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//! An implementation of procedural macro to generate compile-time constants for powers of two.

extern crate proc_macro;

use proc_macro::TokenStream;
use proc_macro2::Span;
use proc_macro_hack::proc_macro_hack;
use quote::quote;
use syn::{
    parse::{Parse, ParseStream, Result},
    parse_macro_input, Error, LitInt,
};

struct Power {
    power: u32,
    base_type: syn::Ident,
}

impl Parse for Power {
    fn parse(input: ParseStream) -> Result<Self> {
        let lit: LitInt = input.parse()?;
        let power = lit.base10_parse::<u32>()?;

        let mut base_type = syn::Ident::new("usize", Span::call_site());
        if let Some(_comma) = input.parse::<Option<syn::Token![,]>>()? {
            if let Some(type_) = input.parse::<Option<syn::Ident>>()? {
                base_type = type_;
            }
        }

        Ok(Power { power, base_type })
    }
}

/// ```no_run
/// const POW2: usize = power_of_two!(2);
/// const POW3: isize = power_of_two!(2, isize);
/// ```
#[proc_macro_hack]
pub fn power_of_two(input: TokenStream) -> TokenStream {
    // Parse the input tokens into a syntax tree
    let Power { power, base_type } = parse_macro_input!(input);

    let expanded = if base_type == "usize" {
        let value = 2usize.pow(power);
        quote!(#value)
    } else if base_type == "u8" {
        let value = 2u8.pow(power);
        quote!(#value)
    } else if base_type == "i8" {
        let value = 2i8.pow(power);
        quote!(#value)
    } else if base_type == "u16" {
        let value = 2u16.pow(power);
        quote!(#value)
    } else if base_type == "i16" {
        let value = 2i16.pow(power);
        quote!(#value)
    } else if base_type == "u32" {
        let value = 2u32.pow(power);
        quote!(#value)
    } else if base_type == "i32" {
        let value = 2i32.pow(power);
        quote!(#value)
    } else if base_type == "u128" {
        let value = 2u128.pow(power);
        quote!(#value)
    } else if base_type == "i128" {
        let value = 2i128.pow(power);
        quote!(#value)
    } else if base_type == "usize" {
        let value = 2usize.pow(power);
        quote!(#value)
    } else if base_type == "isize" {
        let value = 2isize.pow(power);
        quote!(#value)
    } else {
        Error::new_spanned(base_type, "Unknown integer type").to_compile_error()
    };

    // Hand the output tokens back to the compiler
    TokenStream::from(expanded)
}