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[coding] [arithmetic_codec] Convert distr table computation to 64-bit. Improve decode speed by precomputing shifted interval.

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Artyom Polkovnikov 2014-11-18 12:36:21 +03:00 • committed by Alex Zolotarev
parent 5ae31bb3f3
commit c52e60392b
1 changed files with 14 additions and 8 deletions
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22
coding/arithmetic_codec.cpp
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@ -20,9 +20,9 @@ vector<u32> FreqsToDistrTable(vector<u32> const & origFreqs)
while (1)
{
// Resulting distr table is initialized with first zero value.
vector<u32> result(1, 0);
vector<u64> result(1, 0);
vector<u32> freqs;
u32 sum = 0;
u64 sum = 0;
u64 minFreq = ~u64(0);
for (u32 i = 0; i < origFreqs.size(); ++i)
{
@ -39,16 +39,21 @@ vector<u32> FreqsToDistrTable(vector<u32> const & origFreqs)
bool hasDegradedZeroInterval = false;
for (u32 i = 1; i < result.size(); ++i)
{
result[i] = (u64(result[i]) << DISTR_SHIFT) / u64(sum);
result[i] = (result[i] << DISTR_SHIFT) / sum;
if (freqs[i - 1] > 0 && (result[i] - result[i - 1] == 0))
{
hasDegradedZeroInterval = true;
break;
}
}
if (!hasDegradedZeroInterval) return result;
if (!hasDegradedZeroInterval) {
// Convert distr_table to 32-bit vector, although currently only 17 bits are used.
vector<u32> distr_table;
for (u32 i = 0; i < result.size(); ++i) distr_table.push_back(result[i]);
return distr_table;
}
++freqLowerBound;
}
}
}
ArithmeticEncoder::ArithmeticEncoder(vector<u32> const & distrTable)
@ -121,15 +126,16 @@ ArithmeticDecoder::ArithmeticDecoder(Reader & reader, vector<u32> const & distrT
u32 ArithmeticDecoder::Decode()
{
u32 l = 0, r = m_distrTable.size(), m = 0;
u32 shiftedSize = m_size >> DISTR_SHIFT;
while (r - l > 1)
{
m = (l + r) / 2;
u32 intervalBegin = (m_size >> DISTR_SHIFT) * m_distrTable[m];
u32 intervalBegin = shiftedSize * m_distrTable[m];
if (intervalBegin <= m_codeValue) l = m; else r = m;
}
u32 symbol = l;
m_codeValue -= (m_size >> DISTR_SHIFT) * m_distrTable[symbol];
m_size = (m_size >> DISTR_SHIFT) * (m_distrTable[symbol + 1] - m_distrTable[symbol]);
m_codeValue -= shiftedSize * m_distrTable[symbol];
m_size = shiftedSize * (m_distrTable[symbol + 1] - m_distrTable[symbol]);
while (m_size < (u32(1) << 24))
{
m_codeValue <<= 8;