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LibCrypto: Don't compute 2*N remainders in Adler32

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Otherwise Zlib decompression spends half of the time computing the
checksum.
This commit is contained in:
Dan Klishch 2024-05-19 13:43:37 -04:00 • committed by Ali Mohammad Pur
commit 7ae990d2e5
Notes: sideshowbarker 2024-07-16 18:06:41 +09:00
1 changed files with 25 additions and 3 deletions
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28
Userland/Libraries/LibCrypto/Checksum/Adler32.cpp
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@ -12,10 +12,32 @@ namespace Crypto::Checksum {
void Adler32::update(ReadonlyBytes data)
{
for (size_t i = 0; i < data.size(); i++) {
m_state_a = (m_state_a + data.at(i)) % 65521;
m_state_b = (m_state_b + m_state_a) % 65521;
// See https://github.com/SerenityOS/serenity/pull/24408#discussion_r1609051678
constexpr size_t iterations_without_overflow = 380368439;
u64 state_a = m_state_a;
u64 state_b = m_state_b;
while (data.size()) {
// You can verify that no overflow will happen here during at least
// `iterations_without_overflow` iterations using the following Python script:
//
// state_a = 65520
// state_b = 65520
// for i in range(380368439):
// state_a += 255
// state_b += state_a
// print(state_b < 2 ** 64)
auto chunk = data.slice(0, min(data.size(), iterations_without_overflow));
for (u8 byte : chunk) {
state_a += byte;
state_b += state_a;
}
state_a %= 65521;
state_b %= 65521;
data = data.slice(chunk.size());
}
m_state_a = state_a;
m_state_b = state_b;
}
u32 Adler32::digest()