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use std::fmt;
use std::num::NonZeroUsize;

/// An opaque identifier that is associated with AST items.
///
/// The default implementation for an identifier is empty, meaning it does not
/// hold any value, and attempting to perform lookups over it will fail with an
/// error indicating that it's empty with the string `Id(*)`.
#[derive(Clone, Copy, PartialEq, Eq, Hash)]
#[repr(transparent)]
pub struct Id(NonZeroUsize);

impl Id {
    /// Construct the initial (non-empty) id.
    pub fn initial() -> Self {
        Id(NonZeroUsize::new(1).unwrap())
    }

    /// Construct a new opaque identifier.
    pub fn new(index: usize) -> Option<Id> {
        NonZeroUsize::new(index).map(Self)
    }

    /// Get the next id.
    pub fn next(self) -> Option<Id> {
        let n = self.0.get().checked_add(1)?;
        let n = NonZeroUsize::new(n)?;
        Some(Self(n))
    }
}

impl Default for Id {
    fn default() -> Self {
        Self::initial()
    }
}

impl fmt::Debug for Id {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "Id({})", self.0.get())
    }
}