# SPDX-License-Identifier: MIT
"""YS-EN-0004 · check Equation 2 by simulating the fleet event by event.

Failures arrive one at a time across the working cores (a dead core's parts
stop running, unlike the Poisson model). Each failure is fixed from stock
if possible, otherwise by taking the part from a core that is already dead
(it donates its good parts); failing that, the core is laid up.
Prints the simulated fleet POS for the manifests quoted in the note.

Usage: python check_cannibalisation.py   (a few minutes; requires numpy)
"""
from __future__ import annotations

import numpy as np

from sparing import INPUTS, ITEMS

N, T = INPUTS["fleet"], INPUTS["mission_yr"]
RATE = np.array([INPUTS["rate_share"][i] for i in ITEMS]) / INPUTS["core_mtbf_yr"]


def fleet_pos(spares, tolerate=0, reps=20000, seed=1) -> float:
    rng = np.random.default_rng(seed)
    ok = 0
    for _ in range(reps):
        stock = np.array(spares, float)
        dead = []                        # per dead core: which parts it is missing
        working, t = N, 0.0
        while True:
            t += rng.exponential(1 / (working * RATE.sum()))
            if t > T:
                break
            j = rng.choice(len(ITEMS), p=RATE / RATE.sum())
            if stock[j] > 0:
                stock[j] -= 1
                continue
            donor = next((d for d in dead if not d[j]), None)
            if donor is not None:        # strip a dead core for the part
                donor[j] = True
                continue
            miss = np.zeros(len(ITEMS), bool)
            miss[j] = True
            dead.append(miss)
            working -= 1
        ok += (N - working) <= tolerate
    return ok / reps


if __name__ == "__main__":
    print("modules 98/84/71/71/24, m=0 :", fleet_pos([98, 84, 71, 71, 24]))
    print("94 spare cores (kits), m=0   :", fleet_pos([94] * 5))
    print("modules 88/74/61/61/14, m=10:", fleet_pos([88, 74, 61, 61, 14], tolerate=10, reps=40000, seed=7))