Abastro/mathematical-algorithms-cpp
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First optimization: Use span

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Abastro 2025-03-24 19:35:24 +09:00
parent eace365811
commit 213aa29a25
1 changed files with 33 additions and 13 deletions
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46
src/karatsuba.cpp
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@ -67,6 +67,20 @@ template <typename R> vector<R> poly_sub(vector<R> &a, vector<R> &b) {
return res;
}
// Add polynomials in-place assuming the size is allocated
template <typename R>
void poly_add_inplace(span<R> &a, span<R> &b, span<R> &result) {
for (size_t i = 0; i < a.size(); i++)
result[i] = a[i] + b[i];
}
// Subtract polynomials in-place assuming the size is allocated
template <typename R>
void poly_sub_inplace(span<R> &a, span<R> &b, span<R> &result) {
for (size_t i = 0; i < a.size(); i++)
result[i] = a[i] - b[i];
}
// Shift up degrees by n
template <typename R> vector<R> poly_shift_up(int n, vector<R> &p) {
auto result = p;
@ -94,31 +108,34 @@ template <typename R> vector<R> poly_mult_basic(vector<R> &a, vector<R> &b) {
#define THRESHOLD 1
// TODO Reduce allocations
// Send this to professor
/**
* A step of the Karatsuba function.
* @param deg_bnd power-of-2 degree bound
*/
template <typename R>
vector<R> poly_mult_Karatsuba_step(const size_t deg_bnd, vector<R> &a,
vector<R> &b) {
if (deg_bnd <= THRESHOLD)
return poly_mult_basic(a, b);
vector<R> poly_mult_Karatsuba_step(const size_t deg_bnd, span<R> &a,
span<R> &b) {
if (deg_bnd <= THRESHOLD) {
auto vec_a = vector(a.begin(), a.end());
auto vec_b = vector(b.begin(), b.end());
return poly_mult_basic(vec_a, vec_b);
}
const auto next_bnd = deg_bnd >> 1;
const auto next_bnd_in_a = min(next_bnd, a.size());
const auto next_bnd_in_b = min(next_bnd, b.size());
auto a0 = vector(a.begin(), a.begin() + next_bnd_in_a);
auto a1 = vector(a.begin() + next_bnd_in_a, a.end());
auto b0 = vector(b.begin(), b.begin() + next_bnd_in_b);
auto b1 = vector(b.begin() + next_bnd_in_b, b.end());
auto a0 = span(a.begin(), a.begin() + next_bnd_in_a);
auto a1 = span(a.begin() + next_bnd_in_a, a.end());
auto b0 = span(b.begin(), b.begin() + next_bnd_in_b);
auto b1 = span(b.begin() + next_bnd_in_b, b.end());
auto prod0 = poly_mult_Karatsuba_step(next_bnd, a0, b0);
auto prod1 = poly_mult_Karatsuba_step(next_bnd, a1, b1);
auto a01 = poly_add(a0, a1);
auto b01 = poly_add(b0, b1);
auto prod_add = poly_mult_Karatsuba_step(next_bnd, a01, b01);
// Use a0, b0 to store the addition.
poly_add_inplace(a0, a1, a0);
poly_add_inplace(b0, b1, b0);
auto prod_add = poly_mult_Karatsuba_step(next_bnd, a0, b0);
auto tmp1 = poly_sub(prod_add, prod0);
auto tmp2 = poly_sub(tmp1, prod1);
@ -134,7 +151,9 @@ vector<R> poly_mult_Karatsuba(vector<R> &a, vector<R> &b) {
while (deg_bound < max(a.size(), b.size()))
deg_bound = deg_bound << 1;
return poly_mult_Karatsuba_step(deg_bound, a, b);
auto span_a = span(a);
auto span_b = span(b);
return poly_mult_Karatsuba_step(deg_bound, span_a, span_b);
}
void basic_vs_Karatsuba(size_t size) {
@ -184,6 +203,7 @@ int main() {
basic_vs_Karatsuba(2048);
basic_vs_Karatsuba(4096);
basic_vs_Karatsuba(8192);
basic_vs_Karatsuba(16384);
// {
// auto p = random_real_vector(4000);