version 3.0

xcodec/xcodec_hash.h

@@ -0,0 +1,177 @@
+/*
+ * Copyright (c) 2008-2012 Juli Mallett. All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY AUTHOR AND CONTRIBUTORS ``AS IS'' AND
+ * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED.  IN NO EVENT SHALL AUTHOR OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
+ * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+ * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+ * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+ * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+ * SUCH DAMAGE.
+ */
+
+#ifndef	XCODEC_XCODEC_HASH_H
+#define	XCODEC_XCODEC_HASH_H
+
+#include <strings.h>
+
+class XCodecHash {
+	struct RollingHash {
+		uint32_t sum1_;					/* Really <16-bit.  */
+		uint32_t sum2_;					/* Really <32-bit.  */
+		uint32_t buffer_[XCODEC_SEGMENT_LENGTH];	/* Really >8-bit.  */
+
+		RollingHash(void)
+		: sum1_(0),
+		  sum2_(0),
+		  buffer_()
+		{ }
+
+		void add(uint32_t ch, unsigned start)
+		{
+			buffer_[start] = ch;
+
+			sum1_ += ch;
+			sum2_ += sum1_;
+		}
+
+		void reset(void)
+		{
+			sum1_ = 0;
+			sum2_ = 0;
+		}
+
+		void roll(uint32_t ch, unsigned start)
+		{
+			uint32_t dead;
+
+			dead = buffer_[start];
+
+			sum1_ -= dead;
+			sum2_ -= dead * XCODEC_SEGMENT_LENGTH;
+
+			buffer_[start] = ch;
+
+			sum1_ += ch;
+			sum2_ += sum1_;
+		}
+	};
+
+	RollingHash bytes_;
+	RollingHash bits_;
+	unsigned start_;
+#ifndef NDEBUG
+	unsigned length_;
+#endif
+
+public:
+	XCodecHash(void)
+	: bytes_(),
+	  bits_(),
+	  start_(0)
+#ifndef NDEBUG
+	, length_(0)
+#endif
+	{ }
+
+	~XCodecHash()
+	{ }
+
+	void add(uint8_t ch)
+	{
+		unsigned bit = ffs(ch);
+		unsigned word = (unsigned)ch + 1;
+
+#ifndef NDEBUG
+		ASSERT("/xcodec/hash", length_ < XCODEC_SEGMENT_LENGTH);
+#endif
+
+		bytes_.add(word, start_);
+		bits_.add(bit, start_);
+
+#ifndef NDEBUG
+		length_++;
+#endif
+		start_ = (start_ + 1) % XCODEC_SEGMENT_LENGTH;
+	}
+
+	void reset(void)
+	{
+		bytes_.reset();
+		bits_.reset();
+
+#ifndef NDEBUG
+		length_ = 0;
+#endif
+		start_ = 0;
+	}
+
+	void roll(uint8_t ch)
+	{
+		unsigned bit = ffs(ch);
+		unsigned word = (unsigned)ch + 1;
+
+#ifndef NDEBUG
+		ASSERT("/xcodec/hash", length_ == XCODEC_SEGMENT_LENGTH);
+#endif
+
+		bytes_.roll(word, start_);
+		bits_.roll(bit, start_);
+
+		start_ = (start_ + 1) % XCODEC_SEGMENT_LENGTH;
+	}
+
+	/*
+	 * XXX
+	 * Need to write a compression function for this; get rid of the
+	 * completely non-entropic bits, anyway, and try to mix the others.
+	 *
+	 * Need to look at what bits can even possibly be set in sum2_,
+	 * looking at all possible ranges resulting from:
+	 * 	128*data[0] + 127*data[1] + ... 1*data[127]
+	 *
+	 * It seems like since each data[] must have at least 1 bit set,
+	 * there are a great many impossible values, and there is a large
+	 * minimal value.  Should be easy to compress, and would rather
+	 * have the extra bits changing from the normalized (i.e. non-zero)
+	 * input and have to compress than have no bits changing...but
+	 * perhaps those are extensionally-equivalent and that's just
+	 * hocus pocus computer science.  Need to think more clearly and
+	 * fully about it.
+	 */
+	uint64_t mix(void) const
+	{
+#ifndef NDEBUG
+		ASSERT("/xcodec/hash", length_ == XCODEC_SEGMENT_LENGTH);
+#endif
+
+		uint64_t bits_hash = (bits_.sum1_ << 16) + bits_.sum2_;
+		uint64_t bytes_hash = (bytes_.sum1_ << 20) + bytes_.sum2_;
+		return ((bits_hash << 36) + bytes_hash);
+	}
+
+	static uint64_t hash(const uint8_t *data)
+	{
+		XCodecHash xchash;
+		unsigned i;
+
+		for (i = 0; i < XCODEC_SEGMENT_LENGTH; i++)
+			xchash.add(*data++);
+		return (xchash.mix());
+	}
+};
+
+#endif /* !XCODEC_XCODEC_HASH_H */