A crude but servicable growth cycle.

Bio grows, populations consume bio and grow or consume too much and
crash.  THat could set up the first main game dynamic or challenge:
don't let your people eat everything and die.

How to draw off population?  Send them to the stars of course!

We can worry about controlling the growth rate later (either directly
under user control or affected indirectly though factors the user can
control? Something else?)

Remember that by Insect Law populations can only reproduce on their home
worlds!  That constraint isn't in the DB nor the "server" code yet
though.

server.py

@@ -17,3 +17,79 @@
 #    You should have received a copy of the GNU General Public License
 #    along with game.  If not see <http://www.gnu.org/licenses/>.
 #
+import data
+
+
+def turn():
+    conn = data.open_db()
+    cursor = conn.cursor()
+    life_grows(cursor)
+    population_grows(cursor)
+    industry_produces()
+    vessels_move()
+    conn.commit()
+    data.close_db()
+
+
+BIO_GROWTH = 0.03
+POP_GROWTH = 0.0125
+
+def life_grows(db_cursor):
+    print('life_grows')
+    db_cursor.execute('''
+        select id, bio, bio_capacity from planets
+        where bio > 0 and bio < bio_capacity
+        ''')
+    planets = list(db_cursor.fetchall())
+    # Get all the results before updating some in this loop.
+    for planet_id, bio, bio_capacity in planets:
+
+        # We add at least one unit of population per turn.
+        new_bio = round(bio * BIO_GROWTH) or 1
+
+        available_capacity = bio_capacity - bio
+
+        if new_bio > available_capacity:
+            excess_bio = new_bio - available_capacity
+            new_bio = available_capacity
+
+        else:
+            excess_bio = 0
+
+        bio = bio + new_bio
+        assert bio <= bio_capacity
+        print(f'setting planet {planet_id} bio to {bio} increment of {new_bio}, excess_bio {excess_bio}')
+        db_cursor.execute('update planets set bio = ? where id = ?', (bio, planet_id))
+
+
+def population_grows(db_cursor):
+    print('population_grows')
+    db_cursor.execute('''
+        select populations.id, populations.pop, planets.id, planets.bio
+        from populations join planets
+        on populations.planet = planets.id
+        ''')
+    pops = list(db_cursor.fetchall())
+    for pop_id, pop, planet_id, bio in pops:
+        if pop > bio:
+            # famine and death!  Woe!
+            pop = bio >> 1
+            bio //= 10
+        else:
+            bio -= pop  # Everyone eats.
+            new_pop = round(pop * POP_GROWTH)
+            pop += new_pop
+        print(f'population {pop_id} of planet {planet_id} is now {pop}, bio is {bio}')
+        db_cursor.execute('update planets set bio = ? where id = ?', (bio, planet_id))
+        db_cursor.execute('update populations set pop = ? where id = ?', (pop, pop_id))
+
+
+def industry_produces():
+    print('industry_produces')
+
+def vessels_move():
+    print('vessels_move')
+
+
+if __name__ == '__main__':
+    turn()