Adds check to verify the proper order of pitstops

Pitstops must keep the order of date and odometer. That means if
a pitstop's date is between two other pitstops' date than the
odometer values must be as well.

app/forms/checks.py

@@ -2,6 +2,31 @@ from wtforms.validators import ValidationError
 from datetime import date
 
 
+def odometer_date_check(form, field):
+    """
+    Checks that the entered date and odometer of the pitstop is conformant to the existing pitstops. That means, if a
+    pitstops date is between to other pitstops, the odometer should be as well.
+    :param form:
+    :param field:
+    :return:
+    """
+    odometer = form.odometer.data
+    date = form.date.data
+    pitstops = form.pitstops
+
+    if date < pitstops[0].date and odometer >= pitstops[0].odometer:
+        raise ValidationError('The new odometer value must be less than %i km' % pitstops[0].odometer)
+
+    for index in range(0, len(pitstops)-1):
+        if pitstops[index].date <= date < pitstops[index + 1].date:
+            if odometer <= pitstops[index].odometer or odometer >= pitstops[index+1].odometer:
+                raise ValidationError('The new odometer value must be greater than %i km and less than %i km'
+                                      % (pitstops[index].odometer,pitstops[index+1].odometer))
+
+    if date >= pitstops[-1].date and odometer <= pitstops[-1].odometer:
+        raise ValidationError('The new odometer value must be greater than %i km' % pitstops[-1].odometer)
+
+
 def date_check(form, field):
     """
     Checks that the date of the pitstop is not before the date of the latest pitstop and not after today.