package contracts

import (
	"bytes"
	"encoding/json"
	"fmt"
	"io"
	"time"
)

const MaxPayloadBytes = 16 * 1024

// Decode bounds input, rejects unknown fields, requires exactly one JSON value,
// and validates the decoded MQTT contract against its parsed topic.
func Decode[T MQTTMessage](payload []byte, topic Topic, now time.Time) (T, error) {
	var value T
	if len(payload) > MaxPayloadBytes {
		return value, fmt.Errorf("payload exceeds %d bytes", MaxPayloadBytes)
	}
	decoder := json.NewDecoder(bytes.NewReader(payload))
	decoder.DisallowUnknownFields()
	if err := decoder.Decode(&value); err != nil {
		return value, fmt.Errorf("decode JSON: %w", err)
	}
	var trailing any
	if err := decoder.Decode(&trailing); err != io.EOF {
		if err == nil {
			return value, fmt.Errorf("payload contains more than one JSON value")
		}
		return value, fmt.Errorf("trailing data after JSON value: %w", err)
	}
	if err := value.Validate(topic, now); err != nil {
		return value, fmt.Errorf("validate message: %w", err)
	}
	return value, nil
}