GeographicLib 2.4

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DST.cpp
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1/**
2 * \file DST.cpp
3 * \brief Implementation for GeographicLib::DST class
4 *
5 * Copyright (c) Charles Karney (2022) <karney@alum.mit.edu> and licensed under
6 * the MIT/X11 License. For more information, see
7 * https://geographiclib.sourceforge.io/
8 **********************************************************************/
9
10#include <GeographicLib/DST.hpp>
11#include <vector>
12
13#include "kissfft.hh"
14
15 namespace GeographicLib {
16
17 using namespace std;
18
19 DST::DST(int N)
20 : _nN(N < 0 ? 0 : N)
21 , _fft(make_shared<fft_t>(fft_t(2 * _nN, false)))
22 {}
23
24 void DST::reset(int N) {
25 N = N < 0 ? 0 : N;
26 if (N == _nN) return;
27 _nN = N;
28 _fft->assign(2 * _nN, false);
29 }
30
31 void DST::fft_transform(real data[], real F[], bool centerp) const {
32 // Implement DST-III (centerp = false) or DST-IV (centerp = true).
33
34 // Elements (0,N], resp. [0,N), of data should be set on input for centerp
35 // = false, resp. true. F must have a size of at least N and on output
36 // elements [0,N) of F contain the transform.
37 if (_nN == 0) return;
38 if (centerp) {
39 for (int i = 0; i < _nN; ++i) {
40 data[_nN+i] = data[_nN-1-i];
41 data[2*_nN+i] = -data[i];
42 data[3*_nN+i] = -data[_nN-1-i];
43 }
44 } else {
45 data[0] = 0; // set [0]
46 for (int i = 1; i < _nN; ++i)
47 data[_nN+i] = data[_nN-i]; // set [N+1,2*N-1]
48 for (int i = 0; i < 2*_nN; ++i)
49 data[2*_nN+i] = -data[i]; // [2*N, 4*N-1]
50 }
51 vector<complex<real>> ctemp(2*_nN);
52 _fft->transform_real(data, ctemp.data());
53 if (centerp) {
54 real d = -Math::pi()/(4*_nN);
55 for (int i = 0, j = 1; i < _nN; ++i, j+=2)
56 ctemp[j] *= exp(complex<real>(0, j*d));
57 }
58 for (int i = 0, j = 1; i < _nN; ++i, j+=2) {
59 F[i] = -ctemp[j].imag() / (2*_nN);
60 }
61 }
62
63 void DST::fft_transform2(real data[], real F[]) const {
64 // Elements [0,N), of data should be set to the N grid center values and F
65 // should have size of at least 2*N. On input elements [0,N) of F contain
66 // the size N transform; on output elements [0,2*N) of F contain the size
67 // 2*N transform.
68 fft_transform(data, F+_nN, true);
69 // Copy DST-IV order N tx to [0,N) elements of data
70 for (int i = 0; i < _nN; ++i) data[i] = F[i+_nN];
71 for (int i = _nN; i < 2*_nN; ++i)
72 // (DST-IV order N - DST-III order N) / 2
73 F[i] = (data[2*_nN-1-i] - F[2*_nN-1-i])/2;
74 for (int i = 0; i < _nN; ++i)
75 // (DST-IV order N + DST-III order N) / 2
76 F[i] = (data[i] + F[i])/2;
77 }
78
79 void DST::transform(function<real(real)> f, real F[]) const {
80 vector<real> data(4 * _nN);
81 real d = Math::pi()/(2 * _nN);
82 for (int i = 1; i <= _nN; ++i)
83 data[i] = f( i * d );
84 fft_transform(data.data(), F, false);
85 }
86
87 void DST::refine(function<real(real)> f, real F[]) const {
88 vector<real> data(4 * _nN);
89 real d = Math::pi()/(4 * _nN);
90 for (int i = 0; i < _nN; ++i)
91 data[i] = f( (2*i + 1) * d );
92 fft_transform2(data.data(), F);
93 }
94
95 Math::real DST::eval(real sinx, real cosx, const real F[], int N) {
96 // Evaluate
97 // y = sum(F[i] * sin((2*i+1) * x), i, 0, N-1)
98 // using Clenshaw summation.
99 // Approx operation count = (N + 5) mult and (2 * N + 2) add
100 real
101 ar = 2 * (cosx - sinx) * (cosx + sinx), // 2 * cos(2 * x)
102 y0 = N & 1 ? F[--N] : 0, y1 = 0; // accumulators for sum
103 // Now N is even
104 while (N > 0) {
105 // Unroll loop x 2, so accumulators return to their original role
106 y1 = ar * y0 - y1 + F[--N];
107 y0 = ar * y1 - y0 + F[--N];
108 }
109 return sinx * (y0 + y1); // sin(x) * (y0 + y1)
110 }
111
112 Math::real DST::integral(real sinx, real cosx, const real F[], int N) {
113 // Evaluate
114 // y = -sum(F[i]/(2*i+1) * cos((2*i+1) * x), i, 0, N-1)
115 // using Clenshaw summation.
116 // Approx operation count = (N + 5) mult and (2 * N + 2) add
117 real
118 ar = 2 * (cosx - sinx) * (cosx + sinx), // 2 * cos(2 * x)
119 y0 = 0, y1 = 0; // accumulators for sum
120 for (--N; N >= 0; --N) {
121 real t = ar * y0 - y1 + F[N]/(2*N+1);
122 y1 = y0; y0 = t;
123 }
124 return cosx * (y1 - y0); // cos(x) * (y1 - y0)
125 }
126
127 Math::real DST::integral(real sinx, real cosx, real siny, real cosy,
128 const real F[], int N) {
129 // return integral(siny, cosy, F, N) - integral(sinx, cosx, F, N);
130 real
131 // 2*cos(y-x)*cos(y+x) -> 2 * cos(2 * x)
132 ac = +2 * (cosy * cosx + siny * sinx) * (cosy * cosx - siny * sinx),
133 // -2*sin(y-x)*sin(y+x) -> 0
134 as = -2 * (siny * cosx - cosy * sinx) * (siny * cosx + cosy * sinx),
135 y0 = 0, y1 = 0, z0 = 0, z1 = 0; // accumulators for sum
136 for (--N; N >= 0; --N) {
137 real
138 ty = ac * y0 + as * z0 - y1 + F[N]/(2*N+1),
139 tz = as * y0 + ac * z0 - z1;
140 y1 = y0; y0 = ty;
141 z1 = z0; z0 = tz;
142 }
143 // B[0] - B[1] = [y0-y1, z0-z1]
144 // F[0] = [cosy + cosx, cosy - cosx] -> [2 * cosx, 0]
145 // (B[0] - B[1]) . F[0]
146 // = [(y0 - y1) * (cosy + cosx) + (z0 - z1) * (cosy - cosx),
147 // (y0 - y1) * (cosy - cosx) + (z0 - z1) * (cosy + cosx),
148 // return -(2nd element)
149 return (y1 - y0) * (cosy - cosx) + (z1 - z0) * (cosy + cosx);
150 }
151
152} // namespace GeographicLib
DST.hpp
Header for GeographicLib::DST class.
real
GeographicLib::Math::real real
Definition GeodSolve.cpp:28
GeographicLib::DST::reset
void reset(int N)
Definition DST.cpp:24
GeographicLib::DST::transform
void transform(std::function< real(real)> f, real F[]) const
Definition DST.cpp:79
GeographicLib::DST::DST
DST(int N=0)
Definition DST.cpp:19
GeographicLib::DST::eval
static real eval(real sinx, real cosx, const real F[], int N)
Definition DST.cpp:95
GeographicLib::DST::N
int N() const
Definition DST.hpp:93
GeographicLib::DST::refine
void refine(std::function< real(real)> f, real F[]) const
Definition DST.cpp:87
GeographicLib::DST::integral
static real integral(real sinx, real cosx, const real F[], int N)
Definition DST.cpp:112
GeographicLib::Math::pi
static T pi()
Definition Math.hpp:199
GeographicLib::Math::real
double real
Definition Math.hpp:110
GeographicLib
Namespace for GeographicLib.
Definition Accumulator.cpp:12
std
Definition NearestNeighbor.hpp:806