improvement: use a guaranteed-last ulid for balance_as_of calculation

lib/account/calculations/balance_as_of.ex

@@ -12,6 +12,6 @@ defmodule AshDoubleEntry.Account.Calculations.BalanceAsOf do
 
   @doc false
   def ulid(timestamp) do
-    AshDoubleEntry.ULID.generate(timestamp)
+    AshDoubleEntry.ULID.generate_last(timestamp)
   end
 end

lib/ulid.ex

@@ -77,6 +77,31 @@ defmodule AshDoubleEntry.ULID do
     ulid
   end
 
+  @doc """
+  Generates a Crockford Base32 encoded ULID, guaranteed to sort equal to or after any other ULID generated for the same timestamp.
+
+  Do not use this for storage, only for generating comparators, i.e "balance as of a given ulid".
+
+  If a value is provided for `timestamp`, the generated ULID will be for the provided timestamp.
+  Otherwise, a ULID will be generated for the current time.
+
+  Arguments:
+
+  * `timestamp`: A Unix timestamp with millisecond precision.
+  """
+  def generate_last(timestamp \\ System.system_time(:millisecond))
+
+  def generate_last(%DateTime{} = datetime) do
+    datetime
+    |> DateTime.to_unix(:millisecond)
+    |> generate_last()
+  end
+
+  def generate_last(timestamp) do
+    {:ok, ulid} = encode(bingenerate_last(timestamp))
+    ulid
+  end
+
   @doc """
   Generates a binary ULID.
 
@@ -91,6 +116,22 @@ defmodule AshDoubleEntry.ULID do
     <<timestamp::unsigned-size(48), :crypto.strong_rand_bytes(10)::binary>>
   end
 
+  @doc """
+  Generates a binary ULID.
+
+  Do not use this for storage, only for generating comparators, i.e "balance as of a given ulid".
+
+  If a value is provided for `timestamp`, the generated ULID will be for the provided timestamp.
+  Otherwise, a ULID will be generated for the current time.
+
+  Arguments:
+
+  * `timestamp`: A Unix timestamp with millisecond precision.
+  """
+  def bingenerate_last(timestamp \\ System.system_time(:millisecond)) do
+    <<timestamp::unsigned-size(48), 255, 255, 255, 255, 255, 255, 255, 255, 255, 255>>
+  end
+
   @doc false
   def encode(
         <<b1::3, b2::5, b3::5, b4::5, b5::5, b6::5, b7::5, b8::5, b9::5, b10::5, b11::5, b12::5,