datafusion-physical-plan 53.1.0

// Licensed to the Apache Software Foundation (ASF) under one
// or more contributor license agreements.  See the NOTICE file
// distributed with this work for additional information
// regarding copyright ownership.  The ASF licenses this file
// to you under the Apache License, Version 2.0 (the
// "License"); you may not use this file except in compliance
// with the License.  You may obtain a copy of the License at
//
//   http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing,
// software distributed under the License is distributed on an
// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
// KIND, either express or implied.  See the License for the
// specific language governing permissions and limitations
// under the License.

use super::ExecutionPlan;

/// Visit all children of this plan, according to the order defined on `ExecutionPlanVisitor`.
// Note that this would be really nice if it were a method on
// ExecutionPlan, but it can not be because it takes a generic
// parameter and `ExecutionPlan` is a trait
pub fn accept<V: ExecutionPlanVisitor>(
    plan: &dyn ExecutionPlan,
    visitor: &mut V,
) -> Result<(), V::Error> {
    visitor.pre_visit(plan)?;
    for child in plan.children() {
        visit_execution_plan(child.as_ref(), visitor)?;
    }
    visitor.post_visit(plan)?;
    Ok(())
}

/// Trait that implements the [Visitor
/// pattern](https://en.wikipedia.org/wiki/Visitor_pattern) for a
/// depth first walk of `ExecutionPlan` nodes. `pre_visit` is called
/// before any children are visited, and then `post_visit` is called
/// after all children have been visited.
///
/// To use, define a struct that implements this trait and then invoke
/// ['accept'].
///
/// For example, for an execution plan that looks like:
///
/// ```text
/// ProjectionExec: id
///    FilterExec: state = CO
///       DataSourceExec:
/// ```
///
/// The sequence of visit operations would be:
/// ```text
/// visitor.pre_visit(ProjectionExec)
/// visitor.pre_visit(FilterExec)
/// visitor.pre_visit(DataSourceExec)
/// visitor.post_visit(DataSourceExec)
/// visitor.post_visit(FilterExec)
/// visitor.post_visit(ProjectionExec)
/// ```
pub trait ExecutionPlanVisitor {
    /// The type of error returned by this visitor
    type Error;

    /// Invoked on an `ExecutionPlan` plan before any of its child
    /// inputs have been visited. If Ok(true) is returned, the
    /// recursion continues. If Err(..) or Ok(false) are returned, the
    /// recursion stops immediately and the error, if any, is returned
    /// to `accept`
    fn pre_visit(&mut self, plan: &dyn ExecutionPlan) -> Result<bool, Self::Error>;

    /// Invoked on an `ExecutionPlan` plan *after* all of its child
    /// inputs have been visited. The return value is handled the same
    /// as the return value of `pre_visit`. The provided default
    /// implementation returns `Ok(true)`.
    fn post_visit(&mut self, _plan: &dyn ExecutionPlan) -> Result<bool, Self::Error> {
        Ok(true)
    }
}

/// Recursively calls `pre_visit` and `post_visit` for this node and
/// all of its children, as described on [`ExecutionPlanVisitor`]
pub fn visit_execution_plan<V: ExecutionPlanVisitor>(
    plan: &dyn ExecutionPlan,
    visitor: &mut V,
) -> Result<(), V::Error> {
    visitor.pre_visit(plan)?;
    for child in plan.children() {
        visit_execution_plan(child.as_ref(), visitor)?;
    }
    visitor.post_visit(plan)?;
    Ok(())
}