Rename income to flow

Modeling income and expenses separately does not make sense since the
only difference is the sign of the amount. Separate definitions would
lead to a lot of duplicated code.

1 files changed, 33 insertions, 0 deletions

diff --git a/finance/flow.py b/finance/flow.py
new file mode 100644
index 0000000..fb28228
--- /dev/null
+++ b/finance/flow.py
@@ -0,0 +1,33 @@
+import dataclasses
+from datetime import datetime
+from decimal import Decimal
+
+
+@dataclasses.dataclass(kw_only=True, frozen=True)
+class Flow:
+    """Time-discrete flow of money paid on the first day of a month"""
+
+    amount: Decimal
+
+    def integrate(self, start: datetime, end: datetime) -> Decimal:
+        """Integrate the flow between two dates to an amount of money"""
+
+        retval = Decimal(0.0)
+
+        current = datetime(start.year, start.month, 1)
+
+        if start == current:
+            retval += self.amount
+
+        while True:
+            if current.month == 12:
+                current = datetime(current.year + 1, 1, 1)
+            else:
+                current = datetime(current.year, current.month + 1, 1)
+
+            if current <= end:
+                retval += self.amount
+            else:
+                break
+
+        return retval