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use crate::sexp::{SExp, SExp::*, SLeaf::*, util::*};
impl SExp {
/// Evaluates the s-expression one step.
///
/// **Integer operations**
///
/// Addition, subtraction, multiplication and division
/// are supported.
/// ```rust
/// use melisp::sexp::{SExp::*, SLeaf::*, util::*};
///
/// assert_eq!(
/// scons(Add, scons(1, 1)).step().unwrap(),
/// Atom(Int(2))
/// );
///
/// assert_eq!(
/// scons(Sub, scons(1, 2)).step().unwrap(),
/// Atom(Int(-1))
/// );
///
/// assert_eq!(
/// scons(Mul, scons(2, 3)).step().unwrap(),
/// Atom(Int(6))
/// );
///
/// assert_eq!(
/// scons(Div, scons(6, 2)).step().unwrap(),
/// Atom(Int(3))
/// );
/// ```
///
/// Division truncates the decimals.
/// ```rust
/// use melisp::sexp::{SExp::*, SLeaf::*, util::*};
///
/// assert_eq!(
/// scons(Div, scons(5, 2)).step().unwrap(),
/// Atom(Int(2))
/// );
///
/// ```
///
/// Here's an example of a bit more complicated expression:
/// ```rust
/// use melisp::sexp::{SExp::*, SLeaf::*, util::*};
///
/// fn main() {
/// let exp = scons(Mul, scons(
/// 2,
/// scons(Add, scons(
/// scons(Div, scons(5, 2)),
/// 4
/// ))
/// ));
///
/// let exp = exp.step().unwrap();
/// assert_eq!(exp, scons(Mul, scons(2, scons(Add, scons(2, 4)))));
///
/// let exp = exp.step().unwrap();
/// assert_eq!(exp, scons(Mul, scons(2, 6)));
///
/// let exp = exp.step().unwrap();
/// assert_eq!(exp, Atom(Int(12)));
/// }
///
/// ```
pub fn step(self) -> Result<Self, String> {
match self {
SCons(op, e1) if *op == Atom(Add)
|| *op == Atom(Sub)
|| *op == Atom(Mul)
|| *op == Atom(Div) => match *e1 {
SCons(e1, e2) => match (*e1, *e2) {
(Atom(Int(a)), Atom(Int(b))) => match *op {
Atom(Add) => Ok(Atom(Int(a + b))),
Atom(Sub) => Ok(Atom(Int(a - b))),
Atom(Mul) => Ok(Atom(Int(a * b))),
Atom(Div) => Ok(Atom(Int(a / b))),
_ => panic!("unreachable as per match arm"),
}
(Atom(Int(a)), e2) => Ok(scons(op, scons(a, e2.step()?))),
(e1, e2) => Ok(scons(op, scons(e1.step()?, e2))),
},
_ => Err(format!("{:?} should be given two arguments, only one was given: {:?}", op, e1)),
},
t => Err(format!("unimplemented: {:?}.step()", t)),
}
}
}
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