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extern crate proc_macro;
use proc_macro::{TokenStream, TokenTree};
use proc_macro_hack::proc_macro_hack;
use std::iter::FromIterator;
use quote::quote;
#[proc_macro_hack]
pub fn future_union_impl(item: TokenStream) -> TokenStream {
let mut iter = item.into_iter();
let count_arg_token = iter.next().unwrap_or_else(|| panic!("Too few arguments"));
let count_arg = syn::parse::<syn::LitInt>(
TokenStream::from(count_arg_token)
).unwrap_or_else(|_| panic!("Expecting integer literal")).value();
let comma_1 = iter.next().unwrap_or_else(|| panic!("Too few arguments"));
match comma_1 {
TokenTree::Punct(ref p) if p.as_char() == ',' => (),
_ => panic!("Invalid syntax, expected a comma"),
}
let n_arg_token = iter.next().unwrap_or_else(|| panic!("Too few arguments"));
let n_arg = syn::parse::<syn::LitInt>(
TokenStream::from(n_arg_token)
).unwrap_or_else(|_| panic!("Expecting integer literal")).value();
let comma_2 = iter.next().unwrap_or_else(|| panic!("Too few arguments"));
match comma_2 {
TokenTree::Punct(ref p) if p.as_char() == ',' => (),
_ => panic!("Invalid syntax, expected a comma"),
}
let remaining_tokens = proc_macro2::TokenStream::from(TokenStream::from_iter(iter));
TokenStream::from(
future_union_make_tree(count_arg, n_arg, remaining_tokens)
)
}
fn future_union_make_tree(count: u64, n: u64, expr: proc_macro2::TokenStream) -> proc_macro2::TokenStream {
assert!(n < count);
if count <= 0 {
panic!()
} else if count == 1 {
expr
} else if count == 2 {
if n & 1 == 0 {
quote!( futures::future::Either::A(#expr) )
} else {
quote!( futures::future::Either::B(#expr) )
}
} else {
let max_cap = round_up_to_power_of_2(count);
if first_half(max_cap, n) {
let sub_tree = future_union_make_tree(max_cap/2, n, expr);
quote!( futures::future::Either::A(#sub_tree) )
} else {
let sub_tree = future_union_make_tree(count-max_cap/2, n-max_cap/2, expr);
quote!( futures::future::Either::B(#sub_tree) )
}
}
}
#[cfg(test)]
mod tests {
use super::*;
#[test]
fn round_up_to_power_of_2_test() {
assert_eq!(round_up_to_power_of_2(2), 2);
assert_eq!(round_up_to_power_of_2(3), 4);
assert_eq!(round_up_to_power_of_2(4), 4);
assert_eq!(round_up_to_power_of_2(5), 8);
assert_eq!(round_up_to_power_of_2(6), 8);
assert_eq!(round_up_to_power_of_2(7), 8);
assert_eq!(round_up_to_power_of_2(8), 8);
assert_eq!(round_up_to_power_of_2(9), 16);
}
#[test]
fn first_half_test() {
assert!(first_half(2, 0));
assert!(!first_half(2, 1));
assert!(first_half(4, 0));
assert!(first_half(4, 1));
assert!(!first_half(4, 2));
assert!(!first_half(4, 3));
assert!(first_half(8, 0));
assert!(first_half(8, 1));
assert!(first_half(8, 2));
assert!(first_half(8, 3));
assert!(!first_half(8, 4));
assert!(!first_half(8, 5));
assert!(!first_half(8, 6));
assert!(!first_half(8, 7));
}
}
fn first_half(cap: u64, n: u64) -> bool {
assert!(n < cap);
n < cap/2
}
fn round_up_to_power_of_2(n: u64) -> u64 {
(n as f64).log2().ceil().exp2() as u64
}