1 files changed, 59 insertions, 0 deletions
diff --git a/finance/simulate.py b/finance/simulate.py
new file mode 100644
index 0000000..3f8e3ad
--- /dev/null
+++ b/finance/simulate.py
@@ -0,0 +1,59 @@
+from datetime import datetime
+from decimal import Decimal
+from typing import Generator
+
+from finance.model import Flow
+
+
+def integrate(flow: Flow, start: datetime, end: datetime) -> Decimal:
+    """Integrate the flow between two dates to an amount of money"""
+
+    payments: int = 0
+
+    if flow.since is not None:
+        if start < flow.since:
+            start = flow.since
+
+    if flow.until is not None:
+        if end > flow.until:
+            end = flow.until
+
+    for candidate in monthly_candidates(start):
+        if start <= candidate <= end:
+            payments += 1
+        if candidate > end:
+            break
+
+    return flow.amount * Decimal(payments)
+
+
+def monthly_candidates(start: datetime) -> Generator[datetime, None, None]:
+    current = datetime(start.year, start.month, 1)
+    while True:
+        yield current
+        if current.month == 12:
+            current = datetime(current.year + 1, 1, 1)
+        else:
+            current = datetime(current.year, current.month + 1, 1)
+
+
+def simulate(
+    start: datetime, end: datetime, flows: tuple[Flow, ...]
+) -> list[tuple[datetime, Decimal]]:
+    dates: list[datetime] = []
+    values: list[Decimal] = []
+
+    for candidate in monthly_candidates(start):
+        if start <= candidate <= end:
+            dates.append(candidate)
+
+        if candidate > end:
+            break
+
+    for date in dates:
+        value = Decimal(0.0)
+        for flow in flows:
+            value += integrate(flow, start, date)
+        values.append(value)
+
+    return [(date, values[index]) for index, date in enumerate(dates)]