Getting rid of built in vectorised function (Max)

I just wanted to see if its possible to get rid of max in the following whilst still producing the same result.

A=[1,2,3,4,5,6,7,8,9,1,2,3]
idy = [diff(A)>0,true] & [true,diff(A)<0];
[~,idx] = max(A); %get rid of max
idy(idx:end) = false;

[the cyclist]

It might help for you to describe conceptually what you are trying to do with the entire algorithm.