Update the bitextractor to deal with reads other than len=8

He can still deal with READLEN=8 too -- it's easier to test this way.

src/lib/image_hash/bit_extractor.h

@@ -3,12 +3,15 @@
 
 #include <iostream>
 
-template<typename C, size_t N>
+template<typename C, size_t N, size_t READLEN=8>
 class bit_extractor
 {
+protected:
+	static constexpr uint64_t BITMASK = (1 << READLEN) - 1; // e.g. 0xFF, 0x1F, 0xF
+
 public:
 	bit_extractor(const C& bits)
-	    : _bits(bits)
+		: _bits(bits)
 	{}
 
 	template<typename... T>
@@ -22,7 +25,7 @@ class bit_extractor
 	{
 		constexpr auto byte_offset = sizeof...(T);
 
-		uint64_t total = ((uint64_t)(_bits >> (N - bit_offset - 8)) & 0xFF) << (byte_offset << 3); // if byte_offset is 1, we want to shift 8. if 2, 16....
+		uint64_t total = ((uint64_t)(_bits >> (N - bit_offset - READLEN)) & BITMASK) << (byte_offset * READLEN); // if byte_offset is 1, we want to shift 5. if 2, 10....
 		return total | extract(t...);
 	}
 

src/lib/image_hash/test/bitExtractorTest.cpp

@@ -12,17 +12,31 @@
 
 using std::string;
 
-TEST_CASE( "bitExtractorTest/testDefault", "[unit]" )
+TEST_CASE( "bitExtractorTest/testDefault.4", "[unit]" )
 {
-	intx::uint128 bits = 0x11d07e1f;
-	bit_extractor<intx::uint128, 32> be(bits);
+	uint64_t bits = 0x11d07e1f;
+	bit_extractor<uint64_t, 32, 4> be(bits);
+
+	// we're just reading 4 bits at a time.
+	assertEquals( 0x1d, be.extract(0, 8) );    // 0001 1101
+	assertEquals( 0x71, be.extract(16, 24) );  // 0x7 0x1
+	assertEquals( 0x1e, be.extract(0, 20) );
+	assertEquals( 0x0f, be.extract(12, 28) );
+	assertEquals( 0x1d, be.extract(24, 8) );
+}
+
+TEST_CASE( "bitExtractorTest/testDefault.8", "[unit]" )
+{
+	uint64_t bits = 0x11d07e1f;
+	bit_extractor<uint64_t, 32, 8> be(bits);
 
 	assertEquals( 0x11d0, be.extract(0, 8) );
 	assertEquals( 0x7e1f, be.extract(16, 24) );
 	assertEquals( 0x111f, be.extract(0, 24) );
 	assertEquals( 0x1fd0, be.extract(24, 8) );
 }
 
+
 TEST_CASE( "bitExtractorTest/testLargerValue.1", "[unit]" )
 {
 	intx::uint128 bits(0);