// Package env implements encoding and decoding between environment variable and a typed Configuration.
package env

import (
	"fmt"
	"regexp"
	"strings"

	"github.com/containous/traefik/v2/pkg/config/parser"
)

// DefaultNamePrefix is the default prefix for environment variable names.
const DefaultNamePrefix = "TRAEFIK_"

// Decode decodes the given environment variables into the given element.
// The operation goes through four stages roughly summarized as:
// env vars -> map
// map -> tree of untyped nodes
// untyped nodes -> nodes augmented with metadata such as kind (inferred from element)
// "typed" nodes -> typed element
func Decode(environ []string, prefix string, element interface{}) error {
	if err := checkPrefix(prefix); err != nil {
		return err
	}

	vars := make(map[string]string)
	for _, evr := range environ {
		n := strings.SplitN(evr, "=", 2)
		if strings.HasPrefix(strings.ToUpper(n[0]), prefix) {
			key := strings.ReplaceAll(strings.ToLower(n[0]), "_", ".")
			vars[key] = n[1]
		}
	}

	rootName := strings.ToLower(prefix[:len(prefix)-1])
	return parser.Decode(vars, element, rootName)
}

// Encode encodes the configuration in element into the environment variables represented in the returned Flats.
// The operation goes through three stages roughly summarized as:
// typed configuration in element -> tree of untyped nodes
// untyped nodes -> nodes augmented with metadata such as kind (inferred from element)
// "typed" nodes -> environment variables with default values (determined by type/kind)
func Encode(element interface{}) ([]parser.Flat, error) {
	if element == nil {
		return nil, nil
	}

	node, err := parser.EncodeToNode(element, parser.DefaultRootName, false)
	if err != nil {
		return nil, err
	}

	err = parser.AddMetadata(element, node)
	if err != nil {
		return nil, err
	}

	return parser.EncodeToFlat(element, node, parser.FlatOpts{Case: "upper", Separator: "_"})
}

func checkPrefix(prefix string) error {
	prefixPattern := `[a-zA-Z0-9]+_`
	matched, err := regexp.MatchString(prefixPattern, prefix)
	if err != nil {
		return err
	}

	if !matched {
		return fmt.Errorf("invalid prefix %q, the prefix pattern must match the following pattern: %s", prefix, prefixPattern)
	}

	return nil
}