# HG changeset patch
# User Paul Boddie <paul@boddie.org.uk>
# Date 1512423342 -3600
# Node ID 94338c98305bf4bd35566f49505d9015dfa34889
# Parent  8e216b3055d7559113b696ac3cb16406173c4363
Prevent month-level and monthday updates from introducing bad datetime values.

diff -r 8e216b3055d7 -r 94338c98305b vRecurrence.py
--- a/vRecurrence.py	Sun Dec 03 22:56:53 2017 +0100
+++ b/vRecurrence.py	Mon Dec 04 22:35:42 2017 +0100
@@ -681,19 +681,47 @@
     days = (date(*last_day) - weekday0).days
     return days / 7 + 1
 
-def precision(t, level, value=None):
+def constrain_to_month(t):
+
+    "Return 't' corrected to ensure that the day number is within the month."
+
+    year, month, day = t[:3]
+    time = t[3:]
+    max_day = monthrange(year, month)[1]
+    day = min(max_day, day)
+    return tuple((year, month) + (day,) + time)
+
+def within_month(t):
+
+    """
+    Return whether 't' is within the permissible day range of the given month,
+    if 't' describes a day or datetime. Otherwise, return a true value for years
+    and months.
+    """
+
+    if len(t) < 3:
+        return True
+
+    year, month, day = t[:3]
+    max_day = monthrange(year, month)[1]
+    return day <= max_day
+
+def limit_value(t, level):
+
+    "Return 't' trimmed in resolution to the indicated resolution 'level'."
+
+    pos = positions[level]
+    return t[:pos + 1]
+
+def make_value(t, level, value):
 
     """
     Return 't' trimmed in resolution to the indicated resolution 'level',
-    setting 'value' at the given resolution if indicated.
+    extended by setting 'value' at the given resolution.
     """
 
     pos = positions[level]
-
-    if value is None:
-        return t[:pos + 1]
-    else:
-        return t[:pos] + (value,)
+    return t[:pos] + (value,)
 
 def scale(interval, level):
 
@@ -733,18 +761,17 @@
     # Dates and datetimes.
 
     else:
-        updated_for_months = update(t, step[:2])
+        updated_for_months = constrain_to_month(update(t, step[:2]))
+        d = datetime(*updated_for_months)
 
         # Dates only.
 
         if i == 3:
-            d = datetime(*updated_for_months)
             s = timedelta(step[2])
 
         # Datetimes.
 
         else:
-            d = datetime(*updated_for_months)
             s = timedelta(step[2], get_seconds(step))
 
         return to_tuple(d + s)[:len(t)]
@@ -958,13 +985,13 @@
         # qualifier in the selection chain.
 
         if self.first:
-            current = precision(context, self.level)
+            current = limit_value(context, self.level)
 
         # Otherwise, obtain the first value at this resolution within the
         # context period.
 
         else:
-            current = precision(context, self.level, firstvalues[self.level])
+            current = make_value(context, self.level, firstvalues[self.level])
 
         return PatternIterator(self, current, start, end, inclusive,
                                self.step, self.unit_step)
@@ -1307,7 +1334,12 @@
 
         "Return the period indicated by the current value."
 
-        return precision(self.base, self.selector.level, self.value)
+        t = make_value(self.base, self.selector.level, self.value)
+
+        if not within_month(t):
+            raise StopIteration
+
+        return t
 
     def next(self):
 
@@ -1320,15 +1352,15 @@
             if not self.selector.selecting or self.value is None:
                 self.value = self.values.next()
 
-            # Select a period for each value at the current resolution.
-
-            self.current = self.get_selected_period()
-            next = update(self.current, self.step)
+            try:
+                # Select a period for each value at the current resolution.
 
-            # To support setpos, only current and next bound the search, not
-            # the period in addition.
+                self.current = self.get_selected_period()
+                next = update(self.current, self.step)
 
-            try:
+                # To support setpos, only current and next bound the search, not
+                # the period in addition.
+
                 return self.next_item(self.current, next)
 
             # Move on to the next instance.
@@ -1350,7 +1382,7 @@
         value, index = self.value
         offset = timedelta(7 * (index - 1))
         weekday0 = get_first_day(self.base, value)
-        return precision(to_tuple(weekday0 + offset), DAYS)
+        return limit_value(to_tuple(weekday0 + offset), DAYS)
 
 class YearDayFilterIterator(EnumIterator):
 
@@ -1361,7 +1393,7 @@
         "Return the day indicated by the current day value."
 
         offset = timedelta(self.value - 1)
-        return precision(to_tuple(date(*self.base) + offset), DAYS)
+        return limit_value(to_tuple(date(*self.base) + offset), DAYS)
 
 class LimitIterator(SelectorIterator):