// Package env provides a minimal, dependency-free .env file loader.
//
// It reads KEY=VAL pairs from a file and sets them into the process
// environment using os.Setenv, but only for variables that are NOT already
// set. This gives the real environment (system env / shell) priority over
// values in the .env file.
package env

import (
	"bufio"
	"fmt"
	"os"
	"strings"
)

// DefaultPath is the file LoadEnvDefault reads when no path is given.
const DefaultPath = ".env"

// LoadEnvDefault loads environment variables from the default ".env" file
// in the current working directory.
func LoadEnvDefault() error {
	return LoadEnv(DefaultPath)
}

// LoadEnv reads the file at path line by line and sets each KEY=VALUE pair
// into the process environment.
//
// Rules:
//   - Blank lines and lines starting with '#' (comments) are ignored.
//   - Values may be wrapped in single or double quotes, which are stripped.
//   - A variable already present in the environment is never overridden
//     (system env wins; .env is the lowest priority).
func LoadEnv(path string) error {
	f, err := os.Open(path)
	if err != nil {
		return err
	}
	defer f.Close()

	scanner := bufio.NewScanner(f)
	for scanner.Scan() {
		line := strings.TrimSpace(scanner.Text())
		if line == "" || strings.HasPrefix(line, "#") {
			continue
		}

		key, value, ok := strings.Cut(line, "=")
		if !ok {
			// Not KEY=VAL format; skip silently.
			continue
		}
		key = strings.TrimSpace(key)
		if key == "" {
			continue
		}
		value = strings.TrimSpace(value)
		value = unquote(value)

		if _, exists := os.LookupEnv(key); exists {
			// System environment wins over .env.
			continue
		}
		if err := os.Setenv(key, value); err != nil {
			return fmt.Errorf("failed to set env var %q: %w", key, err)
		}
	}
	if err := scanner.Err(); err != nil {
		return fmt.Errorf("failed to read %s: %w", path, err)
	}
	return nil
}

// unquote strips a single matching pair of surrounding single or double
// quotes from value, if present. Any other value is returned unchanged.
func unquote(value string) string {
	if len(value) >= 2 {
		first, last := value[0], value[len(value)-1]
		if (first == '"' && last == '"') || (first == '\'' && last == '\'') {
			return value[1 : len(value)-1]
		}
	}
	return value
}