obikmer/docmd/implementation/obipipeline.md

# obipipeline — parallel pipeline library

`obipipeline` is a generic, multi-threaded data pipeline crate. It connects a **source**, a chain of **stages**, and a **sink** via crossbeam channels, running each stage with a shared worker pool and a biased scheduler.

## Core types

| Type alias | Rust type | Role |
|---|---|---|
| `SourceFn<D>` | `Box<dyn FnMut() -> Result<D, PipelineError> + Send>` | Called repeatedly; `FnMut` because it holds iterator state |
| `SharedFn<D>` | `Arc<dyn Fn(D) -> Result<D, PipelineError> + Send + Sync>` | 1→1 transform shared across workers via `Arc::clone` |
| `SharedFlatFn<D>` | `Arc<dyn Fn(D, &Sender<Result<D, _>>, &Sender<isize>) + Send + Sync>` | 1→N transform; pushes items into channel, sends delta |
| `SinkFn<D>` | `Box<dyn Fn(D) -> Result<(), PipelineError> + Send>` | Final consumer; returns `Result` so errors propagate back |

Stages come in two variants:

```rust
pub enum Stage<D> {
    Transform(SharedFn<D>),   // 1→1
    Flat(SharedFlatFn<D>),    // 1→N
}
```

`Pipeline<D>` holds one `SourceFn`, a `Vec<Stage>`, and one `SinkFn`.  
`WorkerPool<D>` wraps a `Pipeline` with `n_workers` and channel `capacity`.

## WorkerPool

```rust
WorkerPool::new(pipeline: Pipeline<D>, n_workers: usize, capacity: usize) -> Self
WorkerPool::run(self)
```

| Parameter | Role |
|---|---|
| `n_workers` | Number of parallel worker threads. Each worker is generic — it executes whichever transform the scheduler assigns it. |
| `capacity` | Bound on every crossbeam channel in the pipeline. Controls memory and back-pressure: a full channel blocks the sender until a slot frees. |

`run` consumes `self` (all fields are moved into threads). It blocks the calling thread until the pipeline has fully drained — source exhausted and every in-flight item processed by the sink — then joins all threads before returning.

## Data enum

All pipeline stages communicate through a single user-defined enum:

```rust
enum MyData {
    Unsigned(u64),
    Number(f64),
    Text(String),
}
```

Each variant carries the concrete type for one stage's output. The macros pattern-match on this enum to route values between stages.

## Macros

Eight low-level macros build individual stages; one high-level macro (`make_pipeline!`) composes them.

### Low-level

```rust
make_source!(Enum, iterator, OutputVariant)          // iterator yields T
make_source_fallible!(Enum, iterator, OutputVariant) // iterator yields Result<T, E>

make_transform!(Enum, func, InputVariant, OutputVariant)          // func: T -> U
make_transform_fallible!(Enum, func, InputVariant, OutputVariant) // func: T -> Result<U, E>

make_flat_transform!(Enum, func, InputVariant, OutputVariant)          // func: T -> impl IntoIterator<Item=U>
make_flat_transform_fallible!(Enum, func, InputVariant, OutputVariant) // func: T -> Result<impl IntoIterator<Item=U>, E>

make_sink!(Enum, func, InputVariant)           // func: T -> ()
make_sink_fallible!(Enum, func, InputVariant)  // func: T -> Result<(), E>
```

Each macro wraps the closure in the correct smart pointer (`Box` for source/sink, `Arc` for transforms).

### make_pipeline! DSL

```
make_pipeline! {
    DataEnum,
    source   iterator     => OutputVariant,   // or source?  for fallible
    |   func: In => Out,                       // 1→1 non-fallible transform
    |?  func: In => Out,                       // 1→1 fallible transform
    ||  func: In => Out,                       // 1→N non-fallible flat transform
    ||? func: In => Out,                       // 1→N fallible flat transform
    sink     func         @ InputVariant,      // or sink?    for fallible
}
```

`?` marks fallibility on source, individual transforms, or sink independently.  
Implemented as a **TT muncher**: the internal rule `@build` recurses over transform tokens one at a time, accumulating them into a `vec![]`, then terminates on `sink`/`sink?`.

### make_pipe! DSL

`make_pipe!` builds a sourceless/sinkless `Pipe<D, In, Out>` — a reusable, composable stage sequence:

```
make_pipe! {
    DataEnum : InType => OutType,
    |   func: InVariant => OutVariant,
    |?  func: InVariant => OutVariant,
    ||  func: InVariant => OutVariant,
    ||? func: InVariant => OutVariant,
}
```

Two pipes compose with `.then(other)`. Apply to an iterator with `.apply(iter, n_workers, capacity)` to get a `PipeIter<Out>` — an iterator over the pipeline output, backed by a background `WorkerPool`. The scatter step in `obikmer` uses `make_pipe!` and `.apply()` rather than the full `make_pipeline!` / `WorkerPool` pattern.

## Scheduler architecture

```
Source thread ──► [source_rx] ──► Scheduler ──► [worker_tx] ──► Workers (×N)
                                      ▲                               │
                  [stage_rxs] ────────┘◄──────────────────────────────┘
                  [flat_delta_rx] ──► Scheduler  (in_flight adjustment)
                                      │
                              [sink_err_rx]  ← errors from sink (highest priority)
                                      │
                                   Sink thread
```

The scheduler is a single thread running a biased `Select` over all input channels. Priority order (highest first):

```
index 0       sink_err_rx          abort on sink error
index 1       flat_delta_rx        adjust in_flight before dispatching
index 2..=n+1 stage_rxs[n-1..0]   drain last stage first
index n+2     source_rx            pull new data last
```

This back-pressure-friendly ordering ensures downstream stages are drained before new items enter the pipeline.

**Workers** are generic: each receives a `WorkerTask` — either `Transform(data, stage_idx)` or `Flat(data, stage_idx)`. For `Transform`, the worker calls `f(data)` and sends the result to `stage_txs[stage_idx]`. For `Flat`, the worker calls `f(data, &push_tx, &delta_tx)`: the closure pushes N items into `push_tx` then sends `N-1` to `delta_tx`. The scheduler uses the delta to adjust `in_flight` without knowing N in advance.

**Termination** uses an `in_flight: isize` counter and a `flat_workers_active: usize` counter:

- `in_flight` incremented when an item is dispatched from source to workers
- `in_flight` decremented when the item exits the last stage to the sink
- `flat_workers_active` incremented when a `Flat` task is dispatched, decremented when the delta arrives
- the loop exits only when `source_done && in_flight == 0 && flat_workers_active == 0`

This guarantees all in-flight items complete (including all N outputs of a flat stage) before `join()`.

## Error handling

`PipelineError` has four variants:

| Variant | Meaning |
|---|---|
| `EndOfStream` | Source exhausted (normal termination, not sent downstream) |
| `TypeMismatch` | Wrong enum variant arrived at a stage |
| `StepKindMismatch` | Internal routing error |
| `StepError(Box<dyn Error + Send + Sync>)` | Error from user code (wrapped by `make_*_fallible!`) |

Sink errors flow back to the scheduler via a dedicated `Receiver<PipelineError>` registered at index 0 of the Select — the pipeline stops immediately on the first sink error.

## Example

```rust
enum PipelineData { Unsigned(u64), Number(f64), Text(String) }

fn to_f64(x: u64) -> f64 { x as f64 }
fn format_num(n: f64) -> String { format!("{}", n) }
fn reverse(s: String) -> String { s.chars().rev().collect() }
fn hash(s: String) -> u64 { /* djb2 */ }
fn print_hash(h: u64) -> Result<(), std::io::Error> { println!("{}", h); Ok(()) }

let pipeline = make_pipeline! {
    PipelineData,
    source  1u64..=10 => Unsigned,
    | to_f64:     Unsigned => Number,
    | format_num: Number   => Text,
    | reverse:    Text     => Text,
    | hash:       Text     => Unsigned,
    sink?   print_hash     @ Unsigned,
};

WorkerPool::new(pipeline, 4, 64).run();
```