Markov: when looking up the start-of-sentence chain, get one random one

when finding a key for (__start1,__start2), instead of fetcihng all
(which can be a lot, in chatty channels and/or over time), get the
max ID in the table, pick a random ID between 1,max, and pick the
first id >= to it, and use that. just as random, nowhere near as
intensive.

modules/Markov.py

@@ -356,7 +356,15 @@ class Markov(Module):
         values = []
         try:
             db = self.get_db()
-            query = 'SELECT v FROM markov_chain WHERE k1 = ? AND k2 = ?'
+            query = ''
+            if k1 == self.start1 and k2 == self.start2:
+                # hack. get a quasi-random start from the database, in
+                # a faster fashion than selecting all starts
+                max_id = self._get_max_chain_id()
+                rand_id = random.randint(1,max_id)
+                query = 'SELECT v FROM markov_chain WHERE k1 = ? AND k2 = ? AND id >= {0:d} LIMIT 1'.format(rand_id)
+            else:
+                query = 'SELECT v FROM markov_chain WHERE k1 = ? AND k2 = ?'
             cursor = db.execute(query, (k1,k2))
             results = cursor.fetchall()
 
@@ -387,5 +395,23 @@ class Markov(Module):
             print('sqlite error: ' + str(e))
             raise
 
+    def _get_max_chain_id(self):
+        """Get the highest id in the chain table."""
+
+        try:
+            db = self.get_db()
+            query = '''
+                SELECT id FROM markov_chain ORDER BY id DESC LIMIT 1
+                '''
+            cursor = db.execute(query)
+            result = cursor.fetchone()
+            if result:
+                return result['id']
+            else:
+                return None
+        except sqlite3.Error as e:
+            print('sqlite error: ' + str(e))
+            raise
+
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