Design a SIP Calculator and Investment Tracker

Problem

Design an OOP model for a SIP (Systematic Investment Plan) calculator and portfolio tracker.

Requirements

• createSIP(userId, fundId, monthlyAmount, startDate, duration) → sipId
• processMonthlyInstalment(sipId, navOnDate) — buys units at current NAV
• getPortfolioValue(userId, currentNAVMap) — total current value
• getXIRR(sipId) — annualised return
• pauseSIP(sipId) / stopSIP(sipId)

Key classes

SIP, Fund, Transaction, Portfolio, NAVHistory, XIRRCalculator

Design considerations

• Floating point precision for NAV and units (use BigDecimal)
• Immutable transaction records
• XIRR computed via Newton-Raphson iteration

Extension

Add lump sum investment support alongside SIP.