// This file is part of www.nand2tetris.org
// and the book "The Elements of Computing Systems"
// by Nisan and Schocken, MIT Press.
// File name: projects/08/FunctionCalls/FibonacciElement/Main.vm

// Computes the n'th element of the Fibonacci series, recursively.
// n is given in argument[0].  Called by the Sys.init function 
// (part of the Sys.vm file), which also pushes the argument[0] 
// parameter before this code starts running.

function Main.fibonacci 0
push argument 0
push constant 2
lt                     // checks if n<2
if-goto IF_TRUE
goto IF_FALSE
label IF_TRUE          // if n<2, return n
push argument 0        
return
label IF_FALSE         // if n>=2, returns fib(n-2)+fib(n-1)
push argument 0
push constant 2
sub
call Main.fibonacci 1  // computes fib(n-2)
push argument 0
push constant 1
sub
call Main.fibonacci 1  // computes fib(n-1)
add                    // returns fib(n-1) + fib(n-2)
return