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#pragma once
#include "coding/bit_streams.hpp"
#include "coding/byte_stream.hpp"
#include "coding/huffman.hpp"
#include "coding/reader.hpp"
#include "coding/varint.hpp"
#include "coding/writer.hpp"
#include "base/buffer_vector.hpp"
#include "base/checked_cast.hpp"
#include "base/scope_guard.hpp"
#include "base/string_utils.hpp"
#include "std/algorithm.hpp"
#include "std/bind.hpp"
// Trie format:
// -- Serialized Huffman encoding.
// -- Topology of the trie built on Huffman-encoded input strings [2 bits per node, level-order representation].
// -- List of pairs (node id, offset). One pair per final node (i.e. a node where a string ends).
// The lists of node ids and offsets are both non-decreasing and are delta-encoded with
// varuints.
// -- Values of final nodes in level-order. The values for final node |id| start at offset |offset|
// if there is a pair (id, offset) in the list above.
namespace trie
{
// Node is a temporary struct that is used to store the trie in memory
// before it is converted to a succinct representation and put to disk.
// It is rather verbose but hopefully is small enough to fit in memory.
template <class TValueList>
struct Node
{
// The left child is reached by 0, the right by 1.
Node *l, *r;
// A node is final if a key string ends in it.
// In particular, all non-final nodes' valueLists are empty
// and it is expected that a final node's valueList is non-empty.
bool m_isFinal;
// m_valueLists stores all the additional information that is related
// to the key string which leads to this node.
TValueList m_valueList;
Node() : l(nullptr), r(nullptr), m_isFinal(false) {}
};
template <class TNode, class TReader>
TNode * AddToTrie(TNode * root, TReader & bitEncoding, uint32_t numBits)
{
ReaderSource<TReader> src(bitEncoding);
// String size.
ReadVarUint<uint32_t, ReaderSource<TReader>>(src);
BitReader<ReaderSource<TReader>> bitReader(src);
auto cur = root;
for (uint32_t i = 0; i < numBits; ++i)
{
auto nxt = bitReader.Read(1) == 0 ? &cur->l : &cur->r;
if (!*nxt)
*nxt = new TNode();
cur = *nxt;
}
return cur;
}
template <class TNode>
void DeleteTrie(TNode * root)
{
if (!root)
return;
DeleteTrie(root->l);
DeleteTrie(root->r);
delete root;
}
template <typename TNode>
void WriteInLevelOrder(TNode * root, vector<TNode *> & levelOrder)
{
ASSERT(root, ());
levelOrder.push_back(root);
size_t i = 0;
while (i < levelOrder.size())
{
TNode * cur = levelOrder[i++];
if (cur->l)
levelOrder.push_back(cur->l);
if (cur->r)
levelOrder.push_back(cur->r);
}
}
template <typename TWriter, typename TIter, typename TValueList>
void BuildSuccinctTrie(TWriter & writer, TIter const beg, TIter const end)
{
using TrieChar = uint32_t;
using TTrieString = buffer_vector<TrieChar, 32>;
using TNode = Node<TValueList>;
using TEntry = typename TIter::value_type;
TNode * root = new TNode();
MY_SCOPE_GUARD(cleanup, bind(&DeleteTrie<TNode>, root));
TTrieString prevKey;
TEntry prevEntry;
vector<TEntry> entries;
vector<strings::UniString> entryStrings;
for (TIter it = beg; it != end; ++it)
{
TEntry entry = *it;
if (it != beg && entry == prevEntry)
continue;
TrieChar const * const keyData = entry.GetKeyData();
TTrieString key(keyData, keyData + entry.GetKeySize());
using namespace std::rel_ops; // ">=" for keys.
CHECK_GREATER_OR_EQUAL(key, prevKey, (key, prevKey));
entries.push_back(entry);
entryStrings.push_back(strings::UniString(keyData, keyData + entry.GetKeySize()));
prevKey.swap(key);
prevEntry.Swap(entry);
}
coding::HuffmanCoder huffman;
huffman.Init(entryStrings);
huffman.WriteEncoding(writer);
for (size_t i = 0; i < entries.size(); ++i)
{
TEntry const & entry = entries[i];
auto const & key = entryStrings[i];
vector<uint8_t> buf;
MemWriter<vector<uint8_t>> memWriter(buf);
uint32_t numBits = huffman.EncodeAndWrite(memWriter, key);
MemReader bitEncoding(&buf[0], buf.size());
TNode * cur = AddToTrie(root, bitEncoding, numBits);
cur->m_isFinal = true;
cur->m_valueList.Append(entry.GetValue());
}
vector<TNode *> levelOrder;
WriteInLevelOrder(root, levelOrder);
{
// Trie topology.
BitWriter<TWriter> bitWriter(writer);
WriteVarUint(writer, levelOrder.size());
for (size_t i = 0; i < levelOrder.size(); ++i)
{
auto const & cur = levelOrder[i];
bitWriter.Write(cur->l ? 1 : 0, 1 /* numBits */);
bitWriter.Write(cur->r ? 1 : 0, 1 /* numBits */);
}
}
vector<uint32_t> finalNodeIds;
vector<uint32_t> offsetTable;
vector<uint8_t> valueBuf;
MemWriter<vector<uint8_t>> valueWriter(valueBuf);
for (size_t i = 0; i < levelOrder.size(); ++i)
{
TNode const * node = levelOrder[i];
if (!node->m_isFinal)
continue;
finalNodeIds.push_back(static_cast<uint32_t>(i));
offsetTable.push_back(base::asserted_cast<uint32_t>(valueWriter.Pos()));
node->m_valueList.Dump(valueWriter);
}
WriteVarUint(writer, finalNodeIds.size());
for (size_t i = 0; i < finalNodeIds.size(); ++i)
{
if (i == 0)
{
WriteVarUint(writer, finalNodeIds[i]);
WriteVarUint(writer, offsetTable[i]);
}
else
{
WriteVarUint(writer, finalNodeIds[i] - finalNodeIds[i - 1]);
WriteVarUint(writer, offsetTable[i] - offsetTable[i - 1]);
}
}
writer.Write(valueBuf.data(), valueBuf.size());
// todo(@pimenov):
// 1. Investigate the possibility of path compression (short edges + lcp table).
// 2. It is highly probable that valueList will be only a list of feature ids in our
// future implementations of the search.
// We can then dispose of offsetTable and store finalNodeIds as another bit vector.
// Alternatively (and probably better), we can append a special symbol to all those
// strings that have non-empty value lists. We can then get the leaf number
// of a final node by doing rank0.
}
} // namespace trie
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