huffman.cc

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#include "mem/cache/compressors/encoders/huffman.hh"
 
#include <cassert>
 
#include "base/logging.hh"
 
namespace gem5
{
 
namespace compression
{
namespace encoder
{
 
Huffman::Huffman(uint64_t max_code_length)
  : Base(), maxCodeLength(max_code_length)
{
    fatal_if(maxCodeLength > 64,
        "Code length cannot surpass its underlying container");
}
 
void
Huffman::sample(uint64_t value, uint64_t frequency)
{
    if (frequency != 0) {
        trees.push(new Node(value, frequency));
    }
}
 
std::unique_ptr<Huffman::Node>
Huffman::buildTree()
{
    // Construct tree by assigning left and right nodes. The left path leads
    // to the most frequent values
    while (trees.size() > 1) {
        Node* left = trees.top();
        trees.pop();
 
        Node* right = trees.top();
        trees.pop();
 
        Node* parent = new Node(left, right);
        trees.push(parent);
    }
 
    // All queue entries have been merged into a single entry containing
    // the tree
    Node* root = trees.top();
    trees.pop();
    return std::unique_ptr<Node>(root);
}
 
void
Huffman::generateCodeMaps()
{
    valueToCode.clear();
    codeToValue.clear();
    generateCodes(buildTree().get(), Code());
}
 
void
Huffman::generateCodes(const Node* node, const Code& current_code)
{
    // Drop all entries with length greater than maxCodeLength
    if (current_code.length > maxCodeLength) {
        return;
    }
 
    if (node->isLeaf()) {
        valueToCode[node->getValue()] = current_code;
        codeToValue[current_code.code] = node->getValue();
    } else {
        Code right_code = current_code;
        right_code.code = (right_code.code << 1) + 1;
        right_code.length++;
        generateCodes(node->getRightSubTree(), right_code);
 
        Code left_code = current_code;
        left_code.code = left_code.code << 1;
        left_code.length++;
        generateCodes(node->getLeftSubTree(), left_code);
    }
}
 
Code
Huffman::encode(const uint64_t val) const
{
    auto it = valueToCode.find(val);
    if (it == valueToCode.end()) {
        // If the value is unknown, generate a dummy code with invalid
        // length to let the caller know the encoding is invalid
        Code dummy_code;
        dummy_code.code = 0;
        dummy_code.length = 65;
        return dummy_code;
    } else {
        return it->second;
    }
}
 
uint64_t
Huffman::decode(const uint64_t code) const
{
    // A code that does not exist cannot be decoded
    auto it = codeToValue.find(code);
    assert(it != codeToValue.end());
    return it->second;
}
 
} // namespace encoder
} // namespace compression
} // namespace gem5