33 lines
1.1 KiB
TypeScript
33 lines
1.1 KiB
TypeScript
/** Build a valid minimal JVM class file, optionally with an unused UTF-8 constant. */
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export const javaClassFile = (
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extraUtf8Bytes = 0,
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fatCpuLookalike = false,
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): Buffer => {
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const utf8 = (value: string): Buffer => {
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const bytes = Buffer.from(value, "utf8");
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const entry = Buffer.alloc(3 + bytes.length);
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entry[0] = 1;
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entry.writeUInt16BE(bytes.length, 1);
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bytes.copy(entry, 3);
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return entry;
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};
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const constants = [
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Buffer.from([7, 0, 2]),
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utf8("Hello"),
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Buffer.from([7, 0, 4]),
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utf8("java/lang/Object"),
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...(fatCpuLookalike ? [utf8("x"), Buffer.from([3, 0, 0, 0, 7])] : []),
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...(extraUtf8Bytes === 0 ? [] : [utf8("x".repeat(extraUtf8Bytes))]),
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];
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const headerAndPool = Buffer.alloc(10);
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headerAndPool.writeUInt32BE(0xcafebabe, 0);
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headerAndPool.writeUInt16BE(0, 4);
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headerAndPool.writeUInt16BE(65, 6);
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headerAndPool.writeUInt16BE(constants.length + 1, 8);
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const classBody = Buffer.alloc(14);
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[0x21, 1, 3, 0, 0, 0, 0].forEach((value, index) =>
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classBody.writeUInt16BE(value, index * 2),
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);
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return Buffer.concat([headerAndPool, ...constants, classBody]);
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};
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