Compound Interest Calculator — Python source

"""
compound-interest-calculator - compound growth projection.

Display snippet: ports the core project() math from the TypeScript lib
(src/lib/interest.ts). The full projection (contributions, inflation,
per-year rows) lives in the TypeScript/Go sources.

Semantics: monthly simulation with the monthly rate set to the exact
equivalent of the stated compounding frequency — (1 + r/n)**(n/12) - 1 —
so the effective annual rate is preserved for every supported frequency.
Returns the final balance rounded to 2 decimals, or None on invalid input
(negative principal, non-integer years, unsupported frequency,
non-finite values). Standard library only.
"""

import math

VALID_COMPOUNDS = {1, 2, 4, 12, 365}


def round2(n):
    """Round to 2 decimals, half-up to mirror the TS Math.round."""
    return math.floor(n * 100 + 0.5) / 100


def project(principal, rate_pct, years, compounds_per_year):
    values = (principal, rate_pct, years, compounds_per_year)
    if not all(math.isfinite(v) for v in values):
        return None
    if principal < 0 or years < 0 or not isinstance(years, int):
        return None
    if compounds_per_year not in VALID_COMPOUNDS:
        return None

    monthly_rate = (
        (1 + rate_pct / 100 / compounds_per_year) ** (compounds_per_year / 12) - 1
    )

    balance = principal
    for _ in range(years * 12):
        balance *= 1 + monthly_rate
    return round2(balance)


# Example:
#   project(10000, 5, 10, 12) -> 16470.09