BitpackIntegerDecoder sometimes reads past end of input buffer (#87)

src/Decoder.cpp

@@ -731,7 +731,19 @@ size_t BitpackIntegerDecoder<RegisterT>::inputProcessAligned( const char *inbuf,
 #endif
 
       RegisterT w;
-      if ( bitOffset > 0 )
+      if ( bitOffset == 0 )
+      {
+         /// The left shift (used below) is not defined if shift is >= size of
+         /// word
+         w = low;
+      }
+      // Avoid reading the next word, unless it is needed
+      // If the last record finishes on the last bit of input, avoid UMR
+      else if ( bitOffset + bitsPerRecord_ <= RegisterBits )
+      {
+         w = low >> bitOffset;
+      }
+      else
       {
          /// Get upper word (may or may not contain interesting bits),
          RegisterT high = inp[wordPosition + 1];
@@ -743,13 +755,7 @@ size_t BitpackIntegerDecoder<RegisterT>::inputProcessAligned( const char *inbuf,
          /// Shift high to just above the lower bits, shift low LSBit to bit0,
          /// OR together. Note shifts are logical (not arithmetic) because using
          /// unsigned variables.
-         w = ( high << ( 8 * sizeof( RegisterT ) - bitOffset ) ) | ( low >> bitOffset );
-      }
-      else
-      {
-         /// The left shift (used above) is not defined if shift is >= size of
-         /// word
-         w = low;
+         w = ( high << ( RegisterBits - bitOffset ) ) | ( low >> bitOffset );
       }
 
 #ifdef E57_MAX_VERBOSE

src/Decoder.h

@@ -149,6 +149,7 @@ namespace e57
       double offset_;
       unsigned bitsPerRecord_;
       RegisterT destBitMask_;
+      static constexpr size_t RegisterBits = sizeof( RegisterT ) * 8;
    };
 
    class ConstantIntegerDecoder : public Decoder