package runtimecfg

import (
	"fmt"
	"os"
	"strconv"

	"gopkg.in/yaml.v2"
)

// Load substitutes YAML scalars from the service's separately loaded .env.
func Load(path string, target interface{}) error {
	raw, err := os.ReadFile(path)
	if err != nil {
		return err
	}
	var missing string
	expanded := os.Expand(string(raw), func(key string) string {
		value, ok := os.LookupEnv(key)
		if !ok || value == "" {
			missing = key
		}
		return strconv.Quote(value)
	})
	if missing != "" {
		return fmt.Errorf("required environment variable is missing: %s", missing)
	}
	return yaml.Unmarshal([]byte(expanded), target)
}