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// Copyright 2019-2022 PureStake Inc.
// This file is part of Moonbeam.

// Moonbeam 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.

// Moonbeam 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.

#![crate_type = "proc-macro"]
extern crate proc_macro;

use proc_macro::TokenStream;
use proc_macro2::Literal;
use quote::{quote, quote_spanned};
use sha3::{Digest, Keccak256};
use syn::{
    parse_macro_input, spanned::Spanned, Attribute, Expr, ExprLit, Ident, ItemEnum, Lit, LitStr,
};

struct Bytes(Vec<u8>);

impl ::std::fmt::Debug for Bytes {
    #[inline]
    fn fmt(&self, f: &mut std::fmt::Formatter) -> ::std::fmt::Result {
        let data = &self.0;
        write!(f, "[")?;
        if !data.is_empty() {
            write!(f, "{:#04x}u8", data[0])?;
            for unit in data.iter().skip(1) {
                write!(f, ", {:#04x}", unit)?;
            }
        }
        write!(f, "]")
    }
}

#[proc_macro]
pub fn keccak256(input: TokenStream) -> TokenStream {
    let lit_str = parse_macro_input!(input as LitStr);

    let hash = Keccak256::digest(&lit_str.value());

    let bytes = Bytes(hash.to_vec());
    let eval_str = format!("{:?}", bytes);
    let eval_ts: proc_macro2::TokenStream = eval_str.parse().unwrap_or_else(|_| {
        panic!(
            "Failed to parse the string \"{}\" to TokenStream.",
            eval_str
        );
    });
    quote!(#eval_ts).into()
}

/// This macro allows to associate to each variant of an enumeration a discriminant (of type u32
/// whose value corresponds to the first 4 bytes of the Hash Keccak256 of the character string
///indicated by the user of this macro.
///
/// Usage:
///
/// ```ignore
/// #[generate_function_selector]
/// enum Action {
/// 	Toto = "toto()",
/// 	Tata = "tata()",
/// }
/// ```
///
/// Extended to:
///
/// ```rust
/// #[repr(u32)]
/// enum Action {
/// 	Toto = 119097542u32,
/// 	Tata = 1414311903u32,
/// }
/// ```
#[proc_macro_attribute]
pub fn generate_function_selector(_: TokenStream, input: TokenStream) -> TokenStream {
    let item = parse_macro_input!(input as ItemEnum);

    let ItemEnum {
        attrs,
        vis,
        enum_token,
        ident,
        variants,
        ..
    } = item;

    let mut ident_expressions: Vec<Ident> = vec![];
    let mut variant_expressions: Vec<Expr> = vec![];
    let mut variant_attrs: Vec<Vec<Attribute>> = vec![];
    for variant in variants {
        match variant.discriminant {
            Some((_, Expr::Lit(ExprLit { lit, .. }))) =>
                if let Lit::Str(lit_str) = lit {
                    let digest = Keccak256::digest(&lit_str.value());
                    let selector = u32::from_be_bytes([digest[0], digest[1], digest[2], digest[3]]);
                    ident_expressions.push(variant.ident);
                    variant_expressions.push(Expr::Lit(ExprLit {
                        lit: Lit::Verbatim(Literal::u32_suffixed(selector)),
                        attrs: Default::default(),
                    }));
                    variant_attrs.push(variant.attrs);
                } else {
                    return quote_spanned! {
                        lit.span() => compile_error("Expected literal string");
                    }
                    .into()
                },
            Some((_eg, expr)) =>
                return quote_spanned! {
                    expr.span() => compile_error("Expected literal");
                }
                .into(),
            None =>
                return quote_spanned! {
                    variant.span() => compile_error("Each variant must have a discriminant");
                }
                .into(),
        }
    }

    (quote! {
        #(#attrs)*
        #[derive(num_enum::TryFromPrimitive, num_enum::IntoPrimitive)]
        #[repr(u32)]
        #vis #enum_token #ident {
            #(
                #(#variant_attrs)*
                #ident_expressions = #variant_expressions,
            )*
        }
    })
    .into()
}