whatif.py: add --remove for teardown/demolish scenarios

Mirrors --add - removes AMOUNT (default: all) of a building from the
copied spec. Used this to check whether BHP's HWP is still worth
running after its price drop (still +19.8k-28.4k/day marginal,
nowhere near a teardown call) without hand-writing two spec files.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

tests/test_whatif.py

@@ -23,6 +23,18 @@ def test_apply_add_appends_new_building_with_recipe():
     assert spec["buildings"] == [{"building": "CHP", "amount": 1, "recipes": ["MG,S,CU=>IND"]}]
 
 
+def test_apply_remove_drops_building_entirely_by_default():
+    spec = {"buildings": [{"building": "HWP", "amount": 1, "recipes": ["AL,STL,HE=>BHP"]}]}
+    whatif.apply_remove(spec, "HWP")
+    assert spec["buildings"] == []
+
+
+def test_apply_remove_partial_amount():
+    spec = {"buildings": [{"building": "CHP", "amount": 3, "recipes": ["MG,S,CU=>IND"]}]}
+    whatif.apply_remove(spec, "CHP:1")
+    assert spec["buildings"] == [{"building": "CHP", "amount": 2, "recipes": ["MG,S,CU=>IND"]}]
+
+
 def test_apply_infra_adds_delta():
     spec = {"infrastructure": {"HB1": 4}}
     whatif.apply_infra(spec, "HB1:+1,HB2:+2")

tools/whatif.py

@@ -37,6 +37,20 @@ def apply_add(spec: dict, add: str):
     spec["buildings"].append({"building": ticker, "amount": amount, "recipes": recipes})
 
 
+def apply_remove(spec: dict, remove: str):
+    """BUILDING[:AMOUNT] - a teardown scenario: removes AMOUNT (default: all) of that building
+    from the plan. For "should we demolish this" questions - pass the current full base as
+    --after and the --remove result as the plain positional arg (or vice versa; whatif always
+    reports AFTER minus BEFORE, so order which one is which by what you want the sign to mean)."""
+    parts = remove.split(":", 1)
+    ticker = parts[0]
+    amount = int(parts[1]) if len(parts) == 2 else None
+    for b in spec.get("buildings", []):
+        if b["building"] == ticker:
+            b["amount"] = max(0, b["amount"] - amount) if amount else 0
+    spec["buildings"] = [b for b in spec.get("buildings", []) if b["amount"] > 0]
+
+
 def apply_infra(spec: dict, infra: str):
     """HB1:+1,HB2:+2 - deltas added to spec['infrastructure']."""
     inf = spec.setdefault("infrastructure", {})
@@ -56,6 +70,7 @@ def main():
     ap.add_argument("before", help="full-base plan spec as it stands (or as a baseline you want to compare from)")
     ap.add_argument("--after", help="full-base plan spec with the addition already in it; skips --add/--infra")
     ap.add_argument("--add", action="append", default=[], help="BUILDING:AMOUNT[:RECIPE[|RECIPE]], repeatable")
+    ap.add_argument("--remove", action="append", default=[], help="BUILDING[:AMOUNT] - teardown scenario, removes AMOUNT (default: all), repeatable")
     ap.add_argument("--infra", action="append", default=[], help="HB1:+1,HB2:+2, repeatable")
     ap.add_argument("--cx", default=config.DEFAULT_CX)
     ap.add_argument("--basis", default="uni30", choices=["real", "uni30", "vwap30", "vwap7", "ask", "bid", "mid"])
@@ -66,11 +81,13 @@ def main():
     if a.after:
         after_spec = yaml.safe_load(Path(a.after).read_text())
     else:
-        if not a.add and not a.infra:
+        if not a.add and not a.remove and not a.infra:
-            sys.exit("give --after, or at least one --add/--infra")
+            sys.exit("give --after, or at least one --add/--remove/--infra")
         after_spec = copy.deepcopy(before_spec)
         for add in a.add:
             apply_add(after_spec, add)
+        for remove in a.remove:
+            apply_remove(after_spec, remove)
         for infra in a.infra:
             apply_infra(after_spec, infra)