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Shore-up math: borrow target is bills + reserve, no double cash subtract (Closes #119) #128

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robertismo merged 1 commit from 119-shoreup into master 2026-10-01 15:40:30 +00:00
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Winnie commented 2026-10-01 15:40:06 +00:00
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The shore-up borrow target is now bills + reserveNeed. The old math (need = bills + (reserveNeed − cash), borrowed = need − cash) subtracted the bank's own cash twice: a bank whose cash covered its bills was never force-borrowed at all and got bankrupted with the market full, and a cash-short bank under-borrowed and ended below reserve. Now a short bank borrows bills + reserve − cash, pays its bills, and holds the reserve; only a position the market cannot shore up is insolvent.

Regression tests: the bills-covered-but-reserve-short bank with a full market borrows 3050 and survives holding the reserve; the shoreUpNeed math is pinned for the short, covered, and reserve-off cases. Full suite + vet clean.

The shore-up borrow target is now bills + reserveNeed. The old math (need = bills + (reserveNeed − cash), borrowed = need − cash) subtracted the bank's own cash twice: a bank whose cash covered its bills was never force-borrowed at all and got bankrupted with the market full, and a cash-short bank under-borrowed and ended below reserve. Now a short bank borrows bills + reserve − cash, pays its bills, and holds the reserve; only a position the market cannot shore up is insolvent. Regression tests: the bills-covered-but-reserve-short bank with a full market borrows 3050 and survives holding the reserve; the shoreUpNeed math is pinned for the short, covered, and reserve-off cases. Full suite + vet clean.
shore-up math: borrow target is bills + reserve, no double cash subtract (Closes #119)
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The old need = bills + (reserveNeed - cash) subtracted the bank's own
cash twice: a bank whose cash already covered its bills was never
force-borrowed at all and got bankrupted without the bond market being
consulted, and a cash-short bank under-borrowed and ended the turn
below reserve. The borrow target is now bills + reserveNeed: after
paying bills the bank holds the reserve in cash, and only a position
the market cannot shore up is insolvent.

Regression tests: a bills-covered but reserve-short bank with a full
market now borrows 3050 and survives holding the reserve; shoreUpNeed
math pinned for the short, covered, and reserve-off cases.
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