_stream_type<char> & operator<<(debug_stream_type<char> & stream, seqan2::Dna const & data)
+{
+    stream << seqan2::convert<char>(data);
+    return stream;
+}
+
+} // namespace seqan3

test/unit/alphabet/nucleotide/custom_alphabet_seqan2_dna_test.cpp

@@ -0,0 +1,28 @@
+// SPDX-FileCopyrightText: 2006-2026 Knut Reinert & Freie Universität Berlin
+// SPDX-FileCopyrightText: 2016-2026 Knut Reinert & MPI für molekulare Genetik
+// SPDX-License-Identifier: BSD-3-Clause
+
+#include <gtest/gtest.h>
+
+#ifdef SEQAN3_HAS_SEQAN2
+
+// #    include "../alphabet_constexpr_test_template.hpp"
+#    include "../alphabet_test_template.hpp"
+// #    include "../semi_alphabet_constexpr_test_template.hpp"
+#    include "../semi_alphabet_test_template.hpp"
+#    include "custom_alphabet_seqan2_dna.hpp"
+#    include "nucleotide_test_template.hpp"
+
+template <>
+struct nucleotide<seqan2::Dna> : public ::testing::Test
+{
+    static constexpr bool skip_trivial_thirdparty = true;
+};
+
+INSTANTIATE_TYPED_TEST_SUITE_P(seqan2_dna, alphabet, seqan2::Dna, );
+INSTANTIATE_TYPED_TEST_SUITE_P(seqan2_dna, semi_alphabet_test, seqan2::Dna, );
+// INSTANTIATE_TYPED_TEST_SUITE_P(seqan2_dna, alphabet_constexpr, seqan2::Dna, );
+// INSTANTIATE_TYPED_TEST_SUITE_P(seqan2_dna, semi_alphabet_constexpr, seqan2::Dna, );
+INSTANTIATE_TYPED_TEST_SUITE_P(seqan2_dna, nucleotide, seqan2::Dna, );
+
+#endif

test/unit/alphabet/nucleotide/nucleotide_test_template.hpp

@@ -10,13 +10,20 @@
 #include <seqan3/alphabet/nucleotide/concept.hpp>
 
 template <typename t>
-using nucleotide = ::testing::Test;
+struct nucleotide : public ::testing::Test
+{
+    // Do not check the `trivial` concept, because `t` is a third party class and not trivial.
+    static constexpr bool skip_trivial_thirdparty = false;
+};
 
 TYPED_TEST_SUITE_P(nucleotide);
 
 TYPED_TEST_P(nucleotide, concept_check)
 {
-    EXPECT_TRUE(seqan3::trivial<TypeParam>);
+    if constexpr (!TestFixture::skip_trivial_thirdparty)
+    {
+        EXPECT_TRUE(seqan3::trivial<TypeParam>);
+    }
 
     EXPECT_TRUE(seqan3::nucleotide_alphabet<TypeParam>);
     EXPECT_TRUE(seqan3::nucleotide_alphabet<TypeParam &>);
@@ -26,10 +33,10 @@ TYPED_TEST_P(nucleotide, concept_check)
 
 TYPED_TEST_P(nucleotide, complement)
 {
-    EXPECT_EQ(seqan3::complement(TypeParam{}.assign_char('A')), TypeParam{}.assign_char('T'));
-    EXPECT_EQ(seqan3::complement(TypeParam{}.assign_char('C')), TypeParam{}.assign_char('G'));
-    EXPECT_EQ(seqan3::complement(TypeParam{}.assign_char('G')), TypeParam{}.assign_char('C'));
-    EXPECT_EQ(seqan3::complement(TypeParam{}.assign_char('T')), TypeParam{}.assign_char('A'));
+    EXPECT_EQ(seqan3::complement(seqan3::assign_char_to('A', TypeParam{})), seqan3::assign_char_to('T', TypeParam{}));
+    EXPECT_EQ(seqan3::complement(seqan3::assign_char_to('C', TypeParam{})), seqan3::assign_char_to('G', TypeParam{}));
+    EXPECT_EQ(seqan3::complement(seqan3::assign_char_to('G', TypeParam{})), seqan3::assign_char_to('C', TypeParam{}));
+    EXPECT_EQ(seqan3::complement(seqan3::assign_char_to('T', TypeParam{})), seqan3::assign_char_to('A', TypeParam{}));
 
     using vsize_t = std::decay_t<decltype(seqan3::alphabet_size<TypeParam>)>;