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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::{GValue, Primitive},
DegreeDirection,
},
};
/// Physical streaming map that emits `GValue::Scalar(Int64(degree))` for each upstream vertex.
///
/// Produced only by the `degree_pushdown` optimizer — never by the traversal builder.
/// Every `produce()` call is O(1): a single overlay-HashMap lookup (or one CF point read
/// on a cold overlay).
#[derive(Debug)]
pub struct DegreeStep {
upstream: Option<StepRef>,
direction: DegreeDirection,
/// Whether to thread the upstream traverser as parent so `path()` can
/// reconstruct `[Vertex, Int64(degree)]`. Set by the builder when a
/// `path()` step is present anywhere in the physical plan.
track_path: bool,
}
impl DegreeStep {
pub fn new(direction: DegreeDirection, track_path: bool) -> Self {
Self { upstream: None, direction, track_path }
}
}
impl CoreStep for DegreeStep {
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> {
let Some(upstream) = self.upstream.as_ref() else { return Ok(None) };
let Some(t) = upstream.next(ctx)? else { return Ok(None) };
match &t.value {
GValue::Vertex(vk) => {
let degree = ctx.get_degree(*vk, self.direction)?;
Ok(Some(smallvec![Traverser::new_rc_conditional(
GValue::Scalar(Primitive::Int64(degree as i64)),
&t,
self.track_path,
)]))
}
other => {
Err(StoreError::UnexpectedDataType(format!("degree() expects a Vertex traverser, got {:?}", other)))
}
}
}
fn reset(&mut self) {
if let Some(up) = &self.upstream {
up.reset();
}
}
fn upper(&self) -> Option<StepRef> {
self.upstream.clone()
}
fn explain(&self) -> ExplainNode {
let params = vec![("direction", format!("{:?}", self.direction))];
ExplainNode::new("DegreeStep").with_params(params)
}
}