Part 1: Implement database on top of SQLite connection.

We would prefer to talk about a knowledge base on top of a database, but
all the Datomic and DataScript code (and symbols, like :db/add, etc)
refer to the "database of datoms", so let's roll with that nomenclature
and try to be specific that the persistent storage-layer is SQLite.
This will become more clear when we actually use SQLite's unique
capabilities for text indexing.

src/datomish/db.cljc

@@ -0,0 +1,38 @@
+;; This Source Code Form is subject to the terms of the Mozilla Public
+;; License, v. 2.0. If a copy of the MPL was not distributed with this
+;; file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+(ns datomish.db
+  #?(:cljs
+     (:require-macros
+      [datomish.pair-chan :refer [go-pair <?]]
+      [cljs.core.async.macros :refer [go]]))
+  #?(:clj
+     (:require
+      [datomish.sqlite :as s]
+      [datomish.sqlite-schema :as sqlite-schema]
+      [datomish.pair-chan :refer [go-pair <?]]
+      [clojure.core.async :refer [go <! >!]])
+     :cljs
+     (:require
+      [datomish.sqlite :as s]
+      [datomish.sqlite-schema :as sqlite-schema]
+      [datomish.pair-chan]
+      [cljs.core.async :as a :refer [<! >!]])))
+
+(defprotocol IDB
+  (close
+    [db]
+    "Close this database. Returns a pair channel of [nil error]."))
+
+(defrecord DB [sqlite-connection]
+  IDB
+  (close [db] (close (.-sqlite-connection db))))
+
+(declare ensure-current-version)
+
+(defn with-sqlite-connection [sqlite-connection]
+  (go-pair
+    (when-not (= sqlite-schema/current-version (<? (sqlite-schema/ensure-current-version sqlite-connection)))
+      (throw (Exception. "badness ensued"))) ;; TODO: raise
+    (->DB [sqlite-connection])))

src/datomish/sqlite.cljc

@@ -68,3 +68,11 @@
             [nil e])))
       (catch #?(:clj Exception :cljs js/Error) e
         [nil e]))))
+
+(defn get-user-version [db]
+  (go-pair
+    (let [row (first (<? (all-rows db ["PRAGMA user_version"])))]
+      (:user_version row))))
+
+(defn set-user-version [db version]
+  (execute! db [(str "PRAGMA user_version = " version)]))

src/datomish/sqlite_schema.cljc

@@ -0,0 +1,61 @@
+;; This Source Code Form is subject to the terms of the Mozilla Public
+;; License, v. 2.0. If a copy of the MPL was not distributed with this
+;; file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+(ns datomish.sqlite-schema
+  #?(:cljs
+     (:require-macros
+      [datomish.pair-chan :refer [go-pair <?]]
+      [cljs.core.async.macros :refer [go]]))
+  #?(:clj
+     (:require
+      [datomish.sqlite :as s]
+      [datomish.pair-chan :refer [go-pair <?]]
+      [clojure.core.async :refer [go <! >!]])
+     :cljs
+     (:require
+      [datomish.sqlite :as s]
+      [datomish.pair-chan]
+      [cljs.core.async :as a :refer [<! >!]])))
+
+(def current-version 1)
+
+(def v1-statements
+  ["CREATE TABLE datoms (e INTEGER NOT NULL, a TINYINT NOT NULL, v BLOB NOT NULL, tx INTEGER NOT NULL, index_avet TINYINT NOT NULL DEFAULT 0, index_vaet TINYINT NOT NULL DEFAULT 0)"
+   "CREATE TABLE transactions (e INTEGER NOT NULL, a TINYINT NOT NULL, v BLOB NOT NULL, tx INTEGER NOT NULL, added TINYINT NOT NULL DEFAULT 1)"
+   "CREATE INDEX eavt ON datoms (e, a)" ;; No v -- that's an opt-in index.
+   "CREATE INDEX aevt ON datoms (a, e)" ;; No v -- that's an opt-in index.
+   "CREATE INDEX avet ON datoms (a, v, e) WHERE index_avet = 1" ;; Opt-in index: only if a has :db/index true.
+   "CREATE INDEX vaet ON datoms (v, a, e) WHERE index_vaet = 1" ;; Opt-in index: only if a has :db/valueType :db.type/ref
+   "CREATE INDEX tx ON transactions (tx)"])
+
+(defn create-current-version
+  [db]
+  (go-pair
+    (doseq [statement v1-statements]
+      (<? (s/execute! db [statement])))
+    (<? (s/set-user-version db current-version))
+    (<? (s/get-user-version db))))
+
+(defn update-from-version
+  [db from-version]
+  {:pre [(> from-version 0)]} ;; Or we'd create-current-version instead.
+  {:pre [(< from-version current-version)]} ;; Or we wouldn't need to update-from-version.
+  (go-pair
+    (throw (Exception. "No possible migrations!")) ;; TODO: raise.
+    (<? (s/set-user-version db current-version))
+    (<? (s/get-user-version db))))
+
+(defn ensure-current-version
+  [db]
+  (go-pair
+    (let [v (<? (s/get-user-version db))]
+      (cond
+        (= v current-version)
+        v
+
+        (= v 0)
+        (<? (create-current-version db))
+
+        (< v current-version)
+        (<? (update-from-version db v))))))