fn_registry/cpp/functions/datascience/metropolis_hastings.md

name, kind, lang, domain, version, purity, signature, description, tags, uses_functions, uses_types, returns, returns_optional, error_type, imports, tested, tests, test_file_path, file_path, params, output

name	kind	lang	domain	version	purity	signature	description	tags	uses_functions	uses_types	returns	returns_optional	error_type	imports	tested	tests	test_file_path	file_path	params	output
metropolis_hastings	function	cpp	datascience	1.0.0	pure	MHResult mh_run_1d(const std::function<double(double)>& log_pdf, double x0, double sigma, size_t n, double* out, Rng&); MHResult mh_run_nd(const std::function<double(const double*)>& log_pdf, const double* x0, const double* sigma, int d, size_t n, double* out, Rng&)	Metropolis-Hastings 1D y d-dimensional con proposal Gaussian symmetric. Target log-pdf inyectable via std::function (no necesita normalizarse). Devuelve cadena en out[] y accept rate. Pareja CPU del mc_metropolis_hastings_gpu.	mcmc metropolis hastings sampling bayesian datascience pendiente-usar	rng_cpp_datascience			false		cstddef functional cmath vector	false			cpp/functions/datascience/metropolis_hastings.cpp	name desc target_log_pdf Functor con la log-densidad target (no normalizada). 1D recibe double; ND recibe const double* de tamano d. name desc x0 Punto inicial (escalar 1D, array d-dim ND). name desc proposal_sigma Stddev del proposal Gaussian. 1D escalar; ND array de d stddevs (uno por dimension). name desc d (ND) dimension del espacio. name desc n_samples Total de samples a generar (incluyendo x0). name desc out_chain Buffer destino. 1D: double[n]. ND: double[n*d] row-major (sample i, dim k = out[i*d+k]). name desc r Rng (de rng_cpp_datascience). State mutado durante el sampling.	MHResult con n_samples, n_accepted y accept_rate. Tipico target: 0.20-0.40 accept rate (ajustar proposal_sigma).

metropolis_hastings

Random-walk Metropolis estandar. Replica el sampler que usan los 4 calculadores MCMC del set, en formato componible.

Patron 1D (mcmc-bayes / mcmc-full Beta posterior)

double k = 7.0, n_obs = 10.0;     // observados
auto log_post = [k, n_obs](double theta) -> double {
    if (theta <= 0.0 || theta >= 1.0) return -1e300;
    // Beta(2, 2) prior + Binomial likelihood
    return (k + 1.0) * std::log(theta) +
           (n_obs - k + 1.0) * std::log(1.0 - theta);
};

fn::ds::Rng r;
fn::ds::rng_seed(r, 42);

std::vector<double> chain(10000);
auto res = fn::ds::mh_run_1d(log_post,
                             /*x0=*/0.5,
                             /*sigma=*/0.1,
                             chain.size(),
                             chain.data(),
                             r);
// Burn-in: descartar primeros 1000
// res.accept_rate ~ 0.3 si sigma esta bien ajustada

Patron 2D (mcmc-visualizer)

auto log_density = [](const double* xy) -> double {
    double x = xy[0], y = xy[1];
    // Bimodal target del visualizer
    double d1 = (x-2)*(x-2) + (y-2)*(y-2);
    double d2 = (x+2)*(x+2) + (y+2)*(y+2);
    return std::log(std::exp(-0.5*d1) + std::exp(-0.5*d2));
};

double x0[2] = {0.0, 0.0};
double sigma[2] = {1.0, 1.0};
std::vector<double> chain(10000 * 2);

fn::ds::mh_run_nd(log_density, x0, sigma, 2,
                  10000, chain.data(), r);

Notas

• No hay tuning automatico del proposal sigma. Si el accept rate es <0.1, baja sigma; si >0.6, sube. Para tunings serios usar adaptive MH (no incluido aqui).
• target_log_pdf se evalua 2 veces por sample en el peor caso (current cached, proposal nuevo). Si tu log-pdf es caro (10ms+ por eval), N=10^4 cuesta minutos — para esos casos preferir GPU MH (no caben dependencias caras en GLSL — tienes que portar la log-pdf).
• Compatible con rhat_split y ess_basic: corre M cadenas con seeds distintos, apila en chains[m*n + i], llama los diagnosticos.
• 1D y ND comparten la maquinaria pero ND lleva el coste de los std::vector<double> por sample. Si rendimiento importa y conoces d en compile-time, fork-and-specialize.