// Package sidecar stores vr'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       = "vr_log"
	LogFile     = "vr-log.jsonl"
	Description = "private: vr log"
)

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

// Store is a repository's vr 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
	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
	}
	return &Store{
		root:     root,
		lockPath: filepath.Join(filepath.Dir(strings.TrimSpace(configPath)), "vr-agent-logger.lock"),
	}, nil
}

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. 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, found, _, err := s.resolveExisting()
		if err != nil {
			return err
		}
		if found {
			data, err = s.readRevision(rev)
			return err
		}
		data, err = os.ReadFile(filepath.Join(s.root, 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 vrsite 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.resolveExisting()
		if err != nil {
			return err
		}
		if found {
			generation, err = s.logValue(rev, "commit_id")
			return err
		}
		info, err := os.Stat(filepath.Join(s.root, 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 vr 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)
		if err != nil {
			return err
		}
		current := sidecarData
		if len(current) == 0 {
			legacy, legacyErr := os.ReadFile(filepath.Join(s.root, LogFile))
			if legacyErr == nil {
				current = legacy
			} else if !errors.Is(legacyErr, os.ErrNotExist) {
				return legacyErr
			}
		}
		payload, err := makePayload(current)
		if err != nil {
			return err
		}
		if len(payload) == 0 {
			return errors.New("refusing to append an empty vr 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(appendData)
	})
}

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, found, configured, err := s.resolveExisting()
	if err != nil {
		return "", err
	}
	if !found {
		if _, err := s.jj("new", "--no-edit", "root()", "-m", Description); err != nil {
			return "", err
		}
		ids, err := s.descriptionMatches()
		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 = ids[0]
	}
	if !configured {
		if _, err := s.jj("config", "set", "--repo", "revset-aliases."+Alias, strconv.Quote(rev)); err != nil {
			return "", err
		}
	}
	private, err := s.jj("config", "get", "git.private-commits")
	if err != nil {
		return "", err
	}
	if !containsRevsetName(private, Alias) {
		expr := fmt.Sprintf("(%s) | %s", strings.TrimSpace(private), Alias)
		if _, err := s.jj("config", "set", "--repo", "git.private-commits", strconv.Quote(expr)); err != nil {
			return "", err
		}
	}
	return rev, nil
}

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

	ids, err := s.descriptionMatches()
	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 vr sidecar", len(ids), Description)
	}
}

func (s *Store) descriptionMatches() ([]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 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
}

func (s *Store) runAppend(data []byte) error {
	f, err := os.CreateTemp(filepath.Dir(s.lockPath), "vr-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.jj("--config", "snapshot.auto-track=\"all()\"",
		"--config", "snapshot.max-new-file-size=\"1GiB\"",
		"run", "--clean", "--root", "-r", "exactly("+Alias+", 1)", "--",
		"sh", "-c", "cat \"$1\" >> "+LogFile, "sh", tmp)
	return err
}

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
}

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