package provision

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

	"gitea.hostxtra.co.uk/mrhid6/vantage/shared/models"
	"github.com/google/uuid"
	"go.mongodb.org/mongo-driver/v2/bson"
	"go.mongodb.org/mongo-driver/v2/mongo"
)

// ErrNameRejected wraps every reason a name cannot become an instance.
var ErrNameRejected = errors.New("organisation name rejected")

const maxSlugAttempts = 50

// CreateInstance inserts an instance under the first free slug derived from name,
// with a freshly generated ID.
func CreateInstance(ctx context.Context, db *mongo.Database, name string) (*models.Instance, error) {
	return CreateInstanceWithID(ctx, db, uuid.NewString(), name)
}

// CreateInstanceWithID inserts an instance under the first free slug derived from
// name, using a caller-supplied instance ID.
//
// A caller-supplied ID exists for the paid-cloud flow: a placeholder row is
// created before payment and provisioning happens on the confirmed-payment
// webhook. Provisioning with the placeholder's own ID keeps the id stable, so
// the subscription's custom_data never points at a rewritten row and later
// webhooks still resolve it. If an instance with this ID already exists — a
// webhook retried after a partial provision — it is returned as-is rather than
// duplicated.
//
// The count-then-insert loop is racy on its own. It is safe only because
// instances.slug carries a unique index: a lost race surfaces as a duplicate-key
// error, which we treat as "that slug is taken" and retry. Do not remove the
// duplicate-key branch, and do not remove the index.
func CreateInstanceWithID(ctx context.Context, db *mongo.Database, instanceID, name string) (*models.Instance, error) {
	// Idempotency: a retried provision finds its own instance already present.
	var existing models.Instance
	if err := db.Collection("instances").FindOne(ctx,
		bson.M{"instance_id": instanceID}).Decode(&existing); err == nil {
		return &existing, nil
	}

	base, err := BaseSlug(name)
	if err != nil {
		return nil, fmt.Errorf("%w: %s", ErrNameRejected, err.Error())
	}

	for attempt := 1; attempt <= maxSlugAttempts; attempt++ {
		slug := NextSlug(base, attempt)

		n, err := db.Collection("instances").CountDocuments(ctx, bson.M{"slug": slug})
		if err != nil {
			return nil, err
		}
		if n > 0 {
			continue
		}

		inst := models.Instance{
			InstanceID: instanceID,
			Name:       name,
			Slug:       slug,
			CreatedAt:  time.Now().UTC(),
		}
		if _, err := db.Collection("instances").InsertOne(ctx, inst); err != nil {
			if mongo.IsDuplicateKeyError(err) {
				continue // lost the race; try the next slug
			}
			return nil, err
		}
		return &inst, nil
	}
	return nil, fmt.Errorf("%w: could not find a free slug for %q", ErrNameRejected, name)
}

// ErrSlugTaken means the slug a new name derives to already belongs to another
// instance.
//
// Rename refuses rather than appending a counter the way creation does. Creation
// appends because the customer is waiting on an instance and any free slug will
// do; a rename is a request for one specific host, and silently landing them on
// "acme-2" answers a question they did not ask.
var ErrSlugTaken = errors.New("slug taken")

// RenameSlug derives the slug a rename to name would move an instance to, given
// the slug it holds now.
//
// It returns the current slug unchanged when the name still derives to it, so a
// cosmetic edit — capitalisation, punctuation, a trailing "Ltd." — is not a move
// and cannot collide with the instance's own slug.
func RenameSlug(name, currentSlug string) (string, error) {
	base, err := BaseSlug(name)
	if err != nil {
		return "", fmt.Errorf("%w: %s", ErrNameRejected, err.Error())
	}
	if base == currentSlug {
		return currentSlug, nil
	}
	return base, nil
}

// RenameInstance changes an instance's name and re-derives its slug from it.
//
// It returns the name and slug the control plane held BEFORE the write, and
// those are the only correct values to unwind with. The caller's own copy of the
// instance may be stale, and admin's copy stores slug with `omitempty`, so an
// unwind driven from there can write an empty slug — which either mis-restores
// the tenant host or trips the unique index against every other slugless row.
//
// The count-then-update is racy on its own, and is safe for the same reason
// CreateInstanceWithID's loop is: instances.slug carries a unique index, so a
// lost race surfaces as a duplicate-key error. Unlike creation there is nothing
// to retry with — the caller asked for one specific name — so it becomes
// ErrSlugTaken. Do not remove the duplicate-key branch, and do not remove the
// index.
func RenameInstance(ctx context.Context, db *mongo.Database, instanceID, name string) (inst *models.Instance, prevName, prevSlug string, err error) {
	var cur models.Instance
	if err := db.Collection("instances").FindOne(ctx,
		bson.M{"instance_id": instanceID}).Decode(&cur); err != nil {
		return nil, "", "", err
	}
	prevName, prevSlug = cur.Name, cur.Slug

	slug, err := RenameSlug(name, cur.Slug)
	if err != nil {
		return nil, prevName, prevSlug, err
	}

	if slug != cur.Slug {
		n, err := db.Collection("instances").CountDocuments(ctx, bson.M{
			"slug":        slug,
			"instance_id": bson.M{"$ne": instanceID},
		})
		if err != nil {
			return nil, prevName, prevSlug, err
		}
		if n > 0 {
			return nil, prevName, prevSlug, fmt.Errorf("%w: %s", ErrSlugTaken, slug)
		}
	}

	if _, err := db.Collection("instances").UpdateOne(ctx,
		bson.M{"instance_id": instanceID},
		bson.M{"$set": bson.M{"name": name, "slug": slug}}); err != nil {
		if mongo.IsDuplicateKeyError(err) {
			return nil, prevName, prevSlug, fmt.Errorf("%w: %s", ErrSlugTaken, slug)
		}
		return nil, prevName, prevSlug, err
	}

	cur.Name = name
	cur.Slug = slug
	return &cur, prevName, prevSlug, nil
}

// RestoreInstanceIdentity writes an exact name and slug back, unwinding a rename
// whose caller-side bookkeeping then failed.
//
// It derives nothing. The values being restored may include a creation-time
// collision suffix that no name derives to, so re-running RenameInstance with the
// old name would not reproduce them.
func RestoreInstanceIdentity(ctx context.Context, db *mongo.Database, instanceID, name, slug string) error {
	_, err := db.Collection("instances").UpdateOne(ctx,
		bson.M{"instance_id": instanceID},
		bson.M{"$set": bson.M{"name": name, "slug": slug}})
	return err
}

// RollbackInstance deletes an instance that has no users.
//
// It refuses an instance that has users. Rollback exists to clean up a
// half-finished signup, and an instance with users is not half-finished.
func RollbackInstance(ctx context.Context, db *mongo.Database, instanceID string) error {
	n, err := db.Collection("users").CountDocuments(ctx, bson.M{"instance_id": instanceID})
	if err != nil {
		return err
	}
	if n > 0 {
		return fmt.Errorf("refusing to roll back instance %s: it has %d user(s)", instanceID, n)
	}
	_, err = db.Collection("instances").DeleteOne(ctx, bson.M{"instance_id": instanceID})
	return err
}