FA-61129 / Bond day-count conventions / Member archive

Inflation-linked reference index interpolation: the first of the month is already interpolated one day · case 04

Reference CPI on the first of a month is not equal to the lagged monthly index.

Member previewVariant 4 · 3 implementations · 8 checks per implementation

Case contract

Inputs a date [y,m,d], a map "YYYY-MM" to CPI level and a base index. RefCPI = CPI(m-3) + (d-1)/days_in_month(m) * (CPI(m-2) - CPI(m-3)), computed in Decimal and rounded half-up to 5 decimals; the index ratio is RefCPI/base rounded half-up to 5 decimals. Return both as strings.

Why this case matters

Bond accrual and pricing systems depend on exact day-count arithmetic; a single-day error changes settlement cash.

One recorded failure

Sample boundary fixture

This sample comes from the broken implementation of a controlled reproducer.

Boundary fixture	Actual	Expected	Outcome
regression interpolation day offset 1	`["288.60384", "1.51082"]`	`["288.57700", "1.51068"]`	Failed

MEMBER ARCHIVE

The complete case is available to members.

This record includes three runnable implementations, regression fixtures, execution results, and source hashes.

Member access is invitation-based. Sign in with your invited account to inspect the sources.