Fix unaligned memory access UB in compact_vector::access() (#11)

* Fix unaligned memory access UB in compact_vector::access()

Replace reinterpret_cast + dereference with memcpy to avoid undefined
behavior when accessing unaligned uint64_t values. This fixes crashes
and incorrect behavior on ARM64 where unaligned access is not tolerated.

The memcpy approach is standards-compliant and compilers optimize it to
efficient code on all platforms.

* fix UB in append_bits

* Fix assert

Author: adamant-pwn

include/bit_vector.hpp

@@ -97,6 +97,7 @@ struct bit_vector  //
 
         void append_bits(uint64_t bits, uint64_t len) {
             // check there are no spurious bits
+            assert(len <= 64);
             assert(len == 64 || (bits >> len) == 0);
             if (!len) return;
             uint64_t pos_in_word = m_num_bits & 63;

include/compact_vector.hpp

@@ -2,6 +2,7 @@
 
 #include <vector>
 #include <cmath>
+#include <cstring>
 #include <iterator>
 
 #include "essentials.hpp"
@@ -253,7 +254,9 @@ struct compact_vector  //
         assert(i < size());
         uint64_t pos = i * m_width;
         const char* ptr = reinterpret_cast<const char*>(m_data.data());
-        return (*(reinterpret_cast<uint64_t const*>(ptr + (pos >> 3))) >> (pos & 7)) & m_mask;
+        uint64_t word;
+        std::memcpy(&word, ptr + (pos >> 3), sizeof(uint64_t));
+        return (word >> (pos & 7)) & m_mask;
     }
 
     uint64_t back() const { return operator[](size() - 1); }