Add write-ups from hsfzxjy

2020-11-07 13:40:59 +08:00
parent 94b11d2637
commit a5dc4de4e0
3 changed files with 198 additions and 0 deletions
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+# 从零开始的 HTTP 链接
+
+此处提供一个使用 socket 编程完成 TCP 桥接的解法。
+
+运行 `python3 bridge.py :8080 202.38.93.111:0`，浏览器访问 `localhost:8080` 即可。
+
+```python
+"""
+python3 bridge.py :8080 202.38.93.111:0
+"""
+
+import sys
+import socket
+import select
+import logging
+
+
+def address(addr):
+    host, port = addr.split(":", 1)
+    return host, int(port)
+
+
+def main(bind, connect, log_level):
+    logging.basicConfig(level=log_level, format="%(asctime)s:%(levelname)s:%(message)s")
+    serv = socket.create_server(bind)
+    rtol = {}
+    ltor = {}
+    try:
+        while True:
+            readers, _, _ = select.select(
+                tuple(rtol.values()) + tuple(ltor.values()) + (serv,), (), ()
+            )
+            for reader in readers:
+                if reader is serv:
+                    local_client, addr = reader.accept()
+                    remote_client = socket.create_connection(connect)
+                    ltor[local_client.fileno()] = remote_client
+                    rtol[remote_client.fileno()] = local_client
+                elif reader.fileno() in ltor:
+                    data = reader.recv(2 ** 12)
+                    if data:
+                        logging.info("L -> R: %r", data)
+                        ltor[reader.fileno()].sendall(data)
+                    else:
+                        remote = ltor[reader.fileno()]
+                        del ltor[reader.fileno()], rtol[remote.fileno()]
+                        remote.close()
+                        reader.close()
+                else:
+                    data = reader.recv(2 ** 12)
+                    if data:
+                        logging.info("R -> L: %r", data)
+                        rtol[reader.fileno()].sendall(data)
+                    else:
+                        local = rtol[reader.fileno()]
+                        del rtol[reader.fileno()], ltor[local.fileno()]
+                        reader.close()
+                        local.close()
+    except Exception as e:
+        raise e
+    finally:
+        for sock in tuple(rtol.values()) + tuple(ltor.values()) + (serv,):
+            sock.close()
+
+
+if __name__ == "__main__":
+    bind = address(sys.argv[1])
+    connect = address(sys.argv[2])
+    main(bind, connect, "INFO")
+```
@@ -0,0 +1,56 @@
+# 永不溢出的计算器
+
+思路基本同官方。鉴于本题没有提供 TCP 接口，这里给出一个在浏览器里搜索不同二次剩余的脚本。
+
+将以下代码贴在浏览器控制台运行即可。如果脚本停了，最后输出的两个数即是所要的 x 和 y。
+
+此处的原理是利用页面暴露在全局的 WebSocket 对象 `socket` 直接收发数据。
+
+```javascript
+(() => {
+    function rand(a, b) {
+        return Math.round(Math.random() * (b - a) + a)
+    }
+
+    function selectLargeNum() {
+        const len = Math.round(rand(154, 307))
+        let ret = ''
+        for (let i = 0; i < len; i++) ret += rand(0, 9)
+        return ret.replace(/^0+/, '')
+    }
+
+    class Result {
+        constructor() {
+            this.reset()
+        }
+        reset() {
+            this.promise = new Promise(resolve => this.resolve = resolve)
+            this.text = ''
+        }
+    }
+
+    let result = new Result
+    socket.addEventListener('message', (msg) => {
+        result.text += msg.data
+        if (! /\r\n>\s+$/.test(result.text)) return
+        result.resolve(/(\d+)\r\n>\s+$/.exec(result.text)[1])
+    })
+
+
+    async function loop() {
+        let num = selectLargeNum()
+        result.reset()
+        socket.send(num + ' ^ 2\n')
+        let res = await result.promise
+        result.reset()
+        socket.send('sqrt(' + res + ')\n')
+        res = await result.promise
+        console.log(res)
+        console.log(num)
+        if (num !== res) return
+        setTimeout(loop, 100)
+    }
+
+    loop()
+})()
+```
@@ -0,0 +1,72 @@
+# 超基础的数理模拟器
+
+由于学过符号计算软件，本人首先想到用 Mathematica 去算积分，绕了点弯路，但还是可做的。以下给出代码。其中有几个要点：
+
+ 1. Mathematica 的 `ToExpression` 转换后的表达式是 `Integrate[...]` 也即符号积分，此时有可能算出奇怪的答案（比如复数解）。此时应该将 `Integrate` 替换为 `NIntegrate`。
+ 2. 其他一些替换，如把自然对数底 `e` 换成 `E`；最后的 `{d x}` 换成 `dx`；`\` 换成 `\\`；被积函数要用 `{}` 括起来。
+
+偶尔会有几个积分算不出来，但概率不大。半小时左右能拿到 flag。
+
+```python
+import re
+import pexpect
+import requests
+
+
+headers = {
+    "User-Agent": "Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/86.0.4240.111 Safari/537.36",
+    "Referer": "http://202.38.93.111:10190/",
+}
+
+s = requests.session()
+s.cookies.clear()
+
+address = "http://202.38.93.111:10190/"
+
+tex_pattern = re.compile(
+    r"(?P<progress>\d+)\s*题[.\s\S]+<p>\s*\$(?P<expr>.+)\$\s*</p>", re.M | re.X | re.U,
+)
+body_pattern = re.compile(r"(^[^\s]+\s)(?P<body>.*)\\")
+
+
+def process_expr(expr: str):
+    expr = expr.replace("{d x}", "dx")
+    expr = re.sub(r"\be\b", "E", expr)
+    expr = body_pattern.sub(r"\g<1>(\g<body>)\\", expr)
+    expr = expr.replace("\\", "\\\\")
+    return expr
+
+
+def get_code(expr: str):
+    return f'ToExpression["{expr}",TeXForm,Hold]/.Integrate->NIntegrate//ReleaseHold'
+
+
+def next_probem(ans=""):
+    if ans:
+        response = s.post(address + "/submit", data=dict(ans=ans))
+    else:
+        response = s.get(address)
+    print(response.text)
+    matched = tex_pattern.search(response.text)
+    expr = matched.group("expr")
+    progress = matched.group("progress")
+    expr = process_expr(expr)
+    code = get_code(expr)
+    p = pexpect.spawn("wolframscript", ["-code", code])
+    p.wait()
+    if p.exitstatus != 0:
+        return "", progress
+    try:
+        answer = "{:.6f}".format(float(p.read().decode().strip()))
+    except ValueError:
+        return "", progress
+    return answer, progress
+
+
+ans = ""
+while True:
+    ans, round = next_probem(ans)
+    print(ans, round)
+    if round == 0:
+        break
+```