scala.xml.Equality.scala Maven / Gradle / Ivy
/* __ *\
** ________ ___ / / ___ Scala API **
** / __/ __// _ | / / / _ | (c) 2003-2013, LAMP/EPFL **
** __\ \/ /__/ __ |/ /__/ __ | http://scala-lang.org/ **
** /____/\___/_/ |_/____/_/ | | **
** |/ **
\* */
package scala
package xml
/**
* In an attempt to contain the damage being inflicted on consistency by the
* ad hoc `equals` methods spread around `xml`, the logic is centralized and
* all the `xml` classes go through the `xml.Equality trait`. There are two
* forms of `xml` comparison.
*
* 1. `'''def''' strict_==(other: scala.xml.Equality)`
*
* This one tries to honor the little things like symmetry and hashCode
* contracts. The `equals` method routes all comparisons through this.
*
* 1. `xml_==(other: Any)`
*
* This one picks up where `strict_==` leaves off. It might declare any two
* things equal.
*
* As things stood, the logic not only made a mockery of the collections
* equals contract, but also laid waste to that of case classes.
*
* Among the obstacles to sanity are/were:
*
* Node extends NodeSeq extends Seq[Node]
* MetaData extends Iterable[MetaData]
* The hacky "Group" xml node which throws exceptions
* with wild abandon, so don't get too close
* Rampant asymmetry and impossible hashCodes
* Most classes claiming to be equal to "String" if
* some specific stringification of it was the same.
* String was never going to return the favor.
*/
object Equality {
def asRef(x: Any): AnyRef = x.asInstanceOf[AnyRef]
/**
* Note - these functions assume strict equality has already failed.
*/
def compareBlithely(x1: AnyRef, x2: String): Boolean = x1 match {
case x: Atom[_] => x.data == x2
case x: NodeSeq => x.text == x2
case _ => false
}
def compareBlithely(x1: AnyRef, x2: Node): Boolean = x1 match {
case x: NodeSeq if x.length == 1 => x2 == x(0)
case _ => false
}
def compareBlithely(x1: AnyRef, x2: AnyRef): Boolean = {
if (x1 == null || x2 == null)
return (x1 eq x2)
x2 match {
case s: String => compareBlithely(x1, s)
case n: Node => compareBlithely(x1, n)
case _ => false
}
}
}
import Equality._
trait Equality extends scala.Equals {
protected def basisForHashCode: Seq[Any]
def strict_==(other: Equality): Boolean
def strict_!=(other: Equality) = !strict_==(other)
/**
* We insist we're only equal to other `xml.Equality` implementors,
* which heads off a lot of inconsistency up front.
*/
override def canEqual(other: Any): Boolean = other match {
case x: Equality => true
case _ => false
}
/**
* It's be nice to make these final, but there are probably
* people out there subclassing the XML types, especially when
* it comes to equals. However WE at least can pretend they
* are final since clearly individual classes cannot be trusted
* to maintain a semblance of order.
*/
override def hashCode() = basisForHashCode.##
override def equals(other: Any) = doComparison(other, blithe = false)
final def xml_==(other: Any) = doComparison(other, blithe = true)
final def xml_!=(other: Any) = !xml_==(other)
/**
* The "blithe" parameter expresses the caller's unconcerned attitude
* regarding the usual constraints on equals. The method is thereby
* given carte blanche to declare any two things equal.
*/
private def doComparison(other: Any, blithe: Boolean) = {
val strictlyEqual = other match {
case x: AnyRef if this eq x => true
case x: Equality => (x canEqual this) && (this strict_== x)
case _ => false
}
strictlyEqual || (blithe && compareBlithely(this, asRef(other)))
}
}