TRK: add fast getPredictedChi2 - #15696
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Signed-off-by: Felix Schlepper <felix.schlepper@cern.ch>
shahor02
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Aug 19, 2026
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Adds a fast alternative to getPredictedChi2 when only chi2 is required as is the case for the Cells in ITS.
One can skip the full inversion of the cov matrix by using Cholsky decomposition and a fwd substitution.
This reduces the required double operations by 88% and the output stays consistent since the full inversion only catches degenerate cases which produce negative chi2 which are discarded in any case.