Kontera is a Rust library I'm building that turns e-commerce events (sales, refunds, payment fees and payouts) into correct Swedish bookkeeping: balanced verifications on BAS accounts, exported as SIE 4I files. It runs without a database or server, so it can be embedded in any shop or tool.

It came out of a real problem. A payment provider pays out one net amount, but the books need the sales, VAT, refunds and fees posted separately, and those postings have to reconcile to exactly zero.

The core is pure and built so invalid states can't exist: money is stored exactly in öre with defined rounding, account numbers are validated, and a verification can only be created if it balances. It's tested with property-based tests, and CI runs formatting, lints with warnings as errors, an MSRV check and a check that keeps the core free of I/O. The specification and architecture decisions were written first. The domain model is in place, and the posting rules and SIE export are next. It also compiles to WebAssembly for use from Node.