//go:build linux

package proc

import (
	"errors"
	"fmt"
	"os"
	"os/exec"
	"slices"
	"strconv"
	"strings"
	"syscall"

	"github.com/pranshuparmar/witr/pkg/model"
)

// GetFileContext returns file descriptor and lock info for a process
// Will return nil if the context could not be gathered.
func GetFileContext(pid int) *model.FileContext {
	var fileContext model.FileContext

	fdDir := fmt.Sprintf("/proc/%v/fd", pid)
	fdFiles, err := os.ReadDir(fdDir)
	if err != nil {
		return nil
	}

	fileContext.OpenFiles = len(fdFiles)
	fileContext.FileLimit = getFileLimit(pid)
	fileContext.LockedFiles = getLockedFiles(pid)
	fileContext.WatchedDirs = getWatchedDirs(fdDir, fdFiles)

	return &fileContext
}

func getFileLimit(pid int) int {
	var linuxDefaultMaxOpenFile = getDefaultMaxOpenFiles()

	// Read /proc/<pid>/limits for file limit
	data, err := os.ReadFile(fmt.Sprintf("/proc/%v/limits", pid))
	if err != nil {
		return linuxDefaultMaxOpenFile
	}

	dataString := string(data)
	for line := range strings.Lines(dataString) {
		if !strings.HasPrefix(line, "Max open files") {
			continue
		}

		// Data in format: "Max open files $SOFT_LOCK_NUMBER $HARD_LOCK_NUMBER files"
		fields := strings.Fields(line)
		if len(fields) < 4 {
			return linuxDefaultMaxOpenFile
		}
		softLimitString := fields[3]

		if softLimitString == "unlimited" {
			return 0
		}

		softLimit, err := strconv.Atoi(softLimitString)
		if err != nil {
			return linuxDefaultMaxOpenFile
		}

		return softLimit
	}

	return linuxDefaultMaxOpenFile
}

func getDefaultMaxOpenFiles() int {
	// This seems to be a common default for many systems.
	const reasonableDefault int = 1024

	// https://www.man7.org/linux/man-pages/man2/getrlimit.2.html
	var rlimit syscall.Rlimit
	err := syscall.Getrlimit(syscall.RLIMIT_NOFILE, &rlimit)
	if err != nil {
		return reasonableDefault
	}

	return int(rlimit.Max)
}

func getLockedFiles(pid int) []string {
	files, err := getLockedFilesLslocks(pid)
	if errors.Is(err, exec.ErrNotFound) {
		return getLockedFilesProc(pid)
	}
	return files
}

func getLockedFilesLslocks(pid int) ([]string, error) {
	var locked []string
	output, err := exec.Command("lslocks", "-o", "PATH", "-p", strconv.Itoa(pid)).Output()
	if err != nil {
		return nil, err
	}

	// First line of output is PATH (column name) which is not interesting.
	skippedFirst := false
	for fileName := range strings.Lines(string(output)) {
		if !skippedFirst {
			skippedFirst = true
			continue
		}

		locked = append(locked, strings.TrimSpace(fileName))
	}

	return locked, nil
}

// get list of locked files by the process
func getLockedFilesProc(pid int) []string {
	lockedFileData, err := os.ReadFile("/proc/locks")
	if err != nil {
		return nil
	}

	var result []string
	// Output Pattern: <ID>: <TYPE> <ADVISORY/MANDATORY> <ACCESS> <PID> <DEVICE> <START> <END>
	pidStr := strconv.Itoa(pid)

	for _, line := range strings.Split(string(lockedFileData), "\n") {
		if line == "" {
			continue
		}

		fields := strings.Fields(line)
		if len(fields) < 8 {
			continue
		}

		// lockType := fields[1]    // FLOCK, POSIX, or OFDLCK
		lockPid := fields[4]     // PID that owns the lock
		deviceInode := fields[5] // Device:Inode identifier

		// consider POSIX locks (these have valid PIDs)
		// Skip OFDLCK as PID is -1 (owned by multiple processes)
		// skip FLOCK as it may not have valid PID association
		if lockPid == pidStr {
			// Store device:inode as identifier (resolving to file path would require scanning filesystem)
			if !slices.Contains(result, deviceInode) {
				result = append(result, deviceInode)
			}
		}
	}

	return result
}

// get list of directories being accessed by the process
// directories being watched/accessed (detectable via /proc/<pid>/fd)
func getWatchedDirs(fdDir string, entries []os.DirEntry) []string {
	var result []string
	seen := make(map[string]bool)

	for _, entry := range entries {
		path := fmt.Sprintf("%s/%s", fdDir, entry.Name())
		target, err := os.Readlink(path)
		if err != nil {
			continue
		}

		// Check if target is a directory
		info, err := os.Stat(target)
		if err != nil {
			continue
		}

		if info.IsDir() {
			if !seen[target] {
				seen[target] = true
				result = append(result, target)
			}
		}
	}

	return result
}
