// Copyright 2012-2015 The Rust Project Developers. See the COPYRIGHT
// file at the top-level directory of this distribution and at
// http://rust-lang.org/COPYRIGHT.
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.

use std::mem;
use rustc_data_structures::small_vec::SmallVec;
use syntax::ast::CRATE_NODE_ID;
use ty::context::TyCtxt;
use ty::{DefId, DefIdTree};

/// Represents a forest of DefIds closed under the ancestor relation. That is,
/// if a DefId representing a module is contained in the forest then all
/// DefIds defined in that module or submodules are also implicitly contained
/// in the forest.
///
/// This is used to represent a set of modules in which a type is visibly
/// uninhabited.
#[derive(Clone)]
pub struct DefIdForest {
    /// The minimal set of DefIds required to represent the whole set.
    /// If A and B are DefIds in the DefIdForest, and A is a descendant
    /// of B, then only B will be in root_ids.
    /// We use a SmallVec here because (for its use for caching inhabitedness)
    /// its rare that this will contain even two ids.
    root_ids: SmallVec<[DefId; 1]>,
}

impl<'a, 'gcx, 'tcx> DefIdForest {
    /// Create an empty forest.
    pub fn empty() -> DefIdForest {
        DefIdForest {
            root_ids: SmallVec::new(),
        }
    }

    /// Create a forest consisting of a single tree representing the entire
    /// crate.
    #[inline]
    pub fn full(tcx: TyCtxt<'a, 'gcx, 'tcx>) -> DefIdForest {
        let crate_id = tcx.hir.local_def_id(CRATE_NODE_ID);
        DefIdForest::from_id(crate_id)
    }

    /// Create a forest containing a DefId and all its descendants.
    pub fn from_id(id: DefId) -> DefIdForest {
        let mut root_ids = SmallVec::new();
        root_ids.push(id);
        DefIdForest {
            root_ids,
        }
    }

    /// Test whether the forest is empty.
    pub fn is_empty(&self) -> bool {
        self.root_ids.is_empty()
    }

    /// Test whether the forest contains a given DefId.
    pub fn contains(&self,
                    tcx: TyCtxt<'a, 'gcx, 'tcx>,
                    id: DefId) -> bool
    {
        for root_id in self.root_ids.iter() {
            if tcx.is_descendant_of(id, *root_id) {
                return true;
            }
        }
        false
    }

    /// Calculate the intersection of a collection of forests.
    pub fn intersection<I>(tcx: TyCtxt<'a, 'gcx, 'tcx>,
                           iter: I) -> DefIdForest
            where I: IntoIterator<Item=DefIdForest>
    {
        let mut ret = DefIdForest::full(tcx);
        let mut next_ret = SmallVec::new();
        let mut old_ret: SmallVec<[DefId; 1]> = SmallVec::new();
        for next_forest in iter {
            for id in ret.root_ids.drain(..) {
                if next_forest.contains(tcx, id) {
                    next_ret.push(id);
                } else {
                    old_ret.push(id);
                }
            }
            ret.root_ids.extend(old_ret.drain(..));

            for id in next_forest.root_ids {
                if ret.contains(tcx, id) {
                    next_ret.push(id);
                }
            }

            mem::swap(&mut next_ret, &mut ret.root_ids);
            next_ret.drain(..);
        }
        ret
    }

    /// Calculate the union of a collection of forests.
    pub fn union<I>(tcx: TyCtxt<'a, 'gcx, 'tcx>,
                    iter: I) -> DefIdForest
            where I: IntoIterator<Item=DefIdForest>
    {
        let mut ret = DefIdForest::empty();
        let mut next_ret = SmallVec::new();
        for next_forest in iter {
            for id in ret.root_ids.drain(..) {
                if !next_forest.contains(tcx, id) {
                    next_ret.push(id);
                }
            }

            for id in next_forest.root_ids {
                if !next_ret.contains(&id) {
                    next_ret.push(id);
                }
            }

            mem::swap(&mut next_ret, &mut ret.root_ids);
            next_ret.drain(..);
        }
        ret
    }
}