Implement Flow.since and Flow.until

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

2 files changed, 21 insertions, 1 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
diff --git a/finance/test_flow.py b/finance/test_flow.py
index aad4e74..02b225c 100644
--- a/finance/test_flow.py
+++ b/finance/test_flow.py
@@ -5,13 +5,23 @@ from finance import flow
 
 
 def test_income_integration() -> None:
-    inc = flow.Flow(amount=Decimal(3000.0))
+    inc = flow.Flow(
+        amount=Decimal(3000.0),
+        since=datetime(2023, 1, 1),
+        until=datetime(2026, 1, 5),
+    )
 
     tests = (
         (datetime(2024, 3, 12), datetime(2024, 4, 2), Decimal(3000.0)),
         (datetime(2024, 3, 1), datetime(2024, 3, 15), Decimal(3000.0)),
         (datetime(2024, 2, 25), datetime(2024, 3, 1), Decimal(3000.0)),
         (datetime(2024, 2, 25), datetime(2024, 6, 12), Decimal(12000.0)),
+        (datetime(2022, 1, 5), datetime(2022, 9, 14), Decimal(0.0)),
+        (datetime(2022, 1, 5), datetime(2023, 1, 1), Decimal(3000.0)),
+        (datetime(2022, 7, 4), datetime(2024, 12, 8), Decimal(72000.0)),
+        (datetime(2025, 11, 4), datetime(2026, 5, 9), Decimal(6000.0)),
+        (datetime(2026, 1, 5), datetime(2027, 1, 1), Decimal(0.0)),
+        (datetime(2027, 5, 28), datetime(2028, 7, 7), Decimal(0.0)),
     )
 
     for test in tests: