//! This module provides a simplified abstraction for working with
//! code blocks identified by their integer `NodeId`. In particular,
//! it captures a common set of attributes that all "function-like
//! things" (represented by `FnLike` instances) share. For example,
//! all `FnLike` instances have a type signature (be it explicit or
//! inferred). And all `FnLike` instances have a body, i.e., the code
//! that is run when the function-like thing it represents is invoked.
//!
//! With the above abstraction in place, one can treat the program
//! text as a collection of blocks of code (and most such blocks are
//! nested within a uniquely determined `FnLike`), and users can ask
//! for the `Code` associated with a particular NodeId.

use crate::hir::map::Map;
use rustc_hir as hir;
use rustc_hir::intravisit::FnKind;
use rustc_hir::{Expr, FnDecl, Node};
use rustc_span::symbol::Ident;
use rustc_span::Span;

/// An FnLikeNode is a Node that is like a fn, in that it has a decl
/// and a body (as well as a NodeId, a span, etc).
///
/// More specifically, it is one of either:
///
///   - A function item,
///   - A closure expr (i.e., an ExprKind::Closure), or
///   - The default implementation for a trait method.
///
/// To construct one, use the `Code::from_node` function.
#[derive(Copy, Clone, Debug)]
pub struct FnLikeNode<'a> {
    node: Node<'a>,
}

/// MaybeFnLike wraps a method that indicates if an object
/// corresponds to some FnLikeNode.
trait MaybeFnLike {
    fn is_fn_like(&self) -> bool;
}

impl MaybeFnLike for hir::Item<'_> {
    fn is_fn_like(&self) -> bool {
        matches!(self.kind, hir::ItemKind::Fn(..))
    }
}

impl MaybeFnLike for hir::ImplItem<'_> {
    fn is_fn_like(&self) -> bool {
        matches!(self.kind, hir::ImplItemKind::Fn(..))
    }
}

impl MaybeFnLike for hir::TraitItem<'_> {
    fn is_fn_like(&self) -> bool {
        matches!(self.kind, hir::TraitItemKind::Fn(_, hir::TraitFn::Provided(_)))
    }
}

impl MaybeFnLike for hir::Expr<'_> {
    fn is_fn_like(&self) -> bool {
        matches!(self.kind, hir::ExprKind::Closure(..))
    }
}

/// Carries either an FnLikeNode or an Expr, as these are the two
/// constructs that correspond to "code" (as in, something from which
/// we can construct a control-flow graph).
#[derive(Copy, Clone)]
pub enum Code<'a> {
    FnLike(FnLikeNode<'a>),
    Expr(&'a Expr<'a>),
}

impl<'a> Code<'a> {
    pub fn id(&self) -> hir::HirId {
        match *self {
            Code::FnLike(node) => node.id(),
            Code::Expr(block) => block.hir_id,
        }
    }

    /// Attempts to construct a Code from presumed FnLike or Expr node input.
    pub fn from_node(map: &Map<'a>, id: hir::HirId) -> Option<Code<'a>> {
        match map.get(id) {
            Node::Block(_) => {
                //  Use the parent, hopefully an expression node.
                Code::from_node(map, map.get_parent_node(id))
            }
            Node::Expr(expr) => Some(Code::Expr(expr)),
            node => FnLikeNode::from_node(node).map(Code::FnLike),
        }
    }
}

/// These are all the components one can extract from a fn item for
/// use when implementing FnLikeNode operations.
struct ItemFnParts<'a> {
    ident: Ident,
    decl: &'a hir::FnDecl<'a>,
    header: hir::FnHeader,
    vis: &'a hir::Visibility<'a>,
    generics: &'a hir::Generics<'a>,
    body: hir::BodyId,
    id: hir::HirId,
    span: Span,
}

/// These are all the components one can extract from a closure expr
/// for use when implementing FnLikeNode operations.
struct ClosureParts<'a> {
    decl: &'a FnDecl<'a>,
    body: hir::BodyId,
    id: hir::HirId,
    span: Span,
}

impl<'a> ClosureParts<'a> {
    fn new(d: &'a FnDecl<'a>, b: hir::BodyId, id: hir::HirId, s: Span) -> Self {
        ClosureParts { decl: d, body: b, id, span: s }
    }
}

impl<'a> FnLikeNode<'a> {
    /// Attempts to construct a FnLikeNode from presumed FnLike node input.
    pub fn from_node(node: Node<'_>) -> Option<FnLikeNode<'_>> {
        let fn_like = match node {
            Node::Item(item) => item.is_fn_like(),
            Node::TraitItem(tm) => tm.is_fn_like(),
            Node::ImplItem(it) => it.is_fn_like(),
            Node::Expr(e) => e.is_fn_like(),
            _ => false,
        };
        fn_like.then_some(FnLikeNode { node })
    }

    pub fn body(self) -> hir::BodyId {
        self.handle(
            |i: ItemFnParts<'a>| i.body,
            |_, _, _: &'a hir::FnSig<'a>, _, body: hir::BodyId, _| body,
            |c: ClosureParts<'a>| c.body,
        )
    }

    pub fn decl(self) -> &'a FnDecl<'a> {
        self.handle(
            |i: ItemFnParts<'a>| &*i.decl,
            |_, _, sig: &'a hir::FnSig<'a>, _, _, _| &sig.decl,
            |c: ClosureParts<'a>| c.decl,
        )
    }

    pub fn span(self) -> Span {
        self.handle(
            |i: ItemFnParts<'_>| i.span,
            |_, _, _: &'a hir::FnSig<'a>, _, _, span| span,
            |c: ClosureParts<'_>| c.span,
        )
    }

    pub fn id(self) -> hir::HirId {
        self.handle(
            |i: ItemFnParts<'_>| i.id,
            |id, _, _: &'a hir::FnSig<'a>, _, _, _| id,
            |c: ClosureParts<'_>| c.id,
        )
    }

    pub fn constness(self) -> hir::Constness {
        self.kind().header().map_or(hir::Constness::NotConst, |header| header.constness)
    }

    pub fn asyncness(self) -> hir::IsAsync {
        self.kind().header().map_or(hir::IsAsync::NotAsync, |header| header.asyncness)
    }

    pub fn unsafety(self) -> hir::Unsafety {
        self.kind().header().map_or(hir::Unsafety::Normal, |header| header.unsafety)
    }

    pub fn kind(self) -> FnKind<'a> {
        let item = |p: ItemFnParts<'a>| -> FnKind<'a> {
            FnKind::ItemFn(p.ident, p.generics, p.header, p.vis)
        };
        let closure = |_: ClosureParts<'a>| FnKind::Closure;
        let method =
            |_, ident: Ident, sig: &'a hir::FnSig<'a>, vis, _, _| FnKind::Method(ident, sig, vis);
        self.handle(item, method, closure)
    }

    fn handle<A, I, M, C>(self, item_fn: I, method: M, closure: C) -> A
    where
        I: FnOnce(ItemFnParts<'a>) -> A,
        M: FnOnce(
            hir::HirId,
            Ident,
            &'a hir::FnSig<'a>,
            Option<&'a hir::Visibility<'a>>,
            hir::BodyId,
            Span,
        ) -> A,
        C: FnOnce(ClosureParts<'a>) -> A,
    {
        match self.node {
            Node::Item(i) => match i.kind {
                hir::ItemKind::Fn(ref sig, ref generics, block) => item_fn(ItemFnParts {
                    id: i.hir_id(),
                    ident: i.ident,
                    decl: &sig.decl,
                    body: block,
                    vis: &i.vis,
                    span: i.span,
                    header: sig.header,
                    generics,
                }),
                _ => bug!("item FnLikeNode that is not fn-like"),
            },
            Node::TraitItem(ti) => match ti.kind {
                hir::TraitItemKind::Fn(ref sig, hir::TraitFn::Provided(body)) => {
                    method(ti.hir_id(), ti.ident, sig, None, body, ti.span)
                }
                _ => bug!("trait method FnLikeNode that is not fn-like"),
            },
            Node::ImplItem(ii) => match ii.kind {
                hir::ImplItemKind::Fn(ref sig, body) => {
                    method(ii.hir_id(), ii.ident, sig, Some(&ii.vis), body, ii.span)
                }
                _ => bug!("impl method FnLikeNode that is not fn-like"),
            },
            Node::Expr(e) => match e.kind {
                hir::ExprKind::Closure(_, ref decl, block, _fn_decl_span, _gen) => {
                    closure(ClosureParts::new(&decl, block, e.hir_id, e.span))
                }
                _ => bug!("expr FnLikeNode that is not fn-like"),
            },
            _ => bug!("other FnLikeNode that is not fn-like"),
        }
    }
}