added dijkstra algorithm

exam/ex01/dijkstra.py

@@ -0,0 +1,52 @@
+
+def prepare_dijkstra(connection):
+	cursor = connection.cursor()
+	cursor.execute('''INSERT OR IGNORE INTO dijkstra_helper(name) 
+			SELECT destination FROM links
+			''')
+		
+	cursor.execute("UPDATE dijkstra_helper SET value=1e1000")
+	connection.commit()
+
+def dijkstra_one(title, value, connection):
+	cursor = connection.cursor()
+	if(isinstance(title, tuple)):
+		# Idk why this happens.
+		title = title[0]
+	cursor.execute('''SELECT name 
+			FROM dijkstra_helper 
+			LEFT JOIN links ON links.destination=dijkstra_helper.name
+			WHERE links.source=:title 
+				AND dijkstra_helper.value>:value''', {"title": title, "value": value + 1})
+	# This is the list of nodes that have to be updated
+	result = cursor.fetchall()
+
+	cursor.execute('''UPDATE dijkstra_helper 
+			SET value=:value
+			WHERE name IN (
+				SELECT destination
+				FROM links 
+				WHERE source=:title)
+			AND dijkstra_helper.value>:value''', {"value": value + 1, "title": title})
+	connection.commit()
+	return result
+
+def recursive_dijkstra(titles, value, connection):
+	if(not titles):
+		return
+
+	for title in titles:
+		todos = dijkstra_one(title, value, connection)
+		recursive_dijkstra(todos, value + 1, connection)
+
+
+def dijkstra(title, connection):
+	cursor = connection.cursor()
+	cursor.execute("UPDATE dijkstra_helper SET value=0 WHERE name=?", (title,))
+
+	todos = dijkstra_one(title, 1, connection)
+	recursive_dijkstra(todos, 2, connection)
+
+
+
+