feat(rln-relay): integrate get_leaf ffi api (#1790)

tests/v2/waku_rln_relay/test_waku_rln_relay.nim

@@ -147,6 +147,39 @@ suite "Waku rln relay":
     check:
       memberAdded
 
+  test "getMember Nim wrapper":
+      # create an RLN instance which also includes an empty Merkle tree
+    let rlnInstance = createRLNInstance()
+    require:
+      rlnInstance.isOk()
+    let rln = rlnInstance.get()
+    # generate an identity credential
+    let idCredentialRes = membershipKeyGen(rln)
+    require:
+      idCredentialRes.isOk()
+
+    let idCredential = idCredentialRes.get()
+    let pkBuffer = toBuffer(idCredential.idCommitment)
+    let pkBufferPtr = unsafeAddr(pkBuffer)
+
+    let
+      root1 {.noinit.}: Buffer = Buffer()
+      rootPtr1 = unsafeAddr(root1)
+      getRootSuccessful1 = getRoot(rlnInstance.get(), rootPtr1)
+
+    # add the member to the tree
+    let memberAdded = updateNextMember(rln, pkBufferPtr)
+    require:
+      memberAdded
+
+    let leafRes = getMember(rln, 0)
+    require:
+      leafRes.isOk()
+    let leaf = leafRes.get()
+    let leafHex = leaf.inHex()
+    check:
+      leafHex == idCredential.idCommitment.inHex()
+
   test "delete_member Nim wrapper":
     # create an RLN instance which also includes an empty Merkle tree
     let rlnInstance = createRLNInstance()

waku/v2/waku_rln_relay/constants.nim

@@ -1,5 +1,4 @@
 import
-  std/json,
   stint
 
 import

waku/v2/waku_rln_relay/rln/rln_interface.nim

@@ -54,6 +54,11 @@ proc set_leaf*(ctx: ptr RLN, index: uint, input_buffer: ptr Buffer): bool {.impo
 ## the input_buffer holds a serialized leaf of 32 bytes
 ## the return bool value indicates the success or failure of the operation
 
+proc get_leaf*(ctx: ptr RLN, index: uint, output_buffer: ptr Buffer): bool {.importc: "get_leaf".}
+## gets the leaf at position index in the tree stored by ctx
+## the output_buffer holds a serialized leaf of 32 bytes
+## the return bool value indicates the success or failure of the operation
+
 proc init_tree_with_leaves*(ctx: ptr RLN, input_buffer: ptr Buffer): bool {.importc: "init_tree_with_leaves".}
 ## sets multiple leaves in the tree stored by ctx to the value passed by input_buffer
 ## the input_buffer holds a serialized vector of leaves (32 bytes each)

waku/v2/waku_rln_relay/rln/wrappers.nim

@@ -271,6 +271,25 @@ proc insertMember*(rlnInstance: ptr RLN, idComm: IDCommitment): bool =
   let memberAdded = update_next_member(rlnInstance, pkBufferPtr)
   return memberAdded
 
+proc getMember*(rlnInstance: ptr RLN, index: MembershipIndex): RlnRelayResult[IDCommitment] =
+  ## returns the member at the given index
+  ## returns an error if the index is out of bounds
+  ## returns the member if the index is valid
+  var
+    idCommitment {.noinit.}: Buffer = Buffer()
+    idCommitmentPtr = addr(idCommitment)
+    memberRetrieved = get_leaf(rlnInstance, index, idCommitmentPtr)
+
+  if not memberRetrieved:
+    return err("could not get the member")
+
+  if not idCommitment.len == 32:
+    return err("wrong output size")
+
+  let idCommitmentValue = (cast[ptr array[32, byte]](idCommitment.`ptr`))[]
+
+  return ok(@idCommitmentValue)
+
 proc atomicWrite*(rlnInstance: ptr RLN, 
                   index = none(MembershipIndex), 
                   idComms = newSeq[IDCommitment](),