chore: introduce huffman encoder class

Move the code in debugcmd.cc into HuffmanEncoder.

One of the building blocks for #4883

Signed-off-by: Roman Gershman <roman@dragonflydb.io>

src/core/CMakeLists.txt

@@ -21,7 +21,7 @@ add_subdirectory(json)
 set(SEARCH_LIB query_parser)
 
 add_library(dfly_core allocation_tracker.cc bloom.cc compact_object.cc dense_set.cc
-    dragonfly_core.cc extent_tree.cc
+    dragonfly_core.cc extent_tree.cc huff_coder.cc
     interpreter.cc glob_matcher.cc mi_memory_resource.cc qlist.cc sds_utils.cc
     segment_allocator.cc score_map.cc small_string.cc sorted_map.cc task_queue.cc
     tx_queue.cc string_set.cc string_map.cc top_keys.cc detail/bitpacking.cc)

src/core/dfly_core_test.cc

@@ -23,6 +23,7 @@
 #include "base/gtest.h"
 #include "base/logging.h"
 #include "core/glob_matcher.h"
+#include "core/huff_coder.h"
 #include "core/intent_lock.h"
 #include "core/tx_queue.h"
 
@@ -188,6 +189,43 @@ TEST_F(StringMatchTest, Special) {
   EXPECT_TRUE(MatchLen("foo\\", "foo\\", 0));
 }
 
+class HuffCoderTest : public ::testing::Test {
+ protected:
+  HuffmanEncoder encoder_;
+  string error_msg_;
+  const string_view good_table_{
+      "\x1b\x10\xd8\n\n\x19\xc6\x0c\xc3\x30\x0c\x43\x1e\x93\xe4\x11roB\xf6\xde\xbb\x18V\xc2Zk\x03"sv};
+};
+
+TEST_F(HuffCoderTest, Load) {
+  string data("bad");
+
+  ASSERT_FALSE(encoder_.Load(data, &error_msg_));
+
+  data = good_table_;
+  ASSERT_TRUE(encoder_.Load(data, &error_msg_)) << error_msg_;
+
+  data.append("foo");
+  encoder_.Reset();
+  ASSERT_FALSE(encoder_.Load(data, &error_msg_));
+}
+
+TEST_F(HuffCoderTest, Encode) {
+  ASSERT_TRUE(encoder_.Load(good_table_, &error_msg_)) << error_msg_;
+
+  EXPECT_EQ(1, encoder_.BitCount('x'));
+  EXPECT_EQ(3, encoder_.BitCount(':'));
+  EXPECT_EQ(5, encoder_.BitCount('2'));
+  EXPECT_EQ(5, encoder_.BitCount('3'));
+
+  string data("x:23xx");
+
+  uint8_t dest[100];
+  uint32_t dest_size = sizeof(dest);
+  ASSERT_TRUE(encoder_.Encode(data, dest, &dest_size, &error_msg_));
+  ASSERT_EQ(3, dest_size);
+}
+
 using benchmark::DoNotOptimize;
 
 // Parse Double benchmarks

src/core/huff_coder.cc

@@ -0,0 +1,110 @@
+// Copyright 2025, DragonflyDB authors.  All rights reserved.
+// See LICENSE for licensing terms.
+//
+
+#include "core/huff_coder.h"
+
+#include "base/logging.h"
+
+extern "C" {
+#include "huff/huf.h"
+}
+
+using namespace std;
+
+namespace dfly {
+
+constexpr size_t kWspSize = HUF_CTABLE_WORKSPACE_SIZE;
+
+bool HuffmanEncoder::Load(std::string_view binary_data, std::string* error_msg) {
+  CHECK(!huf_ctable_);
+
+  huf_ctable_.reset(new HUF_CElt[HUF_CTABLE_SIZE_ST(255)]);
+  table_max_symbol_ = 255;
+
+  unsigned has_zero_weights = 0;
+  size_t read_size = HUF_readCTable(huf_ctable_.get(), &table_max_symbol_, binary_data.data(),
+                                    binary_data.size(), &has_zero_weights);
+
+  if (HUF_isError(read_size)) {
+    huf_ctable_.reset();
+    *error_msg = HUF_getErrorName(read_size);
+    return false;
+  }
+  if (read_size != binary_data.size()) {
+    *error_msg = "Corrupted data";
+    huf_ctable_.reset();
+    return false;
+  }
+
+  return true;
+}
+
+bool HuffmanEncoder::Build(const unsigned hist[], unsigned max_symbol, std::string* error_msg) {
+  CHECK(!huf_ctable_);
+  huf_ctable_.reset(new HUF_CElt[HUF_CTABLE_SIZE_ST(max_symbol)]);
+
+  unique_ptr<uint32_t[]> wrkspace(new uint32_t[HUF_CTABLE_WORKSPACE_SIZE_U32]);
+
+  size_t num_bits =
+      HUF_buildCTable_wksp(huf_ctable_.get(), hist, max_symbol, 0, wrkspace.get(), kWspSize);
+  if (HUF_isError(num_bits)) {
+    *error_msg = HUF_getErrorName(num_bits);
+    huf_ctable_.reset();
+    return false;
+  }
+  num_bits_ = static_cast<uint8_t>(num_bits);
+  table_max_symbol_ = max_symbol;
+  return true;
+}
+
+void HuffmanEncoder::Reset() {
+  huf_ctable_.reset();
+  table_max_symbol_ = 0;
+}
+
+bool HuffmanEncoder::Encode(std::string_view data, uint8_t* dest, uint32_t* dest_size,
+                            std::string* error_msg) const {
+  DCHECK(huf_ctable_);
+
+  size_t res =
+      HUF_compress1X_usingCTable(dest, *dest_size, data.data(), data.size(), huf_ctable_.get(), 0);
+
+  if (HUF_isError(res)) {
+    *error_msg = HUF_getErrorName(res);
+    return false;
+  }
+  *dest_size = static_cast<uint32_t>(res);
+  return true;
+}
+
+unsigned HuffmanEncoder::BitCount(uint8_t symbol) const {
+  DCHECK(huf_ctable_);
+  return HUF_getNbBitsFromCTable(huf_ctable_.get(), symbol);
+}
+
+size_t HuffmanEncoder::EstimateCompressedSize(const unsigned hist[], unsigned max_symbol) const {
+  DCHECK(huf_ctable_);
+  size_t res = HUF_estimateCompressedSize(huf_ctable_.get(), hist, max_symbol);
+  return res;
+}
+
+string HuffmanEncoder::Export() const {
+  DCHECK(huf_ctable_);
+
+  // Reverse engineered: (maxSymbolValue + 1) / 2 + 1.
+  constexpr unsigned kMaxTableSize = 130;
+  string res;
+  res.resize(kMaxTableSize);
+
+  unique_ptr<uint32_t[]> wrkspace(new uint32_t[HUF_CTABLE_WORKSPACE_SIZE_U32]);
+
+  // Seems we can reuse the same workspace, its capacity is enough.
+  size_t size = HUF_writeCTable_wksp(res.data(), res.size(), huf_ctable_.get(), table_max_symbol_,
+                                     num_bits_, wrkspace.get(), kWspSize);
+  CHECK(!HUF_isError(size));
+  res.resize(size);
+  return res;
+}
+
+}  // namespace dfly

src/core/huff_coder.h

@@ -0,0 +1,50 @@
+// Copyright 2025, DragonflyDB authors.  All rights reserved.
+// See LICENSE for licensing terms.
+//
+
+#pragma once
+
+#include <cstdint>
+#include <memory>
+#include <string_view>
+
+namespace dfly {
+
+class HuffmanEncoder {
+ public:
+  bool Build(const unsigned hist[], unsigned max_symbol, std::string* error_msg);
+
+  bool Encode(std::string_view data, uint8_t* dest, uint32_t* dest_size,
+              std::string* error_msg) const;
+  unsigned BitCount(uint8_t symbol) const;
+
+  size_t EstimateCompressedSize(const unsigned hist[], unsigned max_symbol) const;
+
+  void Reset();
+
+  // Load using the serialized data produced by Export().
+  bool Load(std::string_view binary_data, std::string* error_msg);
+
+  // Exports a binary representation of the table, that can be loaded using Load().
+  std::string Export() const;
+
+  uint8_t num_bits() const {
+    return num_bits_;
+  }
+
+  bool valid() const {
+    return bool(huf_ctable_);
+  }
+
+  unsigned max_symbol() const {
+    return table_max_symbol_;
+  }
+
+ private:
+  using HUF_CElt = size_t;
+  std::unique_ptr<HUF_CElt[]> huf_ctable_;
+  unsigned table_max_symbol_ = 0;
+  uint8_t num_bits_ = 0;
+};
+
+}  // namespace dfly

src/server/debugcmd.cc

@@ -13,6 +13,7 @@ extern "C" {
 
 #include <absl/cleanup/cleanup.h>
 #include <absl/random/random.h>
+#include <absl/strings/escaping.h>
 #include <absl/strings/match.h>
 #include <absl/strings/str_cat.h>
 #include <lz4.h>
@@ -24,6 +25,7 @@ extern "C" {
 #include "base/flags.h"
 #include "base/logging.h"
 #include "core/compact_object.h"
+#include "core/huff_coder.h"
 #include "core/qlist.h"
 #include "core/sorted_map.h"
 #include "core/string_map.h"
@@ -1318,71 +1320,47 @@ void DebugCmd::Compression(CmdArgList args, facade::SinkReplyBuilder* builder) {
   });
 
   size_t num_bits = 0, compressed_size = 0, raw_size = 0;
-  unsigned table_max_symbol = 255;
 
   if (hist.max_symbol) {
-    HUF_CREATE_STATIC_CTABLE(huf_ctable, HufHist::kMaxSymbol);
+    HuffmanEncoder huff_enc;
+    string err_msg;
 
-    unique_ptr<uint32_t[]> wrkspace(new uint32_t[HUF_CTABLE_WORKSPACE_SIZE_U32]);
-    constexpr size_t kWspSize = HUF_CTABLE_WORKSPACE_SIZE;
+    raw_size = 0;
+    for (unsigned i = 0; i <= HufHist::kMaxSymbol; i++) {
+      raw_size += hist.hist[i];
+
+      // force non-zero weights for all symbols.
+      if (hist.hist[i] == 0)
+        hist.hist[i] = 1;
+    }
 
     if (bintable.empty()) {
-      table_max_symbol = hist.max_symbol;
-      num_bits = HUF_buildCTable_wksp(huf_ctable, hist.hist.data(), table_max_symbol, 0,
-                                      wrkspace.get(), kWspSize);
-      if (HUF_isError(num_bits)) {
-        return rb->SendError(StrCat("Internal error: ", HUF_getErrorName(num_bits)));
+      if (!huff_enc.Build(hist.hist.data(), HufHist::kMaxSymbol, &err_msg)) {
+        return rb->SendError(StrCat("Internal error: ", err_msg));
       }
     } else {
       // Try to read the bintable and create a ctable from it.
-      unsigned has_zero_weights = 1;
-
-      size_t read_size = HUF_readCTable(huf_ctable, &table_max_symbol, bintable.data(),
-                                        bintable.size(), &has_zero_weights);
-      if (HUF_isError(read_size)) {
-        return rb->SendError(StrCat("Internal error: ", HUF_getErrorName(read_size)));
-      }
-      if (read_size != bintable.size()) {
-        return rb->SendError("Invalid bintable");
+      if (!huff_enc.Load(bintable, &err_msg)) {
+        return rb->SendError(StrCat("Internal error: ", err_msg));
       }
     }
-
-    compressed_size = HUF_estimateCompressedSize(huf_ctable, hist.hist.data(), table_max_symbol);
-    for (unsigned i = table_max_symbol + 1; i <= hist.max_symbol; i++) {
-      compressed_size += hist.hist[i];
-    }
-    raw_size = 0;
-    for (unsigned i = 0; i <= hist.max_symbol; i++) {
-      raw_size += hist.hist[i];
-    }
+    num_bits = huff_enc.num_bits();
+    compressed_size = huff_enc.EstimateCompressedSize(hist.hist.data(), HufHist::kMaxSymbol);
 
     if (print_bintable) {
-      // Reverse engineered: (maxSymbolValue + 1) / 2 + 1.
-      constexpr unsigned kMaxTableSize = 130;
-      bintable.resize(kMaxTableSize);
-
-      // Seems we can reuse the same workspace, its capacity is enough.
-      size_t res = HUF_writeCTable_wksp(bintable.data(), kMaxTableSize, huf_ctable,
-                                        table_max_symbol, num_bits, wrkspace.get(), kWspSize);
-      if (HUF_isError(res)) {
-        return rb->SendError(StrCat("Internal error: ", HUF_getErrorName(res)));
-      }
-      bintable.resize(res);
+      bintable = huff_enc.Export();
+      VLOG(1) << "bintable: " << absl::CHexEscape(bintable);
     } else {
       bintable.clear();
     }
   }
 
-  unsigned map_len = print_bintable ? 7 : 6;
+  unsigned map_len = print_bintable ? 6 : 5;
 
   rb->StartCollection(map_len, RedisReplyBuilder::CollectionType::MAP);
   rb->SendSimpleString("max_symbol");
   rb->SendLong(hist.max_symbol);
 
-  // in case we load a bintable, table_max_symbol may be different from max_symbol.
-  // if it's smaller, it means our table can not encode all symbols.
-  rb->SendSimpleString("table_max_symbol");
-  rb->SendLong(table_max_symbol);
   rb->SendSimpleString("max_bits");
   rb->SendLong(num_bits);
   rb->SendSimpleString("raw_size");