Matlab/Octave for Parabolic Interpolation

function [p,y,a] = qint(ym1,y0,yp1) 
%QINT - quadratic interpolation of three 
%       uniformly spaced samples
%
% [p,y,a] = qint(ym1,y0,yp1) 
%
% returns the 
%   displacement p, 
%   amplitude y, and
%   half-curvature a
% for a parabola fit through three points. 
% Parabola is given by y(x) = a*(x-p)^2+b, 
% where y(-1)=ym1, y(0)=y0, y(1)=yp1. 

p = (yp1 - ym1)/(2*(2*y0 - yp1 - ym1)); 
y = y0 - 0.25*(ym1-yp1)*p;
a = 0.5*(ym1 - 2*y0 + yp1);