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package scala.scalanative
package linker

import scala.collection.mutable
import scalanative.util.unreachable

/** Our subtyping can be described by a following diagram:
 *
 *  {{{
 *    value kind        ref kind         special kind
 *    |     \           |    \
 *    |     |           |      \
 *    prim  aggr        class  trait
 *    |     |           |      /
 *    |     |           |    /
 *    |     |           null
 *    |     |           /
 *    |     |       /
 *    |     |   /
 *    nothing
 *  }}}
 *
 *  Primitive and aggregate types don't participate in subtyping and they have
 *  to be explicitly boxed to become compatible with a reference type.
 *
 *  Reference types form a simple lattice with java.lang.Object at the top and
 *  null type at the bottom. Subtyping between traits and classes is based on
 *  linearization of the all transitive parents, similarly to scalac.
 *
 *  Nothing is the common bottom type between reference and value types. It
 *  represents computations that may never complete normally (either loops
 *  forever or throws an exception).
 */
object Sub {

  def is(l: nir.Type, r: nir.Type)(implicit
      analysis: ReachabilityAnalysis.Result
  ): Boolean = {
    (l, r) match {
      case (l, r) if l == r =>
        true
      case (nir.Type.Null, (nir.Type.Ptr | _: nir.Type.RefKind)) =>
        true
      case (nir.Type.Nothing, (_: nir.Type.ValueKind | _: nir.Type.RefKind)) =>
        true
      case (_: nir.Type.RefKind, nir.Rt.Object) =>
        true
      case (ScopeRef(linfo), ScopeRef(rinfo)) =>
        linfo.is(rinfo)
      case _ =>
        false
    }
  }

  def is(info: ScopeInfo, ty: nir.Type.RefKind)(implicit
      analysis: ReachabilityAnalysis.Result
  ): Boolean = {
    ty match {
      case ScopeRef(other) =>
        info.is(other)
      case _ =>
        util.unreachable
    }
  }

  def lub(tys: Seq[nir.Type], bound: Option[nir.Type])(implicit
      analysis: ReachabilityAnalysis.Result
  ): nir.Type = {
    tys match {
      case Seq() =>
        unreachable
      case head +: tail =>
        tail.foldLeft[nir.Type](head)(lub(_, _, bound))
    }
  }

  def lub(lty: nir.Type, rty: nir.Type, bound: Option[nir.Type])(implicit
      analysis: ReachabilityAnalysis.Result
  ): nir.Type = {
    (lty, rty) match {
      case _ if lty == rty =>
        lty
      case (ty, nir.Type.Nothing) =>
        ty
      case (nir.Type.Nothing, ty) =>
        ty
      case (nir.Type.Ptr, nir.Type.Null) =>
        nir.Type.Ptr
      case (nir.Type.Null, nir.Type.Ptr) =>
        nir.Type.Ptr
      case (refty: nir.Type.RefKind, nir.Type.Null) =>
        nir.Type.Ref(refty.className, refty.isExact, nullable = true)
      case (nir.Type.Null, refty: nir.Type.RefKind) =>
        nir.Type.Ref(refty.className, refty.isExact, nullable = true)
      case (lty: nir.Type.RefKind, rty: nir.Type.RefKind) =>
        val ScopeRef(linfo) = lty: @unchecked
        val ScopeRef(rinfo) = rty: @unchecked
        val binfo = bound.flatMap(ScopeRef.unapply)
        val lubinfo = lub(linfo, rinfo, binfo)
        val exact =
          lubinfo.name == rinfo.name && rty.isExact &&
            lubinfo.name == linfo.name && lty.isExact
        val nullable =
          lty.isNullable || rty.isNullable
        nir.Type.Ref(lubinfo.name, exact, nullable)
      case _ =>
        util.unsupported(s"lub(${lty.show}, ${rty.show})")
    }
  }

  def lub(linfo: ScopeInfo, rinfo: ScopeInfo, boundInfo: Option[ScopeInfo])(
      implicit analysis: ReachabilityAnalysis.Result
  ): ScopeInfo = {
    if (linfo == rinfo) {
      linfo
    } else if (linfo.is(rinfo)) {
      rinfo
    } else if (rinfo.is(linfo)) {
      linfo
    } else {
      // If bound is not a type of linfo or rinfo
      // it should be ignored. Otherwise java.lang.Object
      // would be returned, which may not be correct
      val correctedBoundInfo = boundInfo.filterNot { bound =>
        (!linfo.is(bound) || !rinfo.is(bound))
      }

      val candidates =
        linfo.linearized.filter { i =>
          rinfo.is(i) && correctedBoundInfo.forall(i.is)
        }

      candidates match {
        case Seq() =>
          analysis.infos(nir.Rt.Object.name).asInstanceOf[ScopeInfo]
        case Seq(cand) =>
          cand
        case _ =>
          def inhabitants(info: ScopeInfo): Int =
            info.implementors.size

          val min = candidates.map(inhabitants).min

          val minimums = candidates.collect {
            case cand if inhabitants(cand) == min =>
              cand
          }

          minimums.headOption.getOrElse {
            analysis.infos(nir.Rt.Object.name).asInstanceOf[ScopeInfo]
          }
      }
    }
  }
}