lar::ComputePi Class Reference

Computes pi (but it does not make it available) More...

Inheritance diagram for lar::ComputePi:

Public Types
using	Counter_t = unsigned long long
	type used for integral counters More...
using	Seed_t = std::default_random_engine::result_type
	type for seed and random numbers More...

Public Member Functions
	ComputePi (fhicl::ParameterSet const &p)
virtual	~ComputePi ()=default
virtual void	analyze (const art::Event &) override
double	best_pi () const
	Returns the current best estimation of pi. More...
Counter_t	best_pi_tries () const
	Returns the current best estimation of pi. More...

Static Public Attributes
static const char *	VersionString = "1.0"
	version of the algorithm More...

Private Attributes
Counter_t	samples
	number of samples to try on each event More...
Seed_t	seed
	number of digits to compute More...
bool	bFixed
	whether the random sequence is always the same More...
bool	bVerbose
	whether to put stuff on screen More...
std::default_random_engine	generator
	random generator More...
Counter_t	hits = 0
	total number of hits More...
Counter_t	tries = 0
	total number of tries (samples) More...

Detailed Description

Computes pi (but it does not make it available)

This module performs a extensive computation whose duration can be indirectly controlled by a parameter. The time taken is supposed to be independent from the framework. This is meant to help establish an absolute time scale.

The module performs some Monte Carlo integration to compute pi. The same number of cycles is used regardless the result. We use a simple pseudo-random generator (std::linear_congruential_engine) with a constant extraction time (and poor randomness quality, and a period so small that in about 20 events the sequence might repeat itself). The fluctuations of the result don't reflect a fluctuation in time.

A test performed on uboonegpvm06,fnal.gov on August 19th, 2014 on 1000 events with Ksamples=50000 (i.e., 50M samples per event), default seed and verbosity on took 0.9179 +/- 0.0009 s, with an RMS of ~3%. It was observed that processing time asymptotically decreased.

Parameters

• Ksamples (integer, default: 10000) number of digits to be computed
• Seed (unsigned integer, default: 314159) chooses the seed for the Monte Carlo integration
• Fixed (boolean, default: false) if true, the same pseudo-random number sequence will be used for all events; otherwise, each event will get its own specific sequence
• Verbose (boolean, default: false) writes the result into the log

Definition at line 52 of file ComputePi_module.cc.

Member Typedef Documentation

using lar::ComputePi::Counter_t = unsigned long long

type used for integral counters

Definition at line 54 of file ComputePi_module.cc.

using lar::ComputePi::Seed_t = std::default_random_engine::result_type

type for seed and random numbers

Definition at line 56 of file ComputePi_module.cc.

Constructor & Destructor Documentation

lar::ComputePi::ComputePi ( fhicl::ParameterSet const &  p )

explicit

Definition at line 100 of file ComputePi_module.cc.

                                                 :
  EDAnalyzer(p),
  samples(p.get<Counter_t>("Ksamples", 10000) * 1000),
  seed(p.get<Seed_t>("Seed", 314159)),
  bFixed(p.get<bool>("Fixed", false)),
  bVerbose(p.get<bool>("Verbose", false)),
  generator(seed)
{
  mf::LogInfo("ComputePi")
    << "version " << VersionString
    << " using " << samples << " samples per event, random seed " << seed;
} // lar::ComputePi::ComputePi()

virtual lar::ComputePi::~ComputePi ( )

virtualdefault

Member Function Documentation

void lar::ComputePi::analyze ( const art::Event &  )

overridevirtual

Definition at line 114 of file ComputePi_module.cc.

                                          {

  // prepare our personal pseudo-random engine;
  // we'll use always the same sequence!
  std::uniform_real_distribution<float> flat(0.0, 1.0);

  // if we want to fix the random sequence, we reseed the generator
  // with the same value over and over again
  if (bFixed) generator.seed(seed);

  Counter_t local_hits = 0, tries_left = samples;
  while (tries_left-- > 0) {
    float x = flat(generator), y = flat(generator);
    if (sqr(x) + sqr(y) < 1.0) ++local_hits;
  } // while

  double local_pi = double(local_hits) / double(samples) * 4.0;
  hits += local_hits;
  tries += samples;

  if (bVerbose) {
    mf::LogInfo("ComputePi") << "today's pi = "
      << std::fixed << std::setprecision(9) << local_pi
      << " (pi = "
      << std::fixed << std::setprecision(12) << best_pi()
      << " after " << best_pi_tries() << " samples)";
  } // if verbose

} // lar::ComputePi::analyze()

double lar::ComputePi::best_pi ( ) const

inline

Returns the current best estimation of pi.

Definition at line 65 of file ComputePi_module.cc.

      { return tries? 4. * double(hits) / double(tries): 3.0; }

Counter_t lar::ComputePi::best_pi_tries ( ) const

inline

Returns the current best estimation of pi.

Definition at line 69 of file ComputePi_module.cc.

{ return tries; }

Member Data Documentation

bool lar::ComputePi::bFixed

private

whether the random sequence is always the same

Definition at line 77 of file ComputePi_module.cc.

bool lar::ComputePi::bVerbose

private

whether to put stuff on screen

Definition at line 78 of file ComputePi_module.cc.

std::default_random_engine lar::ComputePi::generator

private

random generator

Definition at line 80 of file ComputePi_module.cc.

Counter_t lar::ComputePi::hits = 0

private

total number of hits

Definition at line 81 of file ComputePi_module.cc.

Counter_t lar::ComputePi::samples

private

number of samples to try on each event

Definition at line 75 of file ComputePi_module.cc.

Seed_t lar::ComputePi::seed

private

number of digits to compute

Definition at line 76 of file ComputePi_module.cc.

Counter_t lar::ComputePi::tries = 0

private

total number of tries (samples)

Definition at line 82 of file ComputePi_module.cc.

const char * lar::ComputePi::VersionString = "1.0"

static

version of the algorithm

Definition at line 72 of file ComputePi_module.cc.

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The documentation for this class was generated from the following file:

• ComputePi_module.cc