xapian-core  1.4.21
roundestimate.h
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1 
4 /* Copyright 2017,2019 Olly Betts
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20 
21 #ifndef XAPIAN_INCLUDED_ROUNDESTIMATE_H
22 #define XAPIAN_INCLUDED_ROUNDESTIMATE_H
23 
24 #include <algorithm>
25 #include <cmath>
26 #include "exp10.h"
27 
35 template<typename T>
36 inline Xapian::doccount
37 round_estimate(T lb, T ub, T est)
38 {
39  using namespace std;
40 
41  // We round based on the difference between the bounds, or the estimate if
42  // that's smaller - for example, consider lb=11, est=24, ub=1234 where
43  // rounding est to a multiple of 10 is reasonable but rounding it to a
44  // multiple of 1000 isn't.
45  T scale = min(ub - lb, est);
46  if (scale <= 10) {
47  // Estimate is either too close to exact or too small to round.
48  return est;
49  }
50 
51  // Set r to the largest power of 10 <= scale.
52  T r = T(exp10(int(log10(scale))) + 0.5);
53 
54  // Set result to est with less significant digits truncated.
55  T result = est / r * r;
56  if (result < lb) {
57  // We have to round up to be above the lower bound.
58  result += r;
59  } else if (result > ub - r) {
60  // We can't round up as it would exceed the upper bound.
61  } else {
62  // We can choose which way to round so consider whether we're before or
63  // after the mid-point of [result, result+r] and round to the nearer
64  // end of the range. If we're exactly on the middle, pick the rounding
65  // direction which puts the rounded estimate closest to the mid-range
66  // of the bounds.
67  T d = 2 * (est - result);
68  if (d > r || (d == r && result - lb <= ub - r - result)) {
69  result += r;
70  }
71  }
72 
73  return result;
74 }
75 
76 #endif // XAPIAN_INCLUDED_ROUNDESTIMATE_H
Xapian::doccount round_estimate(T lb, T ub, T est)
Round a bounded estimate to an appropriate number of S.F.
Definition: roundestimate.h:37
STL namespace.
static double est(double l, double r, double n)
Definition: orpostlist.cc:306
double exp10(double x)
Definition: exp10.h:35
Define exp10() if not provided by <cmath>
unsigned XAPIAN_DOCID_BASE_TYPE doccount
A count of documents.
Definition: types.h:38