package app

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

// LoadDotEnv loads simple KEY=VALUE entries without replacing variables that
// are already present in the process environment. A missing file is allowed.
func LoadDotEnv(path string) error {
	file, err := os.Open(path)
	if errors.Is(err, os.ErrNotExist) {
		return nil
	}
	if err != nil {
		return fmt.Errorf("open dotenv file: %w", err)
	}
	defer file.Close()

	scanner := bufio.NewScanner(file)
	for lineNumber := 1; scanner.Scan(); lineNumber++ {
		line := strings.TrimSpace(scanner.Text())
		if line == "" || strings.HasPrefix(line, "#") {
			continue
		}
		line = strings.TrimSpace(strings.TrimPrefix(line, "export "))
		key, value, found := strings.Cut(line, "=")
		key = strings.TrimSpace(key)
		if !found || !validEnvironmentKey(key) {
			return fmt.Errorf("%s:%d: invalid environment assignment", path, lineNumber)
		}
		value = strings.TrimSpace(value)
		if len(value) >= 2 && ((value[0] == '\'' && value[len(value)-1] == '\'') || (value[0] == '"' && value[len(value)-1] == '"')) {
			value = value[1 : len(value)-1]
		}
		if _, exists := os.LookupEnv(key); !exists {
			if err := os.Setenv(key, value); err != nil {
				return fmt.Errorf("%s:%d: set environment: %w", path, lineNumber, err)
			}
		}
	}
	if err := scanner.Err(); err != nil {
		return fmt.Errorf("read dotenv file: %w", err)
	}
	return nil
}

func validEnvironmentKey(key string) bool {
	if key == "" || !isEnvironmentKeyStart(key[0]) {
		return false
	}
	for i := 1; i < len(key); i++ {
		if !isEnvironmentKeyStart(key[i]) && (key[i] < '0' || key[i] > '9') {
			return false
		}
	}
	return true
}

func isEnvironmentKeyStart(char byte) bool {
	return char == '_' || char >= 'A' && char <= 'Z' || char >= 'a' && char <= 'z'
}