use proc_macro::{TokenStream, TokenTree};
use quote::quote;

/// Subtracts two numbers and calls a given callback macro with the result.
///
/// # Input
/// `number - number, callback`
///
/// # Examples
/// ```
/// # use std::any::TypeId;
/// use util_macros::sub;
///
/// macro_rules! sub_cb {
///     ($num: literal) => {
///         $num
///     };
/// }
///
/// type Foo = [u8; sub!(5 - 2, sub_cb)];
///
/// assert_eq!(TypeId::of::<Foo>(), TypeId::of::<[u8; 3]>());
/// ```
/// <br>
///
/// The callback is called with the identifier `overflow` if a overflow occurs.
/// ```
/// # use std::any::TypeId;
/// use util_macros::sub;
///
/// macro_rules! sub_cb {
///     ($num: literal) => {
///         $num
///     };
///
///     (overflow) => {
///         128
///     };
/// }
///
/// type Foo = [u8; sub!(3 - 10, sub_cb)];
///
/// assert_eq!(TypeId::of::<Foo>(), TypeId::of::<[u8; 128]>());
/// ```
#[proc_macro]
pub fn sub(input: TokenStream) -> TokenStream
{
    let mut input_tt_iter = input.into_iter();

    let num_a = match input_tt_iter.next().unwrap() {
        TokenTree::Literal(lit) => lit.to_string().parse::<u32>().unwrap(),
        _ => {
            panic!("Expected a number literal");
        }
    };

    match input_tt_iter.next().unwrap() {
        TokenTree::Punct(punct) if punct.as_char() == '-' => {}
        _ => {
            panic!("Expected a '-' token");
        }
    };

    let num_b = match input_tt_iter.next().unwrap() {
        TokenTree::Literal(lit) => lit.to_string().parse::<u32>().unwrap(),
        _ => {
            panic!("Expected a number literal");
        }
    };

    match input_tt_iter.next().unwrap() {
        TokenTree::Punct(punct) if punct.as_char() == ',' => {}
        _ => {
            panic!("Expected a ',' token");
        }
    };

    let cb_ident = match input_tt_iter.next().unwrap() {
        TokenTree::Ident(cb_ident) => {
            proc_macro2::Ident::new(&cb_ident.to_string(), cb_ident.span().into())
        }
        _ => {
            panic!("Expected a identifier");
        }
    };

    let Some(subtracted) = num_a.checked_sub(num_b) else {
        return quote! {
            #cb_ident!(overflow)
        }
        .into();
    };

    let subtracted_lit = proc_macro2::Literal::u32_unsuffixed(subtracted);

    quote! {
        #cb_ident!(#subtracted_lit)
    }
    .into()
}