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Implement 128-bit constant mul in bigint

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Victor Zverovich
2022-04-03 07:12:24 -07:00
parent 02eb215f2f
commit 686de58886
3 changed files with 15 additions and 10 deletions
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11
include/fmt/format-inl.h
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@@ -371,13 +371,16 @@ class bigint {
template <typename UInt, FMT_ENABLE_IF(std::is_same<UInt, uint64_t>::value ||
std::is_same<UInt, uint128_t>::value)>
FMT_CONSTEXPR20 void multiply(UInt value) {
const UInt lower = static_cast<bigit>(value);
const UInt upper = value >> bigit_bits;
using half_uint =
conditional_t<std::is_same<UInt, uint128_t>::value, uint64_t, uint32_t>;
const int shift = num_bits<half_uint>() - bigit_bits;
const UInt lower = static_cast<half_uint>(value);
const UInt upper = value >> num_bits<half_uint>();
UInt carry = 0;
for (size_t i = 0, n = bigits_.size(); i < n; ++i) {
UInt result = lower * bigits_[i] + static_cast<bigit>(carry);
carry =
upper * bigits_[i] + (result >> bigit_bits) + (carry >> bigit_bits);
carry = (upper * bigits_[i] << shift) + (result >> bigit_bits) +
(carry >> bigit_bits);
bigits_[i] = static_cast<bigit>(result);
}
while (carry != 0) {