PW, AW ও ROR পদ্ধতির মধ্যে বাছাই

# PW, AW ও IRR একসাথে গণনা -- তিনটি অল্টারনেটিভের তুলনা

def pw_factor(i, n):
    return ((1 + i) ** n - 1) / (i * (1 + i) ** n)

def ap_factor(i, n):
    return (i * (1 + i) ** n) / ((1 + i) ** n - 1)

def npv(cashflows, i):
    return sum(cf / (1 + i) ** t for t, cf in cashflows)

def find_irr(cashflows, lo=-0.5, hi=1.0, tol=1e-7, max_iter=200):
    f_lo = npv(cashflows, lo)
    f_hi = npv(cashflows, hi)
    if f_lo * f_hi > 0:
        raise ValueError("bracket-এ NPV চিহ্ন বদলাচ্ছে না")
    for _ in range(max_iter):
        mid = (lo + hi) / 2
        f_mid = npv(cashflows, mid)
        if abs(f_mid) < tol or (hi - lo) / 2 < tol:
            return mid
        if f_lo * f_mid < 0:
            hi = mid
        else:
            lo = mid
            f_lo = f_mid
    return (lo + hi) / 2

MARR = 0.10
n = 5

alternatives = {
    "X": {"cost": 15000, "benefit": 4500},
    "Y": {"cost": 20000, "benefit": 5800},
    "Z": {"cost": 25000, "benefit": 7000},
}

results = {}
for name, alt in alternatives.items():
    cost, benefit = alt["cost"], alt["benefit"]
    pw = -cost + benefit * pw_factor(MARR, n)
    aw = pw * ap_factor(MARR, n)
    cashflows = [(0, -cost)] + [(t, benefit) for t in range(1, n + 1)]
    irr = find_irr(cashflows)
    results[name] = {"pw": pw, "aw": aw, "irr": irr}
    print(f"Alt {name}: cost=${cost:,.0f}, benefit=${benefit:,.0f}/yr  ->  "
          f"PW=${pw:,.2f}   AW=${aw:,.2f}   IRR={irr*100:.2f}%")

pw_rank = sorted(results, key=lambda k: results[k]["pw"], reverse=True)
aw_rank = sorted(results, key=lambda k: results[k]["aw"], reverse=True)
irr_rank = sorted(results, key=lambda k: results[k]["irr"], reverse=True)

print()
print(f"PW র‌্যাংকিং (বেশি থেকে কম): {pw_rank}")
print(f"AW র‌্যাংকিং (বেশি থেকে কম): {aw_rank}")
print(f"IRR র‌্যাংকিং (বেশি থেকে কম): {irr_rank}")
print(f"PW ও AW র‌্যাংকিং একই কি না: {pw_rank == aw_rank}")