ulid can take a :time and a :random parameter, optionally

time allows you to fix the time. random allows you to supply a random
function. Primarily useful for testing.

README.md

@@ -18,6 +18,11 @@ one public function, ulid. It returns 3 values
 - a bytes vector of the binary form
 - the binary struct
 
+`ulid` has two optional arguments
+
+`:time` - specify your own timestamp, milliseconds since epoch
+`:random` - specify your own RNG , e.g. `(lambda (x) (random x))`
+
 ```
 CL-USER> (cms-ulid:ulid)
 "01GHGA3GJ65CHWRVQTC8C7AD4P"

ulid.lisp

@@ -39,19 +39,25 @@
            (serapeum:batches bits 5)))))
 
 (let ((last (make-ulid :time 0 :random 0)))
-  (defun ulid ()
+  (defun ulid (&key time  (random #'crypto:strong-random))
     "ulid makes a new ulid
 
      returned as a 3 values tuple
      - string
      - bytes sequence
-     - the binary struct"
-    (let ((now (unix-ts-ms))
+     - the binary struct
+
+     args:
+     :time - specify a timestamp to use
+     :random - a random function that takes an int n and returns  int > 0 and < n
+               (defaults to #'crypto:strong-random"
+    (let ((now (or time (unix-ts-ms)))
           (u last))
       (if (equal now (slot-value last 'time))
           (incf (slot-value u 'random))
           (progn
-            (setf u (make-ulid :time now :random (crypto:strong-random +80-bit-maxint+)))
+            (format t "~a~%" random)
+            (setf u (make-ulid :time now :random (funcall random +80-bit-maxint+)))
             (setf last u)))
       (let ((bytes (ulid->bytes-seq u)))
         (values (bytes->base32 bytes) bytes u)))))