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Move Rabin fingerprinting code to folly.

Summary:
Also generate fingerprint tables every time, so the code doesn't rot.

TODO(tudorb): move benchmark to folly
TODO(tudorb): Include the program used to generate the polynomials
(can't build as it requires NTL from http://www.shoup.net/ntl/)

Test Plan: folly/test

Reviewed By: andrei.alexandrescu@fb.com

FB internal diff: D492455
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1 parent e81459f commit 987da6d2fb6598111f3e1ee3a0f40f35104e6fce @tudor tudor committed Jun 12, 2012
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+/*
+ * Copyright 2012 Facebook, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+/**
+ * Compute 64-, 96-, and 128-bit Rabin fingerprints, as described in
+ * Michael O. Rabin (1981)
+ * Fingerprinting by Random Polynomials
+ * Center for Research in Computing Technology, Harvard University
+ * Tech Report TR-CSE-03-01
+ *
+ * The implementation follows the optimization described in
+ * Andrei Z. Broder (1993)
+ * Some applications of Rabin's fingerprinting method
+ *
+ * extended for fingerprints larger than 64 bits, and modified to use
+ * 64-bit instead of 32-bit integers for computation.
+ *
+ * The precomputed tables are in FingerprintTable.cpp, which is automatically
+ * generated by ComputeFingerprintTable.cpp.
+ *
+ * Benchmarked on 10/13/2009 on a 2.5GHz quad-core Xeon L5420,
+ * - Fingerprint<64>::update64() takes about 12ns
+ * - Fingerprint<96>::update64() takes about 30ns
+ * - Fingerprint<128>::update128() takes about 30ns
+ * (unsurprisingly, Fingerprint<96> and Fingerprint<128> take the
+ * same amount of time, as they both use 128-bit operations; the least
+ * significant 32 bits of Fingerprint<96> will always be 0)
+ *
+ * @author Tudor Bosman (tudorb@facebook.com)
+ */
+
+#ifndef FOLLY_FINGERPRINT_H_
+#define FOLLY_FINGERPRINT_H_
+
+#include <cstdint>
+
+#include "folly/Range.h"
+
+namespace folly {
+
+namespace detail {
+template <int BITS>
+struct FingerprintTable {
+ static const uint64_t poly[1 + (BITS-1)/64];
+ static const uint64_t table[8][256][1 + (BITS-1)/64];
+};
+} // namespace detail
+
+/**
+ * Compute the Rabin fingerprint.
+ *
+ * TODO(tudorb): Extend this to allow removing values from the computed
+ * fingerprint (so we can fingerprint a sliding window, as in the Rabin-Karp
+ * string matching algorithm)
+ *
+ * update* methods return *this, so you can chain them together:
+ * Fingerprint<96>().update8(x).update(str).update64(val).write(output);
+ */
+template <int BITS>
+class Fingerprint {
+ public:
+ Fingerprint() {
+ // Use a non-zero starting value. We'll use (1 << (BITS-1))
+ fp_[0] = 1UL << 63;
+ for (int i = 1; i < size(); i++)
+ fp_[i] = 0;
+ }
+
+ Fingerprint& update8(uint8_t v) {
+ uint8_t out = shlor8(v);
+ xortab(detail::FingerprintTable<BITS>::table[0][out]);
+ return *this;
+ }
+
+ // update32 and update64 are convenience functions to update the fingerprint
+ // with 4 and 8 bytes at a time. They are faster than calling update8
+ // in a loop. They process the bytes in big-endian order.
+ Fingerprint& update32(uint32_t v) {
+ uint32_t out = shlor32(v);
+ for (int i = 0; i < 4; i++) {
+ xortab(detail::FingerprintTable<BITS>::table[i][out&0xff]);
+ out >>= 8;
+ }
+ return *this;
+ }
+
+ Fingerprint& update64(uint64_t v) {
+ uint64_t out = shlor64(v);
+ for (int i = 0; i < 8; i++) {
+ xortab(detail::FingerprintTable<BITS>::table[i][out&0xff]);
+ out >>= 8;
+ }
+ return *this;
+ }
+
+ Fingerprint& update(StringPiece str) {
+ // TODO(tudorb): We could be smart and do update64 or update32 if aligned
+ for (auto c : str) {
+ update8(uint8_t(c));
+ }
+ return *this;
+ }
+
+ /**
+ * Return the number of uint64s needed to hold the fingerprint value.
+ */
+ static int size() {
+ return 1 + (BITS-1)/64;
+ }
+
+ /**
+ * Write the computed fingeprint to an array of size() uint64_t's.
+ * For Fingerprint<64>, size()==1; we write 64 bits in out[0]
+ * For Fingerprint<96>, size()==2; we write 64 bits in out[0] and
+ * the most significant 32 bits of out[1]
+ * For Fingerprint<128>, size()==2; we write 64 bits in out[0] and
+ * 64 bits in out[1].
+ */
+ void write(uint64_t* out) const {
+ for (int i = 0; i < size(); i++) {
+ out[i] = fp_[i];
+ }
+ }
+
+ private:
+ // XOR the fingerprint with a value from one of the tables.
+ void xortab(const uint64_t* tab) {
+ for (int i = 0; i < size(); i++) {
+ fp_[i] ^= tab[i];
+ }
+ }
+
+ // Helper functions: shift the fingerprint value left by 8/32/64 bits,
+ // return the "out" value (the bits that were shifted out), and add "v"
+ // in the bits on the right.
+ uint8_t shlor8(uint8_t v);
+ uint32_t shlor32(uint32_t v);
+ uint64_t shlor64(uint64_t v);
+
+ uint64_t fp_[1 + (BITS-1)/64];
+};
+
+// Convenience functions
+
+/**
+ * Return the 64-bit Rabin fingerprint of a string.
+ */
+inline uint64_t fingerprint64(StringPiece str) {
+ uint64_t fp;
+ Fingerprint<64>().update(str).write(&fp);
+ return fp;
+}
+
+/**
+ * Compute the 96-bit Rabin fingerprint of a string.
+ * Return the 64 most significant bits in *msb, and the 32 least significant
+ * bits in *lsb.
+ */
+inline void fingerprint96(StringPiece str,
+ uint64_t* msb, uint32_t* lsb) {
+ uint64_t fp[2];
+ Fingerprint<96>().update(str).write(fp);
+ *msb = fp[0];
+ *lsb = (uint32_t)(fp[1] >> 32);
+}
+
+/**
+ * Compute the 128-bit Rabin fingerprint of a string.
+ * Return the 64 most significant bits in *msb, and the 64 least significant
+ * bits in *lsb.
+ */
+inline void fingerprint128(StringPiece str,
+ uint64_t* msb, uint64_t* lsb) {
+ uint64_t fp[2];
+ Fingerprint<128>().update(str).write(fp);
+ *msb = fp[0];
+ *lsb = fp[1];
+}
+
+
+template <>
+inline uint8_t Fingerprint<64>::shlor8(uint8_t v) {
+ uint8_t out = (uint8_t)(fp_[0] >> 56);
+ fp_[0] = (fp_[0] << 8) | ((uint64_t)v);
+ return out;
+}
+
+template <>
+inline uint32_t Fingerprint<64>::shlor32(uint32_t v) {
+ uint32_t out = (uint32_t)(fp_[0] >> 32);
+ fp_[0] = (fp_[0] << 32) | ((uint64_t)v);
+ return out;
+}
+
+template <>
+inline uint64_t Fingerprint<64>::shlor64(uint64_t v) {
+ uint64_t out = fp_[0];
+ fp_[0] = v;
+ return out;
+}
+
+template <>
+inline uint8_t Fingerprint<96>::shlor8(uint8_t v) {
+ uint8_t out = (uint8_t)(fp_[0] >> 56);
+ fp_[0] = (fp_[0] << 8) | (fp_[1] >> 56);
+ fp_[1] = (fp_[1] << 8) | ((uint64_t)v << 32);
+ return out;
+}
+
+template <>
+inline uint32_t Fingerprint<96>::shlor32(uint32_t v) {
+ uint32_t out = (uint32_t)(fp_[0] >> 32);
+ fp_[0] = (fp_[0] << 32) | (fp_[1] >> 32);
+ fp_[1] = ((uint64_t)v << 32);
+ return out;
+}
+
+template <>
+inline uint64_t Fingerprint<96>::shlor64(uint64_t v) {
+ uint64_t out = fp_[0];
+ fp_[0] = fp_[1] | (v >> 32);
+ fp_[1] = v << 32;
+ return out;
+}
+
+template <>
+inline uint8_t Fingerprint<128>::shlor8(uint8_t v) {
+ uint8_t out = (uint8_t)(fp_[0] >> 56);
+ fp_[0] = (fp_[0] << 8) | (fp_[1] >> 56);
+ fp_[1] = (fp_[1] << 8) | ((uint64_t)v);
+ return out;
+}
+
+template <>
+inline uint32_t Fingerprint<128>::shlor32(uint32_t v) {
+ uint32_t out = (uint32_t)(fp_[0] >> 32);
+ fp_[0] = (fp_[0] << 32) | (fp_[1] >> 32);
+ fp_[1] = (fp_[1] << 32) | ((uint64_t)v);
+ return out;
+}
+
+template <>
+inline uint64_t Fingerprint<128>::shlor64(uint64_t v) {
+ uint64_t out = fp_[0];
+ fp_[0] = fp_[1];
+ fp_[1] = v;
+ return out;
+}
+
+} // namespace folly
+
+#endif /* FOLLY_FINGERPRINT_H_ */
+
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