The ranks of n parts are a permutation of 0..n-1 (being ahead is a strict
total order; pigeonhole by hand, core Lean has no subperm), so exactly
u parts get a unit and one gets the fraction. apportion_sum and
apportion_sum_of_shortfall; the audit shows only the standard axioms.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GWVo7x3UHw5kwCUaWxiejX
Each close into members' capital rounded on its own, ties to the
earlier member, so twelve monthly 50/50 closes of odd-cent income could
leave one member several cents ahead of half the year (docs/testing.md
#12, found with the members' equity fix).
- CloseTo::Members now splits cumulatively over a run (closes back to
back with the same members and shares): each close gives a member
allocate(run total so far) - allocate(run total before). Periods
record their close order and a cumulative close's members and net
(Period::close_seq, member_close) so the next close continues the run
and replay recomputes it exactly. A close to anything else, or other
shares, ends the run.
- Journaled "rounding": "cumulative". The per-close rule is kept as
CloseTo::MembersPerClose for closes journaled "largest_remainder";
older closes still replay with the last member absorbing it.
- Lean: runPart_telescopes (a member's total from a run is their
allocation of the run's total) and closePart_total (each close hands
out exactly its net), audited to the standard axioms.
- Tests: hand-computed three one-cent months (per close A 0.03 / B 0;
cumulative A 0.02 / B 0.01), a run ending, a property that each member
stays within a cent of their share (fails on the per-close rule), and
a store contract that each rule journals and replays as itself.
Wanderland's books have no closes yet, so all of theirs use the new rule.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GWVo7x3UHw5kwCUaWxiejX
The money arithmetic every invariant rests on now lives in one kernel,
ledger-core/src/kernel.rs: signing a line, totals, net, per-account
movements, reversal, normal balances, trial-balance columns, closing
lines, and the rounding plug for member splits. It is generic over a
checked `Amount` trait. The engine runs it on Decimal (validate,
movements, reverse_entry, account_balance, close_period_to, allocate,
trial_balance): there is one copy of the arithmetic, and it is the one
under proof.
Three layers, docs/verification.md maps every law to its proofs:
- Kani (src/proofs.rs): 17 harnesses prove the kernel at i64 for every
input up to 4 lines over 3 accounts. An entry is accepted exactly
when it is two or more positive, in-range lines whose debits equal
credits; accepted entries net to zero; grouping by account loses
nothing; a reversal mirrors and cancels its original; trial-balance
columns balance iff the books net to zero; closing lines zero their
accounts and a close balances however the net is split; the plug
makes parts sum to the whole. No panics, no overflow. kani::cover!
shows the proofs are not vacuous.
- Lean 4 (verify/lean): 24 headline theorems for books of any size. The
ledger is a state machine (add account, draft, post, reverse, open,
close), and in every reachable state the books net to zero, the trial
balance balances (as of any date), and the accounting equation holds.
Posting only appends ("posted is forever"), closed periods are
frozen, a close zeroes revenue and expenses and credits exactly the net
income, and period statements add up. Audit.lean shows standard axioms
only, no sorry.
- Bridge (tests/kernel_bridge.rs): the Decimal and i64 kernels agree.
What the proofs found (each one missed by every test, fuzzer, and
mutant):
- The engine accepted negative line amounts; only MCP refused them. Now
NegativeAmountLine: direction is the side's job.
- Summing lines near Decimal::MAX panicked. Lines are now capped at one
quadrillion and ten decimal places (AmountOutOfRange), which keeps
every realistic sum exact, and the kernel's sums are checked.
- FX conversion (foreign_amount * fx_rate) could panic the same way; it
is now a checked multiply.
- Kani itself: 0.68 mis-models symbolic-length copy_from_slice
(model-checking/kani#4904) and reported a reversal that doesn't net to
zero. The counterexample, replayed natively, passes, and a 15-line
standalone crate reproduces the bug. Harnesses avoid bulk copies, and
a should_panic canary will flag the upstream fix.
`just prove` runs it all (Kani memory- and time-capped, fails on an
unsatisfiable cover or a non-standard Lean axiom). AGENTS.md now makes
formal coverage part of every core change.
Also, docs: GnuCash is *an* oracle, never *the* oracle (Stephen). The
spec is accounting principles, proved. Golden files are recomputed from
principles. External implementations (GnuCash, the old Sheet, QBO/Xero)
are cross-checks whose disagreements get adjudicated, never obeyed.
docs/oracle.md gains "Who is right"; AGENTS, README, architecture,
roadmap, parity, and STATUS are reworded to match.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GWVo7x3UHw5kwCUaWxiejX
swaitsandClaude Opus 5.5