Compute fingerprint tables at constexpr time

Summary: [Folly] Compute fingerprint tables at constexpr time, v.s. as a separate build step which generates a source file.

Reviewed By: Orvid

Differential Revision: D10081043

fbshipit-source-id: f01f7587aafd29fc1c44515730b759325e1566d8

CMakeLists.txt

@@ -119,8 +119,6 @@ list(REMOVE_ITEM files
   ${FOLLY_DIR}/python/GILAwareManualExecutor.cpp
 )
 list(REMOVE_ITEM hfiles
-  ${FOLLY_DIR}/detail/SlowFingerprint.h
-  ${FOLLY_DIR}/detail/FingerprintPolynomial.h
   ${FOLLY_DIR}/python/fibers.h
   ${FOLLY_DIR}/python/GILAwareManualExecutor.h
 )
@@ -194,9 +192,6 @@ auto_source_group(folly ${FOLLY_DIR} ${files} ${hfiles})
 apply_folly_compile_options_to_target(folly_base)
 # Add the generated files to the correct source group.
 source_group("folly" FILES ${CMAKE_CURRENT_BINARY_DIR}/folly/folly-config.h)
-source_group("folly\\build" FILES
-  ${CMAKE_CURRENT_BINARY_DIR}/folly/build/FingerprintTables.cpp
-)
 
 # Generate pkg-config variables from folly_deps before we add our own
 # build/install-time include directory generator expressions
@@ -219,51 +214,10 @@ target_compile_definitions(folly_base
     $<TARGET_PROPERTY:folly_deps,INTERFACE_COMPILE_DEFINITIONS>
 )
 
-# Now to generate the fingerprint tables
-add_executable(GenerateFingerprintTables
-  ${FOLLY_DIR}/build/GenerateFingerprintTables.cpp
-  $<TARGET_OBJECTS:folly_base>
-)
-target_link_libraries(GenerateFingerprintTables PRIVATE folly_deps)
-apply_folly_compile_options_to_target(GenerateFingerprintTables)
-set_property(TARGET GenerateFingerprintTables PROPERTY FOLDER "Build")
-source_group("" FILES ${FOLLY_DIR}/build/GenerateFingerprintTables.cpp)
-
-# Compile the fingerprint tables.
-add_custom_command(
-  OUTPUT ${CMAKE_CURRENT_BINARY_DIR}/folly/build/FingerprintTables.cpp
-  COMMAND
-    ${CMAKE_COMMAND} -E make_directory ${CMAKE_CURRENT_BINARY_DIR}/folly/build
-  COMMAND
-    GenerateFingerprintTables
-    --install_dir ${CMAKE_CURRENT_BINARY_DIR}/folly/build
-  DEPENDS GenerateFingerprintTables
-  COMMENT "Generating the fingerprint tables..."
-)
-add_library(folly_fingerprint STATIC
-  ${CMAKE_CURRENT_BINARY_DIR}/folly/build/FingerprintTables.cpp
-  ${FOLLY_DIR}/Fingerprint.h
-  ${FOLLY_DIR}/detail/SlowFingerprint.h
-  ${FOLLY_DIR}/detail/FingerprintPolynomial.h
-  $<TARGET_OBJECTS:folly_base>
-)
-target_link_libraries(folly_fingerprint PRIVATE folly_deps)
-apply_folly_compile_options_to_target(folly_fingerprint)
-
-# We want to generate a single library and target for folly, but we needed a
-# two-stage compile for the fingerprint tables, so we create a phony source
-# file that we modify whenever the base libraries change, causing folly to be
-# re-linked, making things happy.
-add_custom_command(OUTPUT ${CMAKE_CURRENT_BINARY_DIR}/folly_dep.cpp
-  COMMAND ${CMAKE_COMMAND} -E touch ${CMAKE_CURRENT_BINARY_DIR}/folly_dep.cpp
-  DEPENDS folly_base folly_fingerprint
-)
 add_library(folly
-  ${CMAKE_CURRENT_BINARY_DIR}/folly_dep.cpp
   $<TARGET_OBJECTS:folly_base>
 )
 apply_folly_compile_options_to_target(folly)
-source_group("" FILES ${CMAKE_CURRENT_BINARY_DIR}/folly_dep.cpp)
 
 target_link_libraries(folly PUBLIC folly_deps)
 
@@ -274,9 +228,6 @@ install(TARGETS folly folly_deps
   ARCHIVE DESTINATION ${LIB_INSTALL_DIR})
 auto_install_files(folly ${FOLLY_DIR}
   ${hfiles}
-  ${FOLLY_DIR}/Fingerprint.h
-  ${FOLLY_DIR}/detail/SlowFingerprint.h
-  ${FOLLY_DIR}/detail/FingerprintPolynomial.h
 )
 install(
   FILES ${CMAKE_CURRENT_BINARY_DIR}/folly/folly-config.h

folly/Fingerprint.cpp

+/*
+ * Copyright 2018-present 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.
+ */
+
+#include <folly/Fingerprint.h>
+
+#include <folly/Utility.h>
+#include <folly/detail/FingerprintPolynomial.h>
+
+namespace folly {
+namespace detail {
+
+namespace {
+
+// The polynomials below were generated by a separate program that requires the
+// NTL (Number Theory Library) from http://www.shoup.net/ntl/
+//
+// Briefly: randomly generate a polynomial of degree D, test for
+// irreducibility, repeat until you find an irreducible polynomial
+// (roughly 1/D of all polynomials of degree D are irreducible, so
+// this will succeed in D/2 tries on average; D is small (64..128) so
+// this simple method works well)
+//
+// DO NOT REPLACE THE POLYNOMIALS USED, EVER, as that would change the value
+// of every single fingerprint in existence.
+template <size_t Deg>
+struct FingerprintTablePoly;
+template <>
+struct FingerprintTablePoly<63> {
+  static constexpr uint64_t data[1] = {0xbf3736b51869e9b7};
+};
+template <>
+struct FingerprintTablePoly<95> {
+  static constexpr uint64_t data[2] = {0x51555cb0aa8d39c3, 0xb679ec3700000000};
+};
+template <>
+struct FingerprintTablePoly<127> {
+  static constexpr uint64_t data[2] = {0xc91bff9b8768b51b, 0x8c5d5853bd77b0d3};
+};
+
+template <typename D, size_t S0, size_t... I0>
+constexpr auto copy_table(D const (&table)[S0], index_sequence<I0...>) {
+  using array = std::array<D, S0>;
+  return array{{table[I0]...}};
+}
+template <typename D, size_t S0>
+constexpr auto copy_table(D const (&table)[S0]) {
+  return copy_table(table, make_index_sequence<S0>{});
+}
+
+template <typename D, size_t S0, size_t S1, size_t... I0>
+constexpr auto copy_table(D const (&table)[S0][S1], index_sequence<I0...>) {
+  using array = std::array<std::array<D, S1>, S0>;
+  return array{{copy_table(table[I0])...}};
+}
+template <typename D, size_t S0, size_t S1>
+constexpr auto copy_table(D const (&table)[S0][S1]) {
+  return copy_table(table, make_index_sequence<S0>{});
+}
+
+template <typename D, size_t S0, size_t S1, size_t S2, size_t... I0>
+constexpr auto copy_table(D const (&table)[S0][S1][S2], index_sequence<I0...>) {
+  using array = std::array<std::array<std::array<D, S2>, S1>, S0>;
+  return array{{copy_table(table[I0])...}};
+}
+template <typename D, size_t S0, size_t S1, size_t S2>
+constexpr auto copy_table(D const (&table)[S0][S1][S2]) {
+  return copy_table(table, make_index_sequence<S0>{});
+}
+
+template <size_t Deg>
+constexpr poly_table<Deg> make_poly_table() {
+  FingerprintPolynomial<Deg> const poly(FingerprintTablePoly<Deg>::data);
+  uint64_t table[8][256][poly_size(Deg)] = {};
+  // table[i][q] is Q(X) * X^(k+8*i) mod P(X),
+  // where k is the number of bits in the fingerprint (and deg(P)) and
+  // Q(X) = q7*X^7 + q6*X^6 + ... + q1*X + q0 is a degree-7 polyonomial
+  // whose coefficients are the bits of q.
+  for (uint16_t x = 0; x < 256; x++) {
+    FingerprintPolynomial<Deg> t;
+    t.setHigh8Bits(uint8_t(x));
+    for (int i = 0; i < 8; i++) {
+      t.mulXkmod(8, poly);
+      for (size_t j = 0; j < poly_size(Deg); ++j) {
+        table[i][x][j] = t.get(j);
+      }
+    }
+  }
+  return copy_table(table);
+}
+
+} // namespace
+
+template <>
+FOLLY_STORAGE_CONSTEXPR const uint64_t
+    FingerprintTable<64>::poly[poly_size(64)] = {
+        FingerprintTablePoly<63>::data[0]};
+template <>
+FOLLY_STORAGE_CONSTEXPR const uint64_t
+    FingerprintTable<96>::poly[poly_size(96)] = {
+        FingerprintTablePoly<95>::data[0],
+        FingerprintTablePoly<95>::data[1]};
+template <>
+FOLLY_STORAGE_CONSTEXPR const uint64_t
+    FingerprintTable<128>::poly[poly_size(128)] = {
+        FingerprintTablePoly<127>::data[0],
+        FingerprintTablePoly<127>::data[1]};
+
+template <>
+FOLLY_STORAGE_CONSTEXPR const poly_table<64> FingerprintTable<64>::table =
+    make_poly_table<63>();
+template <>
+FOLLY_STORAGE_CONSTEXPR const poly_table<96> FingerprintTable<96>::table =
+    make_poly_table<95>();
+template <>
+FOLLY_STORAGE_CONSTEXPR const poly_table<128> FingerprintTable<128>::table =
+    make_poly_table<127>();
+
+} // namespace detail
+} // namespace folly

folly/Fingerprint.h

@@ -28,8 +28,7 @@
  * 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.
+ * The precomputed tables are in Fingerprint.cpp.
  *
  * Benchmarked on 10/13/2009 on a 2.5GHz quad-core Xeon L5420,
  * - Fingerprint<64>::update64() takes about 12ns
@@ -44,6 +43,7 @@
 
 #pragma once
 
+#include <array>
 #include <cstdint>
 
 #include <folly/Range.h>
@@ -52,27 +52,35 @@ namespace folly {
 
 namespace detail {
 
+constexpr size_t poly_size(size_t bits) {
+  return 1 + (bits - 1) / 64;
+}
+
+template <size_t Deg>
+using poly_table =
+    std::array<std::array<std::array<uint64_t, poly_size(Deg)>, 256>, 8>;
+
 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];
+  static const uint64_t poly[poly_size(BITS)];
+  static const poly_table<BITS> table;
 };
 
 template <int BITS>
-const uint64_t FingerprintTable<BITS>::poly[1 + (BITS - 1) / 64] = {};
+const uint64_t FingerprintTable<BITS>::poly[poly_size(BITS)] = {};
 template <int BITS>
-const uint64_t FingerprintTable<BITS>::table[8][256][1 + (BITS - 1) / 64] = {};
+const poly_table<BITS> FingerprintTable<BITS>::table = {};
 
 #ifndef _MSC_VER
 // MSVC 2015 can't handle these extern specialization declarations,
 // but they aren't needed for things to work right, so we just don't
 // declare them in the header for MSVC.
 
-#define FOLLY_DECLARE_FINGERPRINT_TABLES(BITS)                      \
-  template <>                                                       \
-  const uint64_t FingerprintTable<BITS>::poly[1 + (BITS - 1) / 64]; \
-  template <>                                                       \
-  const uint64_t FingerprintTable<BITS>::table[8][256][1 + (BITS - 1) / 64]
+#define FOLLY_DECLARE_FINGERPRINT_TABLES(BITS)                  \
+  template <>                                                   \
+  const uint64_t FingerprintTable<BITS>::poly[poly_size(BITS)]; \
+  template <>                                                   \
+  const poly_table<BITS> FingerprintTable<BITS>::table
 
 FOLLY_DECLARE_FINGERPRINT_TABLES(64);
 FOLLY_DECLARE_FINGERPRINT_TABLES(96);
@@ -142,8 +150,8 @@ class Fingerprint {
   /**
    * Return the number of uint64s needed to hold the fingerprint value.
    */
-  static int size() {
-    return 1 + (BITS - 1) / 64;
+  constexpr static int size() {
+    return detail::poly_size(BITS);
   }
 
   /**
@@ -162,7 +170,7 @@ class Fingerprint {
 
  private:
   // XOR the fingerprint with a value from one of the tables.
-  void xortab(const uint64_t* tab) {
+  void xortab(std::array<uint64_t, detail::poly_size(BITS)> const& tab) {
     for (int i = 0; i < size(); i++) {
       fp_[i] ^= tab[i];
     }
@@ -175,7 +183,7 @@ class Fingerprint {
   uint32_t shlor32(uint32_t v);
   uint64_t shlor64(uint64_t v);
 
-  uint64_t fp_[1 + (BITS - 1) / 64];
+  uint64_t fp_[detail::poly_size(BITS)];
 };
 
 // Convenience functions

folly/build/GenerateFingerprintTables.cpp

-/*
- * Copyright 2012-present 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.
- */
-
-#ifndef __STDC_FORMAT_MACROS
-#define __STDC_FORMAT_MACROS 1
-#endif
-
-#include <cinttypes>
-#include <cstdio>
-
-#include <string>
-
-#include <glog/logging.h>
-
-#include <folly/Portability.h>
-#include <folly/portability/GFlags.h>
-
-#include <folly/detail/FingerprintPolynomial.h>
-
-using namespace folly;
-using namespace folly::detail;
-
-// The defaults were generated by a separate program that requires the
-// NTL (Number Theory Library) from http://www.shoup.net/ntl/
-//
-// Briefly: randomly generate a polynomial of degree D, test for
-// irreducibility, repeat until you find an irreducible polynomial
-// (roughly 1/D of all polynomials of degree D are irreducible, so
-// this will succeed in D/2 tries on average; D is small (64..128) so
-// this simple method works well)
-//
-// DO NOT REPLACE THE POLYNOMIALS USED, EVER, as that would change the value
-// of every single fingerprint in existence.
-DEFINE_int64(
-    poly64,
-    0xbf3736b51869e9b7,
-    "Generate 64-bit tables using this polynomial");
-DEFINE_int64(
-    poly96_m,
-    0x51555cb0aa8d39c3,
-    "Generate 96-bit tables using this polynomial "
-    "(most significant 64 bits)");
-DEFINE_int32(
-    poly96_l,
-    0xb679ec37,
-    "Generate 96-bit tables using this polynomial "
-    "(least significant 32 bits)");
-DEFINE_int64(
-    poly128_m,
-    0xc91bff9b8768b51b,
-    "Generate 128-bit tables using this polynomial "
-    "(most significant 64 bits)");
-DEFINE_int64(
-    poly128_l,
-    0x8c5d5853bd77b0d3,
-    "Generate 128-bit tables using this polynomial "
-    "(least significant 64 bits)");
-DEFINE_string(install_dir, ".", "Direectory to place output files in");
-DEFINE_string(fbcode_dir, "", "fbcode directory (ignored)");
-
-namespace {
-
-template <int DEG>
-void computeTables(FILE* file, const FingerprintPolynomial<DEG>& poly) {
-  uint64_t table[8][256][FingerprintPolynomial<DEG>::size()];
-  // table[i][q] is Q(X) * X^(k+8*i) mod P(X),
-  // where k is the number of bits in the fingerprint (and deg(P)) and
-  // Q(X) = q7*X^7 + q6*X^6 + ... + q1*X + q0 is a degree-7 polyonomial
-  // whose coefficients are the bits of q.
-  for (uint16_t x = 0; x < 256; x++) {
-    FingerprintPolynomial<DEG> t;
-    t.setHigh8Bits(uint8_t(x));
-    for (int i = 0; i < 8; i++) {
-      t.mulXkmod(8, poly);
-      t.write(&(table[i][x][0]));
-    }
-  }
-
-  // Write the actual polynomial used; this isn't needed during fast
-  // fingerprint calculation, but it's useful for reference and unittesting.
-  uint64_t poly_val[FingerprintPolynomial<DEG>::size()];
-  poly.write(poly_val);
-  CHECK_ERR(fprintf(
-      file,
-      "template <>\n"
-      "const uint64_t FingerprintTable<%d>::poly[%d] = {",
-      DEG + 1,
-      FingerprintPolynomial<DEG>::size()));
-  for (int j = 0; j < FingerprintPolynomial<DEG>::size(); j++) {
-    CHECK_ERR(fprintf(file, "%s%" PRIu64 "LLU", j ? ", " : "", poly_val[j]));
-  }
-  CHECK_ERR(fprintf(file, "};\n\n"));
-
-  // Write the tables.
-  CHECK_ERR(fprintf(
-      file,
-      "template <>\n"
-      "const uint64_t FingerprintTable<%d>::table[8][256][%d] = {\n",
-      DEG + 1,
-      FingerprintPolynomial<DEG>::size()));
-  for (int i = 0; i < 8; i++) {
-    CHECK_ERR(fprintf(
-        file,
-        "  // Table %d"
-        "\n"
-        "  {\n",
-        i));
-    for (int x = 0; x < 256; x++) {
-      CHECK_ERR(fprintf(file, "    {"));
-      for (int j = 0; j < FingerprintPolynomial<DEG>::size(); j++) {
-        CHECK_ERR(
-            fprintf(file, "%s%" PRIu64 "LLU", (j ? ", " : ""), table[i][x][j]));
-      }
-      CHECK_ERR(fprintf(file, "},\n"));
-    }
-    CHECK_ERR(fprintf(file, "  },\n"));
-  }
-  CHECK_ERR(fprintf(file, "\n};\n\n"));
-}
-
-} // namespace
-
-int main(int argc, char* argv[]) {
-  gflags::ParseCommandLineFlags(&argc, &argv, true);
-  google::InitGoogleLogging(argv[0]);
-
-  std::string name = FLAGS_install_dir + "/" + "FingerprintTables.cpp";
-  FILE* file = fopen(name.c_str(), "w");
-  PCHECK(file);
-
-  CHECK_ERR(fprintf(
-      file,
-      "/**\n"
-      " * Fingerprint tables for 64-, 96-, and 128-bit Rabin fingerprints.\n"
-      " *\n"
-      " * AUTOMATICALLY GENERATED.  DO NOT EDIT.\n"
-      " */\n"
-      "\n"
-      "#include <folly/Fingerprint.h>\n"
-      "\n"
-      "namespace folly {\n"
-      "namespace detail {\n"
-      "\n"));
-
-  FingerprintPolynomial<63> poly64((const uint64_t*)&FLAGS_poly64);
-  computeTables(file, poly64);
-
-  uint64_t poly96_val[2];
-  poly96_val[0] = (uint64_t)FLAGS_poly96_m;
-  poly96_val[1] = (uint64_t)FLAGS_poly96_l << 32;
-  FingerprintPolynomial<95> poly96(poly96_val);
-  computeTables(file, poly96);
-
-  uint64_t poly128_val[2];
-  poly128_val[0] = (uint64_t)FLAGS_poly128_m;
-  poly128_val[1] = (uint64_t)FLAGS_poly128_l;
-  FingerprintPolynomial<127> poly128(poly128_val);
-  computeTables(file, poly128);
-
-  CHECK_ERR(fprintf(
-      file,
-      "}  // namespace detail\n"
-      "}  // namespace folly\n"));
-  CHECK_ERR(fclose(file));
-
-  return 0;
-}

folly/detail/FingerprintPolynomial.h

@@ -18,8 +18,6 @@
 
 #include <cstdint>
 
-#include <glog/logging.h>
-
 namespace folly {
 namespace detail {
 
@@ -33,33 +31,30 @@ namespace detail {
 template <int DEG>
 class FingerprintPolynomial {
  public:
-  FingerprintPolynomial() {
-    for (int i = 0; i < size(); i++) {
-      val_[i] = 0;
-    }
+  static constexpr int size() {
+    return 1 + DEG / 64;
   }
 
-  explicit FingerprintPolynomial(const uint64_t* vals) {
+  constexpr FingerprintPolynomial() {}
+
+  constexpr explicit FingerprintPolynomial(const uint64_t (&vals)[size()]) {
     for (int i = 0; i < size(); i++) {
       val_[i] = vals[i];
     }
   }
 
-  void write(uint64_t* out) const {
-    for (int i = 0; i < size(); i++) {
-      out[i] = val_[i];
-    }
+  constexpr uint64_t get(size_t i) const {
+    return val_[i];
   }
 
-  void add(const FingerprintPolynomial<DEG>& other) {
+  constexpr void add(const FingerprintPolynomial<DEG>& other) {
     for (int i = 0; i < size(); i++) {
       val_[i] ^= other.val_[i];
     }
   }
 
   // Multiply by X.  The actual degree must be < DEG.
-  void mulX() {
-    CHECK_EQ(0u, val_[0] & (1ULL << 63));
+  constexpr void mulX() {
     uint64_t b = 0;
     for (int i = size() - 1; i >= 0; i--) {
       uint64_t nb = val_[i] >> 63;
@@ -92,7 +87,7 @@ class FingerprintPolynomial {
   // So A(X) * X mod P(X) is:
   //   the binary representation of A, left shift by 1,
   //   XOR p if a_(k-1) == 1
-  void mulXmod(const FingerprintPolynomial<DEG>& p) {
+  constexpr void mulXmod(const FingerprintPolynomial<DEG>& p) {
     bool needXOR = (val_[0] & (1ULL << 63));
     val_[0] &= ~(1ULL << 63);
     mulX();
@@ -102,16 +97,14 @@ class FingerprintPolynomial {
   }
 
   // Compute (this * X^k) mod P(X) by repeatedly multiplying by X (see above)
-  void mulXkmod(int k, const FingerprintPolynomial<DEG>& p) {
+  constexpr void mulXkmod(int k, const FingerprintPolynomial<DEG>& p) {
     for (int i = 0; i < k; i++) {
       mulXmod(p);
     }
   }
 
   // add X^k, where k <= DEG
-  void addXk(int k) {
-    DCHECK_GE(k, 0);
-    DCHECK_LE(k, DEG);
+  constexpr void addXk(int k) {
     int word_offset = (DEG - k) / 64;
     int bit_offset = 63 - (DEG - k) % 64;
     val_[word_offset] ^= (1ULL << bit_offset);
@@ -120,17 +113,13 @@ class FingerprintPolynomial {
   // Set the highest 8 bits to val.
   // If val is interpreted as polynomial of degree 7, then this sets *this
   // to val * X^(DEG-7)
-  void setHigh8Bits(uint8_t val) {
+  constexpr void setHigh8Bits(uint8_t val) {
     val_[0] = ((uint64_t)val) << (64 - 8);
     for (int i = 1; i < size(); i++) {
       val_[i] = 0;
     }
   }
 
-  static constexpr int size() {
-    return 1 + DEG / 64;
-  }
-
  private:
   // Internal representation: big endian
   // val_[0] contains the highest order coefficients, with bit 63 as the
@@ -138,7 +127,7 @@ class FingerprintPolynomial {
   //
   // If DEG+1 is not a multiple of 64,  val_[size()-1] only uses the highest
   // order (DEG+1)%64 bits (the others are always 0)
-  uint64_t val_[1 + DEG / 64];
+  uint64_t val_[size()] = {};
 };
 
 } // namespace detail

folly/detail/SlowFingerprint.h

@@ -61,7 +61,9 @@ class SlowFingerprint {
   }
 
   void write(uint64_t* out) const {
-    fp_.write(out);
+    for (int i = 0; i < fp_.size(); ++i) {
+      out[i] = fp_.get(i);
+    }
   }
 
  private: