Implement Flow.since and Flow.until

That way a flow can be limited to a certain time frame.

1 files changed, 10 insertions, 0 deletions

diff --git a/finance/flow.py b/finance/flow.py
index a5817af..9ea72c8 100644
--- a/finance/flow.py
+++ b/finance/flow.py
@@ -9,12 +9,22 @@ class Flow:
     """Time-discrete flow of money paid on the first day of a month"""
 
     amount: Decimal
+    since: None | datetime
+    until: None | datetime
 
     def integrate(self, start: datetime, end: datetime) -> Decimal:
         """Integrate the flow between two dates to an amount of money"""
 
         payments: int = 0
 
+        if self.since is not None:
+            if start < self.since:
+                start = self.since
+
+        if self.until is not None:
+            if end > self.until:
+                end = self.until
+
         for candidate in monthly_candidates(start):
             if start <= candidate <= end:
                 payments += 1