[FIX] Add size function to kmer_hash view

CHANGELOG.md

@@ -175,6 +175,8 @@ Note that 3.1.0 will be the first API stable release and interfaces in this rele
   ([\#1410](https://github.com/seqan/seqan3/pull/1410)).
 * Renamed `seqan3::views::all` to `seqan3::views::type_reduce`
   ([\#1410](https://github.com/seqan/seqan3/pull/1410)).
+* Added size() function to `seqan3::views::kmer_hash`
+  ([\#1722](https://github.com/seqan/seqan3/pull/1722)).
 
 #### Search
 

include/seqan3/range/views/kmer_hash.hpp

@@ -174,6 +174,17 @@ class kmer_hash_view : public std::ranges::view_interface<kmer_hash_view<urng_t>
         return end();
     }
     //!\}
+
+    /*!\brief Returns the size of the range, if the underlying range is a std::ranges::sized_range.
+     * \returns Size of range.
+     */
+    uint64_t size()
+        requires std::ranges::sized_range<urng_t>
+    {
+        if (std::ranges::size(urange) >= shape_.size())
+            return std::ranges::size(urange) - shape_.size() + 1;
+        return 0;
+    }
 };
 
 /*!\brief Iterator for calculating hash values via a given seqan3::shape.

test/unit/range/views/view_kmer_hash_test.cpp

@@ -137,3 +137,16 @@ TEST_F(kmer_hash_test, issue1643)
     auto k_mer_size_25_view = text_23_elements | seqan3::views::kmer_hash(seqan3::ungapped{25u});
     EXPECT_TRUE(k_mer_size_25_view.empty());
 }
+
+// https://github.com/seqan/seqan3/issues/1719
+TEST_F(kmer_hash_test, issue1719)
+{
+    uint64_t const expected = 0;
+    std::vector<seqan3::dna5> sequence{""_dna5};
+    auto v = sequence | seqan3::views::kmer_hash(seqan3::ungapped{25});
+    EXPECT_EQ(expected, v.size());
+
+    std::vector<seqan3::dna5> sequence2{"ACGATCGATCGTAGCTACTGAGC"_dna5};
+    auto v2 = sequence2 | seqan3::views::kmer_hash(seqan3::ungapped{25});
+    EXPECT_EQ(expected, v2.size());
+}