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extern crate quote;
extern crate proc_macro2;
extern crate syn;
use proc_macro::TokenStream;
use proc_macro2::TokenStream as TokenStream2;
use proc_macro2::TokenTree as TokenTree2;
use std::str::FromStr;
use std::any::type_name;
use quote::quote;
#[proc_macro_derive(GenRandom, attributes(prob, scale, bias))]
pub fn gen_random_derive(input: TokenStream) -> TokenStream {
// Construct a representation of Rust code as a syntax tree
// that we can manipulate
let ast = syn::parse(input).unwrap();
// Build the trait implementation
impl_gen_random(&ast)
}
fn get_attribute_literal(attrs: &[syn::Attribute], name: &str) -> Option<proc_macro2::Literal> {
let attr = attrs.iter().find(|a| {
let path = &a.path;
if let Some(ident) = path.get_ident() {
ident == name
} else {
false
}
})?;
let tokens: TokenStream2 = attr.tokens.clone().into();
let mut tokens: Vec<TokenTree2> = tokens.into_iter().collect();
if tokens.len() != 2 {
panic!("Expected {name} = <value>");
}
use TokenTree2::{Punct, Literal};
match &tokens[0] {
Punct(equals) if equals.as_char() == '=' => {}
_ => panic!("Expected = after {name} attribute"),
};
let Literal(literal) = tokens.remove(1) else {
panic!("Bad value for {name} attribute.");
};
Some(literal)
}
fn parse_attribute_value<T: FromStr>(attrs: &[syn::Attribute], name: &str) -> Option<T> {
let literal = get_attribute_literal(attrs, name)?;
let Ok(value) = literal.to_string().parse() else {
panic!("Bad {} for {name} attribute", type_name::<T>())
};
Some(value)
}
fn generate_fields(fields: &syn::Fields) -> impl quote::ToTokens {
let mut field_values = quote! {};
for field in fields.iter() {
if let Some(name) = &field.ident {
field_values.extend(quote! {#name: });
}
let ty = &field.ty;
field_values.extend(quote! { <#ty as GenRandom>::gen_random(rng) });
if let Some(scale) = get_attribute_literal(&field.attrs, "scale") {
field_values.extend(quote! { * #scale });
}
if let Some(bias) = get_attribute_literal(&field.attrs, "bias") {
field_values.extend(quote! { + #bias });
}
field_values.extend(quote! { , });
}
// surround the field values with either () or {} brackets
match fields {
syn::Fields::Named(_) => {
Some(proc_macro2::Group::new(
proc_macro2::Delimiter::Brace,
field_values
))
},
syn::Fields::Unnamed(_) => {
Some(proc_macro2::Group::new(
proc_macro2::Delimiter::Parenthesis,
field_values
))
},
syn::Fields::Unit => None,
}
}
// very very precise summation algorithm
// see https://en.wikipedia.org/wiki/Kahan_summation_algorithm
fn kahan_sum(it: impl IntoIterator<Item = f64>) -> f64 {
let mut it = it.into_iter();
let mut sum = 0.0;
let mut c = 0.0;
while let Some(x) = it.next() {
let y = x - c;
let t = sum + y;
c = (t - sum) - y;
sum = t;
}
sum
}
fn impl_gen_random(ast: &syn::DeriveInput) -> TokenStream {
let name = &ast.ident;
let mut function_body;
match &ast.data {
syn::Data::Enum(enumeration) => {
let variants = &enumeration.variants;
let prob_sum = kahan_sum(variants.iter().map(|variant| {
match parse_attribute_value(&variant.attrs, "prob") {
Some(prob) => if prob >= 0.0 {
prob
} else {
panic!("Variant {} has negative probability", variant.ident)
},
None => panic!("Variant {} has no probability", variant.ident)
}
}));
if prob_sum <= f64::EPSILON {
panic!("Sum of probabilties is (basically) zero.");
}
// ideally we would just do
// let mut variant: f64 = rng.gen_range(0.0..1.0);
// variant -= variant1_probability;
// if variant < 0.0 { bla bla bla }
// variant -= variant2_probability;
// if variant < 0.0 { bla bla bla }
// etc.
// but because of floating point imprecision, it's possible
// that all if conditions are false.
// however we know that for each subtraction at most one ULP is lost.
// so we'll be fine as long as we put the end of the range at
// prob_sum * (1.0 - (variant_count + 2) * ULP)
// the + 2 is for the imprecision lost in kahan_sum and one more just to be sure.
let variant_max = prob_sum * (1.0 - f64::EPSILON * (variants.len() + 2) as f64);
function_body = quote! {
let mut variant: f64 = rng.gen_range(0.0..=#variant_max);
};
// this test value ensures that the gen_random function never panicks.
let mut test_variant = variant_max;
// parse enum fields
for variant in variants.iter() {
// Note: None case was checked above when computing prob_sum
let probability: f64 = parse_attribute_value(&variant.attrs, "prob").unwrap();
let name = &variant.ident;
let field_values = generate_fields(&variant.fields);
function_body.extend(quote! {
variant -= #probability;
if variant < 0.0 { return Self::#name #field_values; }
});
test_variant -= probability;
}
if test_variant >= 0.0 {
panic!("i did floating-point math wrong. this should never happen. (test_variant = {test_variant})");
}
function_body.extend(quote! {
panic!("RNG returned value outside of range.")
});
},
syn::Data::Struct(structure) => {
let field_values = generate_fields(&structure.fields);
function_body = quote! {
Self #field_values
};
},
syn::Data::Union(_) => panic!("derive(GenRandom) cannot be applied to unions."),
};
let gen = quote! {
impl GenRandom for #name {
fn gen_random(rng: &mut impl rand::Rng) -> Self {
#function_body
}
}
};
//println!("{gen}");
gen.into()
}
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