1 //===--- Random.h - Utilities for random sampling -------------------------===//
2 //
3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4 // See https://llvm.org/LICENSE.txt for license information.
5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6 //
7 //===----------------------------------------------------------------------===//
8 //
9 // Utilities for random sampling.
10 //
11 //===----------------------------------------------------------------------===//
12 
13 #ifndef LLVM_FUZZMUTATE_RANDOM_H
14 #define LLVM_FUZZMUTATE_RANDOM_H
15 
16 #include <random>
17 #include "llvm/Support/raw_ostream.h"
18 namespace llvm {
19 
20 /// Return a uniformly distributed random value between \c Min and \c Max
21 template <typename T, typename GenT> T uniform(GenT &Gen, T Min, T Max) {
22   return std::uniform_int_distribution<T>(Min, Max)(Gen);
23 }
24 
25 /// Return a uniformly distributed random value of type \c T
26 template <typename T, typename GenT> T uniform(GenT &Gen) {
27   return uniform<T>(Gen, std::numeric_limits<T>::min(),
28                     std::numeric_limits<T>::max());
29 }
30 
31 /// Randomly selects an item by sampling into a set with an unknown number of
32 /// elements, which may each be weighted to be more likely choices.
33 template <typename T, typename GenT> class ReservoirSampler {
34   GenT &RandGen;
35   std::remove_const_t<T> Selection = {};
36   uint64_t TotalWeight = 0;
37 
38 public:
39   ReservoirSampler(GenT &RandGen) : RandGen(RandGen) {}
40 
41   uint64_t totalWeight() const { return TotalWeight; }
42   bool isEmpty() const { return TotalWeight == 0; }
43 
44   const T &getSelection() const {
45     assert(!isEmpty() && "Nothing selected");
46     return Selection;
47   }
48 
49   explicit operator bool() const { return !isEmpty();}
50   const T &operator*() const { return getSelection(); }
51 
52   /// Sample each item in \c Items with unit weight
53   template <typename RangeT> ReservoirSampler &sample(RangeT &&Items) {
54     for (auto &I : Items)
55       sample(I, 1);
56     return *this;
57   }
58 
59   /// Sample a single item with the given weight.
60   ReservoirSampler &sample(const T &Item, uint64_t Weight) {
61     if (!Weight)
62       // If the weight is zero, do nothing.
63       return *this;
64     TotalWeight += Weight;
65     // Consider switching from the current element to this one.
66     if (uniform<uint64_t>(RandGen, 1, TotalWeight) <= Weight)
67       Selection = Item;
68     return *this;
69   }
70 };
71 
72 template <typename GenT, typename RangeT,
73           typename ElT = std::remove_reference_t<
74               decltype(*std::begin(std::declval<RangeT>()))>>
75 ReservoirSampler<ElT, GenT> makeSampler(GenT &RandGen, RangeT &&Items) {
76   ReservoirSampler<ElT, GenT> RS(RandGen);
77   RS.sample(Items);
78   return RS;
79 }
80 
81 template <typename GenT, typename T>
82 ReservoirSampler<T, GenT> makeSampler(GenT &RandGen, const T &Item,
83                                       uint64_t Weight) {
84   ReservoirSampler<T, GenT> RS(RandGen);
85   RS.sample(Item, Weight);
86   return RS;
87 }
88 
89 template <typename T, typename GenT>
90 ReservoirSampler<T, GenT> makeSampler(GenT &RandGen) {
91   return ReservoirSampler<T, GenT>(RandGen);
92 }
93 
94 } // End llvm namespace
95 
96 #endif // LLVM_FUZZMUTATE_RANDOM_H
97