package config

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

// encoding/json accepts repeated object keys. Inspect the token stream before
// typed decoding so deployment settings cannot silently override earlier values.
func uniqueJSONKeys(decoder *json.Decoder, depth int) error {
	if depth > 16 {
		return errors.New("configuration nesting is too deep")
	}
	token, err := decoder.Token()
	if err != nil {
		return err
	}
	delimiter, compound := token.(json.Delim)
	if !compound {
		return nil
	}
	switch delimiter {
	case '{':
		seen := make(map[string]bool)
		for decoder.More() {
			keyToken, err := decoder.Token()
			if err != nil {
				return err
			}
			key, ok := keyToken.(string)
			if !ok {
				return errors.New("configuration object key must be a string")
			}
			if seen[key] {
				return fmt.Errorf("duplicate configuration field %q", key)
			}
			seen[key] = true
			if err := uniqueJSONKeys(decoder, depth+1); err != nil {
				return err
			}
		}
	case '[':
		for decoder.More() {
			if err := uniqueJSONKeys(decoder, depth+1); err != nil {
				return err
			}
		}
	default:
		return errors.New("unexpected configuration JSON delimiter")
	}
	_, err = decoder.Token()
	return err
}