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//! Macro for [scientific](https://docs.rs/scientific/latest/scientific/), the arbitrary precision scientific number. Not intended to be used directly.

use crate::parser::parse_scientific;
use proc_macro::TokenStream;
use proc_macro2::{Delimiter, Group, Punct, Spacing, Span, TokenStream as TokenStream2};
use quote::{quote, ToTokens, TokenStreamExt};
use syn::parse::{Parse, ParseStream};
use syn::{parse_macro_input, Error, LitFloat, LitInt, Token};

mod parser;

// Helper: allow parsing Int or Float
enum LitIntOrLitFloat {
  LitInt(LitInt),
  LitFloat(LitFloat),
}

impl LitIntOrLitFloat {
  fn base10_digits(&self) -> &str {
    match self {
      LitIntOrLitFloat::LitInt(num) => num.base10_digits(),
      LitIntOrLitFloat::LitFloat(num) => num.base10_digits(),
    }
  }

  fn suffix(&self) -> &str {
    match self {
      LitIntOrLitFloat::LitInt(num) => num.suffix(),
      LitIntOrLitFloat::LitFloat(num) => num.suffix(),
    }
  }

  fn span(&self) -> Span {
    match self {
      LitIntOrLitFloat::LitInt(num) => num.span(),
      LitIntOrLitFloat::LitFloat(num) => num.span(),
    }
  }
}

impl Parse for LitIntOrLitFloat {
  fn parse(input: ParseStream) -> Result<Self, Error> {
    let lookahead = input.lookahead1();
    if lookahead.peek(LitInt) {
      input.parse().map(LitIntOrLitFloat::LitInt)
    } else if lookahead.peek(LitFloat) {
      input.parse().map(LitIntOrLitFloat::LitFloat)
    } else {
      Err(lookahead.error())
    }
  }
}

// Helper: Parse LitScientific (optional minus sign and LitIntOrLitFloat)
struct LitScientific {
  neg: Option<Token![-]>,
  num: LitIntOrLitFloat,
}

impl Parse for LitScientific {
  fn parse(input: ParseStream) -> Result<Self, Error> {
    Ok(LitScientific {
      neg: input.parse()?,
      num: input.parse()?,
    })
  }
}

// Helper: Convert to Tokens as a slice
struct AsSlice<'a>(&'a [u8]);

impl<'a> ToTokens for AsSlice<'a> {
  fn to_tokens(&self, stream: &mut TokenStream2) {
    let mut body_stream = TokenStream2::new();
    for d in self.0.iter() {
      d.to_tokens(&mut body_stream);
      body_stream.append(Punct::new(',', Spacing::Alone));
    }
    stream.append(Group::new(Delimiter::Bracket, body_stream));
  }
}

// Actual macro
/// Macro for Scientific (Arbitrary precision scientific number)
///
/// Create a constant `Scientific` number.
#[allow(non_snake_case)]
#[proc_macro]
pub fn Scientific(item: TokenStream) -> TokenStream {
  let LitScientific { neg, num } = parse_macro_input!(item as LitScientific);
  if !num.suffix().is_empty() {
    return TokenStream::from(Error::new(num.span(), "No suffix allowed").to_compile_error());
  }
  match parse_scientific(num.base10_digits()) {
    Err(()) => TokenStream::from(Error::new(num.span(), "Parse error").to_compile_error()),
    Ok(None) => quote!(::scientific::Scientific::ZERO).into(),
    Ok(Some((mantissa, exponent))) => {
      let neg = neg.is_some();
      let len = mantissa.len();
      let mantissa = AsSlice(&mantissa);
      quote!(
        {
          const MANTISSA: [u8; #len] = #mantissa;
          ::scientific::Scientific::unchecked_non_zero_static_new(#neg, &MANTISSA, #exponent)
        }
      )
      .into()
    }
  }
}