[SPARK-40599][SQL] Add multiTransform methods to TreeNode to generate alternatives

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/trees/TreeNode.scala

@@ -618,6 +618,134 @@ abstract class TreeNode[BaseType <: TreeNode[BaseType]] extends Product with Tre
     }
   }
 
+  /**
+   * Returns alternative copies of this node where `rule` has been recursively applied to it and all
+   * of its children (pre-order).
+   *
+   * @param rule a function used to generate alternatives for a node
+   * @return     the stream of alternatives
+   */
+  def multiTransformDown(
+      rule: PartialFunction[BaseType, Stream[BaseType]]): Stream[BaseType] = {
+    multiTransformDownWithPruning(AlwaysProcess.fn, UnknownRuleId)(rule)
+  }
+
+  /**
+   * Returns alternative copies of this node where `rule` has been recursively applied to it and all
+   * of its children (pre-order).
+   *
+   * As it is very easy to generate enormous number of alternatives when the input tree is huge or
+   * when the rule returns many alternatives for many nodes, this function returns the alternatives
+   * as a lazy `Stream` to be able to limit the number of alternatives generated at the caller side
+   * as needed.
+   *
+   * The rule should not apply or can return a one element stream of original node to indicate that
+   * the original node without any transformation is a valid alternative.
+   *
+   * The rule can return `Stream.empty` to indicate that the original node should be pruned. In this
+   * case `multiTransform()` returns an empty `Stream`.
+   *
+   * Please consider the following examples of `input.multiTransformDown(rule)`:
+   *
+   * We have an input expression:
+   *    `Add(a, b)`
+   *
+   * 1.
+   * We have a simple rule:
+   *   `a` => `Stream(1, 2)`
+   *   `b` => `Stream(10, 20)`
+   *   `Add(a, b)` => `Stream(11, 12, 21, 22)`
+   *
+   * The output is:
+   *   `Stream(11, 12, 21, 22)`
+   *
+   * 2.
+   * In the previous example if we want to generate alternatives of `a` and `b` too then we need to
+   * explicitly add the original `Add(a, b)` expression to the rule:
+   *   `a` => `Stream(1, 2)`
+   *   `b` => `Stream(10, 20)`
+   *   `Add(a, b)` => `Stream(11, 12, 21, 22, Add(a, b))`
+   *
+   * The output is:
+   *   `Stream(11, 12, 21, 22, Add(1, 10), Add(2, 10), Add(1, 20), Add(2, 20))`
+   *
+   * @param rule   a function used to generate alternatives for a node
+   * @param cond   a Lambda expression to prune tree traversals. If `cond.apply` returns false
+   *               on a TreeNode T, skips processing T and its subtree; otherwise, processes
+   *               T and its subtree recursively.
+   * @param ruleId is a unique Id for `rule` to prune unnecessary tree traversals. When it is
+   *               UnknownRuleId, no pruning happens. Otherwise, if `rule` (with id `ruleId`)
+   *               has been marked as in effective on a TreeNode T, skips processing T and its
+   *               subtree. Do not pass it if the rule is not purely functional and reads a
+   *               varying initial state for different invocations.
+   * @return       the stream of alternatives
+   */
+  def multiTransformDownWithPruning(
+      cond: TreePatternBits => Boolean,
+      ruleId: RuleId = UnknownRuleId
+    )(rule: PartialFunction[BaseType, Stream[BaseType]]): Stream[BaseType] = {
+    if (!cond.apply(this) || isRuleIneffective(ruleId)) {
+      return Stream(this)
+    }
+
+    // We could return `Stream(this)` if the `rule` doesn't apply and handle both
+    // - the doesn't apply
+    // - and the rule returns a one element `Stream(originalNode)`
+    // cases together. But, unfortunately it doesn't seem like there is a way to match on a one
+    // element stream without eagerly computing the tail head. So this contradicts with the purpose
+    // of only taking the necessary elements from the alternatives. I.e. the
+    // "multiTransformDown is lazy" test case in `TreeNodeSuite` would fail.
+    // Please note that this behaviour has a downside as well that we can only mark the rule on the
+    // original node ineffective if the rule didn't match.
+    var ruleApplied = true
+    val afterRules = CurrentOrigin.withOrigin(origin) {
+      rule.applyOrElse(this, (_: BaseType) => {
+        ruleApplied = false
+        Stream.empty
+      })
+    }
+
+    val afterRulesStream = if (afterRules.isEmpty) {
+      if (ruleApplied) {
+        // If the rule returned with empty alternatives then prune
+        Stream.empty
+      } else {
+        // If the rule was not applied then keep the original node
+        this.markRuleAsIneffective(ruleId)
+        Stream(this)
+      }
+    } else {
+      // If the rule was applied then use the returned alternatives
+      afterRules.map { afterRule =>
+        if (this fastEquals afterRule) {
+          this
+        } else {
+          afterRule.copyTagsFrom(this)
+          afterRule
+        }
+      }
+    }
+
+    afterRulesStream.flatMap { afterRule =>
+      if (afterRule.containsChild.nonEmpty) {
+        generateChildrenSeq(
+            afterRule.children.map(_.multiTransformDownWithPruning(cond, ruleId)(rule)))
+          .map(afterRule.withNewChildren)
+      } else {
+        Stream(afterRule)
+      }
+    }
+  }
+
+  private def generateChildrenSeq[T](childrenStreams: Seq[Stream[T]]): Stream[Seq[T]] = {
+    childrenStreams.foldRight(Stream(Seq.empty[T]))((childrenStream, childrenSeqStream) =>
+      for {
+        childrenSeq <- childrenSeqStream
+        child <- childrenStream
+      } yield child +: childrenSeq
+    )
+  }
+
   /**
    * Returns a copy of this node where `f` has been applied to all the nodes in `children`.
    */

sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/trees/TreeNodeSuite.scala

@@ -977,4 +977,108 @@ class TreeNodeSuite extends SparkFunSuite with SQLHelper {
       assert(origin.context.summary.isEmpty)
     }
   }
+
+  private def newErrorAfterStream(es: Expression*) = {
+    es.toStream.append(
+      throw new NoSuchElementException("Stream should not return more elements")
+    )
+  }
+
+  test("multiTransformDown generates all alternatives") {
+    val e = Add(Add(Literal("a"), Literal("b")), Add(Literal("c"), Literal("d")))
+    val transformed = e.multiTransformDown {
+      case StringLiteral("a") => Stream(Literal(1), Literal(2), Literal(3))
+      case StringLiteral("b") => Stream(Literal(10), Literal(20), Literal(30))
+      case Add(StringLiteral("c"), StringLiteral("d"), _) =>
+        Stream(Literal(100), Literal(200), Literal(300))
+    }
+    val expected = for {
+      cd <- Seq(Literal(100), Literal(200), Literal(300))
+      b <- Seq(Literal(10), Literal(20), Literal(30))
+      a <- Seq(Literal(1), Literal(2), Literal(3))
+    } yield Add(Add(a, b), cd)
+    assert(transformed === expected)
+  }
+
+  test("multiTransformDown is lazy") {
+    val e = Add(Add(Literal("a"), Literal("b")), Add(Literal("c"), Literal("d")))
+    val transformed = e.multiTransformDown {
+      case StringLiteral("a") => Stream(Literal(1), Literal(2), Literal(3))
+      case StringLiteral("b") => newErrorAfterStream(Literal(10))
+      case Add(StringLiteral("c"), StringLiteral("d"), _) => newErrorAfterStream(Literal(100))
+    }
+    val expected = for {
+      a <- Seq(Literal(1), Literal(2), Literal(3))
+    } yield Add(Add(a, Literal(10)), Literal(100))
+    // We don't access alternatives for `b` after 10 and for `c` after 100
+    assert(transformed.take(3) == expected)
+    intercept[NoSuchElementException] {
+      transformed.take(3 + 1).toList
+    }
+
+    val transformed2 = e.multiTransformDown {
+      case StringLiteral("a") => Stream(Literal(1), Literal(2), Literal(3))
+      case StringLiteral("b") => Stream(Literal(10), Literal(20), Literal(30))
+      case Add(StringLiteral("c"), StringLiteral("d"), _) => newErrorAfterStream(Literal(100))
+    }
+    val expected2 = for {
+      b <- Seq(Literal(10), Literal(20), Literal(30))
+      a <- Seq(Literal(1), Literal(2), Literal(3))
+    } yield Add(Add(a, b), Literal(100))
+    // We don't access alternatives for `c` after 100
+    assert(transformed2.take(3 * 3) === expected2)
+    intercept[NoSuchElementException] {
+      transformed.take(3 * 3 + 1).toList
+    }
+  }
+
+  test("multiTransformDown rule return this") {
+    val e = Add(Add(Literal("a"), Literal("b")), Add(Literal("c"), Literal("d")))
+    val transformed = e.multiTransformDown {
+      case s @ StringLiteral("a") => Stream(Literal(1), Literal(2), s)
+      case s @ StringLiteral("b") => Stream(Literal(10), Literal(20), s)
+      case a @ Add(StringLiteral("c"), StringLiteral("d"), _) =>
+        Stream(Literal(100), Literal(200), a)
+    }
+    val expected = for {
+      cd <- Seq(Literal(100), Literal(200), Add(Literal("c"), Literal("d")))
+      b <- Seq(Literal(10), Literal(20), Literal("b"))
+      a <- Seq(Literal(1), Literal(2), Literal("a"))
+    } yield Add(Add(a, b), cd)
+    assert(transformed == expected)
+  }
+
+  test("multiTransformDown doesn't stop generating alternatives of descendants when non-leaf is " +
+    "transformed and itself is in the alternatives") {
+    val e = Add(Add(Literal("a"), Literal("b")), Add(Literal("c"), Literal("d")))
+    val transformed = e.multiTransformDown {
+      case a @ Add(StringLiteral("a"), StringLiteral("b"), _) =>
+        Stream(Literal(11), Literal(12), Literal(21), Literal(22), a)
+      case StringLiteral("a") => Stream(Literal(1), Literal(2))
+      case StringLiteral("b") => Stream(Literal(10), Literal(20))
+      case Add(StringLiteral("c"), StringLiteral("d"), _) => Stream(Literal(100), Literal(200))
+    }
+    val expected = for {
+      cd <- Seq(Literal(100), Literal(200))
+      ab <- Seq(Literal(11), Literal(12), Literal(21), Literal(22)) ++
+        (for {
+          b <- Seq(Literal(10), Literal(20))
+          a <- Seq(Literal(1), Literal(2))
+        } yield Add(a, b))
+    } yield Add(ab, cd)
+    assert(transformed == expected)
+  }
+
+  test("multiTransformDown can prune") {
+    val e = Add(Add(Literal("a"), Literal("b")), Add(Literal("c"), Literal("d")))
+    val transformed = e.multiTransformDown {
+      case StringLiteral("a") => Stream.empty
+    }
+    assert(transformed.isEmpty)
+
+    val transformed2 = e.multiTransformDown {
+      case Add(StringLiteral("c"), StringLiteral("d"), _) => Stream.empty
+    }
+    assert(transformed2.isEmpty)
+  }
 }