Plain Ibor amortizing bonds

This additive factory uses the existing Ibor coupon leg, Bond principal inference and discounting engine. The scalar FloatingRateBond constructor is unchanged.

Boundary	Entry point
Rust	FloatingRateBond::new_amortizing
C	itofin_amortizing_floating_rate_bond_new
Go	Session.NewAmortizingFloatingRateBond
Python	itofin.instruments.AmortizingFloatingRateBond

Bounded contract

• Supply exactly one positive, finite, nonincreasing outstanding balance for each schedule period. Inputs are copied. A constant vector is permitted.
• Coupons have unit gearing. Empty spreads mean zero; shorter spread vectors repeat their last element. Spreads must be finite and cannot exceed the period count.
• Principal is repaid at par when balances decrease, on the preceding coupon's adjusted payment date. The final outstanding balance is repaid on the final adjusted payment date. Coupons precede principal on coincident dates.
• Contractual maturity remains the schedule end date, even when the last payment is moved to a later business day.
• The existing native Bond near-equality rule coalesces numerically close balance changes. Coupon balances still come from the supplied vector. This bounded factory preserves that native behavior rather than introducing different math.
• Payment convention, optional fixing lag/convention, issue date and ex-coupon terms use the existing calendar and supported-date guards. The schedule calendar is the payment calendar; there is no separate payment lag or redemption vector.
• Bond and index must share the same settings object, not merely equal dates.
• In this new factory, Rust/C/Go/Python settlement days, effective coupon fixing lag and retained index fixing lag are bounded by the actual supported Date maximum-minus-minimum span (1901-2199). Oversized lags fail before leg/calendar arithmetic. A particular date/calendar walk can still have a tighter valid range. This does not change day-bound behavior of the existing scalar constructor.

There are no caps, floors, in-arrears, overnight, callable, accreting or structured payoff terms. Amortizing AssetSwap support is not part of this contract.

Pricing and retained ownership

The result supports the common Bond NPV, settlement value, current notional, clean/dirty price, accrued amount, cash-flow and redemption queries. Clean/dirty prices and accrued amounts retain the existing per-100 current-notional units. Yield and yield-to-price analytics preserve the existing BondFunctions contract: zero outstanding settlement notional is a typed nontradable error. This differs from native QuantLib Bond::yield and Bond::dirtyPrice, which short-circuit to zero in that state. The native fixture records retain those zero results; tests explicitly qualify this inherited API-layer difference and assert nontradable errors, including unchanged C outputs. Tradable yield comparisons retain the original numerical budgets. The scalar bullet analytics obey the same unchanged rule.

Attach the existing DiscountingBondEngine; no separate coupon pricer is needed for ordinary plain Ibor coupons.

Historical coupons require their actual past fixings. An unfixed fixing today uses the existing settings policy; later historical observations supersede that forecast. Construction itself does not require a fixing history or an evaluation date. Future forecast-calendar checks remain deferred until the relevant amount is requested.

The bond, its legs and individual cash-flow views retain the live index/settings/ curve graph. Forward and discount quote updates remain observable after their original wrappers are released. Freeze/unfreeze and explicit recalculation retain the existing Bond behavior.

Amortization interleaves principal with coupons. Each retained cash-flow view therefore maps its actual full-leg position to its original coupon metadata. Principal entries have no fixing/forecast guard. Releasing the bond or original index wrapper does not change that map.

Errors and language boundaries

• C preserves caller-owned outputs on failure, requires correctly typed handles from the same context, and confines context operations to the creating thread.
• Go returns the existing *FloatingRateBond, rejects mixed sessions and closed inputs, and runs native operations through its locked-thread session worker. Closing a session invalidates all its views.
• Python exposes a named Bond subclass with retained index() and set_coupon_pricer() methods. It is unsendable, as are the existing bond classes.
• Nonfinite amounts are errors, not successful NaN/Inf results. Invalid balances, spread vectors, settings identity, supported dates and calendar walks are rejected.

Examples

• Python example
• Go example

See bond bindings for common pricing/lifecycle contracts and Go ownership for context and session rules.