Add function for calculating sqlite batch size

Add default sqlite variable limit constant

tests/test_calc_batch_size.py

@@ -0,0 +1,45 @@
+# pylint: disable=redefined-outer-name
+# pylint: disable=missing-class-docstring
+# pylint: disable=too-few-public-methods
+# pylint: disable=unused-import
+import peewee
+
+import peewee_plus
+from .fixtures import fakedb
+
+
+def test_sqlite(fakedb):
+    """Test the calculation of batch sizes on SQLite"""
+
+    class TestModel(peewee.Model):
+        class Meta:
+            database = fakedb
+
+        data = peewee.IntegerField()
+
+    models = [TestModel(item) for item in range(500)]
+    assert (
+        peewee_plus.calc_batch_size(models) <= peewee_plus.SQLITE_DEFAULT_VARIABLE_LIMIT
+    )
+    assert peewee_plus.calc_batch_size(models) < len(models)
+
+    assert peewee_plus.calc_batch_size([]) == 0
+
+
+def test_non_sqlite():
+    """Test the calculation of batch sizes on non-SQLite"""
+
+    class TestModel(peewee.Model):
+        class Meta:
+            database = peewee.DatabaseProxy()
+
+        data = peewee.IntegerField()
+
+    # Three is just chosen as an arbitrary multiplier to ensure the value is larger than the
+    # sqlite variable limit
+    assert peewee_plus.calc_batch_size(
+        [
+            TestModel(item)
+            for item in range(peewee_plus.SQLITE_DEFAULT_VARIABLE_LIMIT * 3)
+        ]
+    ) == (peewee_plus.SQLITE_DEFAULT_VARIABLE_LIMIT * 3)