Single-variable linear level finding ("inverse" INTERP1): findX

XI = findX(X,Y,YI) estimates the XI values at which the dependent
variables Y reach or cross a specified target level YI (scalar value). If there are multiple solutions, findX finds all of them. It may be seen as the "inverse" operation of the MATLAB's function
INTERP1, under 'linear' method.

X is a vector containing a sequence of monotone increasing values of the independent variable. Y is a vector containing the discrete values of the dependent variable (underlying function).

[XI,IDEXACT] = findX(X,Y,YI) returns in IDEXACT the indices where the original values of Y exactly reach the target level YI, i.e. Y(IDEXACT)=YI, so interpolation was not needed.