refactor(proof)!: set tree depth = 1 when a group has 1 member (#831)

re #812

packages/proof/src/generate-proof.ts

@@ -22,6 +22,7 @@ import type { SemaphoreProof } from "./types"
  * If it is not defined, it will be inferred from the group or Merkle proof passed as the second parameter.
  * Finally, the artifacts themselves can be passed manually with file paths,
  * or they will be automatically fetched.
+ * Please keep in mind that groups with 1 member or 2 members cannot be considered anonymous.
  * @param identity The Semaphore identity.
  * @param groupOrMerkleProof The Semaphore group or its Merkle proof.
  * @param message The Semaphore message.
@@ -79,7 +80,7 @@ export default async function generateProof(
             throw new TypeError(`The tree depth must be a number between ${MIN_DEPTH} and ${MAX_DEPTH}`)
         }
     } else {
-        merkleTreeDepth = merkleProofLength
+        merkleTreeDepth = merkleProofLength !== 0 ? merkleProofLength : 1
     }
 
     // If the Snark artifacts are not defined they will be automatically downloaded.

packages/proof/tests/index.test.ts

@@ -48,6 +48,15 @@ describe("Proof", () => {
             expect(BigInt(proof.merkleTreeRoot)).toBe(group.root)
         }, 80000)
 
+        it("Should generate a Semaphore proof for a group with 1 member", async () => {
+            const group = new Group([identity.commitment])
+
+            const proof = await generateProof(identity, group, message, scope)
+
+            expect(typeof proof).toBe("object")
+            expect(BigInt(proof.merkleTreeRoot)).toBe(group.root)
+        }, 80000)
+
         it("Should generate a Semaphore proof passing a Merkle proof instead of a group", async () => {
             const group = new Group([1n, 2n, identity.commitment])