43 lines
948 B
C
43 lines
948 B
C
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#include <math.h>
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#include "nrutil.h"
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#define SAFETY 0.9
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#define PGROW -0.2
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#define PSHRNK -0.25
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#define ERRCON 1.89e-4
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void rkqs(y,dydx,n,x,htry,eps,yscal,hdid,hnext,derivs)
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float *hdid,*hnext,*x,dydx[],eps,htry,y[],yscal[];
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int n;
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void (*derivs)();
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{
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void rkck();
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int i;
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float errmax,h,htemp,xnew,*yerr,*ytemp;
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yerr=vector(1,n);
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ytemp=vector(1,n);
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h=htry;
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for (;;) {
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rkck(y,dydx,n,*x,h,ytemp,yerr,derivs);
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errmax=0.0;
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for (i=1;i<=n;i++) errmax=FMAX(errmax,fabs(yerr[i]/yscal[i]));
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errmax /= eps;
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if (errmax <= 1.0) break;
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htemp=SAFETY*h*pow(errmax,PSHRNK);
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h=(h >= 0.0 ? FMAX(htemp,0.1*h) : FMIN(htemp,0.1*h));
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xnew=(*x)+h;
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if (xnew == *x) nrerror("stepsize underflow in rkqs");
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}
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if (errmax > ERRCON) *hnext=SAFETY*h*pow(errmax,PGROW);
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else *hnext=5.0*h;
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*x += (*hdid=h);
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for (i=1;i<=n;i++) y[i]=ytemp[i];
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free_vector(ytemp,1,n);
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free_vector(yerr,1,n);
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}
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#undef SAFETY
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#undef PGROW
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#undef PSHRNK
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#undef ERRCON
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