rill-graph

Static DAG signal graph — topology and port connections only. Processing is driven by Port::propagate (not an external engine).

Key components

Graph — immutable DAG container, topology is fixed at build time
GraphBuilder — the only way to build a graph (Source → Processor → Sink), fills downstream_input_ptrs, parent, upstream_buffer for zero-copy routing
Kahn's algorithm — topological sort with cycle detection
Port::propagate — recursive signal propagation:
1. Copy data to downstream input ports (skipped for zero-copy upstream_buffer ports)
2. Run port algorithm (run_action)
3. Call pre_process (feedback mix)
4. Call the downstream node's process_block (generate/process/consume)
5. snapshot_feedback on output ports
6. Recurse through output ports' downstream_input_ptrs
Zero-copy routing — 1:1 and fan-out connections read directly from upstream output buffer via upstream_buffer. Copy only for fan-in and feedback.
Hard-RT safe — no heap allocations, no locks, no syscalls in the signal path. All Port::propagate data structures are pre-allocated at graph construction time (downstream_nodes, downstream_input_ptrs). Communication with the control thread is exclusively through lock-free MpscQueue<ParameterCommand>.
SIMD-friendly — fixed buffer position in memory for the graph's lifetime
Port routing — connections and feedback buffers live on ports
Feedback support — port.pre_process / port.snapshot_feedback
Port types — Signal, Control, Clock, Feedback, Param

Source::generate, then Port::propagate to cascade through the DAG.