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// Copyright (c) 2026 Austin Han <austinhan1024@gmail.com>
//
// This file is part of RocksGraph.
//
// RocksGraph is free software: you can redistribute it and/or modify
// it under the terms of the GNU General Public License as published by
// the Free Software Foundation, either version 2 of the License, or
// (at your option) any later version.
//
// RocksGraph is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
// GNU General Public License for more details.
//
// You should have received a copy of the GNU General Public License
// along with RocksGraph. If not, see <https://www.gnu.org/licenses/>.
use crate::types::PIPELINE_PRODUCE_SIZE;
use std::rc::Rc;
use smallvec::{smallvec, SmallVec};
use crate::engine::volcano::steps::traits::ExplainNode;
use crate::{
engine::{
context::GraphCtx,
traverser::Traverser,
volcano::steps::traits::{CoreStep, StepRef},
},
types::error::StoreError,
};
/// A physical step that limits the number of traversers emitted by its upstream.
#[derive(Debug)]
pub struct LimitStep {
// ── Upstream link ──
upstream: Option<StepRef>,
// ── Static/Fixed configuration ──
/// The maximum number of elements to yield.
limit: i64,
// ── Dynamic/Runtime execution state ──
/// The number of elements yielded so far.
current_idx: usize,
}
/// Creates a new `LimitStep` with the specified limit.
impl LimitStep {
pub fn new(limit: i64) -> Self {
Self { upstream: None, limit, current_idx: 0 }
}
}
impl CoreStep for LimitStep {
fn add_upper(&mut self, upstream: StepRef) {
// Sets the upstream step for this limit.
self.upstream = Some(upstream);
}
fn produce(
&mut self,
ctx: &mut dyn GraphCtx,
) -> Result<Option<SmallVec<[Rc<Traverser>; PIPELINE_PRODUCE_SIZE]>>, StoreError> {
// Produces traversers from its upstream until the limit is reached.
if self.current_idx >= self.limit as usize {
return Ok(None);
}
let Some(upstream) = self.upstream.as_ref() else { return Ok(None) };
let Some(t) = upstream.next(ctx)? else { return Ok(None) };
self.current_idx += 1;
Ok(Some(smallvec![t]))
}
fn reset(&mut self) {
// Resets the step's internal counter and its upstream.
self.current_idx = 0;
if let Some(up) = &self.upstream {
up.reset();
}
}
fn upper(&self) -> Option<StepRef> {
// Returns a clone of the upstream step reference.
self.upstream.clone()
}
fn explain(&self) -> ExplainNode {
let params = vec![("limit", self.limit.to_string())];
ExplainNode::new("LimitStep").with_params(params)
}
}