package main

// This file holds the default excludes (README "Rules for the walk"):
// the directories a scan's walk does not enter below an operand, unless
// scan is given --no-default-excludes.

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

// The systems that have default excludes, as runtime.GOOS names them.
const (
	goosLinux  = "linux"
	goosDarwin = "darwin"
)

// mountInfoPath is the file in which Linux lists the mounts the reading
// process sees.
const mountInfoPath = "/proc/self/mountinfo"

// The fields of a line of mountInfoPath that kernelMounts reads, counted
// from 0 (see proc(5)): the mount point is the fifth field, and after
// the sixth come optional fields, then a field holding "-", then the
// filesystem type.
const (
	mountPointField     = 4
	optionalFieldsStart = 6
)

// typesPerHelpLine is how many filesystem types the scan command's help
// lists on each line.
const typesPerHelpLine = 6

// errMountInfoLine reports a line of mountInfoPath without the fields
// kernelMounts reads.
var errMountInfoLine = errors.New("unexpected line")

// defaultExcludes returns the directories that are default excludes on
// the system goos names, as runtime.GOOS does: a fixed list on Linux and
// on macOS, and none on any other system. On Linux the mount points
// kernelMounts finds are default excludes as well.
func defaultExcludes(goos string) []string {
	switch goos {
	case goosLinux:
		return []string{"/proc", "/sys", "/dev", "/run", "/tmp", "/var/tmp"}
	case goosDarwin:
		// A walk of / reaches everything in /System/Volumes/Data through
		// the firmlinks, so walking it as well would record each of
		// those files twice.
		return []string{
			"/dev", "/private/tmp", "/private/var/tmp", "/private/var/vm",
			"/System/Volumes/VM", "/System/Volumes/Data",
		}
	default:
		return nil
	}
}

// kernelFilesystemTypes returns the Linux filesystem types whose mount
// points are default excludes: filesystems that the kernel makes up or
// that are kept in memory.
func kernelFilesystemTypes() []string {
	return []string{
		"proc", "sysfs", "devtmpfs", "devpts", "tmpfs", "ramfs",
		"cgroup", "cgroup2", "debugfs", "tracefs", "securityfs", "pstore",
		"bpf", "configfs", "fusectl", "mqueue", "hugetlbfs", "efivarfs",
		"binfmt_misc", "autofs", "nsfs", "rpc_pipefs", "selinuxfs",
	}
}

// findDefaultExcludes returns the default excludes of the system goos
// names, each with what the scan says about it on the line it prints
// when its walk does not enter it. On Linux it reads the mount points
// from mountInfoPath.
func findDefaultExcludes(goos string) (map[string]string, error) {
	excludes := make(map[string]string)

	for _, dir := range defaultExcludes(goos) {
		excludes[dir] = "default exclude"
	}

	if goos != goosLinux {
		return excludes, nil
	}

	f, err := os.Open(mountInfoPath)
	if err != nil {
		return nil, err
	}

	defer func() { _ = f.Close() }()

	mounts, err := kernelMounts(f)
	if err != nil {
		return nil, fmt.Errorf("read %s: %w", mountInfoPath, err)
	}

	for dir, fsType := range mounts {
		excludes[dir] = "default exclude, " + fsType + " filesystem"
	}

	return excludes, nil
}

// kernelMounts reads mountinfo, laid out as mountInfoPath is, and
// returns the mount point of every filesystem whose type is one of
// kernelFilesystemTypes, with that type. Where one filesystem is mounted
// over another, a later line names the one on top, so the last line for
// a mount point decides.
func kernelMounts(mountinfo io.Reader) (map[string]string, error) {
	types := kernelFilesystemTypes()

	// The kernel writes a space, tab, newline or backslash in a mount
	// point as an octal escape.
	unescape := strings.NewReplacer(
		`\040`, " ", `\011`, "\t", `\012`, "\n", `\134`, `\`)

	mounts := make(map[string]string)
	lines := bufio.NewScanner(mountinfo)

	for lines.Scan() {
		fields := strings.Fields(lines.Text())

		sep := slices.Index(fields, "-")
		if sep < optionalFieldsStart || sep+1 >= len(fields) {
			return nil, fmt.Errorf("%w: %q", errMountInfoLine, lines.Text())
		}

		mountPoint := unescape.Replace(fields[mountPointField])

		fsType := fields[sep+1]
		if slices.Contains(types, fsType) {
			mounts[mountPoint] = fsType
		} else {
			delete(mounts, mountPoint)
		}
	}

	return mounts, lines.Err()
}

// defaultExcludesHelp describes the default excludes of the system goos
// names, for the scan command's help.
func defaultExcludesHelp(goos string) string {
	dirs := defaultExcludes(goos)
	if len(dirs) == 0 {
		return "This system has no default excludes."
	}

	var help strings.Builder

	help.WriteString("Below an operand, the walk does not enter these " +
		"directories, the default\nexcludes, unless --no-default-excludes " +
		"is given:\n\n")

	for _, dir := range dirs {
		fmt.Fprintf(&help, "  %s\n", dir)
	}

	if goos == goosLinux {
		help.WriteString("\nnor the mount point of any filesystem of " +
			"these types:\n\n")

		types := kernelFilesystemTypes()
		for line := range slices.Chunk(types, typesPerHelpLine) {
			fmt.Fprintf(&help, "  %s\n", strings.Join(line, ", "))
		}
	}

	return help.String()
}