// Package sidecar stores notevi's append-only JSONL log in a private jj change.
//
// The change is an anonymous direct child of root(), so it is visible and
// durable in the jj repository without becoming part of project history or the
// working copy. A repo-local revset alias names it and git.private-commits keeps
// accidental --change pushes from publishing it.
package sidecar

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

const (
	Alias       = "notevi_log"
	LogFile     = "notevi-log.jsonl"
	Description = "private: notevi log"
	lockFile    = "notevi.lock"
)

// naming is the set of repo-visible names one generation of the tool gave the
// sidecar: the revset alias, the file inside the change, and the change's own
// description.
type naming struct{ alias, logFile, description string }

var current = naming{Alias, LogFile, Description}

// legacy is what the tool called itself before the notevi rebrand. Every read
// path still recognizes it so an un-migrated repository opens; Migrate renames
// it in place so nothing in the repository keeps the old name.
var legacy = naming{"vr_log", "vr-log.jsonl", "private: vr log"}

const legacyLockFile = "vr-agent-logger.lock"

var ErrNoLog = errors.New("notevi log has not been initialized")

// Store is a repository's notevi log. Open is read-only with respect to jj; the
// sidecar change and its config are created lazily by Append.
type Store struct {
	root      string
	configDir string
	lockPath  string
}

// Open finds the jj workspace containing dir.
func Open(dir string) (*Store, error) {
	root, err := jjOutput(dir, "workspace", "root")
	if err != nil {
		return nil, err
	}
	root = strings.TrimSpace(root)
	configPath, err := jjOutput(root, "config", "path", "--repo")
	if err != nil {
		return nil, err
	}
	configDir := filepath.Dir(strings.TrimSpace(configPath))
	return &Store{
		root:      root,
		configDir: configDir,
		lockPath:  filepath.Join(configDir, lockFile),
	}, nil
}

// Descriptions lists every change subject that has ever named a notevi sidecar,
// current first. Callers that discover sidecars by description need the older
// ones too, so an un-migrated repository is still recognized as having a log.
func Descriptions() []string { return []string{current.description, legacy.description} }

func (s *Store) Root() string { return s.root }

func (s *Store) String() string {
	return fmt.Sprintf("jj sidecar %s:%s", Alias, LogFile)
}

// Read returns the current log. It reads a sidecar that still carries the
// pre-rebrand names without migrating it, and before the first sidecar write it
// falls back to a repo-root legacy log, which makes migration transparent.
func (s *Store) Read() ([]byte, error) {
	var data []byte
	err := s.withLock(syscall.LOCK_SH, func() error {
		rev, n, found, _, err := s.locate()
		if err != nil {
			return err
		}
		if found {
			data, err = s.readRevision(rev, n.logFile)
			return err
		}
		data, err = os.ReadFile(filepath.Join(s.root, legacy.logFile))
		if errors.Is(err, os.ErrNotExist) {
			return ErrNoLog
		}
		return err
	})
	return data, err
}

// Generation is a cheap identity for the current contents, used by the web app
// to notice appends. A sidecar rewrite gets a new commit id.
func (s *Store) Generation() (string, error) {
	var generation string
	err := s.withLock(syscall.LOCK_SH, func() error {
		rev, _, found, _, err := s.locate()
		if err != nil {
			return err
		}
		if found {
			generation, err = s.logValue(rev, "commit_id")
			return err
		}
		info, err := os.Stat(filepath.Join(s.root, legacy.logFile))
		if errors.Is(err, os.ErrNotExist) {
			generation = "empty"
			return nil
		}
		if err != nil {
			return err
		}
		generation = fmt.Sprintf("legacy:%d:%d", info.Size(), info.ModTime().UnixNano())
		return nil
	})
	return generation, err
}

// Append serializes sidecar rewrites across notevi processes. makePayload runs
// while holding that lock and receives the complete current log, allowing note
// numbering and reply validation to be atomic with the append.
func (s *Store) Append(makePayload func(current []byte) ([]byte, error)) error {
	return s.withLock(syscall.LOCK_EX, func() error {
		rev, err := s.ensure()
		if err != nil {
			return err
		}
		sidecarData, err := s.readRevision(rev, LogFile)
		if err != nil {
			return err
		}
		current := sidecarData
		if len(current) == 0 {
			old, oldErr := os.ReadFile(filepath.Join(s.root, legacy.logFile))
			if oldErr == nil {
				current = old
			} else if !errors.Is(oldErr, os.ErrNotExist) {
				return oldErr
			}
		}
		payload, err := makePayload(current)
		if err != nil {
			return err
		}
		if len(payload) == 0 {
			return errors.New("refusing to append an empty notevi log payload")
		}

		var appendData []byte
		if len(sidecarData) == 0 && len(current) > 0 {
			appendData = append(appendData, current...)
		}
		if len(current) > 0 && current[len(current)-1] != '\n' {
			appendData = append(appendData, '\n')
		}
		appendData = append(appendData, payload...)
		if appendData[len(appendData)-1] != '\n' {
			appendData = append(appendData, '\n')
		}
		return s.runAppend(rev, appendData)
	})
}

// Migrate renames a pre-rebrand sidecar in place: the file inside the change,
// the change description, the repo-local revset alias, and the alias mentioned
// in git.private-commits. It reports whether anything was renamed and is a
// no-op on a repository that is already current or has no sidecar at all.
func (s *Store) Migrate() (migrated bool, err error) {
	err = s.withLock(syscall.LOCK_EX, func() error {
		rev, n, found, configured, err := s.locate()
		if err != nil {
			return err
		}
		if !found || n != legacy {
			// Still adopt an unconfigured current sidecar, so that a repo whose
			// jj config was lost (moving a workspace orphans it) is repaired.
			if found && !configured {
				return s.configure(rev)
			}
			return nil
		}
		if err := s.rename(rev); err != nil {
			return err
		}
		migrated = true
		return nil
	})
	return migrated, err
}

// rename performs the legacy -> current renaming for an already-located sidecar.
// The move is two runs, not one `mv`: see ensureLogFile.
func (s *Store) rename(rev string) error {
	listed, err := s.jj("file", "list", "-r", rev, "--", legacy.logFile)
	if err != nil {
		return err
	}
	if strings.TrimSpace(listed) != "" {
		if err := s.ensureLogFile(rev); err != nil {
			return err
		}
		if _, err := s.jjRun(rev, "cat "+legacy.logFile+" >> "+LogFile+" && rm "+legacy.logFile); err != nil {
			return err
		}
	}
	if _, err := s.jj("describe", "-r", rev, "-m", Description); err != nil {
		return err
	}
	if err := s.configure(rev); err != nil {
		return err
	}
	if _, err := s.jj("config", "get", "revset-aliases."+legacy.alias); err == nil {
		if _, err := s.jj("config", "unset", "--repo", "revset-aliases."+legacy.alias); err != nil {
			return err
		}
	}
	os.Remove(filepath.Join(s.configDir, legacyLockFile)) //nolint:errcheck
	return nil
}

// configure points the repo-local alias at rev and refreshes the push guard.
func (s *Store) configure(rev string) error {
	if _, err := s.jj("config", "set", "--repo", "revset-aliases."+Alias, strconv.Quote(rev)); err != nil {
		return err
	}
	return s.guard()
}

// guard keeps git.private-commits naming the current alias, rewriting a legacy
// mention rather than leaving both names in the expression.
func (s *Store) guard() error {
	private, err := s.jj("config", "get", "git.private-commits")
	if err != nil {
		return err
	}
	expr := strings.TrimSpace(private)
	next := expr
	if containsRevsetName(next, legacy.alias) {
		next = replaceRevsetName(next, legacy.alias, Alias)
	}
	if !containsRevsetName(next, Alias) {
		next = fmt.Sprintf("(%s) | %s", next, Alias)
	}
	if next == expr {
		return nil
	}
	_, err = s.jj("config", "set", "--repo", "git.private-commits", strconv.Quote(next))
	return err
}

func (s *Store) withLock(kind int, fn func() error) error {
	if err := os.MkdirAll(filepath.Dir(s.lockPath), 0o755); err != nil {
		return err
	}
	f, err := os.OpenFile(s.lockPath, os.O_CREATE|os.O_RDWR, 0o600)
	if err != nil {
		return err
	}
	defer f.Close()
	if err := syscall.Flock(int(f.Fd()), kind); err != nil {
		return err
	}
	defer syscall.Flock(int(f.Fd()), syscall.LOCK_UN) //nolint:errcheck
	return fn()
}

func (s *Store) ensure() (string, error) {
	rev, n, found, configured, err := s.locate()
	if err != nil {
		return "", err
	}
	if found && n == legacy {
		// The first append to a pre-rebrand repository renames it, so no
		// repository keeps writing under the old name once notevi touches it.
		return rev, s.rename(rev)
	}
	if !found {
		if _, err := s.jj("new", "--no-edit", "root()", "-m", Description); err != nil {
			return "", err
		}
		ids, err := s.descriptionMatches(Description)
		if err != nil {
			return "", err
		}
		if len(ids) != 1 {
			return "", fmt.Errorf("created %q sidecar but found %d matching root children", Description, len(ids))
		}
		rev, configured = ids[0], false
	}
	if !configured {
		return rev, s.configure(rev)
	}
	return rev, s.guard()
}

// locate finds this repository's sidecar under the current names, falling back
// to the pre-rebrand ones, and reports which generation named it.
func (s *Store) locate() (rev string, n naming, found, configured bool, err error) {
	rev, found, configured, err = s.resolveExisting(current)
	if err != nil || found {
		return rev, current, found, configured, err
	}
	rev, found, configured, err = s.resolveExisting(legacy)
	if !found {
		return rev, current, false, false, err
	}
	return rev, legacy, found, configured, err
}

func (s *Store) resolveExisting(n naming) (rev string, found, configured bool, err error) {
	configuredValue, configErr := s.jj("config", "get", "revset-aliases."+n.alias)
	if configErr == nil {
		rev, err = s.logValue("exactly("+n.alias+", 1)", "change_id")
		if err != nil {
			return "", false, true, fmt.Errorf("repo-local revset alias %s is invalid: %w", n.alias, err)
		}
		valid := fmt.Sprintf("exactly(%s & root()+ & subject(exact:%s), 1)", n.alias, strconv.Quote(n.description))
		if _, err := s.logValue(valid, "change_id"); err != nil {
			return "", false, true, fmt.Errorf("repo-local revset alias %s=%q does not name notevi's sidecar: %w",
				n.alias, strings.TrimSpace(configuredValue), err)
		}
		return rev, true, true, nil
	}

	ids, err := s.descriptionMatches(n.description)
	if err != nil {
		return "", false, false, err
	}
	switch len(ids) {
	case 0:
		return "", false, false, nil
	case 1:
		return ids[0], true, false, nil
	default:
		return "", false, false, fmt.Errorf("found %d root children described %q; cannot choose a notevi sidecar", len(ids), n.description)
	}
}

func (s *Store) descriptionMatches(description string) ([]string, error) {
	revset := fmt.Sprintf("root()+ & subject(exact:%s)", strconv.Quote(description))
	out, err := s.jj("log", "-G", "-r", revset, "-T", "change_id ++ \"\\n\"")
	if err != nil {
		return nil, err
	}
	var ids []string
	for _, id := range strings.Fields(out) {
		ids = append(ids, id)
	}
	return ids, nil
}

func (s *Store) logValue(revset, template string) (string, error) {
	out, err := s.jj("log", "-G", "-r", revset, "-T", template)
	if err != nil {
		return "", err
	}
	value := strings.TrimSpace(out)
	if value == "" {
		return "", fmt.Errorf("revision %q produced no %s", revset, template)
	}
	return value, nil
}

func (s *Store) readRevision(rev, logFile string) ([]byte, error) {
	listed, err := s.jj("file", "list", "-r", rev, "--", logFile)
	if err != nil {
		return nil, err
	}
	if strings.TrimSpace(listed) == "" {
		return nil, nil
	}
	out, err := s.jj("file", "show", "-r", rev, "--", logFile)
	return []byte(out), err
}

// ensureLogFile creates the log file, empty, if the sidecar does not have it.
//
// This is load-bearing, not tidiness. jj only auto-tracks a *new* file in the
// run working copy while it is under snapshot.max-new-file-size, and the run
// working copy does not take that limit from the command line, so writing a
// whole log under a name the change has never held is silently dropped — the
// commit is rewritten with the file missing and jj reports success. Writing the
// file empty first is always under the limit; from then on every byte is a
// modification of a tracked file, which snapshots at any size.
func (s *Store) ensureLogFile(rev string) error {
	listed, err := s.jj("file", "list", "-r", rev, "--", LogFile)
	if err != nil {
		return err
	}
	if strings.TrimSpace(listed) != "" {
		return nil
	}
	_, err = s.jjRun(rev, ": > "+LogFile)
	return err
}

func (s *Store) runAppend(rev string, data []byte) error {
	if err := s.ensureLogFile(rev); err != nil {
		return err
	}
	f, err := os.CreateTemp(s.configDir, "notevi-log-append-*")
	if err != nil {
		return err
	}
	tmp := f.Name()
	defer os.Remove(tmp) //nolint:errcheck
	if _, err := f.Write(data); err != nil {
		f.Close()
		return err
	}
	if err := f.Close(); err != nil {
		return err
	}
	_, err = s.jjRun(rev, "cat \"$1\" >> "+LogFile, tmp)
	return err
}

// jjRun rewrites the sidecar change by running script against a clean checkout
// of it. rev is a change id or alias; the working copy is never involved.
func (s *Store) jjRun(rev, script string, scriptArgs ...string) (string, error) {
	args := []string{"--config", "snapshot.auto-track=\"all()\"",
		"--config", "snapshot.max-new-file-size=\"1GiB\"",
		"run", "--clean", "--root", "-r", "exactly((" + rev + "), 1)", "--",
		"sh", "-c", script, "sh"}
	return s.jj(append(args, scriptArgs...)...)
}

func (s *Store) jj(args ...string) (string, error) {
	return jjOutput(s.root, args...)
}

func jjOutput(dir string, args ...string) (string, error) {
	full := append([]string{"--ignore-working-copy", "--no-pager"}, args...)
	cmd := exec.Command("jj", full...)
	cmd.Dir = dir
	out, err := cmd.Output()
	if err != nil {
		message := strings.TrimSpace(string(out))
		if ee, ok := err.(*exec.ExitError); ok {
			message = strings.TrimSpace(string(ee.Stderr))
		}
		if message != "" {
			return "", fmt.Errorf("jj %s: %w: %s", strings.Join(args, " "), err, message)
		}
		return "", fmt.Errorf("jj %s: %w", strings.Join(args, " "), err)
	}
	return string(out), nil
}

func containsRevsetName(expr, name string) bool {
	for i := 0; i+len(name) <= len(expr); i++ {
		if expr[i:i+len(name)] != name {
			continue
		}
		before := i == 0 || !isNameByte(expr[i-1])
		after := i+len(name) == len(expr) || !isNameByte(expr[i+len(name)])
		if before && after {
			return true
		}
	}
	return false
}

// replaceRevsetName swaps whole revset names, so that rewriting an expression
// mentioning vr_log cannot corrupt an unrelated name that merely contains it.
func replaceRevsetName(expr, name, replacement string) string {
	var b strings.Builder
	for i := 0; i < len(expr); {
		if expr[i:min(i+len(name), len(expr))] == name {
			before := i == 0 || !isNameByte(expr[i-1])
			after := i+len(name) == len(expr) || !isNameByte(expr[i+len(name)])
			if before && after {
				b.WriteString(replacement)
				i += len(name)
				continue
			}
		}
		b.WriteByte(expr[i])
		i++
	}
	return b.String()
}

func isNameByte(b byte) bool {
	return b == '_' || b == '-' || b >= 'a' && b <= 'z' || b >= 'A' && b <= 'Z' || b >= '0' && b <= '9'
}