package engine

import (
	"crypto/sha256"
	"encoding/hex"
	"errors"
	"fmt"
	"io/fs"
	"os"
	"os/user"
	"path/filepath"
	"sort"
	"strconv"
	"strings"

	udiff "github.com/aymanbagabas/go-udiff"

	"jarl/internal/model"
	"jarl/internal/render"
)

// Deployer writes rendered files to disk.
type Deployer struct {
	// Group, if set, is the group generated files and directories belong
	// to (named must be able to read them; on Debian this is "bind").
	Group string
	// NamedWritable lists directories (and their subdirectories) named
	// must be able to write to: zone files (for journals and DNSSEC
	// signing) and secondary zone files. Other directories are only
	// readable by the group.
	NamedWritable []string
	gid           int
}

func (d *Deployer) dirMode(dir string) fs.FileMode {
	for _, w := range d.NamedWritable {
		if w != "" && (dir == w || strings.HasPrefix(dir, w+string(filepath.Separator))) {
			return 0o770 | fs.ModeSetgid
		}
	}
	return 0o750 | fs.ModeSetgid
}

func (d *Deployer) groupID() (int, error) {
	if d.Group == "" {
		return -1, nil
	}
	if d.gid != 0 {
		return d.gid, nil
	}
	if n, err := strconv.Atoi(d.Group); err == nil {
		d.gid = n
		return n, nil
	}
	g, err := user.LookupGroup(d.Group)
	if err != nil {
		return -1, fmt.Errorf("group %q: %w", d.Group, err)
	}
	n, _ := strconv.Atoi(g.Gid)
	d.gid = n
	return n, nil
}

type fileBackup struct {
	path    string
	existed bool
	content []byte
	mode    fs.FileMode
}

// Backup holds the previous contents of files changed by Install.
type Backup struct {
	d     *Deployer
	files []fileBackup
}

func readFile(path string) ([]byte, bool, error) {
	b, err := os.ReadFile(path)
	if errors.Is(err, fs.ErrNotExist) {
		return nil, false, nil
	}
	if err != nil {
		return nil, false, err
	}
	return b, true, nil
}

// EnsureDir creates dir (and missing parents) with mode and the deploy
// group. Existing directories are left alone.
func (d *Deployer) EnsureDir(dir string, mode fs.FileMode) error {
	if _, err := os.Stat(dir); err == nil {
		return nil
	}
	if parent := filepath.Dir(dir); parent != dir {
		if err := d.EnsureDir(parent, d.dirMode(parent)); err != nil {
			return err
		}
	}
	if err := os.Mkdir(dir, mode.Perm()); err != nil && !errors.Is(err, fs.ErrExist) {
		return err
	}
	gid, err := d.groupID()
	if err != nil {
		return err
	}
	if gid >= 0 {
		if err := os.Chown(dir, -1, gid); err != nil {
			return fmt.Errorf("chgrp %s: %w", dir, err)
		}
	}
	fi, err := os.Stat(dir)
	if err != nil {
		return err
	}
	// mkdir applies the umask, and a setgid parent passes its bit on, so
	// the mode usually needs fixing. Under systemd's RestrictSUIDSGID (as in
	// jarl.service) any chmod that includes the setgid bit fails with
	// EPERM; the permission bits matter more (named writes journals here),
	// and every file Jarl writes gets the group explicitly, so fall back to
	// them alone.
	if fi.Mode()&(fs.ModePerm|fs.ModeSetgid) == mode&(fs.ModePerm|fs.ModeSetgid) {
		return nil
	}
	err = chmod(dir, mode)
	if errors.Is(err, fs.ErrPermission) && mode&fs.ModeSetgid != 0 {
		err = chmod(dir, mode.Perm())
	}
	if err != nil {
		return fmt.Errorf("chmod %s: %w", dir, err)
	}
	return nil
}

// chmod is os.Chmod; tests replace it to simulate a sandbox.
var chmod = os.Chmod

func (d *Deployer) writeAtomic(path string, content []byte, mode fs.FileMode) error {
	dir := filepath.Dir(path)
	if err := d.EnsureDir(dir, d.dirMode(dir)); err != nil {
		return err
	}
	tmp, err := os.CreateTemp(dir, "."+filepath.Base(path)+".jarl-*")
	if err != nil {
		return err
	}
	name := tmp.Name()
	ok := false
	defer func() {
		if !ok {
			tmp.Close()
			os.Remove(name)
		}
	}()
	if _, err := tmp.Write(content); err != nil {
		return err
	}
	if err := tmp.Sync(); err != nil {
		return err
	}
	if err := tmp.Chmod(mode); err != nil {
		return err
	}
	gid, err := d.groupID()
	if err != nil {
		return err
	}
	if gid >= 0 {
		if err := tmp.Chown(-1, gid); err != nil {
			return fmt.Errorf("chgrp %s: %w", path, err)
		}
	}
	if err := tmp.Close(); err != nil {
		return err
	}
	if err := os.Rename(name, path); err != nil {
		return err
	}
	ok = true
	if df, err := os.Open(dir); err == nil {
		df.Sync()
		df.Close()
	}
	return nil
}

// writeNew creates a file that doesn't exist yet (fs.ErrExist if it
// does), with the deploy group.
func (d *Deployer) writeNew(path string, content []byte, mode fs.FileMode) error {
	dir := filepath.Dir(path)
	if err := d.EnsureDir(dir, d.dirMode(dir)); err != nil {
		return err
	}
	f, err := os.OpenFile(path, os.O_WRONLY|os.O_CREATE|os.O_EXCL, mode)
	if err != nil {
		return err
	}
	_, err = f.Write(content)
	if gid, gerr := d.groupID(); err == nil && gerr == nil && gid >= 0 {
		err = f.Chown(-1, gid)
	}
	if cerr := f.Close(); err == nil {
		err = cerr
	}
	if err != nil {
		os.Remove(path)
	}
	return err
}

// finish gives a temporary file written next to path its mode and the
// deploy group, and moves it into place.
func (d *Deployer) finish(tmp, path string, mode fs.FileMode) error {
	if err := os.Chmod(tmp, mode); err != nil {
		return err
	}
	gid, err := d.groupID()
	if err != nil {
		return err
	}
	if gid >= 0 {
		if err := os.Chown(tmp, -1, gid); err != nil {
			return fmt.Errorf("chgrp %s: %w", path, err)
		}
	}
	return os.Rename(tmp, path)
}

// Install writes files and removes paths, returning a backup that undoes
// the changes. On error the files already written are restored.
func (d *Deployer) Install(files []*render.File, remove []string, dirs []string) (*Backup, error) {
	b := &Backup{d: d}
	for _, dir := range dirs {
		// Directories named writes into (secondary zone files).
		if err := d.EnsureDir(dir, 0o770|fs.ModeSetgid); err != nil {
			return b, err
		}
	}
	save := func(path string) error {
		old, existed, err := readFile(path)
		if err != nil {
			return err
		}
		mode := fs.FileMode(0o640)
		if fi, err := os.Stat(path); err == nil {
			mode = fi.Mode().Perm()
		}
		b.files = append(b.files, fileBackup{path: path, existed: existed, content: old, mode: mode})
		return nil
	}
	for _, f := range files {
		if err := save(f.Path); err != nil {
			b.Restore()
			return nil, err
		}
		if err := d.writeAtomic(f.Path, f.Content, f.Mode); err != nil {
			b.Restore()
			return nil, fmt.Errorf("writing %s: %w", f.Path, err)
		}
	}
	for _, p := range remove {
		if err := save(p); err != nil {
			b.Restore()
			return nil, err
		}
		if err := os.Remove(p); err != nil && !errors.Is(err, fs.ErrNotExist) {
			b.Restore()
			return nil, fmt.Errorf("removing %s: %w", p, err)
		}
	}
	return b, nil
}

// Restore puts back the previous file contents.
func (b *Backup) Restore() error {
	if b == nil {
		return nil
	}
	var errs []error
	for i := len(b.files) - 1; i >= 0; i-- {
		f := b.files[i]
		if f.existed {
			if err := b.d.writeAtomic(f.path, f.content, f.mode); err != nil {
				errs = append(errs, err)
			}
		} else if err := os.Remove(f.path); err != nil && !errors.Is(err, fs.ErrNotExist) {
			errs = append(errs, err)
		}
	}
	return errors.Join(errs...)
}

func sha(b []byte) string {
	h := sha256.Sum256(b)
	return hex.EncodeToString(h[:])
}

// DriftEntry is a managed file that was changed outside Jarl.
type DriftEntry struct {
	Path   string `json:"path"`
	Reason string `json:"reason"` // modified, missing
}

// CheckDrift compares managed files on disk with what Jarl last wrote.
func CheckDrift(deployed map[string]*model.DeployedFile) []DriftEntry {
	var out []DriftEntry
	for path, f := range deployed {
		b, exists, err := readFile(path)
		switch {
		case err != nil:
			out = append(out, DriftEntry{Path: path, Reason: "unreadable: " + err.Error()})
		case !exists:
			out = append(out, DriftEntry{Path: path, Reason: "missing"})
		case sha(b) != f.SHA256:
			out = append(out, DriftEntry{Path: path, Reason: "modified"})
		}
	}
	sort.Slice(out, func(i, j int) bool { return out[i].Path < out[j].Path })
	return out
}

// fileDiff returns a unified diff from old to new for display. Secret
// files only show that they changed.
func fileDiff(path string, old []byte, existed bool, new []byte, removed, secret bool) string {
	from, to := path, path
	if !existed {
		from = "/dev/null"
	}
	if removed {
		to = "/dev/null"
	}
	if secret {
		return fmt.Sprintf("--- %s\n+++ %s\n@@ contents hidden: this file holds TSIG secrets @@\n", from, to)
	}
	d := udiff.Unified(from, to, string(old), string(new))
	if d != "" && !strings.HasSuffix(d, "\n") {
		d += "\n"
	}
	return d
}