%% Cubic interpolation translated to HP by L.R.Linares
%% from a C program by Paul Burke, 2007.
%% This program is provided 'as is' and no guarantee is
%% made in as much as its accuracy or correctness.
%%
%% You need two points of the curve to the left your your 
%% 'quest' and two points to the right of your 'quest'.
%% x0 x1 x2 x3 are assumed to be equally spaced, so only
%% the two x1 and x2 (the closest to the quest) are 
%% necessary.
%% The program returns the same group of numbers it
%% received (see the prompt at the top of the program below)
%% but replaces 'x' by the corresponding 'y' value.

<< "y0 y1 y2 y3 x1 x2 x" PROMPT
   -> y0 y1 y2 y3 x1 x2 x
   <<
      '(x-x1)/(x2-x1)' EVAL 
      'y3-y2-y0+y1' EVAL DUP
      'y0-y1' EVAL SWAP -
      'y2-y0' EVAL
      y1
      -> mu a0 a1 a2 a3
      <<
         '(((a0*mu+a1)*mu+a2)*mu+a3)' EVAL
         y0 y1 y2 y3 x1 x2 6 ROLL
      >>
   >>
>>