package skillpack

import (
	"context"
	"errors"
	"fmt"
	"time"

	"github.com/google/uuid"
)

// Syncer ties a Store, a PackCache, and Sources together into the subscription
// lifecycle: subscribe (initial pin), check (record a PENDING update, never move
// the pin), and apply (the explicit re-pin). It owns the supply-chain invariant
// — the only call that changes the bytes an agent runs is Apply, always with an
// actor recorded.
type Syncer struct {
	Cache PackCache // content store for pinned trees
	Subs  Store     // subscription metadata store

	// SourceFor builds the Source for a stored subscription. A host overrides
	// this to enforce its allow-list (reject a disallowed URL/kind before any
	// fetch). Nil uses DefaultSourceFor (dir + git, no allow-list).
	SourceFor func(*Subscription) (Source, error)

	// Now/NewID are injectable for deterministic tests.
	Now   func() time.Time
	NewID func() string
}

func (y *Syncer) now() time.Time {
	if y.Now != nil {
		return y.Now()
	}
	return time.Now()
}

func (y *Syncer) newID() string {
	if y.NewID != nil {
		return y.NewID()
	}
	return uuid.NewString()
}

func (y *Syncer) sourceFor(s *Subscription) (Source, error) {
	if y.SourceFor != nil {
		return y.SourceFor(s)
	}
	return DefaultSourceFor(s)
}

// DefaultSourceFor reconstructs a Source from a subscription's stored
// coordinates, with no allow-list. A host that cares about which sources are
// permitted should set Syncer.SourceFor instead of using this.
func DefaultSourceFor(s *Subscription) (Source, error) {
	switch s.SourceKind {
	case "dir":
		return DirSource{Path: s.SourceURL}, nil
	case "git":
		return GitSource{URL: s.SourceURL, Subpath: s.Subpath}, nil
	default:
		return nil, fmt.Errorf("skillpack: unknown source kind %q", s.SourceKind)
	}
}

// fetchPack fetches src at ref, caches the resulting tree, and returns the
// parsed pack plus the source's resolved ref.
func (y *Syncer) fetchPack(ctx context.Context, src Source, ref string) (*Pack, string, error) {
	tree, sourceRef, err := src.Fetch(ctx, ref)
	if err != nil {
		return nil, "", err
	}
	pack, err := LoadPack(tree)
	if err != nil {
		return nil, "", err
	}
	if err := y.Cache.Put(ctx, pack.Digest, pack.Tree); err != nil {
		return nil, "", err
	}
	return pack, sourceRef, nil
}

// Subscribe fetches a pack from src at trackRef, caches it, and persists a new
// Subscription pinned to that exact content, attributed to by. It rejects a
// second subscription to the same pack name.
func (y *Syncer) Subscribe(ctx context.Context, src Source, trackRef, by string) (*Subscription, error) {
	pack, sourceRef, err := y.fetchPack(ctx, src, trackRef)
	if err != nil {
		return nil, err
	}
	existing, err := y.Subs.GetByName(ctx, pack.Manifest.Name)
	if err == nil {
		return nil, fmt.Errorf("skillpack: already subscribed to %q (id %s)", pack.Manifest.Name, existing.ID)
	}
	if !errors.Is(err, ErrNotFound) {
		// A transient store error must NOT fall through to creating a row — that
		// would produce a duplicate subscription the uniqueness check missed.
		return nil, fmt.Errorf("skillpack: checking for existing subscription %q: %w", pack.Manifest.Name, err)
	}

	sub := &Subscription{
		ID:         y.newID(),
		Name:       pack.Manifest.Name,
		SourceKind: src.Kind(),
		SourceURL:  src.String(),
		TrackRef:   trackRef,
		Enabled:    true,
	}
	// Store the raw URL + subpath separately (String() may combine them for
	// display). GitSource methods have value receivers, so a caller may pass
	// either GitSource or *GitSource — handle both.
	switch gs := src.(type) {
	case GitSource:
		sub.SourceURL, sub.Subpath = gs.URL, gs.Subpath
	case *GitSource:
		sub.SourceURL, sub.Subpath = gs.URL, gs.Subpath
	}
	sub.pinTo(pack, sourceRef, by, y.now())
	if err := y.Subs.Save(ctx, sub); err != nil {
		return nil, err
	}
	return sub, nil
}

// Check fetches the subscription's tracked ref and, if the content digest
// differs from the current pin, caches the new tree and records it as PENDING —
// it never moves the pin. If the tracked ref matches the pin, any stale pending
// state is cleared. The updated subscription is saved and returned.
func (y *Syncer) Check(ctx context.Context, id string) (*Subscription, error) {
	sub, err := y.Subs.Get(ctx, id)
	if err != nil {
		return nil, err
	}
	src, err := y.sourceFor(sub)
	if err != nil {
		return nil, err
	}
	pack, sourceRef, err := y.fetchPack(ctx, src, sub.TrackRef)
	if err != nil {
		return nil, err
	}
	if pack.Digest == sub.PinnedDigest {
		// No change upstream; drop any previously-recorded pending update.
		sub.PendingDigest, sub.PendingSourceRef, sub.PendingAt = "", "", time.Time{}
	} else {
		sub.PendingDigest = pack.Digest
		sub.PendingSourceRef = sourceRef
		sub.PendingAt = y.now()
	}
	if err := y.Subs.Save(ctx, sub); err != nil {
		return nil, err
	}
	return sub, nil
}

// CheckAll runs Check on every subscription and returns the ones that now have a
// pending update. Errors on individual subscriptions are collected, not fatal —
// one unreachable source shouldn't stop the sweep. A host calls this on its own
// ticker (skillpack has no cron opinion; the update is never auto-applied so the
// cadence only affects how fresh the "pending" signal is).
func (y *Syncer) CheckAll(ctx context.Context) (pending []Subscription, errs []error) {
	subs, err := y.Subs.List(ctx)
	if err != nil {
		return nil, []error{err}
	}
	for i := range subs {
		updated, err := y.Check(ctx, subs[i].ID)
		if err != nil {
			errs = append(errs, fmt.Errorf("skillpack: check %q: %w", subs[i].Name, err))
			continue
		}
		if updated.HasPending() {
			pending = append(pending, *updated)
		}
	}
	return pending, errs
}

// Apply promotes a subscription's pending update to the active pin, attributed
// to by. This is the ONLY call that changes what an agent runs. It errors if
// there is no pending update or the pending tree is missing from the cache.
func (y *Syncer) Apply(ctx context.Context, id, by string) (*Subscription, error) {
	sub, err := y.Subs.Get(ctx, id)
	if err != nil {
		return nil, err
	}
	if !sub.HasPending() {
		return nil, fmt.Errorf("skillpack: %q has no pending update to apply", sub.Name)
	}
	tree, err := y.Cache.Get(ctx, sub.PendingDigest)
	if err != nil {
		return nil, fmt.Errorf("skillpack: pending tree for %q missing from cache: %w", sub.Name, err)
	}
	pack, err := LoadPack(tree)
	if err != nil {
		return nil, err
	}
	sub.pinTo(pack, sub.PendingSourceRef, by, y.now())
	if err := y.Subs.Save(ctx, sub); err != nil {
		return nil, err
	}
	return sub, nil
}