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. It MUST ignore an event whose Seq is not greater than the
	// stored high-water mark, and it MUST ignore an unknown Type.
	Apply(ctx context.Context, ev Envelope) error
}

// Ingest reads a stream to its end, feeding a sink.
//
// 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.
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); err != nil {
			return err
		}
	}
}