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simple_continued_fraction: added protected non-const accessor to part…
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…ial denominators (#970)
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Tomáš Kolárik committed Mar 31, 2023
1 parent 5376689 commit 0101051
Showing 1 changed file with 50 additions and 35 deletions.
85 changes: 50 additions & 35 deletions include/boost/math/tools/simple_continued_fraction.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -32,17 +32,28 @@ namespace boost::math::tools {
template<typename Real, typename Z = int64_t>
class simple_continued_fraction {
public:
simple_continued_fraction(Real x) : x_{x} {
typedef Z int_type;

simple_continued_fraction(Real x) {
using std::floor;
using std::abs;
using std::sqrt;
using std::isfinite;
const Real orig_x = x;
if (!isfinite(x)) {
throw std::domain_error("Cannot convert non-finites into continued fractions.");
throw std::domain_error("Cannot convert non-finites into continued fractions.");
}

constexpr int p = std::numeric_limits<Real>::max_digits10;
if constexpr (p == 2147483647) {
precision_ = orig_x.backend().precision();
} else {
precision_ = p;
}

b_.reserve(50);
Real bj = floor(x);
b_.push_back(static_cast<Z>(bj));
b_.push_back(static_cast<int_type>(bj));
if (bj == x) {
b_.shrink_to_fit();
return;
Expand All @@ -54,14 +65,13 @@ class simple_continued_fraction {
}
Real C = f;
Real D = 0;
int i = 0;
// the "1 + i++" lets the error bound grow slowly with the number of convergents.
// the "1 + i" lets the error bound grow slowly with the number of convergents.
// I have not worked out the error propagation of the Modified Lentz's method to see if it does indeed grow at this rate.
// Numerical Recipes claims that no one has worked out the error analysis of the modified Lentz's method.
while (abs(f - x_) >= (1 + i++)*std::numeric_limits<Real>::epsilon()*abs(x_))
{
const Real eps_abs_orig_x = std::numeric_limits<Real>::epsilon()*abs(orig_x);
for (int i = 0; abs(f - orig_x) >= (1 + i)*eps_abs_orig_x; ++i) {
bj = floor(x);
b_.push_back(static_cast<Z>(bj));
b_.push_back(static_cast<int_type>(bj));
x = 1/(x-bj);
D += bj;
if (D == 0) {
Expand All @@ -78,29 +88,31 @@ class simple_continued_fraction {
// The shorter representation is considered the canonical representation,
// so if we compute a non-canonical representation, change it to canonical:
if (b_.size() > 2 && b_.back() == 1) {
b_[b_.size() - 2] += 1;
b_.resize(b_.size() - 1);
b_.pop_back();
b_.back() += 1;
}
b_.shrink_to_fit();

for (size_t i = 1; i < b_.size(); ++i) {

const size_t size_ = b_.size();
for (size_t i = 1; i < size_; ++i) {
if (b_[i] <= 0) {
std::ostringstream oss;
oss << "Found a negative partial denominator: b[" << i << "] = " << b_[i] << "."
#ifndef BOOST_MATH_STANDALONE
<< " This means the integer type '" << boost::core::demangle(typeid(Z).name())
<< " This means the integer type '" << boost::core::demangle(typeid(int_type).name())
#else
<< " This means the integer type '" << typeid(Z).name()
<< " This means the integer type '" << typeid(int_type).name()
#endif
<< "' has overflowed and you need to use a wider type,"
<< " or there is a bug.";
throw std::overflow_error(oss.str());
}
}
}

Real khinchin_geometric_mean() const {
if (b_.size() == 1) {
const size_t size_ = b_.size();
if (size_ == 1) {
return std::numeric_limits<Real>::quiet_NaN();
}
using std::log;
Expand All @@ -110,53 +122,56 @@ class simple_continued_fraction {
// A random partial denominator has ~80% chance of being in this table:
const std::array<Real, 7> logs{std::numeric_limits<Real>::quiet_NaN(), Real(0), log(static_cast<Real>(2)), log(static_cast<Real>(3)), log(static_cast<Real>(4)), log(static_cast<Real>(5)), log(static_cast<Real>(6))};
Real log_prod = 0;
for (size_t i = 1; i < b_.size(); ++i) {
if (b_[i] < static_cast<Z>(logs.size())) {
for (size_t i = 1; i < size_; ++i) {
if (b_[i] < static_cast<int_type>(logs.size())) {
log_prod += logs[b_[i]];
}
else
{
log_prod += log(static_cast<Real>(b_[i]));
}
}
log_prod /= (b_.size()-1);
log_prod /= (size_-1);
return exp(log_prod);
}

Real khinchin_harmonic_mean() const {
if (b_.size() == 1) {
const size_t size_ = b_.size();
if (size_ == 1) {
return std::numeric_limits<Real>::quiet_NaN();
}
Real n = b_.size() - 1;
Real n = size_ - 1;
Real denom = 0;
for (size_t i = 1; i < b_.size(); ++i) {
for (size_t i = 1; i < size_; ++i) {
denom += 1/static_cast<Real>(b_[i]);
}
return n/denom;
}

const std::vector<Z>& partial_denominators() const {

// Note that this also includes the integer part (i.e. all the coefficients)
const std::vector<int_type>& partial_denominators() const {
return b_;
}

template<typename T, typename Z2>
friend std::ostream& operator<<(std::ostream& out, simple_continued_fraction<T, Z2>& scf);
protected:
simple_continued_fraction() = default;

// Note that non-const operations may result in invalid simple continued fraction
std::vector<int_type>& partial_denominators() {
return b_;
}
private:
const Real x_;
std::vector<Z> b_;
std::vector<int_type> b_{};

int precision_{};
};


template<typename Real, typename Z2>
std::ostream& operator<<(std::ostream& out, simple_continued_fraction<Real, Z2>& scf) {
constexpr const int p = std::numeric_limits<Real>::max_digits10;
if constexpr (p == 2147483647) {
out << std::setprecision(scf.x_.backend().precision());
} else {
out << std::setprecision(p);
}

out << std::setprecision(scf.precision_);
out << "[" << scf.b_.front();
if (scf.b_.size() > 1)
{
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