// Package txn provides functions for running transactions.
package txn

import (
	"context"
	"fmt"
)

type Manager interface {
	Begin(ctx context.Context, writable bool) (context.Context, error)
	Commit(ctx context.Context) error
	Rollback(ctx context.Context) error

	IsLocked(err error) bool
}

type DatabaseProvider interface {
	WithDatabase(ctx context.Context) (context.Context, error)
}

// TxnFunc is a function that is used in transaction hooks.
// It should return an error if something went wrong.
type TxnFunc func(ctx context.Context) error

// MustFunc is a function that is used in transaction hooks.
// It does not return an error.
type MustFunc func(ctx context.Context)

// WithTxn executes fn in a transaction. If fn returns an error then
// the transaction is rolled back. Otherwise it is committed.
// This function will call m.Begin with writable = true.
// This function should be used for making changes to the database.
func WithTxn(ctx context.Context, m Manager, fn TxnFunc) error {
	const (
		execComplete = true
		writable     = true
	)
	return withTxn(ctx, m, fn, writable, execComplete)
}

// WithReadTxn executes fn in a transaction. If fn returns an error then
// the transaction is rolled back. Otherwise it is committed.
// This function will call m.Begin with writable = false.
func WithReadTxn(ctx context.Context, m Manager, fn TxnFunc) error {
	const (
		execComplete = true
		writable     = false
	)
	return withTxn(ctx, m, fn, writable, execComplete)
}

func withTxn(ctx context.Context, m Manager, fn TxnFunc, writable bool, execCompleteOnLocked bool) error {
	// post-hooks should be executed with the outside context
	txnCtx, err := begin(ctx, m, writable)
	if err != nil {
		return err
	}

	hookMgr := hookManagerCtx(txnCtx)

	defer func() {
		if p := recover(); p != nil {
			// a panic occurred, rollback and repanic
			rollback(txnCtx, m)
			panic(p)
		}

		if err != nil {
			// something went wrong, rollback
			rollback(txnCtx, m)

			// execute post-hooks with outside context
			hookMgr.executePostRollbackHooks(ctx)

			if execCompleteOnLocked || !m.IsLocked(err) {
				hookMgr.executePostCompleteHooks(ctx)
			}
		} else {
			// all good, commit
			err = commit(txnCtx, m)

			// execute post-hooks with outside context
			hookMgr.executePostCommitHooks(ctx)
			hookMgr.executePostCompleteHooks(ctx)
		}

	}()

	err = fn(txnCtx)
	return err
}

func begin(ctx context.Context, m Manager, writable bool) (context.Context, error) {
	var err error
	ctx, err = m.Begin(ctx, writable)
	if err != nil {
		return nil, err
	}

	hm := hookManager{}
	ctx = hm.register(ctx)

	return ctx, nil
}

func commit(ctx context.Context, m Manager) error {
	hookMgr := hookManagerCtx(ctx)
	if err := hookMgr.executePreCommitHooks(ctx); err != nil {
		return err
	}

	if err := m.Commit(ctx); err != nil {
		return err
	}

	return nil
}

func rollback(ctx context.Context, m Manager) {
	if err := m.Rollback(ctx); err != nil {
		return
	}
}

// WithDatabase executes fn with the context provided by p.WithDatabase.
// It does not run inside a transaction, so all database operations will be
// executed in their own transaction.
func WithDatabase(ctx context.Context, p DatabaseProvider, fn TxnFunc) error {
	var err error
	ctx, err = p.WithDatabase(ctx)
	if err != nil {
		return err
	}

	return fn(ctx)
}

// Retryer is a provides WithTxn function that retries the transaction
// if it fails with a locked database error.
// Transactions are run in exclusive mode.
type Retryer struct {
	Manager Manager
	// use value < 0 to retry forever
	Retries int
	OnFail  func(ctx context.Context, err error, attempt int) error
}

func (r Retryer) WithTxn(ctx context.Context, fn TxnFunc) error {
	var attempt int
	var err error
	for attempt = 1; attempt <= r.Retries || r.Retries < 0; attempt++ {
		const (
			execComplete = false
			exclusive    = true
		)
		err = withTxn(ctx, r.Manager, fn, exclusive, execComplete)

		if err == nil {
			return nil
		}

		if !r.Manager.IsLocked(err) {
			return err
		}

		if r.OnFail != nil {
			if err := r.OnFail(ctx, err, attempt); err != nil {
				return err
			}
		}
	}

	return fmt.Errorf("failed after %d attempts: %w", attempt, err)
}