update comments

README.md

@@ -1,9 +1,9 @@
 # UUIDv7
 
 [![CI](https://github.com/ryanwinchester/uuidv7/actions/workflows/ci.yml/badge.svg)](https://github.com/ryanwinchester/uuidv7/actions/workflows/ci.yml)
- [![Hex.pm](https://img.shields.io/hexpm/v/uuid_v7)](https://hex.pm/packages/uuid_v7)
- [![Hex.pm](https://img.shields.io/hexpm/dt/uuid_v7)](https://hex.pm/packages/uuid_v7)
- [![Hex.pm](https://img.shields.io/hexpm/l/uuid_v7)](https://github.com/ryanwinchester/uuidv7/blob/main/LICENSE)
+[![Hex.pm](https://img.shields.io/hexpm/v/uuid_v7)](https://hex.pm/packages/uuid_v7)
+[![Hex.pm](https://img.shields.io/hexpm/dt/uuid_v7)](https://hex.pm/packages/uuid_v7)
+[![Hex.pm](https://img.shields.io/hexpm/l/uuid_v7)](https://github.com/ryanwinchester/uuidv7/blob/main/LICENSE)
 
 UUIDv7 for Elixir and (optionally) Ecto, using an 18-bit randomly-seeded counter.
 
@@ -72,6 +72,15 @@ defmodule MyApp.Blog.Post do
 end
 ```
 
+To use UUIDs for everything in your migrations, it's easiest to just add that as the
+default type in your `config.exs`. e.g.:
+
+```elixir
+config :app, App.Repo,
+  migration_primary_key: [type: :binary_id],
+  migration_foreign_key: [type: :binary_id]
+```
+
 ## Benchmarks
 
 Run benchmarks with
@@ -83,6 +92,7 @@ MIX_ENV=bench mix run bench/filename.exs
 Where `filename.exs` is the name of one of the benchmark files in the `bench` directory.
 
 ### Compared to `Uniq.UUID`
+
 (which has no counter or time-leap protection. millisecond precision.)
 
 #### String:

lib/uuidv7.ex

@@ -29,6 +29,9 @@ defmodule UUIDv7 do
   """
   @type raw :: <<_::128>>
 
+  @version 7
+  @variant 2
+
   @doc """
   Generates a version 7 UUID using submilliseconds for increased clock precision.
 
@@ -51,14 +54,23 @@ defmodule UUIDv7 do
 
   """
   def bingenerate do
+    # Generate the random seed for the counter, and `rand_b` for the UUID at
+    # the same time. This minimizes the number of random bytes needed.
     <<rand_a::17, _::1, rand_b::38>> = :crypto.strong_rand_bytes(7)
 
-    {time, clock} = Clock.next(<<rand_a::17>>)
+    {time_ms, clock} = Clock.next(<<rand_a::17>>)
 
+    # Split up the counter to fit into the UUIDv7 format.
     <<clock_a::big-unsigned-12, clock_b::big-unsigned-6>> = clock
 
-    <<time::big-unsigned-48, 7::4, clock_a::big-unsigned-12, 2::2, clock_b::big-unsigned-6,
-      rand_b::56>>
+    <<
+      time_ms::big-unsigned-48,
+      @version::4,
+      clock_a::big-unsigned-12,
+      @variant::2,
+      clock_b::big-unsigned-6,
+      rand_b::56
+    >>
   end
 
   @doc """
@@ -71,12 +83,12 @@ defmodule UUIDv7 do
 
   """
   @spec extract_timestamp(t | raw) :: integer
-  def extract_timestamp(<<_::288>> = uuid) do
-    decode(uuid) |> extract_timestamp()
+  def extract_timestamp(<<timestamp_ms::big-unsigned-48, @version::4, _::76>>) do
+    timestamp_ms
   end
 
-  def extract_timestamp(<<ms::big-unsigned-48, 7::4, _::76>>) do
-    ms
+  def extract_timestamp(<<_::288>> = uuid) do
+    decode(uuid) |> extract_timestamp()
   end
 
   @doc """