var async = require('async');
var fs = require('fs');
var pg = require('pg');

// Connect to the bank database.

var config = {
    user: 'maxroach',
    host: 'localhost',
    database: 'bank',
    port: 26257
};

// Wrapper for a transaction.  This automatically re-calls "op" with
// the client as an argument as long as the database server asks for
// the transaction to be retried.

function txnWrapper(client, op, next) {
    client.query('BEGIN; SAVEPOINT cockroach_restart', function (err) {
        if (err) {
            return next(err);
        }

        var released = false;
        async.doWhilst(function (done) {
                var handleError = function (err) {
                    // If we got an error, see if it's a retryable one
                    // and, if so, restart.
                    if (err.code === '40001') {
                        // Signal the database that we'll retry.
                        return client.query('ROLLBACK TO SAVEPOINT cockroach_restart', done);
                    }
                    // A non-retryable error; break out of the
                    // doWhilst with an error.
                    return done(err);
                };

                // Attempt the work.
                op(client, function (err) {
                    if (err) {
                        return handleError(err);
                    }
                    var opResults = arguments;

                    // If we reach this point, release and commit.
                    client.query('RELEASE SAVEPOINT cockroach_restart', function (err) {
                        if (err) {
                            return handleError(err);
                        }
                        released = true;
                        return done.apply(null, opResults);
                    });
                });
            },
            function () {
                return !released;
            },
            function (err) {
                if (err) {
                    client.query('ROLLBACK', function () {
                        next(err);
                    });
                } else {
                    var txnResults = arguments;
                    client.query('COMMIT', function (err) {
                        if (err) {
                            return next(err);
                        } else {
                            return next.apply(null, txnResults);
                        }
                    });
                }
            });
    });
}

// The transaction we want to run.

function transferFunds(client, from, to, amount, next) {
    // Check the current balance.
    client.query('SELECT balance FROM accounts WHERE id = $1', [from], function (err, results) {
        if (err) {
            return next(err);
        } else if (results.rows.length === 0) {
            return next(new Error('account not found in table'));
        }

        var acctBal = results.rows[0].balance;
        if (acctBal >= amount) {
            // Perform the transfer.
            async.waterfall([
                function (next) {
                    // Subtract amount from account 1.
                    client.query('UPDATE accounts SET balance = balance - $1 WHERE id = $2', [amount, from], next);
                },
                function (updateResult, next) {
                    // Add amount to account 2.
                    client.query('UPDATE accounts SET balance = balance + $1 WHERE id = $2', [amount, to], next);
                },
                function (updateResult, next) {
                    // Fetch account balances after updates.
                    client.query('SELECT id, balance FROM accounts', function (err, selectResult) {
                        next(err, selectResult ? selectResult.rows : null);
                    });
                }
            ], next);
        } else {
            next(new Error('insufficient funds'));
        }
    });
}

// Create a pool.
var pool = new pg.Pool(config);

pool.connect(function (err, client, done) {
    // Closes communication with the database and exits.
    var finish = function () {
        done();
        process.exit();
    };

    if (err) {
        console.error('could not connect to cockroachdb', err);
        finish();
    }

    // Execute the transaction.
    txnWrapper(client,
        function (client, next) {
            transferFunds(client, 1, 2, 100, next);
        },
        function (err, results) {
            if (err) {
                console.error('error performing transaction', err);
                finish();
            }

            console.log('Balances after transfer:');
            results.forEach(function (result) {
                console.log(result);
            });

            finish();
        });
});