Utilities to construct or decompose datomic entity ids on on-prem systems using bit arithmetic

eid_utils.clj

(ns favila.eid-utils
  "Utilities to construct or decompose entity ids on on-prem systems.

  Many of these utilities replicate datomic.api functions because the peer api
  lacks them.

  The entity id format is:

      sign-bit, reserved-bit, 20-partition-bits, 42-t-bits

  1. The sign-bit is on for tempids, off otherwise.
  2. Reserved-bit must be 0.
  3. The 20 partition bits are the t-bits of the partition eid this eid
     belongs in.
  4. The lowest 42 bits are \"t\" or \"idx\" bits. These bits are unique
     across all entity ids.")

(def ^:const ^long MASK-20
  2r000000000000000000000000000000000000000000011111111111111111111)
(def ^:const ^long MASK-42
  2r000000000000000000000111111111111111111111111111111111111111111)
(def ^:const ^long MASK-PART
  2r011111111111111111111000000000000000000000000000000000000000000)
(def ^:const ^long TX-PART-BITS
  2r000000000000000000011000000000000000000000000000000000000000000)

(defn make-eid ^long [^long partition-eid ^long t]
  (bit-or
   (bit-shift-left partition-eid 42)
   (bit-and MASK-42 t)))

(defn part ^long [^long eid]
  (bit-shift-right (bit-and eid MASK-PART) 42))

(defn tx->t ^long [^long t]
  (bit-and MASK-42 t))

(defn t->tx ^long [^long t]
  (bit-or TX-PART-BITS (bit-and MASK-42 t)))

(defn entid-at ^long [^long part-eid ^long t]
  {:pre [(== (bit-and part-eid MASK-20) part-eid)]}
  (make-eid part-eid t))

(defn partition-start ^long [^long part-eid]
  (entid-at part-eid 0))

(defn partition-end ^long [^long part-eid]
  (entid-at part-eid MASK-42))