1 #include <gsl/gsl_math.h>
2 #include <gsl/gsl_errno.h>
3 #include <gsl/gsl_poly.h>
5 /* Find a minimum in x=[0,1] of the interpolating quadratic through
6 * (0,f0) (1,f1) with derivative fp0 at x=0. The interpolating
7 * polynomial is q(x) = f0 + fp0 * z + (f1-f0-fp0) * z^2
11 interp_quad (double f0, double fp0, double f1, double zl, double zh)
13 double fl = f0 + zl*(fp0 + zl*(f1 - f0 -fp0));
14 double fh = f0 + zh*(fp0 + zh*(f1 - f0 -fp0));
15 double c = 2 * (f1 - f0 - fp0); /* curvature */
17 double zmin = zl, fmin = fl;
19 if (fh < fmin) { zmin = zh; fmin = fh; }
21 if (c > 0) /* positive curvature required for a minimum */
23 double z = -fp0 / c; /* location of minimum */
24 if (z > zl && z < zh) {
25 double f = f0 + z*(fp0 + z*(f1 - f0 -fp0));
26 if (f < fmin) { zmin = z; fmin = f; };
33 /* Find a minimum in x=[0,1] of the interpolating cubic through
34 * (0,f0) (1,f1) with derivatives fp0 at x=0 and fp1 at x=1.
36 * The interpolating polynomial is:
38 * c(x) = f0 + fp0 * z + eta * z^2 + xi * z^3
40 * where eta=3*(f1-f0)-2*fp0-fp1, xi=fp0+fp1-2*(f1-f0).
44 cubic (double c0, double c1, double c2, double c3, double z)
46 return c0 + z * (c1 + z * (c2 + z * c3));
50 check_extremum (double c0, double c1, double c2, double c3, double z,
51 double *zmin, double *fmin)
53 /* could make an early return by testing curvature >0 for minimum */
55 double y = cubic (c0, c1, c2, c3, z);
59 *zmin = z; /* accepted new point*/
65 interp_cubic (double f0, double fp0, double f1, double fp1, double zl, double zh)
67 double eta = 3 * (f1 - f0) - 2 * fp0 - fp1;
68 double xi = fp0 + fp1 - 2 * (f1 - f0);
69 double c0 = f0, c1 = fp0, c2 = eta, c3 = xi;
73 zmin = zl; fmin = cubic(c0, c1, c2, c3, zl);
74 check_extremum (c0, c1, c2, c3, zh, &zmin, &fmin);
77 int n = gsl_poly_solve_quadratic (3 * c3, 2 * c2, c1, &z0, &z1);
79 if (n == 2) /* found 2 roots */
81 if (z0 > zl && z0 < zh)
82 check_extremum (c0, c1, c2, c3, z0, &zmin, &fmin);
83 if (z1 > zl && z1 < zh)
84 check_extremum (c0, c1, c2, c3, z1, &zmin, &fmin);
86 else if (n == 1) /* found 1 root */
88 if (z0 > zl && z0 < zh)
89 check_extremum (c0, c1, c2, c3, z0, &zmin, &fmin);
98 interpolate (double a, double fa, double fpa,
99 double b, double fb, double fpb, double xmin, double xmax,
102 /* Map [a,b] to [0,1] */
103 double z, alpha, zmin, zmax;
105 zmin = (xmin - a) / (b - a);
106 zmax = (xmax - a) / (b - a);
115 if (order > 2 && GSL_IS_REAL(fpb)) {
116 z = interp_cubic (fa, fpa * (b - a), fb, fpb * (b - a), zmin, zmax);
118 z = interp_quad (fa, fpa * (b - a), fb, zmin, zmax);
121 alpha = a + z * (b - a);
126 /* recommended values from Fletcher are
127 rho = 0.01, sigma = 0.1, tau1 = 9, tau2 = 0.05, tau3 = 0.5 */
130 minimize (gsl_function_fdf * fn, double rho, double sigma,
131 double tau1, double tau2, double tau3,
132 int order, double alpha1, double *alpha_new)
134 double f0, fp0, falpha, falpha_prev, fpalpha, fpalpha_prev, delta,
136 double alpha = alpha1, alpha_prev = 0.0;
137 double a, b, fa, fb, fpa, fpb;
138 const size_t bracket_iters = 100, section_iters = 100;
141 GSL_FN_FDF_EVAL_F_DF (fn, 0.0, &f0, &fp0);
145 /* Avoid uninitialized variables morning */
148 fpa = fp0; fpb = 0.0;
150 /* Begin bracketing */
152 while (i++ < bracket_iters)
154 falpha = GSL_FN_FDF_EVAL_F (fn, alpha);
156 /* Fletcher's rho test */
158 if (falpha > f0 + alpha * rho * fp0 || falpha >= falpha_prev)
160 a = alpha_prev; fa = falpha_prev; fpa = fpalpha_prev;
161 b = alpha; fb = falpha; fpb = GSL_NAN;
162 break; /* goto sectioning */
165 fpalpha = GSL_FN_FDF_EVAL_DF (fn, alpha);
167 /* Fletcher's sigma test */
169 if (fabs (fpalpha) <= -sigma * fp0)
177 a = alpha; fa = falpha; fpa = fpalpha;
178 b = alpha_prev; fb = falpha_prev; fpb = fpalpha_prev;
179 break; /* goto sectioning */
182 delta = alpha - alpha_prev;
185 double lower = alpha + delta;
186 double upper = alpha + tau1 * delta;
188 alpha_next = interpolate (alpha_prev, falpha_prev, fpalpha_prev,
189 alpha, falpha, fpalpha, lower, upper, order);
194 falpha_prev = falpha;
195 fpalpha_prev = fpalpha;
199 /* Sectioning of bracket [a,b] */
201 while (i++ < section_iters)
206 double lower = a + tau2 * delta;
207 double upper = b - tau3 * delta;
209 alpha = interpolate (a, fa, fpa, b, fb, fpb, lower, upper, order);
212 falpha = GSL_FN_FDF_EVAL_F (fn, alpha);
214 if ((a-alpha)*fpa <= GSL_DBL_EPSILON) {
215 /* roundoff prevents progress */
219 if (falpha > f0 + rho * alpha * fp0 || falpha >= fa)
222 b = alpha; fb = falpha; fpb = GSL_NAN;
226 fpalpha = GSL_FN_FDF_EVAL_DF (fn, alpha);
228 if (fabs(fpalpha) <= -sigma * fp0)
231 return GSL_SUCCESS; /* terminate */
234 if ( ((b-a) >= 0 && fpalpha >= 0) || ((b-a) <=0 && fpalpha <= 0))
236 b = a; fb = fa; fpb = fpa;
237 a = alpha; fa = falpha; fpa = fpalpha;
241 a = alpha; fa = falpha; fpa = fpalpha;