How to do Not-a-Knot spline in MATLAB without MATLAB's "spline" function?

I have the following code for a spline with natural end conditions. I am tasked with modifying it to work with not-a-knot and zero slope clamped end conditions. I am having trouble figuring out how to do that. Can anyone help me??

    function [yy,dy,d2] = my_spline(x,y,xx)
    n=length(x);
    if length(y)~=n, error('x and y must have the same length'); end
    if any(diff(x)<=0), error('x not strictly ascending'); end
    m=length(xx);
    aa(1,1)=1; aa(n,n)=1;
    bb=zeros(n,1);
    for i=2:n-1
        aa(i,i-1)=h(x,i-1);
        aa(i,i)=2*(h(x,i-1)+h(x,i));
        aa(i,i+1)=h(x,i);
        bb(i)=3*(fd(i+1,i,x,y)-fd(i,i-1,x,y));
    end
    c=aa\bb;
    for i=1:n-1
        a(i)=y(i);
        b(i)=fd(i+1,i,x,y)-h(x,i)/3*(2*c(i)+c(i+1));
        d(i)=(c(i+1)-c(i))/3/h(x,i);
    end
    for i=1:m
        [yy(i),dy(i),d2(i)]=SplineInterp(x,n,a,b,c,d,xx(i));
    end
    end
    function hh=h(x,i)
    hh=x(i+1)-x(i);
    end
    function fdd=fd(i,j,x,y)
    fdd=(y(i)-y(j))/(x(i)-x(j));
    end
    function [yyy,dyy,d2y]=SplineInterp(x,n,a,b,c,d,xi)
    for ii=1:n-1
        if xi>=x(ii)-0.000001 & xi<=x(ii+1)+0.000001
            yyy=a(ii)+b(ii)*(xi-x(ii))+c(ii)*(xi-x(ii))^2+d(ii)*(xi-x(ii))^3;
            dyy=b(ii)+2*c(ii)*(xi-x(ii))+3*d(ii)*(xi-x(ii))^2;
            d2y=2*c(ii)+6*d(ii)*(xi-x(ii));
            break
        end
    end
    end

[Joost]

Could you please present the code in a code block for better readibility?