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Fix spline extrapolation #81

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Merged
toastedcrumpets merged 4 commits into from
Aug 22, 2026
+7 −8
Merged

Fix spline extrapolation #81

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69c73bc
fix: Use low BC value for low extrapolation with linear splines
kcooley-cvd Aug 21, 2026
be4a995
fix: First-derivative calculation for low extrapolation of splines
kcooley-cvd Aug 21, 2026
1fe0bb1
fix: First-derivative calculation for high extrapolation of splines
kcooley-cvd Aug 21, 2026
4362b38
fix: Quadratic terms for spline extrapolation
kcooley-cvd Aug 21, 2026
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15 changes: 7 additions & 8 deletions src/magnet/magnet/math/spline.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -135,18 +135,17 @@ class Spline : private std::vector<std::pair<double, double>> {
const double lx = xval - x(0);

if (_type == LINEAR)
return lx * _BCHighVal + y(0);
return lx * _BCLowVal + y(0);

const double firstDeriv =
(y(1) - y(0)) / h(0) - 2 * h(0) * (_data[0].b + 2 * _data[1].b) / 6;
const double firstDeriv = _data[0].c;

switch (_BCLow) {
case FIXED_1ST_DERIV_BC:
return lx * _BCLowVal + y(0);
case FIXED_2ND_DERIV_BC:
return lx * lx * _BCLowVal + firstDeriv * lx + y(0);
return 0.5 * lx * lx * _BCLowVal + firstDeriv * lx + y(0);
case PARABOLIC_RUNOUT_BC:
return lx * lx * _ddy[0] + lx * firstDeriv + y(0);
return 0.5 * lx * lx * _ddy[0] + lx * firstDeriv + y(0);
}
throw std::runtime_error("Unknown BC");
}
Expand All @@ -158,16 +157,16 @@ class Spline : private std::vector<std::pair<double, double>> {
return lx * _BCHighVal + y(size() - 1);

const double firstDeriv =
2 * h(size() - 2) * (_ddy[size() - 2] + 2 * _ddy[size() - 1]) / 6 +
h(size() - 2) * (_ddy[size() - 2] + 2 * _ddy[size() - 1]) / 6 +
(y(size() - 1) - y(size() - 2)) / h(size() - 2);

switch (_BCHigh) {
case FIXED_1ST_DERIV_BC:
return lx * _BCHighVal + y(size() - 1);
case FIXED_2ND_DERIV_BC:
return lx * lx * _BCHighVal + firstDeriv * lx + y(size() - 1);
return 0.5 * lx * lx * _BCHighVal + firstDeriv * lx + y(size() - 1);
case PARABOLIC_RUNOUT_BC:
return lx * lx * _ddy[size() - 1] + lx * firstDeriv + y(size() - 1);
return 0.5 * lx * lx * _ddy[size() - 1] + lx * firstDeriv + y(size() - 1);
}
throw std::runtime_error("Unknown BC");
}
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