package outpost

import (
	"context"
	"encoding/json"
	"errors"
	"fmt"
	"time"
)

// EmitOption adjusts a message before it is appended.
type EmitOption func(*Message)

// ID sets the message ID. By default Emit generates one; set it when the
// business object already has a stable identifier to reuse as the dedup key.
func ID(id string) EmitOption { return func(m *Message) { m.ID = id } }

// Key sets the grouping key, for example the recipient or user id.
func Key(key string) EmitOption { return func(m *Message) { m.Key = key } }

// Header sets one header. Headers are frozen after Emit.
func Header(key, value string) EmitOption {
	return func(m *Message) {
		if m.Headers == nil {
			m.Headers = make(map[string]string, 1)
		}
		m.Headers[key] = value
	}
}

// AvailableAt delays delivery until t, for scheduled sends.
func AvailableAt(t time.Time) EmitOption { return func(m *Message) { m.AvailableAt = t } }

// Emit encodes v as JSON and appends it through a under the given name.
func Emit(ctx context.Context, a Appender, name string, v any, opts ...EmitOption) error {
	if name == "" {
		return errors.New("outpost: empty message name")
	}
	if a == nil {
		return errors.New("outpost: nil appender")
	}
	payload, err := json.Marshal(v)
	if err != nil {
		return fmt.Errorf("outpost: encode %q: %w", name, err)
	}
	msg := Message{Name: name, Payload: payload, CreatedAt: time.Now()}
	for _, opt := range opts {
		opt(&msg)
	}
	if msg.ID == "" {
		if msg.ID, err = newID(); err != nil {
			return fmt.Errorf("outpost: generate id: %w", err)
		}
	}
	return a.Append(ctx, msg)
}