Use a combined cache derivation by default

src/narUtils.ts

@@ -0,0 +1,51 @@
+import type { Readable, Writable } from "stream";
+
+const MAX_UINT32 = 2 ** 32 - 1;
+
+/** Write one or more NAR strings to the output. */
+export const writeNarStrings = (out: Writable, ...input: string[]) => {
+  let size = 0;
+  const bufs = input.map((str) => {
+    const buf = Buffer.from(str);
+    // TODO: Support up to MAX_SAFE_INTEGER
+    if (buf.byteLength > MAX_UINT32) {
+      throw Error(`NAR string too long: ${buf.byteLength}`);
+    }
+    size += 8 + Math.ceil(buf.byteLength / 8) * 8;
+    return buf;
+  });
+
+  const res = Buffer.alloc(size);
+  let pos = 0;
+  for (const buf of bufs) {
+    res.writeUInt32LE(buf.byteLength, pos);
+    buf.copy(res, pos + 8);
+    pos += 8 + Math.ceil(buf.byteLength / 8) * 8;
+  }
+
+  out.write(res);
+};
+
+/** Write the contents of a stream as a NAR string to the output. */
+export const writeNarStream = async (
+  out: Writable,
+  size: number,
+  input: Readable,
+) => {
+  // TODO: Support up to MAX_SAFE_INTEGER
+  if (size > MAX_UINT32) {
+    throw Error(`NAR string too long: ${size}`);
+  }
+  const sizeBuf = Buffer.alloc(8);
+  sizeBuf.writeUInt32LE(size);
+  out.write(sizeBuf);
+
+  for await (const chunk of input) {
+    out.write(chunk);
+  }
+
+  const padding = 8 - (size % 8);
+  if (padding !== 8) {
+    out.write(Buffer.alloc(padding));
+  }
+};