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Add constexpr shift_right mp helper #4008

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Apr 12, 2024
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Add constexpr shift_right mp helper #4008

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20 changes: 17 additions & 3 deletions src/lib/math/mp/mp_core.h
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Original file line number Diff line number Diff line change
Expand Up @@ -845,15 +845,29 @@ inline constexpr W shift_left(std::array<W, N>& x) {
static_assert(S < WordInfo<W>::bits, "Shift too large");

W carry = 0;
for(size_t j = 0; j != N; ++j) {
const W w = x[j];
x[j] = (w << S) | carry;
for(size_t i = 0; i != N; ++i) {
const W w = x[i];
x[i] = (w << S) | carry;
carry = w >> (WordInfo<W>::bits - S);
}

return carry;
}

template <size_t S, WordType W, size_t N>
inline consteval W shift_right(std::array<W, N>& x) {
static_assert(S < WordInfo<W>::bits, "Shift too large");

W carry = 0;
for(size_t i = 0; i != N; ++i) {
const W w = x[N - 1 - i];
x[N - 1 - i] = (w >> S) | carry;
carry = w << (WordInfo<W>::bits - S);
}
Comment on lines +861 to +866
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If we'd let the loop run backwards from i = N - 1 to i >0, would that make the indices easier to understand?

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I agree the indices would be clearer going in the other direction. But for whatever reason I find the pattern of consistently mapping from i \in [0..N) to an action on that i easier to follow.


return carry;
}

template <WordType W, size_t N>
consteval auto hex_to_words(const char (&s)[N]) {
// Char count includes null terminator which we ignore
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