package ingest

import (
	"context"
	"errors"
	"io"
)

// Sink materializes a stream into the reporting database.
//
// Idempotency lives HERE and not in the reader, because the effect and the high-water mark
// (run_attempts.last_seq) have to move in ONE transaction: an implementation that applies an event
// and then records it has a crash window that duplicates work.
//
// This is a reporting database, not an event store (research/23 §3): the events are not kept.
type Sink interface {
	// Begin binds the stream to an attempt — the engine's run id is the idempotency namespace.
	Begin(ctx context.Context, h Hello) error
	// Apply materializes one event AND the cursor it moves, in one transaction. It MUST ignore an
	// event whose Seq is not greater than the stored high-water mark, and it MUST ignore an unknown
	// Type. The reader applies the same rule before calling, so the two agree even when a re-read
	// races a live writer; the sink's copy is the one that holds under a crash.
	Apply(ctx context.Context, ev Envelope, c Cursor) error
}

// Ingest reads a PIPE to its end, feeding a sink. That is the DEV path (D39.106 §3): production
// tails the book's journal with Tail, and there is no pipe at all.
//
// It stops at the first error and returns it: a gap or a malformed line is not recoverable by
// reading further — the caller reconciles from `tmctl status --json`, which is the whole reason
// that channel is part of the ratified seam. The cursor it hands the sink is empty, because a pipe
// has no position to resume from — which is exactly why the ratified transport is a file.
func Ingest(ctx context.Context, r io.Reader, sink Sink) error {
	d := NewDecoder(r)
	h, err := d.Hello()
	if err != nil {
		return err
	}
	if err := sink.Begin(ctx, h); err != nil {
		return err
	}
	for {
		if err := ctx.Err(); err != nil {
			return err
		}
		ev, err := d.Next()
		if errors.Is(err, io.EOF) {
			return nil
		}
		if err != nil {
			return err
		}
		if err := sink.Apply(ctx, ev, Cursor{}); err != nil {
			return err
		}
	}
}