⚠️ 自己写：去掉 TIME-WAIT

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本节的内存版 TCP 状态机（判题机不联网；真机上 ss -tan 看到的 State 就是这些名字）：

TABLE[(状态, 事件)] = (新状态, 要发出去的段)   事件：listen / connect / close / timeout / retrans 是本机动作，SYN / SYN-ACK / ACK / FIN / RST 是收到的段
talk(script, drop=())     script 形如 ["B:listen", "A:connect", "A:close", "B:close"]；一台主机发出的段立刻送到对面（序号在 drop 里的丢掉，轨迹里标 x），对面的回信再送回来
                          交回 (轨迹 ["A>B SYN", "B>A SYN-ACK", ...], {"A": 状态, "B": 状态})

最后那个 ACK（轨迹第 7 段）丢了，B 在 LAST-ACK 里会重发 FIN（事件 B:retrans）。补全 last_ack_lost(keep)：keep 为 True 时 A 老老实实等在 TIME-WAIT；为 False 时 A 收到最后的 FIN 后立刻 A:timeout 进 CLOSED、再让 B 重发。交回「最后一段的名字,A 的状态」。两种各跑一遍：

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TABLE = {
    ("CLOSED", "listen"):        ("LISTEN", None),
    ("CLOSED", "connect"):       ("SYN-SENT", "SYN"),
    ("LISTEN", "SYN"):           ("SYN-RECV", "SYN-ACK"),
    ("SYN-SENT", "SYN-ACK"):     ("ESTAB", "ACK"),
    ("SYN-RECV", "ACK"):         ("ESTAB", None),
    ("ESTAB", "close"):          ("FIN-WAIT-1", "FIN"),
    ("ESTAB", "FIN"):            ("CLOSE-WAIT", "ACK"),
    ("FIN-WAIT-1", "ACK"):       ("FIN-WAIT-2", None),
    ("FIN-WAIT-1", "FIN"):       ("CLOSING", "ACK"),
    ("FIN-WAIT-2", "FIN"):       ("TIME-WAIT", "ACK"),
    ("CLOSING", "ACK"):          ("TIME-WAIT", None),
    ("CLOSE-WAIT", "close"):     ("LAST-ACK", "FIN"),
    ("LAST-ACK", "ACK"):         ("CLOSED", None),
    ("LAST-ACK", "retrans"):     ("LAST-ACK", "FIN"),
    ("TIME-WAIT", "FIN"):        ("TIME-WAIT", "ACK"),
    ("TIME-WAIT", "timeout"):    ("CLOSED", None),
    ("CLOSED", "FIN"):           ("CLOSED", "RST"),
    ("LAST-ACK", "RST"):         ("CLOSED", None),
}

class Host:
    def __init__(self, name):
        self.name = name
        self.state = "CLOSED"

def step(self, event):
        """交回要发出去的段（没有就 None）；没这一行的 (状态, 事件) 一律不动。"""
        key = (self.state, event)
        if key not in TABLE:
            return None
        self.state, out = TABLE[key]
        return out

def talk(script, drop=()):
    """script: ["B:listen", "A:connect", "A:close", "B:close", "A:timeout", ...]。
    一台主机发出的段立刻送到对面（除非它的序号在 drop 里），对面的回信再送回来。
    交回 (段的轨迹 ["A>B SYN", ...], {"A": 状态, "B": 状态})。"""
    hosts = {"A": Host("A"), "B": Host("B")}
    trace = []

def send(frm, seg):
        to = "B" if frm == "A" else "A"
        trace.append(frm + ">" + to + " " + seg)
        if len(trace) in drop:
            trace[-1] += " x"
            return
        reply = hosts[to].step(seg)
        if reply:
            send(to, reply)

for item in script:
        who, event = item.split(":")
        seg = hosts[who].step(event)
        if seg:
            send(who, seg)
    return trace, {k: h.state for k, h in hosts.items()}

def last_ack_lost(keep):
    script = ["B:listen", "A:connect", "A:close", "B:close"]
    # TODO: keep 为 False 时先 A:timeout；然后 B:retrans；talk(script, drop=(7,))
    return ""

print(last_ack_lost(True) + "/" + last_ack_lost(False))

本次输入：

输出：

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