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//===--- Random.h - Utilities for random sampling -------------------------===//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
// Utilities for random sampling.
#include <random>
#include "llvm/Support/raw_ostream.h"
namespace llvm {
/// Return a uniformly distributed random value between \c Min and \c Max
template <typename T, typename GenT> T uniform(GenT &Gen, T Min, T Max) {
return std::uniform_int_distribution<T>(Min, Max)(Gen);
/// Return a uniformly distributed random value of type \c T
template <typename T, typename GenT> T uniform(GenT &Gen) {
return uniform<T>(Gen, std::numeric_limits<T>::min(),
/// Randomly selects an item by sampling into a set with an unknown number of
/// elements, which may each be weighted to be more likely choices.
template <typename T, typename GenT> class ReservoirSampler {
GenT &RandGen;
typename std::remove_const<T>::type Selection = {};
uint64_t TotalWeight = 0;
ReservoirSampler(GenT &RandGen) : RandGen(RandGen) {}
uint64_t totalWeight() const { return TotalWeight; }
bool isEmpty() const { return TotalWeight == 0; }
const T &getSelection() const {
assert(!isEmpty() && "Nothing selected");
return Selection;
explicit operator bool() const { return !isEmpty();}
const T &operator*() const { return getSelection(); }
/// Sample each item in \c Items with unit weight
template <typename RangeT> ReservoirSampler &sample(RangeT &&Items) {
for (auto &I : Items)
sample(I, 1);
return *this;
/// Sample a single item with the given weight.
ReservoirSampler &sample(const T &Item, uint64_t Weight) {
if (!Weight)
// If the weight is zero, do nothing.
return *this;
TotalWeight += Weight;
// Consider switching from the current element to this one.
if (uniform<uint64_t>(RandGen, 1, TotalWeight) <= Weight)
Selection = Item;
return *this;
template <typename GenT, typename RangeT,
typename ElT = typename std::remove_reference<
ReservoirSampler<ElT, GenT> makeSampler(GenT &RandGen, RangeT &&Items) {
ReservoirSampler<ElT, GenT> RS(RandGen);
return RS;
template <typename GenT, typename T>
ReservoirSampler<T, GenT> makeSampler(GenT &RandGen, const T &Item,
uint64_t Weight) {
ReservoirSampler<T, GenT> RS(RandGen);
RS.sample(Item, Weight);
return RS;
template <typename T, typename GenT>
ReservoirSampler<T, GenT> makeSampler(GenT &RandGen) {
return ReservoirSampler<T, GenT>(RandGen);
} // End llvm namespace