feat: add fixedScriptToDescriptor with tests

This is useful in testing

Issue: BTC-1348

Author: OttoAllmendinger

package-lock.json

@@ -43,6 +43,7 @@
     "check-fmt": "prettier --check ."
   },
   "devDependencies": {
+    "@bitgo/utxo-lib": "^10.1.0",
     "@types/mocha": "^10.0.7",
     "@types/node": "^20.14.10",
     "mocha": "^10.6.0",

packages/wasm-miniscript/package.json

@@ -0,0 +1,109 @@
+import * as assert from "assert";
+import * as utxolib from "@bitgo/utxo-lib";
+import { Descriptor } from "../js";
+
+/** Expand a template with the given root wallet keys and chain code */
+function expand(template: string, rootWalletKeys: utxolib.bitgo.RootWalletKeys, chainCode: number) {
+  return template.replace(/\$([0-9])/g, (_, i) => {
+    const keyIndex = parseInt(i, 10);
+    if (keyIndex !== 0 && keyIndex !== 1 && keyIndex !== 2) {
+      throw new Error("Invalid key index");
+    }
+    const xpub = rootWalletKeys.triple[keyIndex].neutered().toBase58();
+    const prefix = rootWalletKeys.derivationPrefixes[keyIndex];
+    return xpub + "/" + prefix + "/" + chainCode + "/*";
+  });
+}
+
+/**
+ * Get a standard output descriptor that corresponds to the proprietary HD wallet setup
+ * used in BitGo wallets.
+ * Only supports a subset of script types.
+ */
+function getDescriptorForScriptType(
+  rootWalletKeys: utxolib.bitgo.RootWalletKeys,
+  scriptType: utxolib.bitgo.outputScripts.ScriptType2Of3,
+  scope: "internal" | "external",
+): string {
+  const chain =
+    scope === "external"
+      ? utxolib.bitgo.getExternalChainCode(scriptType)
+      : utxolib.bitgo.getInternalChainCode(scriptType);
+  switch (scriptType) {
+    case "p2sh":
+      return expand("sh(multi(2,$0,$1,$2))", rootWalletKeys, chain);
+    case "p2shP2wsh":
+      return expand("sh(wsh(multi(2,$0,$1,$2)))", rootWalletKeys, chain);
+    case "p2wsh":
+      return expand("wsh(multi(2,$0,$1,$2))", rootWalletKeys, chain);
+    default:
+      throw new Error(`Unsupported script type ${scriptType}`);
+  }
+}
+
+const rootWalletKeys = new utxolib.bitgo.RootWalletKeys(utxolib.testutil.getKeyTriple("wasm"));
+const scriptTypes = ["p2sh", "p2shP2wsh", "p2wsh"] as const;
+const scope = ["external", "internal"] as const;
+const index = [0, 1, 2];
+
+function runTest(
+  scriptType: utxolib.bitgo.outputScripts.ScriptType2Of3,
+  index: number,
+  scope: "internal" | "external",
+) {
+  describe(`scriptType=${scriptType}, index=${index}, scope=${scope}`, function () {
+    const chainCode =
+      scope === "external"
+        ? utxolib.bitgo.getExternalChainCode(scriptType)
+        : utxolib.bitgo.getInternalChainCode(scriptType);
+    const derivedKeys = rootWalletKeys.deriveForChainAndIndex(chainCode, index);
+    const scriptUtxolib = utxolib.bitgo.outputScripts.createOutputScript2of3(
+      derivedKeys.publicKeys,
+      scriptType,
+    ).scriptPubKey;
+
+    it("descriptor should have expected format", function () {
+      const descriptor = Descriptor.fromString(
+        getDescriptorForScriptType(rootWalletKeys, scriptType, scope),
+        "derivable",
+      );
+      const [x1, x2, x3] = rootWalletKeys.triple.map((xpub) => xpub.neutered().toBase58());
+      if (scriptType === "p2sh" && scope === "external") {
+        // spot check
+        assert.ok(
+          descriptor
+            .toString()
+            .startsWith(`sh(multi(2,${x1}/0/0/0/*,${x2}/0/0/0/*,${x3}/0/0/0/*))`),
+        );
+      }
+      if (scriptType === "p2shP2wsh" && scope === "internal") {
+        // spot check
+        assert.ok(
+          descriptor
+            .toString()
+            .startsWith(`sh(wsh(multi(2,${x1}/0/0/11/*,${x2}/0/0/11/*,${x3}/0/0/11/*)))`),
+        );
+      }
+    });
+
+    it("address should match descriptor", function () {
+      const scriptFromDescriptor = Buffer.from(
+        Descriptor.fromString(
+          getDescriptorForScriptType(rootWalletKeys, scriptType, scope),
+          "derivable",
+        )
+          .atDerivationIndex(index)
+          .scriptPubkey(),
+      );
+      assert.deepStrictEqual(scriptUtxolib.toString("hex"), scriptFromDescriptor.toString("hex"));
+    });
+  });
+}
+
+scriptTypes.forEach((scriptType) => {
+  index.forEach((index) => {
+    scope.forEach((scope) => {
+      runTest(scriptType, index, scope);
+    });
+  });
+});