//go:build linux

package system

import (
	"fmt"
	"os"
	"os/exec"
	"path/filepath"
	"strings"
	"syscall"
)

// IsMountPoint checks if a path is on a different device than its parent.
// Returns true if the path is a mount point (different device ID from parent).
func IsMountPoint(path string) bool {
	var pathStat, parentStat syscall.Stat_t
	if err := syscall.Stat(path, &pathStat); err != nil {
		return false
	}
	parent := filepath.Dir(path)
	if err := syscall.Stat(parent, &parentStat); err != nil {
		return false
	}
	return pathStat.Dev != parentStat.Dev
}

// IsWritable checks if the given path is writable by attempting to create+remove a temp file.
func IsWritable(path string) bool {
	testFile := filepath.Join(path, ".felhom-write-test")
	f, err := os.Create(testFile)
	if err != nil {
		return false
	}
	f.Close()
	os.Remove(testFile)
	return true
}

// PathsOverlap returns true if one path is a parent or child of the other.
func PathsOverlap(a, b string) bool {
	a = filepath.Clean(a)
	b = filepath.Clean(b)
	if a == b {
		return true
	}
	aSep := a + string(os.PathSeparator)
	bSep := b + string(os.PathSeparator)
	return strings.HasPrefix(aSep, bSep) || strings.HasPrefix(bSep, aSep)
}

// DiskUsageInfo holds disk usage statistics for a path.
type DiskUsageInfo struct {
	TotalGB     float64
	UsedGB      float64
	AvailGB     float64
	UsedPercent float64
	TotalHuman  string
	UsedHuman   string
}

// GetDiskUsage returns disk usage info for a path, or nil on error.
func GetDiskUsage(path string) *DiskUsageInfo {
	var stat syscall.Statfs_t
	if err := syscall.Statfs(path, &stat); err != nil {
		return nil
	}

	bsize := uint64(stat.Bsize)
	total := stat.Blocks * bsize
	avail := stat.Bavail * bsize
	used := total - (stat.Bfree * bsize)

	const gb = 1024 * 1024 * 1024
	info := &DiskUsageInfo{
		TotalGB: float64(total) / float64(gb),
		UsedGB:  float64(used) / float64(gb),
		AvailGB: float64(avail) / float64(gb),
	}
	if total > 0 {
		info.UsedPercent = float64(used) / float64(total) * 100
	}
	info.TotalHuman = formatGB(info.TotalGB)
	info.UsedHuman = formatGB(info.UsedGB)
	return info
}

func formatGB(gb float64) string {
	if gb >= 1000 {
		return fmt.Sprintf("%.1f TB", gb/1024)
	}
	return fmt.Sprintf("%.1f GB", gb)
}

// FSInfo holds filesystem type, device, and disk model info.
type FSInfo struct {
	FSType string // "ext4", "btrfs"
	Device string // "/dev/sda1"
	Model  string // "WD Elements 25A2" (best-effort from sysfs)
}

// GetFSInfo returns filesystem info for a path using findmnt, or nil on error.
func GetFSInfo(path string) *FSInfo {
	out, err := exec.Command("findmnt", "-n", "-o", "SOURCE,FSTYPE", "--target", path).Output()
	if err != nil {
		return nil
	}
	fields := strings.Fields(strings.TrimSpace(string(out)))
	if len(fields) < 2 {
		return nil
	}
	info := &FSInfo{
		Device: fields[0],
		FSType: fields[1],
	}
	// Try to get disk model from sysfs
	info.Model = diskModel(info.Device)
	return info
}

// DestinationHealth holds the result of a tiered backup destination check.
type DestinationHealth struct {
	Exists      bool
	Writable    bool
	MountPoint  bool    // true if path is on a different device from its parent
	SystemDrive bool    // true if path is on the same device as /
	UsedPercent float64 // disk usage percentage (0 if unknown)
	FreeGB      float64
	Warning     string // human-readable warning message in Hungarian (empty = ok)
	Blocked     bool   // if true, backup must not run
	Severity    string // "ok", "warning", "critical"
}

// CheckBackupDestination performs tiered validation of a cross-drive backup destination.
// Returns a DestinationHealth describing any issues found.
func CheckBackupDestination(path string) DestinationHealth {
	h := DestinationHealth{Severity: "ok"}

	// Tier 1: path must exist
	if _, err := os.Stat(path); os.IsNotExist(err) {
		h.Warning = "A cél tárhely (" + path + ") nem létezik!"
		h.Blocked = true
		h.Severity = "critical"
		return h
	}
	h.Exists = true

	// Tier 2: path must be writable
	if !IsWritable(path) {
		h.Warning = "A cél tárhely (" + path + ") nem írható! Ellenőrizd a jogosultságokat."
		h.Blocked = true
		h.Severity = "critical"
		return h
	}
	h.Writable = true

	// Tier 3: detect if source and destination are on the same block device
	// (stronger than IsMountPoint — catches e.g. bind mounts within same device)
	if isSameBlockDevice(path, "/") {
		h.SystemDrive = true
		// This is a warning, not a block — user data still protected against software errors
		h.Warning = "A cél tárhely (" + path + ") a rendszermeghajtón van. " +
			"Meghajtóhiba esetén az eredeti adat és a mentés is elveszhet. " +
			"Külső meghajtó használata javasolt."
		h.Severity = "warning"
		// Don't return early — also check disk usage
	} else {
		h.MountPoint = true
	}

	// Tier 4: disk usage checks
	if di := GetDiskUsage(path); di != nil {
		h.UsedPercent = di.UsedPercent
		h.FreeGB = di.AvailGB
		if di.UsedPercent >= 95 {
			h.Warning = fmt.Sprintf("A mentési meghajtó megtelt (%.0f%% használt)!", di.UsedPercent)
			h.Blocked = true
			h.Severity = "critical"
		} else if di.UsedPercent >= 90 && h.Severity == "ok" {
			h.Warning = fmt.Sprintf("A mentési meghajtó majdnem megtelt (%.0f%% használt).", di.UsedPercent)
			h.Severity = "warning"
		}
	}

	return h
}

// isSameBlockDevice returns true if pathA and pathB are on the same block device.
func isSameBlockDevice(pathA, pathB string) bool {
	var statA, statB syscall.Stat_t
	if err := syscall.Stat(pathA, &statA); err != nil {
		return false
	}
	if err := syscall.Stat(pathB, &statB); err != nil {
		return false
	}
	return statA.Dev == statB.Dev
}

// diskModel reads the disk model from /sys/block/<dev>/device/model.
func diskModel(device string) string {
	// /dev/sda1 → sda, /dev/nvme0n1p1 → nvme0n1
	base := filepath.Base(device)
	// Strip partition number: sda1 → sda, nvme0n1p1 → nvme0n1
	disk := base
	if strings.HasPrefix(base, "nvme") {
		// nvme0n1p1 → find last 'p' followed by digits
		if idx := strings.LastIndex(base, "p"); idx > 4 {
			disk = base[:idx]
		}
	} else {
		// sda1 → sda: strip trailing digits
		disk = strings.TrimRight(base, "0123456789")
	}
	modelPath := "/sys/block/" + disk + "/device/model"
	data, err := os.ReadFile(modelPath)
	if err != nil {
		return ""
	}
	return strings.TrimSpace(string(data))
}