Fix: uint32_t to uint40_t cast (#404)

Co-authored-by: Ash Vardanian <1983160+ashvardanian@users.noreply.github.com>

Author: Ngalstyan4

cpp/test.cpp

@@ -57,6 +57,39 @@ template <typename index_at> struct aligned_wrapper_gt {
     }
 };
 
+/**
+ * Tests the functionality of the custom uint40_t type ensuring consistent 
+ * behavior across various constructors from uint32_t, uint64_t, and size_t types.
+ */
+void test_uint40() {
+    // Constants for tests
+    std::uint64_t max_uint40_k = (1ULL << 40) - 1;
+
+    for (std::uint64_t original_value : {
+        42ull,                        // Typical small number
+        4242ull,                      // Larger number still within uint40 range
+        1ull << 40,                   // Exactly at the boundary of uint40
+        (1ull << 40) + 1,             // Just beyond the boundary of uint40
+        1ull << 63                    // Well beyond the uint40 boundary, tests masking
+    }) {
+        std::uint32_t v_32 = static_cast<std::uint32_t>(original_value);
+        std::uint64_t v_64 = original_value;
+        std::size_t v_size = static_cast<std::size_t>(original_value);
+
+        // Create uint40_t instances from different types
+        uint40_t n_40_from_32(v_32);
+        uint40_t n_40_from_64(v_64);
+        uint40_t n_40_from_size(v_size);
+
+        // Expected value after masking
+        std::uint64_t expected_value = original_value & max_uint40_k;
+
+        // Check if all conversions are equal to the masked value
+        expect(n_40_from_32 == expected_value);
+        expect(n_40_from_64 == expected_value);
+        expect(n_40_from_size == expected_value);
+    }
+}
 /**
  * Tests the behavior of various move-constructors and move-assignment operators for the index.
  *
@@ -700,6 +733,7 @@ template <typename key_at, typename slot_at> void test_replacing_update() {
 }
 
 int main(int, char**) {
+    test_uint40();
 
     // Weird corner cases
     test_replacing_update<std::int64_t, std::uint32_t>();

include/usearch/index.hpp

@@ -785,7 +785,7 @@ class usearch_pack_m uint40_t {
 
   public:
     inline uint40_t() noexcept { broadcast(0); }
-    inline uint40_t(std::uint32_t n) noexcept { std::memcpy(&octets[1], &n, 4); }
+    inline uint40_t(std::uint32_t n) noexcept { std::memcpy(&octets, &n, 4); }
 
 #ifdef USEARCH_64BIT_ENV
     inline uint40_t(std::uint64_t n) noexcept { std::memcpy(octets, &n, 5); }
@@ -811,7 +811,7 @@ class usearch_pack_m uint40_t {
 #ifdef USEARCH_64BIT_ENV
         std::memcpy(&result, octets, 5);
 #else
-        std::memcpy(&result, octets + 1, 4);
+        std::memcpy(&result, octets, 4);
 #endif
         return result;
     }