fix(reconcile): fill earliest month deficit first in multi-month allocations #35

kacerr · 2026-05-12T00:00:08+02:00

kacerr commented

2026-05-12 00:00:08 +02:00

What

Multi-month payment allocation was splitting each transaction proportionally across matched months based on total expected fees, ignoring what prior transactions had already paid into those months.

Surfaced by: Matyáš Thér paid 200 CZK (for 2026-02) then 550 CZK (for 2026-02 + 2026-03). The 550 was being split 366.67/183.33 by the proportional formula instead of filling 2026-02's remaining deficit (300) and spilling 250 into 2026-03. Result: 566/183 instead of the correct 500/250.

Fix

Replace the greedy + proportional branches in both Python and Go reconcilers with a single fill-first loop:

Iterate matched months in user-supplied order (chronological by convention).
For each month: alloc = min(remaining, max(0, expected − paid_so_far)).
Any surplus after all deficits are covered → credit bucket.

Files

scripts/match_payments.py — Python reconciler (canonical)
go/internal/domain/reconcile/reconcile.go — Go reconciler
Tests and two parity fixtures updated to match new behavior.

## What Multi-month payment allocation was splitting each transaction proportionally across matched months based on total expected fees, ignoring what prior transactions had already paid into those months. **Surfaced by**: Matyáš Thér paid 200 CZK (for 2026-02) then 550 CZK (for 2026-02 + 2026-03). The 550 was being split 366.67/183.33 by the proportional formula instead of filling 2026-02's remaining deficit (300) and spilling 250 into 2026-03. Result: 566/183 instead of the correct 500/250. ## Fix Replace the greedy + proportional branches in both Python and Go reconcilers with a single fill-first loop: - Iterate matched months in user-supplied order (chronological by convention). - For each month: `alloc = min(remaining, max(0, expected − paid_so_far))`. - Any surplus after all deficits are covered → credit bucket. ## Files - `scripts/match_payments.py` — Python reconciler (canonical) - `go/internal/domain/reconcile/reconcile.go` — Go reconciler - Tests and two parity fixtures updated to match new behavior.

kacerr added 1 commit 2026-05-12 00:00:08 +02:00

fix(reconcile): fill earliest month deficit first in multi-month allocations

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Details

8734089223

Replace proportional split with a fill-first loop that allocates
min(remaining, deficit) to each matched month in user-supplied order,
where deficit = expected - already_paid. Prior transactions' contributions
are now properly accounted for, so a second payment on overlapping months
fills only what's still owed instead of splitting proportionally by total
expected. Surplus after all deficits are covered goes to the credit bucket.

Fixes: Matyáš Thér 200+550 showing 566/183 instead of 500/250.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

kacerr merged commit 10e2e9dc04 into main

2026-05-12 00:01:37 +02:00

kacerr referenced this issue from a commit

2026-05-12 00:01:38 +02:00

Merge pull request 'fix(reconcile): fill earliest month deficit first in multi-month allocations' (#35) from fix/fill-first-multi-month-allocation into main

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Reference: kacerr/fuj-management#35