Add support for filtering todos.

At this point there are two filters:

* A grep filter that finds a pattern in a todo text
* A relevance filter, which selects todos that are relevant at this
  moment.

Filter.py

+class Filter(object):
+    # def __init__(self):
+    #     pass
+
+    def filter(self, p_todos, p_limit=None):
+        """
+        Filters a list of todos. Truncates the list after p_limit todo
+        items (or no maximum limit if omitted).
+        """
+
+        result = [t for t in p_todos if self.match(t)]
+        return result[:p_limit]
+
+    def match(self, p_todo):
+        """ Default match value. """
+        return True
+
+class GrepFilter(Filter):
+    """ Matches when the todo text contains a text. """
+
+    def __init__(self, p_expression, p_case_sensitive=None):
+        self.expression = p_expression
+
+        if p_case_sensitive:
+            self.case_sensitive = p_case_sensitive
+        else:
+            # only be case sensitive when the expression contains at least one
+            # capital character.
+            self.case_sensitive = \
+                len([c for c in p_expression if c.isupper()]) > 0
+
+    def match(self, p_todo):
+        expr = self.expression
+        string = p_todo.source()
+        if not self.case_sensitive:
+            expr = expr.lower()
+            string = string.lower()
+
+        return string.find(expr) != -1
+
+class RelevanceFilter(Filter):
+    """
+    Matches when the todo is relevant, i.e.:
+
+    The item has not been completed AND
+    The start date is blank, today or in the past, AND
+    The priority is 'A' or the priority is B with due date within 30 days or
+    the priority is C with due date within 14 days.
+    """
+
+    def match(self, p_todo):
+        is_due = p_todo.is_active()
+        is_due |= p_todo.due_date() == None
+        is_due |= p_todo.priority() == 'A'
+        is_due |= p_todo.priority() == 'B' and p_todo.days_till_due() <= 30
+        is_due |= p_todo.priority() == 'C' and p_todo.days_till_due() <= 14
+
+        return p_todo.is_active() and is_due

test/FilterTest.py

+""" Tests for the filter functionality. """
+
+import pdb
+import datetime
+import unittest
+
+import Filter
+from TestFacilities import load_file, todolist_to_string
+import Todo
+
+class FilterTest(unittest.TestCase):
+    def setUp(self):
+        # self.today = datetime.date.today()
+        pass
+
+    def test_filter1(self):
+        todo = Todo.Todo("(C) Relevant")
+        relevance = Filter.RelevanceFilter()
+        result = relevance.filter([todo], 0)
+
+        self.assertEquals(result, [])
+
+    def test_filter2(self):
+        todo = Todo.Todo("(C) Relevant")
+        relevance = Filter.RelevanceFilter()
+        result = relevance.filter([todo], 100)
+
+        self.assertEquals(result, [todo])
+
+    def test_filter3(self):
+        todo = Todo.Todo("(C) Relevant")
+        relevance = Filter.RelevanceFilter()
+        result = relevance.filter([todo])
+
+        self.assertEquals(result, [todo])
+
+    def test_filter4(self):
+        """ Test case insensitive match. """
+        todos = load_file('data/FilterTest1.txt')
+        grep = Filter.GrepFilter('+project')
+
+        filtered_todos = grep.filter(todos)
+        reference = load_file('data/FilterTest1a-result.txt')
+
+        self.assertEquals(todolist_to_string(filtered_todos), \
+            todolist_to_string(reference))
+
+    def test_filter5(self):
+        """ Test case sensitive match. """
+        todos = load_file('data/FilterTest1.txt')
+        grep = Filter.GrepFilter('+Project')
+
+        filtered_todos = grep.filter(todos)
+        reference = load_file('data/FilterTest1b-result.txt')
+
+        self.assertEquals(todolist_to_string(filtered_todos), \
+            todolist_to_string(reference))
+
+    def test_filter6(self):
+        """ Test case sensitive match (forced, with lowercase). """
+        todos = load_file('data/FilterTest1.txt')
+        grep = Filter.GrepFilter('+project', True)
+
+        filtered_todos = grep.filter(todos)
+        reference = load_file('data/FilterTest1c-result.txt')
+
+        self.assertEquals(todolist_to_string(filtered_todos), \
+            todolist_to_string(reference))

test/data/FilterTest1.txt

+(C) This is part of some +Project
+(D) Just an arbitrary task
+(C) Another mildly important task for the +Project
+(C) And here's one with a lowercase +project

test/data/FilterTest1a-result.txt

+(C) This is part of some +Project
+(C) Another mildly important task for the +Project
+(C) And here's one with a lowercase +project

test/data/FilterTest1b-result.txt

+(C) This is part of some +Project
+(C) Another mildly important task for the +Project

test/data/FilterTest1c-result.txt

+(C) And here's one with a lowercase +project

test/run.sh

@@ -5,7 +5,7 @@ PYTHONPATH=..
 if [ -n "$1" ]; then
   TESTS=$1
 else
-  TESTS="SorterTest TodoBaseTest TodoFileTest TodoTest TodoListTest"
+  TESTS="FilterTest SorterTest TodoBaseTest TodoFileTest TodoTest TodoListTest"
 fi
 
 for TEST in $TESTS; do