package domain

import (
	"errors"
	"fmt"
	"hash/fnv"
	"strings"
	"unicode/utf8"
)

const maxInstanceNameRunes = 80

var errInvalidInstanceName = errors.New("invalid instance name")

// NormalizeInstanceName validates the user-visible instance name before it is
// stored or used to derive a Windows profile directory. The validation keeps
// the display name readable instead of silently rewriting user input.
func NormalizeInstanceName(value string) (string, error) {
	name := strings.TrimSpace(value)
	if name == "" {
		return "", fmt.Errorf("%w: name is required", errInvalidInstanceName)
	}
	if utf8.RuneCountInString(name) > maxInstanceNameRunes {
		return "", fmt.Errorf("%w: name is too long", errInvalidInstanceName)
	}
	if strings.HasSuffix(name, ".") {
		return "", fmt.Errorf("%w: name cannot end with a period", errInvalidInstanceName)
	}
	for _, value := range name {
		if value < 0x20 || strings.ContainsRune(`<>:"/\\|?*`, value) {
			return "", fmt.Errorf("%w: name contains a Windows-reserved character", errInvalidInstanceName)
		}
	}
	base := strings.ToUpper(strings.Split(name, ".")[0])
	if isReservedWindowsDeviceName(base) {
		return "", fmt.Errorf("%w: name is a reserved Windows device name", errInvalidInstanceName)
	}
	return name, nil
}

// InstanceProfileDirectoryName derives a compact, readable, and unique
// profile directory component. The stable hash keeps paths distinct even
// when display names share a truncated prefix.
func InstanceProfileDirectoryName(name, instanceID string) (string, error) {
	name, err := NormalizeInstanceName(name)
	if err != nil {
		return "", err
	}
	instanceID = strings.TrimSpace(instanceID)
	if instanceID == "" {
		return "", fmt.Errorf("%w: instance ID is required", errInvalidInstanceName)
	}
	segment := truncateUTF8(name, 64)
	hash := fnv.New32a()
	_, _ = hash.Write([]byte(instanceID))
	return fmt.Sprintf("instance-%s-%08x", segment, hash.Sum32()), nil
}

func truncateUTF8(value string, limit int) string {
	if len(value) <= limit {
		return value
	}
	count := 0
	for index := range value {
		if index > limit {
			break
		}
		count = index
	}
	if count == 0 {
		return value
	}
	return value[:count]
}

func isReservedWindowsDeviceName(value string) bool {
	switch value {
	case "CON", "PRN", "AUX", "NUL", "COM1", "COM2", "COM3", "COM4", "COM5", "COM6", "COM7", "COM8", "COM9", "LPT1", "LPT2", "LPT3", "LPT4", "LPT5", "LPT6", "LPT7", "LPT8", "LPT9":
		return true
	default:
		return false
	}
}