reminder-bot/command_attributes/src/attributes.rs

use std::fmt::{self, Write};

use proc_macro2::Span;
use syn::{
    parse::{Error, Result},
    spanned::Spanned,
    Attribute, Ident, Lit, LitStr, Meta, NestedMeta, Path,
};

use crate::{
    structures::{ApplicationCommandOptionType, Arg},
    util::{AsOption, LitExt},
};

#[derive(Debug, Clone, Copy, PartialEq)]
pub enum ValueKind {
    // #[<name>]
    Name,

    // #[<name> = <value>]
    Equals,

    // #[<name>([<value>, <value>, <value>, ...])]
    List,

    // #[<name>([<prop> = <value>, <prop> = <value>, ...])]
    EqualsList,

    // #[<name>(<value>)]
    SingleList,
}

impl fmt::Display for ValueKind {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        match self {
            ValueKind::Name => f.pad("`#[<name>]`"),
            ValueKind::Equals => f.pad("`#[<name> = <value>]`"),
            ValueKind::List => f.pad("`#[<name>([<value>, <value>, <value>, ...])]`"),
            ValueKind::EqualsList => {
                f.pad("`#[<name>([<prop> = <value>, <prop> = <value>, ...])]`")
            }
            ValueKind::SingleList => f.pad("`#[<name>(<value>)]`"),
        }
    }
}

fn to_ident(p: Path) -> Result<Ident> {
    if p.segments.is_empty() {
        return Err(Error::new(p.span(), "cannot convert an empty path to an identifier"));
    }

    if p.segments.len() > 1 {
        return Err(Error::new(p.span(), "the path must not have more than one segment"));
    }

    if !p.segments[0].arguments.is_empty() {
        return Err(Error::new(p.span(), "the singular path segment must not have any arguments"));
    }

    Ok(p.segments[0].ident.clone())
}

#[derive(Debug)]
pub struct Values {
    pub name: Ident,
    pub literals: Vec<(Option<String>, Lit)>,
    pub kind: ValueKind,
    pub span: Span,
}

impl Values {
    #[inline]
    pub fn new(
        name: Ident,
        kind: ValueKind,
        literals: Vec<(Option<String>, Lit)>,
        span: Span,
    ) -> Self {
        Values { name, literals, kind, span }
    }
}

pub fn parse_values(attr: &Attribute) -> Result<Values> {
    fn is_list_or_named_list(meta: &NestedMeta) -> ValueKind {
        match meta {
            // catch if the nested value is a literal value
            NestedMeta::Lit(_) => ValueKind::List,
            // catch if the nested value is a meta value
            NestedMeta::Meta(m) => match m {
                // path => some quoted value
                Meta::Path(_) => ValueKind::List,
                Meta::List(_) | Meta::NameValue(_) => ValueKind::EqualsList,
            },
        }
    }

    let meta = attr.parse_meta()?;

    match meta {
        Meta::Path(path) => {
            let name = to_ident(path)?;

            Ok(Values::new(name, ValueKind::Name, Vec::new(), attr.span()))
        }
        Meta::List(meta) => {
            let name = to_ident(meta.path)?;
            let nested = meta.nested;

            if nested.is_empty() {
                return Err(Error::new(attr.span(), "list cannot be empty"));
            }

            if is_list_or_named_list(nested.first().unwrap()) == ValueKind::List {
                let mut lits = Vec::with_capacity(nested.len());

                for meta in nested {
                    match meta {
                        // catch if the nested value is a literal value
                        NestedMeta::Lit(l) => lits.push((None, l)),
                        // catch if the nested value is a meta value
                        NestedMeta::Meta(m) => match m {
                            // path => some quoted value
                            Meta::Path(path) => {
                                let i = to_ident(path)?;
                                lits.push((None, Lit::Str(LitStr::new(&i.to_string(), i.span()))))
                            }
                            Meta::List(_) | Meta::NameValue(_) => {
                                return Err(Error::new(attr.span(), "cannot nest a list; only accept literals and identifiers at this level"))
                            }
                        },
                    }
                }

                let kind = if lits.len() == 1 { ValueKind::SingleList } else { ValueKind::List };

                Ok(Values::new(name, kind, lits, attr.span()))
            } else {
                let mut lits = Vec::with_capacity(nested.len());

                for meta in nested {
                    match meta {
                        // catch if the nested value is a literal value
                        NestedMeta::Lit(_) => {
                            return Err(Error::new(attr.span(), "key-value pairs expected"))
                        }
                        // catch if the nested value is a meta value
                        NestedMeta::Meta(m) => match m {
                            Meta::NameValue(n) => {
                                let name = to_ident(n.path)?.to_string();
                                let value = n.lit;

                                lits.push((Some(name), value));
                            }
                            Meta::List(_) | Meta::Path(_) => {
                                return Err(Error::new(attr.span(), "key-value pairs expected"))
                            }
                        },
                    }
                }

                Ok(Values::new(name, ValueKind::EqualsList, lits, attr.span()))
            }
        }
        Meta::NameValue(meta) => {
            let name = to_ident(meta.path)?;
            let lit = meta.lit;

            Ok(Values::new(name, ValueKind::Equals, vec![(None, lit)], attr.span()))
        }
    }
}

#[derive(Debug, Clone)]
struct DisplaySlice<'a, T>(&'a [T]);

impl<'a, T: fmt::Display> fmt::Display for DisplaySlice<'a, T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        let mut iter = self.0.iter().enumerate();

        match iter.next() {
            None => f.write_str("nothing")?,
            Some((idx, elem)) => {
                write!(f, "{}: {}", idx, elem)?;

                for (idx, elem) in iter {
                    f.write_char('\n')?;
                    write!(f, "{}: {}", idx, elem)?;
                }
            }
        }

        Ok(())
    }
}

#[inline]
fn is_form_acceptable(expect: &[ValueKind], kind: ValueKind) -> bool {
    if expect.contains(&ValueKind::List) && kind == ValueKind::SingleList {
        true
    } else {
        expect.contains(&kind)
    }
}

#[inline]
fn validate(values: &Values, forms: &[ValueKind]) -> Result<()> {
    if !is_form_acceptable(forms, values.kind) {
        return Err(Error::new(
            values.span,
            // Using the `_args` version here to avoid an allocation.
            format_args!("the attribute must be in of these forms:\n{}", DisplaySlice(forms)),
        ));
    }

    Ok(())
}

#[inline]
pub fn parse<T: AttributeOption>(values: Values) -> Result<T> {
    T::parse(values)
}

pub trait AttributeOption: Sized {
    fn parse(values: Values) -> Result<Self>;
}

impl AttributeOption for Vec<String> {
    fn parse(values: Values) -> Result<Self> {
        validate(&values, &[ValueKind::List])?;

        Ok(values.literals.into_iter().map(|(_, l)| l.to_str()).collect())
    }
}

impl AttributeOption for String {
    #[inline]
    fn parse(values: Values) -> Result<Self> {
        validate(&values, &[ValueKind::Equals, ValueKind::SingleList])?;

        Ok(values.literals[0].1.to_str())
    }
}

impl AttributeOption for bool {
    #[inline]
    fn parse(values: Values) -> Result<Self> {
        validate(&values, &[ValueKind::Name, ValueKind::SingleList])?;

        Ok(values.literals.get(0).map_or(true, |(_, l)| l.to_bool()))
    }
}

impl AttributeOption for Ident {
    #[inline]
    fn parse(values: Values) -> Result<Self> {
        validate(&values, &[ValueKind::SingleList])?;

        Ok(values.literals[0].1.to_ident())
    }
}

impl AttributeOption for Vec<Ident> {
    #[inline]
    fn parse(values: Values) -> Result<Self> {
        validate(&values, &[ValueKind::List])?;

        Ok(values.literals.into_iter().map(|(_, l)| l.to_ident()).collect())
    }
}

impl AttributeOption for Option<String> {
    fn parse(values: Values) -> Result<Self> {
        validate(&values, &[ValueKind::Name, ValueKind::Equals, ValueKind::SingleList])?;

        Ok(values.literals.get(0).map(|(_, l)| l.to_str()))
    }
}

impl AttributeOption for Arg {
    fn parse(values: Values) -> Result<Self> {
        validate(&values, &[ValueKind::EqualsList])?;

        let mut arg: Arg = Default::default();

        for (key, value) in &values.literals {
            match key {
                Some(s) => match s.as_str() {
                    "name" => {
                        arg.name = value.to_str();
                    }
                    "description" => {
                        arg.description = value.to_str();
                    }
                    "required" => {
                        arg.required = value.to_bool();
                    }
                    "kind" => arg.kind = ApplicationCommandOptionType::from_str(value.to_str()),
                    _ => {
                        return Err(Error::new(key.span(), "unexpected attribute"));
                    }
                },
                _ => {
                    return Err(Error::new(key.span(), "unnamed attribute"));
                }
            }
        }

        Ok(arg)
    }
}

impl<T: AttributeOption> AttributeOption for AsOption<T> {
    #[inline]
    fn parse(values: Values) -> Result<Self> {
        Ok(AsOption(Some(T::parse(values)?)))
    }
}

macro_rules! attr_option_num {
    ($($n:ty),*) => {
        $(
            impl AttributeOption for $n {
                fn parse(values: Values) -> Result<Self> {
                    validate(&values, &[ValueKind::SingleList])?;

                    Ok(match &values.literals[0].1 {
                        Lit::Int(l) => l.base10_parse::<$n>()?,
                        l => {
                            let s = l.to_str();
                            // Use `as_str` to guide the compiler to use `&str`'s parse method.
                            // We don't want to use our `parse` method here (`impl AttributeOption for String`).
                            match s.as_str().parse::<$n>() {
                                Ok(n) => n,
                                Err(_) => return Err(Error::new(l.span(), "invalid integer")),
                            }
                        }
                    })
                }
            }

            impl AttributeOption for Option<$n> {
                #[inline]
                fn parse(values: Values) -> Result<Self> {
                    <$n as AttributeOption>::parse(values).map(Some)
                }
            }
        )*
    }
}

attr_option_num!(u16, u32, usize);