feat: add LengthDistance

CMakeLists.txt

@@ -2,7 +2,7 @@
 cmake_minimum_required(VERSION 3.16..3.23)
 
 project(bk-tree
-    VERSION 0.1.1
+    VERSION 0.1.3
     LANGUAGES CXX
     DESCRIPTION "Header-only Burkhard-Keller tree library"
     HOMEPAGE_URL "https://github.com/poyea/bk-tree")

bktree/bktree.hpp

@@ -65,6 +65,22 @@ class Distance {
   }
 };
 
+/**
+ * @brief Length metric
+ *
+ * \f$d(x, y) = |m - n|\f$ where
+ * \f$m\f$ is the length of \f$x\f$, and \f$n\f$ is the length of \f$y\f$, for any \f$x,
+ * y.\f$
+ */
+class LengthDistance final : public Distance<LengthDistance> {
+public:
+  explicit LengthDistance(){};
+  integer_type compute_distance(std::string_view s, std::string_view t) const {
+    std::int64_t const d = s.length() - t.length();
+    return d >= 0 ? d : -d;
+  }
+};
+
 /**
  * @brief Identity metric
  *

tests/distance_length.cpp

@@ -0,0 +1,34 @@
+#include "gtest/gtest.h"
+
+#include "bktree.hpp"
+
+namespace bk_tree_test {
+
+class Distance_Length_TEST : public ::testing::Test {
+protected:
+  Distance_Length_TEST() {}
+
+  virtual ~Distance_Length_TEST() {}
+
+  virtual void SetUp() {
+    // post-construction
+  }
+
+  virtual void TearDown() {
+    // pre-destruction
+  }
+
+  bk_tree::metrics::LengthDistance dist;
+};
+
+TEST_F(Distance_Length_TEST, LengthDistances) {
+  EXPECT_TRUE(dist("", "") == 0);
+  EXPECT_TRUE(dist("peter", "piter") == 0);
+  EXPECT_TRUE(dist("peter", "pitar") == 0);
+  EXPECT_TRUE(dist("peter", "pitat") == 0);
+  EXPECT_TRUE(dist("ca", "abc") == 1);
+  EXPECT_TRUE(dist("peter", "") == 5);
+  EXPECT_TRUE(dist("", "peter") == 5);
+}
+
+} // namespace bk_tree_test