#! perl -slw
use strict;
use futures;

sub fibonacci {
    my $n = shift;
    return $n if $n < 2;
    my $r1 = futures->new( \&fib_t1, $n -2 ); #1
    my $r2 = futures->new( \&fib_t1, $n -1 ); #2
    return $r1 + $r2;
}

sub fib_t1 {
    my $n = shift;
    return $n if $n < 2;
    my $r1 = futures->new( \&fib_t2, $n -2 ); #3 & #5
    my $r2 = futures->new( \&fib_t2, $n -1 ); #4 & #6
    return $r1 + $r2;
}

sub fib_t2 {
    my $n = shift;
    return $n if $n < 2;
    return fib_t2( $n -2 ) + fib_t2( $n -1 );
}