50 lines
1.1 KiB
C++
50 lines
1.1 KiB
C++
/**********************************************************
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This software is part of J.-S. Caux's ABACUS library.
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Copyright (c) J.-S. Caux.
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-----------------------------------------------------------
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File: polint.cc
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Purpose: Polynomial interpolation
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***********************************************************/
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#include "ABACUS.h"
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using namespace std;
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void ABACUS::polint(Vect_DP& xa, Vect_DP& ya, const DP x, DP& y, DP& dy)
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{
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// Polynomial interpolation/extrapolation, NR page 113.
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int i, m, ns=0;
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DP den, dif, dift, ho, hp, w;
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int n = xa.size();
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Vect_DP c(n), d(n);
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dif = fabs(x-xa[0]);
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for (i = 0; i < n; i++) {
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if ((dift = fabs(x-xa[i])) < dif) {
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ns = i;
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dif = dift;
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}
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c[i] = ya[i];
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d[i] = ya[i];
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}
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y = ya[ns--];
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for (m = 1; m < n; m++) {
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for (i = 0; i < n-m; i++) {
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ho = xa[i] - x;
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hp = xa[i+m] - x;
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w = c[i+1] - d[i];
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if ((den = ho-hp) == 0.0) ABACUSerror("Error in routine polint.");
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den = w/den;
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d[i] = hp * den;
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c[i] = ho * den;
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}
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y += (dy = (2 * (ns + 1) < (n-m) ? c[ns + 1] : d[ns--]));
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}
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}
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