Stan Math Library  2.15.0
reverse mode automatic differentiation
inv_Phi.hpp
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1 #ifndef STAN_MATH_PRIM_SCAL_FUN_INV_PHI_HPP
2 #define STAN_MATH_PRIM_SCAL_FUN_INV_PHI_HPP
3 
8 
9 namespace stan {
10  namespace math {
11 
26  inline double inv_Phi(double p) {
27  check_bounded("inv_Phi", "Probability variable", p, 0, 1);
28 
29  if (p < 8e-311)
30  return NEGATIVE_INFTY;
31  if (p == 1)
32  return INFTY;
33 
34  static const double a[6] = {
35  -3.969683028665376e+01, 2.209460984245205e+02,
36  -2.759285104469687e+02, 1.383577518672690e+02,
37  -3.066479806614716e+01, 2.506628277459239e+00
38  };
39  static const double b[5] = {
40  -5.447609879822406e+01, 1.615858368580409e+02,
41  -1.556989798598866e+02, 6.680131188771972e+01,
42  -1.328068155288572e+01
43  };
44  static const double c[6] = {
45  -7.784894002430293e-03, -3.223964580411365e-01,
46  -2.400758277161838e+00, -2.549732539343734e+00,
47  4.374664141464968e+00, 2.938163982698783e+00
48  };
49  static const double d[4] = {
50  7.784695709041462e-03, 3.224671290700398e-01,
51  2.445134137142996e+00, 3.754408661907416e+00
52  };
53 
54  static const double p_low = 0.02425;
55  static const double p_high = 0.97575;
56 
57  double x;
58  if ((p_low <= p) && (p <= p_high)) {
59  double q = p - 0.5;
60  double r = q * q;
61  x = (((((a[0]*r + a[1])*r + a[2])*r + a[3])*r + a[4])*r + a[5])*q
62  / (((((b[0]*r + b[1])*r + b[2])*r + b[3])*r + b[4])*r + 1.0);
63  } else if (p < p_low) {
64  double q = std::sqrt(-2.0*std::log(p));
65  x = (((((c[0]*q + c[1])*q + c[2])*q + c[3])*q + c[4])*q + c[5])
66  / ((((d[0]*q + d[1])*q + d[2])*q + d[3])*q + 1.0);
67  } else {
68  double q = std::sqrt(-2.0 * log1m(p));
69  x = -(((((c[0]*q + c[1])*q + c[2])*q + c[3])*q + c[4])*q + c[5])
70  / ((((d[0]*q + d[1])*q + d[2])*q + d[3])*q + 1.0);
71  }
72 
73  if (x < 37.6) { // gradient blows up past here
74  double e = Phi(x) - p;
75  double u = e * SQRT_2_TIMES_SQRT_PI * std::exp(0.5 * x * x);
76  x -= u / (1.0 + 0.5 * x * u);
77  }
78 
79  return x;
80  }
81 
82  }
83 }
84 #endif
fvar< T > sqrt(const fvar< T > &x)
Definition: sqrt.hpp:14
fvar< T > inv_Phi(const fvar< T > &p)
Definition: inv_Phi.hpp:14
void check_bounded(const char *function, const char *name, const T_y &y, const T_low &low, const T_high &high)
Check if the value is between the low and high values, inclusively.
fvar< T > log(const fvar< T > &x)
Definition: log.hpp:14
const double SQRT_2_TIMES_SQRT_PI
Definition: constants.hpp:156
fvar< T > exp(const fvar< T > &x)
Definition: exp.hpp:10
fvar< T > Phi(const fvar< T > &x)
Definition: Phi.hpp:12
double e()
Return the base of the natural logarithm.
Definition: constants.hpp:94
const double INFTY
Positive infinity.
Definition: constants.hpp:43
const double NEGATIVE_INFTY
Negative infinity.
Definition: constants.hpp:49
fvar< T > log1m(const fvar< T > &x)
Definition: log1m.hpp:13

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