//! Infrastructure for contextual error messages
use std::fmt;

use proc_macro2::Span;

use syn::{spanned::Spanned, Ident, Member, Path};

/// Reference to a compound's parent
///
/// This reference can be converted to a hopefully-useful human-readable
/// string via [`std::fmt::Display`].
#[derive(Clone, Debug)]
pub(super) enum ParentRef {
    /// The parent is addressable by a path, e.g. a struct type or enum
    /// variant.
    Named(Path),

    /// The parent is not addressable.
    ///
    /// This is typically the case for compounds created for `ExtractDef`.
    Unnamed {
        /// The parent's ref.
        ///
        /// For extracts, this refers to the compound where the field with
        /// the extract is declared.
        parent: Box<ParentRef>,

        /// The field inside that parent.
        ///
        /// For extracts, this refers to the compound field where the extract
        /// is declared.
        field: Member,
    },

    /// The parent is not addressable, but it is also not the child of another
    /// addressable thing.
    ///
    /// This is typically the case for compounds created for `Wrapped`.
    Wrapper {
        /// A reference to something nameable
        inner: Box<ParentRef>,
    },
}

impl From<Path> for ParentRef {
    fn from(other: Path) -> Self {
        Self::Named(other)
    }
}

impl<'x> From<&'x Path> for ParentRef {
    fn from(other: &'x Path) -> Self {
        Self::Named(other.clone())
    }
}

impl ParentRef {
    /// Create a new `ParentRef` for a member inside this one.
    ///
    /// Returns a [`Self::Unnamed`] with `self` as parent and `member` as
    /// field.
    pub(crate) fn child(&self, member: Member) -> Self {
        Self::Unnamed {
            parent: Box::new(self.clone()),
            field: member,
        }
    }

    /// Create a new `ParentRef` for a wrapper of this one.
    ///
    /// Returns a [`Self::Wrapper`] with `self` as inner element.
    pub(crate) fn wrapper(&self) -> Self {
        Self::Wrapper {
            inner: Box::new(self.clone()),
        }
    }

    /// Return a span which can be used for error messages.
    ///
    /// This points at the closest [`Self::Named`] variant in the parent
    /// chain.
    #[allow(dead_code)]
    pub(crate) fn span(&self) -> Span {
        match self {
            Self::Named(p) => p.span(),
            Self::Unnamed { parent, .. } => parent.span(),
            Self::Wrapper { inner, .. } => inner.span(),
        }
    }

    /// Try to extract an ident from this ParentRef.
    pub(crate) fn try_as_ident(&self) -> Option<&Ident> {
        match self {
            Self::Named(path) => {
                if path.leading_colon.is_some() {
                    return None;
                }
                if path.segments.len() != 1 {
                    return None;
                }
                let segment = &path.segments[0];
                if !segment.arguments.is_empty() {
                    return None;
                }
                Some(&segment.ident)
            }
            _ => None,
        }
    }
}

impl fmt::Display for ParentRef {
    fn fmt<'f>(&self, f: &'f mut fmt::Formatter) -> fmt::Result {
        match self {
            Self::Named(name) => {
                let mut first = true;
                for segment in name.segments.iter() {
                    if !first {
                        write!(f, "::")?;
                    }
                    first = false;
                    write!(f, "{}", segment.ident)?;
                }
                write!(f, " element")
            }
            Self::Unnamed { parent, field } => {
                write!(f, "extraction for {} in {}", FieldName(field), parent)
            }
            Self::Wrapper { inner } => {
                write!(f, "wrapper element of {}", inner)
            }
        }
    }
}

/// Ephemeral struct to create a nice human-readable representation of
/// [`syn::Member`].
///
/// It implements [`std::fmt::Display`] for that purpose and is otherwise of
/// little use.
#[repr(transparent)]
struct FieldName<'x>(&'x Member);

impl<'x> fmt::Display for FieldName<'x> {
    fn fmt<'f>(&self, f: &'f mut fmt::Formatter) -> fmt::Result {
        match self.0 {
            Member::Named(v) => write!(f, "field '{}'", v),
            Member::Unnamed(v) => write!(f, "unnamed field {}", v.index),
        }
    }
}

/// Create a string error message for a missing child element.
///
/// `parent_name` should point at the compound which is being parsed and
/// `field` should be the field to which the child belongs.
pub(super) fn on_missing_child(parent_name: &ParentRef, field: &Member) -> String {
    format!("Missing child {} in {}.", FieldName(&field), parent_name)
}

/// Create a string error message for a duplicate child element.
///
/// `parent_name` should point at the compound which is being parsed and
/// `field` should be the field to which the child belongs.
pub(super) fn on_duplicate_child(parent_name: &ParentRef, field: &Member) -> String {
    format!(
        "{} must not have more than one child in {}.",
        parent_name,
        FieldName(&field)
    )
}

/// Create a string error message for a missing attribute.
///
/// `parent_name` should point at the compound which is being parsed and
/// `field` should be the field to which the attribute belongs.
pub(super) fn on_missing_attribute(parent_name: &ParentRef, field: &Member) -> String {
    format!(
        "Required attribute {} on {} missing.",
        FieldName(&field),
        parent_name
    )
}