Add makePathBuilder to make path builders

packages/faucet/src/pathbuilder.spec.ts

@@ -0,0 +1,64 @@
+import { Slip10RawIndex } from "@cosmjs/crypto";
+import { makeCosmoshubPath } from "@cosmjs/proto-signing";
+
+import { makePathBuilder, PathBuilder } from "./pathbuilder";
+
+describe("pathbuilder", () => {
+  describe("PathBuilder", () => {
+    it("is compatible to makeCosmoshubPath", () => {
+      const _builder: PathBuilder = makeCosmoshubPath;
+    });
+  });
+
+  describe("makePathBuilder", () => {
+    it("works", () => {
+      const hub = makePathBuilder("m/44'/118'/0'/0/a");
+      expect(hub(0)).toEqual([
+        Slip10RawIndex.hardened(44),
+        Slip10RawIndex.hardened(118),
+        Slip10RawIndex.hardened(0),
+        Slip10RawIndex.normal(0),
+        Slip10RawIndex.normal(0),
+      ]);
+      expect(hub(25)).toEqual([
+        Slip10RawIndex.hardened(44),
+        Slip10RawIndex.hardened(118),
+        Slip10RawIndex.hardened(0),
+        Slip10RawIndex.normal(0),
+        Slip10RawIndex.normal(25),
+      ]);
+
+      const stellar = makePathBuilder("m/44'/148'/a'");
+      expect(stellar(0)).toEqual([
+        Slip10RawIndex.hardened(44),
+        Slip10RawIndex.hardened(148),
+        Slip10RawIndex.hardened(0),
+      ]);
+      expect(stellar(42)).toEqual([
+        Slip10RawIndex.hardened(44),
+        Slip10RawIndex.hardened(148),
+        Slip10RawIndex.hardened(42),
+      ]);
+    });
+
+    it("throws for invalid patterns", () => {
+      // No `a`
+      expect(() => makePathBuilder("m/44'/118'/0'/0/30")).toThrowError(
+        /Missing account index variable `a` in pattern./i,
+      );
+
+      // Two `a`
+      expect(() => makePathBuilder("m/44'/118'/a'/0/a")).toThrowError(
+        /More than one account index variable `a` in pattern/i,
+      );
+
+      // Stray character
+      expect(() => makePathBuilder("m/44'?/118'/0'/0/a")).toThrowError(
+        /Syntax error while reading path component/i,
+      );
+
+      // Missing m/
+      expect(() => makePathBuilder("44'/118'/0'/0/a")).toThrowError(/Path string must start with 'm'/i);
+    });
+  });
+});

packages/faucet/src/pathbuilder.ts

@@ -0,0 +1,26 @@
+import { HdPath, stringToPath } from "@cosmjs/crypto";
+
+export type PathBuilder = (account_index: number) => HdPath;
+
+/**
+ * Insert a BIP32 path that contains a valiable `a` for the numeric account index.
+ * This variable will be replaces when the path builder is used.
+ *
+ * @param pattern, e.g. m/44'/148'/a' for Stellar paths
+ */
+export function makePathBuilder(pattern: string): PathBuilder {
+  if (pattern.indexOf("a") === -1) throw new Error("Missing account index variable `a` in pattern.");
+  if (pattern.indexOf("a") !== pattern.lastIndexOf("a")) {
+    throw new Error("More than one account index variable `a` in pattern.");
+  }
+
+  const builder: PathBuilder = function (a: number): HdPath {
+    const path = pattern.replace("a", a.toString());
+    return stringToPath(path);
+  };
+
+  // test builder
+  const _path = builder(0);
+
+  return builder;
+}