// Package instance is the lifecycle service behind run -d, ps, stop,
// logs, rm, and exec: instance state lives on disk, one directory per
// instance, so the CLI verbs and a later API server (RFC 0005
// milestone 3) read and write the same thing and neither owns it.
//
// Layout: $WERKDOCK_HOME/instances/<id>/ with config.json (what to
// run), state.json (whether and how it ran), stdout and stderr (the
// command's output, for logs), run/ (bound at /run so sockets and pid
// files survive a stop and are visible to exec), and volumes/<path>
// for the image's anonymous VOLUMEs. A `.tmp` suffix marks a directory
// being created or removed.
package instance

import (
	"crypto/rand"
	"encoding/hex"
	"encoding/json"
	"errors"
	"fmt"
	"os"
	"path/filepath"
	"regexp"
	"sort"
	"strings"
	"syscall"
	"time"

	"werkdock/internal/engine"
)

// ConfigVersion is the format of config.json; the contract between the
// CLI and the API server (RFC 0005).
const ConfigVersion = 1

// Config is what an instance runs, written once at create.
type Config struct {
	Version int    `json:"version"`
	ID      string `json:"id"`
	Name    string `json:"name"`
	// Image is the reference the instance was created from, in short form.
	Image   string         `json:"image"`
	Created time.Time      `json:"created"`
	Spec    engine.RunSpec `json:"spec"`
	// Volumes are the image's anonymous VOLUME paths, bound from
	// volumes/<path> below the instance directory (Spec.Mounts carries
	// the binds; this lists them for inspect).
	Volumes []string `json:"volumes,omitempty"`
	// AutoRemove is --rm: the instance directory goes when the command exits.
	AutoRemove bool `json:"autoRemove,omitempty"`
	// Labels are docker's container labels; the API server's clients
	// (Testcontainers) set them at create and filter on them later.
	Labels map[string]string `json:"labels,omitempty"`
	// Ports is what the service listens on. Werkdock has no network
	// namespace, so ContainerPort and HostPort are the same number
	// unless the port was moved by the environment variable the image
	// honours; either way this is what an API client is told
	// (RFC 0005, Ports).
	Ports []PortMapping `json:"ports,omitempty"`
}

// PortMapping is one published port.
type PortMapping struct {
	ContainerPort int    `json:"containerPort"`
	HostPort      int    `json:"hostPort"`
	Protocol      string `json:"protocol"`
}

// Status is the lifecycle state, docker's words.
type Status string

const (
	StatusCreated Status = "created"
	StatusRunning Status = "running"
	StatusExited  Status = "exited"
)

// State is state.json, rewritten by the supervisor at every transition.
type State struct {
	Status Status `json:"status"`
	// PID is the supervisor process; ChildPID the sandboxed command as
	// the host sees it, the target of stop and kill.
	PID        int       `json:"pid,omitempty"`
	ChildPID   int       `json:"childPid,omitempty"`
	StartedAt  time.Time `json:"startedAt,omitempty"`
	FinishedAt time.Time `json:"finishedAt,omitempty"`
	ExitCode   int       `json:"exitCode"`
	// Error is set when the command could not be started, or when the
	// supervisor vanished without recording an exit.
	Error string `json:"error,omitempty"`
}

// ExitCodeLost is recorded when the supervisor died without writing an
// exit code (a reboot, a kill -9): docker's 255 for the same case.
const ExitCodeLost = 255

// Instance is one instance directory, loaded.
type Instance struct {
	Dir    string
	Config Config
	State  State
}

// Manager is the instance store below the werkdock home.
type Manager struct {
	// Root is $WERKDOCK_HOME; instances live in Root/instances.
	Root string
}

func (m Manager) instancesDir() string { return filepath.Join(m.Root, "instances") }

// ShortID is the 12-character form ps prints.
func (i *Instance) ShortID() string { return i.Config.ID[:12] }

// Running reports whether the command is running (as far as the state
// file knows; Load reconciles it with the supervisor's liveness).
func (i *Instance) Running() bool { return i.State.Status == StatusRunning }

func (i *Instance) StdoutPath() string { return filepath.Join(i.Dir, "stdout") }
func (i *Instance) StderrPath() string { return filepath.Join(i.Dir, "stderr") }

var namePattern = regexp.MustCompile(`^[a-zA-Z0-9][a-zA-Z0-9_.-]*$`)

// Create writes a new instance directory for cfg and returns it in
// state created. ID and Created are assigned here; Name defaults to
// the image's repository basename plus the ID's first six characters.
// The anonymous volumes and /run are bound from the instance
// directory, in front of cfg.Spec.Mounts (an explicit mount on the
// same path already excluded them, see the CLI).
func (m Manager) Create(cfg Config) (*Instance, error) {
	if cfg.Name != "" && !namePattern.MatchString(cfg.Name) {
		return nil, fmt.Errorf("invalid instance name %q: [a-zA-Z0-9][a-zA-Z0-9_.-]* is allowed", cfg.Name)
	}
	if cfg.Name != "" {
		if existing, err := m.Get(cfg.Name); err == nil {
			return nil, fmt.Errorf("the name %q is already in use by instance %s (werkdock rm %s to reuse it)", cfg.Name, existing.ShortID(), cfg.Name)
		}
	}
	raw := make([]byte, 32)
	if _, err := rand.Read(raw); err != nil {
		return nil, err
	}
	cfg.Version = ConfigVersion
	cfg.ID = hex.EncodeToString(raw)
	cfg.Created = time.Now()
	if cfg.Name == "" {
		cfg.Name = defaultName(cfg.Image, cfg.ID)
	}
	dir := filepath.Join(m.instancesDir(), cfg.ID)
	tmp := dir + ".tmp"
	if err := os.MkdirAll(tmp, 0o755); err != nil {
		return nil, err
	}
	var binds []engine.Mount
	runDir := filepath.Join(tmp, "run")
	if err := os.Mkdir(runDir, 0o755); err != nil {
		return nil, err
	}
	binds = append(binds, engine.Mount{Mode: engine.MountBind, Source: filepath.Join(dir, "run"), Dest: "/run"})
	sort.Strings(cfg.Volumes)
	for _, v := range cfg.Volumes {
		rel := filepath.Join("volumes", filepath.FromSlash(strings.TrimPrefix(filepath.Clean(v), "/")))
		if err := os.MkdirAll(filepath.Join(tmp, rel), 0o755); err != nil {
			return nil, err
		}
		binds = append(binds, engine.Mount{Mode: engine.MountBind, Source: filepath.Join(dir, rel), Dest: filepath.Clean(v)})
	}
	cfg.Spec.Mounts = append(binds, cfg.Spec.Mounts...)
	inst := &Instance{Dir: tmp, Config: cfg, State: State{Status: StatusCreated}}
	if err := writeJSON(filepath.Join(tmp, "config.json"), cfg); err != nil {
		return nil, err
	}
	if err := inst.SaveState(); err != nil {
		return nil, err
	}
	if err := os.Rename(tmp, dir); err != nil {
		return nil, err
	}
	inst.Dir = dir
	return inst, nil
}

func defaultName(image, id string) string {
	repo := image
	if i := strings.LastIndex(repo, ":"); i > strings.LastIndex(repo, "/") {
		repo = repo[:i]
	}
	if i := strings.LastIndex(repo, "/"); i >= 0 {
		repo = repo[i+1:]
	}
	repo = strings.TrimLeft(repo, "_.-")
	if repo == "" {
		repo = "instance"
	}
	return repo + "-" + id[:6]
}

// SaveState rewrites state.json atomically.
func (i *Instance) SaveState() error {
	return writeJSON(filepath.Join(i.Dir, "state.json"), i.State)
}

// Reload re-reads state.json.
func (i *Instance) Reload() error {
	return readJSON(filepath.Join(i.Dir, "state.json"), &i.State)
}

func writeJSON(path string, v interface{}) error {
	data, err := json.MarshalIndent(v, "", "  ")
	if err != nil {
		return err
	}
	tmp := path + ".tmp"
	if err := os.WriteFile(tmp, append(data, '\n'), 0o644); err != nil {
		return err
	}
	return os.Rename(tmp, path)
}

func readJSON(path string, v interface{}) error {
	data, err := os.ReadFile(path)
	if err != nil {
		return err
	}
	return json.Unmarshal(data, v)
}

// ErrNotFound is returned by Get for an unknown id, prefix, or name.
var ErrNotFound = errors.New("no such instance")

// load reads one instance directory and reconciles its state: a
// "running" instance whose supervisor is gone is recorded as exited
// with ExitCodeLost, and an exited --rm instance is removed on sight
// (the supervisor normally does both; this covers its death).
func (m Manager) load(dir string) (*Instance, error) {
	inst := &Instance{Dir: dir}
	if err := readJSON(filepath.Join(dir, "config.json"), &inst.Config); err != nil {
		return nil, err
	}
	if err := inst.Reload(); err != nil {
		return nil, err
	}
	if inst.Running() && !processAlive(inst.State.PID) {
		inst.State.Status = StatusExited
		inst.State.ExitCode = ExitCodeLost
		inst.State.FinishedAt = time.Now()
		inst.State.Error = "the supervisor process exited without recording the exit code"
		if err := inst.SaveState(); err != nil {
			return nil, err
		}
	}
	if inst.Config.AutoRemove && inst.State.Status == StatusExited {
		if err := m.remove(inst); err != nil {
			return nil, err
		}
		return nil, fmt.Errorf("%w: %s", ErrNotFound, inst.ShortID())
	}
	return inst, nil
}

func processAlive(pid int) bool {
	if pid <= 0 {
		return false
	}
	err := syscall.Kill(pid, 0)
	return err == nil || errors.Is(err, syscall.EPERM)
}

// List returns every instance, newest first.
func (m Manager) List() ([]*Instance, error) {
	entries, err := os.ReadDir(m.instancesDir())
	if os.IsNotExist(err) {
		return nil, nil
	}
	if err != nil {
		return nil, err
	}
	var all []*Instance
	for _, e := range entries {
		if !e.IsDir() || strings.HasSuffix(e.Name(), ".tmp") {
			continue
		}
		inst, err := m.load(filepath.Join(m.instancesDir(), e.Name()))
		if errors.Is(err, ErrNotFound) {
			continue
		}
		if err != nil {
			return nil, fmt.Errorf("instance %s: %w", e.Name(), err)
		}
		all = append(all, inst)
	}
	sort.Slice(all, func(a, b int) bool { return all[a].Config.Created.After(all[b].Config.Created) })
	return all, nil
}

// HasLabels reports whether the instance carries every label in want
// (docker's label filter: all of them must match).
func (i *Instance) HasLabels(want map[string]string) bool {
	for k, v := range want {
		got, ok := i.Config.Labels[k]
		if !ok || (v != "" && got != v) {
			return false
		}
	}
	return true
}

// Get resolves key the docker way: a full id, a name, or a unique id
// prefix.
func (m Manager) Get(key string) (*Instance, error) {
	if key == "" {
		return nil, fmt.Errorf("%w: empty name", ErrNotFound)
	}
	all, err := m.List()
	if err != nil {
		return nil, err
	}
	for _, inst := range all {
		if inst.Config.ID == key || inst.Config.Name == key {
			return inst, nil
		}
	}
	var matches []*Instance
	for _, inst := range all {
		if strings.HasPrefix(inst.Config.ID, key) {
			matches = append(matches, inst)
		}
	}
	switch len(matches) {
	case 0:
		return nil, fmt.Errorf("%w: %s", ErrNotFound, key)
	case 1:
		return matches[0], nil
	}
	return nil, fmt.Errorf("instance id prefix %q is ambiguous (%d matches)", key, len(matches))
}

// Remove deletes the instance directory. A running instance is refused
// unless force, which kills it first (docker rm -f).
func (m Manager) Remove(inst *Instance, force bool) error {
	if inst.Running() {
		if !force {
			return fmt.Errorf("instance %s is running: stop it first, or rm -f", inst.Config.Name)
		}
		if err := m.Kill(inst, syscall.SIGKILL); err != nil {
			return err
		}
		if _, err := m.Wait(inst, 10*time.Second); err != nil {
			return err
		}
	}
	return m.remove(inst)
}

func (m Manager) remove(inst *Instance) error {
	tmp := inst.Dir + ".rm.tmp"
	if err := os.Rename(inst.Dir, tmp); err != nil {
		if os.IsNotExist(err) {
			return nil
		}
		return err
	}
	return os.RemoveAll(tmp)
}