package refscan

import (
	"encoding/json"
	"errors"
	"fmt"
	"os"
	"os/exec"
	"path/filepath"
	"sort"
	"strings"
	"sync"
	"sync/atomic"
	"time"

	"bugabinga.net/luci/internal/inbox"
	"bugabinga.net/luci/internal/trigger"
)

const zeroRev = "0000000000000000000000000000000000000000"

var (
	pollMu      sync.Mutex
	lastEventID atomic.Int64
	writeInbox  = inbox.WriteID
	saveRefs    = save
)

type RepoErrors struct {
	errs map[string]error
}

func (e *RepoErrors) Error() string {
	names := make([]string, 0, len(e.errs))
	for name := range e.errs {
		names = append(names, name)
	}
	sort.Strings(names)
	messages := make([]string, 0, len(names))
	for _, name := range names {
		messages = append(messages, e.errs[name].Error())
	}
	return strings.Join(messages, "\n")
}

func (e *RepoErrors) Failed(repo string) bool {
	_, failed := e.errs[repo]
	return failed
}

type PollConfig struct {
	DataDir   string
	RepoRoots []string
	InboxDir  string
}

type Ref struct {
	Repo string `json:"repo"`
	Name string `json:"name"`
	Rev  string `json:"rev"`
}

type outboxEvent struct {
	ID    string            `json:"id"`
	Event trigger.PushEvent `json:"event"`
}

type outbox struct {
	Current []Ref         `json:"current"`
	Events  []outboxEvent `json:"events"`
}

func Poll(cfg PollConfig) ([]string, error) {
	pollMu.Lock()
	defer pollMu.Unlock()
	statePath := filepath.Join(cfg.DataDir, "state", "refs.json")
	outboxPath := filepath.Join(cfg.DataDir, "state", "refs.outbox.json")
	previous, existed, err := load(statePath)
	if err != nil {
		return nil, err
	}
	ids, err := recoverOutbox(cfg, statePath, outboxPath)
	if err != nil {
		return ids, err
	}
	if existed {
		previous, _, err = load(statePath)
		if err != nil {
			return ids, err
		}
	}
	current, scanErr := Scan(cfg.RepoRoots)
	if current == nil {
		return ids, scanErr
	}
	var repoErrs *RepoErrors
	if errors.As(scanErr, &repoErrs) {
		for _, ref := range previous {
			if repoErrs.Failed(ref.Repo) {
				current = append(current, ref)
			}
		}
		sortRefs(current)
	}
	if !existed {
		return ids, errors.Join(scanErr, saveRefs(statePath, current))
	}
	events := Events(previous, current)
	if len(events) == 0 {
		return ids, errors.Join(scanErr, saveRefs(statePath, current))
	}
	pending := outbox{Current: current, Events: make([]outboxEvent, len(events))}
	for i, event := range events {
		pending.Events[i] = outboxEvent{ID: nextEventID(), Event: event}
	}
	if err := saveOutbox(outboxPath, pending); err != nil {
		return ids, errors.Join(scanErr, err)
	}
	published, err := recoverOutbox(cfg, statePath, outboxPath)
	ids = append(ids, published...)
	return ids, errors.Join(scanErr, err)
}

// recoverOutbox publishes durable occurrences before advancing refs.json.
// Replaying an outbox reuses its IDs, so an interrupted A-B-A-B sequence
// retains every transition without creating a second run for one occurrence.
func recoverOutbox(cfg PollConfig, statePath, outboxPath string) ([]string, error) {
	pending, found, err := loadOutbox(outboxPath)
	if err != nil || !found {
		return nil, err
	}
	ids := make([]string, 0, len(pending.Events))
	for _, item := range pending.Events {
		err := writeInbox(cfg.InboxDir, item.ID, item.Event)
		if err == nil {
			ids = append(ids, item.ID)
			continue
		}
		if !errors.Is(err, os.ErrExist) {
			return ids, err
		}
	}
	if err := inbox.Sync(cfg.InboxDir); err != nil {
		return ids, err
	}
	if err := saveRefs(statePath, pending.Current); err != nil {
		return ids, err
	}
	if err := os.Remove(outboxPath); err != nil && !errors.Is(err, os.ErrNotExist) {
		return ids, err
	}
	return ids, syncDir(filepath.Dir(outboxPath))
}

func nextEventID() string {
	for {
		previous := lastEventID.Load()
		next := time.Now().UnixNano()
		if next <= previous {
			next = previous + 1
		}
		if lastEventID.CompareAndSwap(previous, next) {
			return fmt.Sprintf("%d", next)
		}
	}
}

func Scan(roots []string) ([]Ref, error) {
	refs := []Ref{}
	repoErrs := &RepoErrors{errs: map[string]error{}}
	for _, root := range roots {
		entries, err := os.ReadDir(root)
		if err != nil {
			if os.IsNotExist(err) {
				continue
			}
			return nil, err
		}
		for _, entry := range entries {
			if !entry.IsDir() {
				continue
			}
			slug := strings.TrimSuffix(entry.Name(), ".git")
			if !cleanSlug(slug) {
				continue
			}
			repoPath := filepath.Join(root, entry.Name())
			if _, err := os.Stat(filepath.Join(repoPath, "HEAD")); err != nil {
				continue
			}
			repoRefs, err := scanRepo(repoPath, slug)
			if err != nil {
				repoErrs.errs[slug] = err
				continue
			}
			refs = append(refs, repoRefs...)
		}
	}
	sortRefs(refs)
	if len(repoErrs.errs) != 0 {
		return refs, repoErrs
	}
	return refs, nil
}

func Events(previous []Ref, current []Ref) []trigger.PushEvent {
	prev := map[string]Ref{}
	for _, ref := range previous {
		prev[key(ref)] = ref
	}
	events := []trigger.PushEvent{}
	for _, ref := range current {
		old := zeroRev
		if before, ok := prev[key(ref)]; ok {
			old = before.Rev
		}
		if old == ref.Rev {
			continue
		}
		events = append(events, trigger.PushEvent{Kind: "push", Repo: ref.Repo, Old: old, New: ref.Rev, Ref: ref.Name})
	}
	return events
}

func scanRepo(repoPath, slug string) ([]Ref, error) {
	cmd := exec.Command("git", "--git-dir", repoPath, "for-each-ref", "--format=%(refname) %(objectname) %(*objectname)", "refs/heads", "refs/tags")
	out, err := cmd.CombinedOutput()
	if err != nil {
		return nil, fmt.Errorf("git scan %s: %w: %s", repoPath, err, strings.TrimSpace(string(out)))
	}
	refs := []Ref{}
	for _, line := range strings.Split(strings.TrimSpace(string(out)), "\n") {
		if strings.TrimSpace(line) == "" {
			continue
		}
		fields := strings.Fields(line)
		if len(fields) < 2 || len(fields) > 3 {
			return nil, fmt.Errorf("bad ref line %q", line)
		}
		rev := fields[1]
		if len(fields) == 3 {
			rev = fields[2]
		}
		refs = append(refs, Ref{Repo: slug, Name: fields[0], Rev: rev})
	}
	return refs, nil
}

func load(path string) ([]Ref, bool, error) {
	data, err := os.ReadFile(path)
	if err != nil {
		if os.IsNotExist(err) {
			return nil, false, nil
		}
		return nil, false, err
	}
	var refs []Ref
	if err := json.Unmarshal(data, &refs); err != nil {
		return nil, false, err
	}
	return refs, true, nil
}

func loadOutbox(path string) (outbox, bool, error) {
	data, err := os.ReadFile(path)
	if errors.Is(err, os.ErrNotExist) {
		return outbox{}, false, nil
	}
	if err != nil {
		return outbox{}, false, err
	}
	var pending outbox
	if err := json.Unmarshal(data, &pending); err != nil {
		return outbox{}, false, err
	}
	for _, item := range pending.Events {
		if item.ID == "" || filepath.Base(item.ID) != item.ID {
			return outbox{}, false, fmt.Errorf("unsafe ref outbox id %q", item.ID)
		}
		if err := item.Event.Validate(); err != nil {
			return outbox{}, false, err
		}
	}
	return pending, true, nil
}

func saveOutbox(path string, pending outbox) error { return saveJSON(path, pending) }

func save(path string, refs []Ref) error { return saveJSON(path, refs) }

func saveJSON(path string, value any) error {
	if err := os.MkdirAll(filepath.Dir(path), 0o755); err != nil {
		return err
	}
	data, err := json.MarshalIndent(value, "", "  ")
	if err != nil {
		return err
	}
	data = append(data, '\n')
	file, err := os.OpenFile(path+".tmp", os.O_CREATE|os.O_TRUNC|os.O_WRONLY, 0o644)
	if err != nil {
		return err
	}
	tmp := file.Name()
	if _, err := file.Write(data); err != nil {
		file.Close()
		_ = os.Remove(tmp)
		return err
	}
	if err := file.Sync(); err != nil {
		file.Close()
		_ = os.Remove(tmp)
		return err
	}
	if err := file.Close(); err != nil {
		_ = os.Remove(tmp)
		return err
	}
	if err := os.Rename(tmp, path); err != nil {
		_ = os.Remove(tmp)
		return err
	}
	return syncDir(filepath.Dir(path))
}

func syncDir(path string) error {
	dir, err := os.Open(path)
	if err != nil {
		return err
	}
	defer dir.Close()
	return dir.Sync()
}

func sortRefs(refs []Ref) {
	sort.Slice(refs, func(i, j int) bool {
		if refs[i].Repo != refs[j].Repo {
			return refs[i].Repo < refs[j].Repo
		}
		return refs[i].Name < refs[j].Name
	})
}

func key(ref Ref) string { return ref.Repo + "\x00" + ref.Name }

func cleanSlug(slug string) bool {
	return strings.TrimSpace(slug) != "" && slug == filepath.Base(slug) && slug != "." && slug != ".." && !strings.Contains(slug, string(filepath.Separator))
}