//
// Try some recursion with pattern matching... especially in
// conjunction with match to get a similar behavious as in e.g.
// haskell...
//
// Georg Hopp <georg@steffers.org>
//
// Copyright © 2019 Georg Hopp
//
// This program is free software: you can redistribute it and/or modify
// it under the terms of the GNU General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.
//
// This program is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU General Public License for more details.
//
// You should have received a copy of the GNU General Public License
// along with this program.  If not, see <http://www.gnu.org/licenses/>.
//

// Borrowing rules do not apply here because everything lives on the stack.
fn faculty(x: u32) -> u32 {
    match x {
        0 => 1,
        _ => x * faculty(x - 1),
    }
}

// for a tail recursive version we need to pass the currently calculated
// faculty to an inner tail recursive function.
// This will probably be optimized better, because the compiler would be
// able to unroll the complete recursion.
fn tail_faculty(x: u32) -> u32 {
    fn faculty(x: u32, f: u32) -> u32 {
        match x {
            0 => f,
            _ => faculty(x - 1, x * f),
        }
    };
    faculty(x, 1)
}

fn main() {
    for i in 0..10 {
        println!("Fakultät                {} = {}", i, faculty(i));
        println!("tail recursive Fakultät {} = {}", i, tail_faculty(i));
    }
}