[DOC] clarification how shapes are applied in kmer-hash (#2981)

* [DOC] clarification how shapes are applied in kmer-hash * Apply suggestions from code review * [misc] improved doc and snippet for kmer_hash Update test/snippet/search/views/kmer_hash.cpp Update include/seqan3/search/views/kmer_hash.hpp Co-authored-by: Svenja Mehringer <[email protected]> Co-authored-by: Enrico Seiler <[email protected]> Co-authored-by: Svenja Mehringer <[email protected]>
seqan · May 17, 2022 · d3aa8be · d3aa8be · vercel · May 17, 2022
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diff --git a/include/seqan3/search/views/kmer_hash.hpp b/include/seqan3/search/views/kmer_hash.hpp
@@ -774,6 +774,9 @@ namespace seqan3::views
  *
  * See the \link views views submodule documentation \endlink for detailed descriptions of the view properties.
  *
+ * \attention The Shape is defined from right to left! The mask 0b11111101 applied to "AGAAAATA" is 
+ * interpreted as "A.AAAATA" (and not "AGAAAA.A") and will return the hash value for "AAAAATA".
+ *
  * ### Example
  *
  * \include test/snippet/search/views/kmer_hash.cpp

diff --git a/test/snippet/search/views/kmer_hash.cpp b/test/snippet/search/views/kmer_hash.cpp
@@ -14,4 +14,14 @@ int main()
     seqan3::debug_stream << (text | seqan3::views::kmer_hash(seqan3::ungapped{3})) << '\n'; // [6,27,44,50,9]
 
     seqan3::debug_stream << (text | seqan3::views::kmer_hash(0b101_shape)) << '\n'; // [2,7,8,14,1]
+
+    // Attention: the Shape is defined from right to left! 
+    // The mask 0b11111101 applied to "AGAAAATA" ("A.AAAATA") will yield
+    // the same hash value as mask 0b1111111 applied to "AAAAATA".
+    {
+        auto text1 = "AGAAAATA"_dna4;
+        auto text2 = "AAAAATA"_dna4;
+        seqan3::debug_stream << (text1 | seqan3::views::kmer_hash(0b11111101_shape)) << '\n'; // [12]
+        seqan3::debug_stream << (text2 | seqan3::views::kmer_hash(0b1111111_shape)) << '\n';  // [12]
+    }
 }
diff --git a/test/snippet/search/views/kmer_hash.err b/test/snippet/search/views/kmer_hash.err
@@ -1,3 +1,5 @@
 [6,27,44,50,9]
 [6,27,44,50,9]
 [2,7,8,14,1]
+[12]
+[12]