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Fix undefined behavior in Huffman encoder for single-symbol input #133

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Fix undefined behavior in Huffman encoder for single-symbol input #133

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7 changes: 5 additions & 2 deletions include/SZ3/encoder/HuffmanEncoder.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -479,11 +479,14 @@ class HuffmanEncoder : public concepts::EncoderInterface<T> {
if (n->t) {
huffmanTree->code[n->c] = static_cast<uint64_t *>(malloc(2 * sizeof(uint64_t)));
if (len <= 64) {
(huffmanTree->code[n->c])[0] = out1 << (64 - len);
// A Huffman tree with a single symbol gives a zero-length code (build_code is called with
// len == 0 for the root). Shifting a 64-bit value by 64 is undefined behavior, so guard it.
(huffmanTree->code[n->c])[0] = (len == 0) ? 0 : (out1 << (64 - len));
(huffmanTree->code[n->c])[1] = out2;
} else {
(huffmanTree->code[n->c])[0] = out1;
(huffmanTree->code[n->c])[1] = out2 << (128 - len);
// Likewise, len >= 128 would shift by >= 64; such codes do not fit in 128 bits anyway.
(huffmanTree->code[n->c])[1] = (len >= 128) ? out2 : (out2 << (128 - len));
}
huffmanTree->cout[n->c] = static_cast<unsigned char>(len);
// std::cout << "build_code: c = " << n->c << ", len = " << len << ", out1 = " << out1 << ", out2 = " << out2
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