// Copyright 2012 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 middle::ty::{BuiltinBounds};
use middle::ty::RegionVid;
use middle::ty;
use middle::typeck::infer::combine::*;
use middle::typeck::infer::glb::Glb;
use middle::typeck::infer::lattice::*;
use middle::typeck::infer::sub::Sub;
use middle::typeck::infer::to_str::InferStr;
use middle::typeck::infer::{cres, InferCtxt};
use middle::typeck::infer::fold_regions_in_sig;
use middle::typeck::infer::{TypeTrace, Subtype};
use middle::typeck::isr_alist;
use util::ppaux::mt_to_str;

use extra::list;
use syntax::ast::{Many, Once, extern_fn, impure_fn};
use syntax::ast::{unsafe_fn};
use syntax::ast::{Onceness, purity};

pub struct Lub(CombineFields);  // least-upper-bound: common supertype

impl Lub {
    pub fn bot_ty(&self, b: ty::t) -> cres<ty::t> { Ok(b) }
    pub fn ty_bot(&self, b: ty::t) -> cres<ty::t> {
        self.bot_ty(b) // commutative
    }
}

impl Combine for Lub {
    fn infcx(&self) -> @mut InferCtxt { self.infcx }
    fn tag(&self) -> ~str { ~"lub" }
    fn a_is_expected(&self) -> bool { self.a_is_expected }
    fn trace(&self) -> TypeTrace { self.trace }

    fn sub(&self) -> Sub { Sub(**self) }
    fn lub(&self) -> Lub { Lub(**self) }
    fn glb(&self) -> Glb { Glb(**self) }

    fn mts(&self, a: &ty::mt, b: &ty::mt) -> cres<ty::mt> {
        let tcx = self.infcx.tcx;

        debug!("%s.mts(%s, %s)",
               self.tag(),
               mt_to_str(tcx, a),
               mt_to_str(tcx, b));

        if a.mutbl != b.mutbl {
            return Err(ty::terr_mutability)
        }

        let m = a.mutbl;
        match m {
          MutImmutable => {
            self.tys(a.ty, b.ty).chain(|t| Ok(ty::mt {ty: t, mutbl: m}) )
          }

          MutMutable => {
            self.infcx.try(|| {
                eq_tys(self, a.ty, b.ty).then(|| {
                    Ok(ty::mt {ty: a.ty, mutbl: m})
                })
            }).chain_err(|e| Err(e))
          }
        }
    }

    fn contratys(&self, a: ty::t, b: ty::t) -> cres<ty::t> {
        Glb(**self).tys(a, b)
    }

    fn purities(&self, a: purity, b: purity) -> cres<purity> {
        match (a, b) {
          (unsafe_fn, _) | (_, unsafe_fn) => Ok(unsafe_fn),
          (impure_fn, _) | (_, impure_fn) => Ok(impure_fn),
          (extern_fn, extern_fn) => Ok(extern_fn),
        }
    }

    fn oncenesses(&self, a: Onceness, b: Onceness) -> cres<Onceness> {
        match (a, b) {
            (Once, _) | (_, Once) => Ok(Once),
            (Many, Many) => Ok(Many)
        }
    }

    fn bounds(&self, a: BuiltinBounds, b: BuiltinBounds) -> cres<BuiltinBounds> {
        // More bounds is a subtype of fewer bounds, so
        // the LUB (mutual supertype) is the intersection.
        Ok(a.intersection(b))
    }

    fn contraregions(&self, a: ty::Region, b: ty::Region)
                    -> cres<ty::Region> {
        return Glb(**self).regions(a, b);
    }

    fn regions(&self, a: ty::Region, b: ty::Region) -> cres<ty::Region> {
        debug!("%s.regions(%?, %?)",
               self.tag(),
               a.inf_str(self.infcx),
               b.inf_str(self.infcx));

        Ok(self.infcx.region_vars.lub_regions(Subtype(self.trace), a, b))
    }

    fn fn_sigs(&self, a: &ty::FnSig, b: &ty::FnSig) -> cres<ty::FnSig> {
        // Note: this is a subtle algorithm.  For a full explanation,
        // please see the large comment in `region_inference.rs`.

        // Take a snapshot.  We'll never roll this back, but in later
        // phases we do want to be able to examine "all bindings that
        // were created as part of this type comparison", and making a
        // snapshot is a convenient way to do that.
        let snapshot = self.infcx.region_vars.start_snapshot();

        // Instantiate each bound region with a fresh region variable.
        let (a_with_fresh, a_isr) =
            self.infcx.replace_bound_regions_with_fresh_regions(
                self.trace, a);
        let (b_with_fresh, _) =
            self.infcx.replace_bound_regions_with_fresh_regions(
                self.trace, b);

        // Collect constraints.
        let sig0 = if_ok!(super_fn_sigs(self, &a_with_fresh, &b_with_fresh));
        debug!("sig0 = %s", sig0.inf_str(self.infcx));

        // Generalize the regions appearing in sig0 if possible
        let new_vars =
            self.infcx.region_vars.vars_created_since_snapshot(snapshot);
        let sig1 =
            fold_regions_in_sig(
                self.infcx.tcx,
                &sig0,
                |r, _in_fn| generalize_region(self, snapshot, new_vars,
                                              a_isr, r));
        return Ok(sig1);

        fn generalize_region(this: &Lub,
                             snapshot: uint,
                             new_vars: &[RegionVid],
                             a_isr: isr_alist,
                             r0: ty::Region) -> ty::Region {
            // Regions that pre-dated the LUB computation stay as they are.
            if !is_var_in_set(new_vars, r0) {
                debug!("generalize_region(r0=%?): not new variable", r0);
                return r0;
            }

            let tainted = this.infcx.region_vars.tainted(snapshot, r0);

            // Variables created during LUB computation which are
            // *related* to regions that pre-date the LUB computation
            // stay as they are.
            if !tainted.iter().all(|r| is_var_in_set(new_vars, *r)) {
                debug!("generalize_region(r0=%?): \
                        non-new-variables found in %?",
                       r0, tainted);
                return r0;
            }

            // Otherwise, the variable must be associated with at
            // least one of the variables representing bound regions
            // in both A and B.  Replace the variable with the "first"
            // bound region from A that we find it to be associated
            // with.
            let mut ret = None;
            do list::each(a_isr) |pair| {
                let (a_br, a_r) = *pair;
                if tainted.iter().any(|x| x == &a_r) {
                    debug!("generalize_region(r0=%?): \
                            replacing with %?, tainted=%?",
                           r0, a_br, tainted);
                    ret = Some(ty::re_bound(a_br));
                    false
                } else {
                    true
                }
            };

            match ret {
                Some(x) => x,
                None => this.infcx.tcx.sess.span_bug(
                            this.trace.origin.span(),
                            fmt!("Region %? is not associated with \
                                  any bound region from A!", r0))
            }
        }
    }

    fn tys(&self, a: ty::t, b: ty::t) -> cres<ty::t> {
        super_lattice_tys(self, a, b)
    }
}
