Add allocate_index helper function

src/lib/mina_base/sparse_ledger_base.ml

@@ -152,6 +152,7 @@ M.
   , get_exn
   , path_exn
   , set_exn
+  , allocate_index
   , find_index
   , find_index_exn
   , add_path

src/lib/mina_base/sparse_ledger_base.mli

@@ -42,6 +42,8 @@ val set_exn : t -> int -> Account.t -> t
 val path_exn :
   t -> int -> [ `Left of Ledger_hash.t | `Right of Ledger_hash.t ] list
 
+val allocate_index : t -> Account_id.t -> int * t
+
 val find_index : t -> Account_id.t -> int option
 
 val find_index_exn : t -> Account_id.t -> int

src/lib/sparse_ledger_lib/sparse_ledger.ml

@@ -65,6 +65,8 @@ module type S = sig
 
   val of_hash : depth:int -> current_location:int option -> hash -> t
 
+  val allocate_index : t -> account_id -> int * t
+
   val get_exn : t -> int -> account
 
   val path_exn : t -> int -> [ `Left of hash | `Right of hash ] list
@@ -209,6 +211,16 @@ end = struct
           (List.map t.indexes ~f:fst)
           ()
 
+  let allocate_index ({ T.current_location; indexes; _ } as t) account_id =
+    let new_location =
+      match current_location with None -> 0 | Some x -> x + 1
+    in
+    ( new_location
+    , { t with
+        current_location = Some new_location
+      ; indexes = (account_id, new_location) :: indexes
+      } )
+
   let get_exn ({ T.tree; depth; _ } as t) idx =
     let rec go i tree =
       match (i < 0, tree) with