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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, GValue},
};
/// A physical step that collects all upstream traversers into a single `GValue::List`.
///
/// This implements the Gremlin `fold()` step: it drains the upstream pipeline
/// completely, wraps every value into a `Vec<GValue>`, and emits it as one
/// `GValue::List` traverser downstream. It emits exactly once and then signals
/// exhaustion.
#[derive(Debug, Default)]
pub struct FoldStep {
// ── Upstream link ──
upstream: Option<StepRef>,
// ── Dynamic/Runtime execution state ──
/// Whether the folded list has already been emitted.
emitted: bool,
}
impl CoreStep for FoldStep {
fn add_upper(&mut self, upstream: StepRef) {
self.upstream = Some(upstream);
}
fn produce(
&mut self,
ctx: &mut dyn GraphCtx,
) -> Result<Option<SmallVec<[Rc<Traverser>; PIPELINE_PRODUCE_SIZE]>>, StoreError> {
if self.emitted {
return Ok(None);
}
let Some(upstream) = self.upstream.as_ref() else { return Ok(None) };
let mut list = Vec::new();
while let Some(t) = upstream.next(ctx)? {
list.push(t.value.clone());
}
self.emitted = true;
Ok(Some(smallvec![Traverser::new_rc(GValue::List(list))]))
}
fn reset(&mut self) {
self.emitted = false;
if let Some(up) = &self.upstream {
up.reset();
}
}
fn upper(&self) -> Option<StepRef> {
self.upstream.clone()
}
fn explain(&self) -> ExplainNode {
ExplainNode::new("FoldStep")
}
}