BNPL Platform

01// Money is int64 cents, never float. The installment sum is an invariant.02func (e *Engine) PayInstallment(ctx context.Context, id InstallmentID) error {03    tx, err := e.db.BeginTx(ctx, &sql.TxOptions{Isolation: sql.LevelSerializable})04    if err != nil {05        return err06    }07    defer tx.Rollback()08 09    // Row lock prevents a double-payment race under concurrent load.10    var status string11    err = tx.QueryRowContext(ctx,12        `SELECT status FROM installments WHERE id = $1 FOR UPDATE`,13        id).Scan(&status)14    if err != nil {15        return err16    }17    if status == "paid" {18        return ErrAlreadyPaid          // idempotent: retries are safe19    }20 21    if _, err = tx.ExecContext(ctx,22        `UPDATE installments SET status = 'paid', paid_at = now() WHERE id = $1`,23        id); err != nil {24        return err25    }26    return tx.Commit()27}28 29// splitCents distributes a total so the parts ALWAYS sum back to the original.30func splitCents(total int64, n int) []int64 {31    base, rem := total/int64(n), total%int64(n)32    out := make([]int64, n)33    for i := range out {34        out[i] = base35        if int64(i) < rem {            // remainder to the earliest installments36            out[i]++37        }38    }39    return out                          // sum(out) == total, guaranteed40}