Add machine-aware maximum product length

tests/test_roll_calculation.py

@@ -9,6 +9,7 @@ from roll_calculation import (
     _canonical_name,
     article_production_site,
     calculate_material_weight,
+    calculate_material_length_for_weight,
     calculate_product_length,
     calculate_roll,
     get_article,
@@ -196,6 +197,41 @@ class RollCalculationTests(unittest.TestCase):
         self.assertAlmostEqual(result["material_weight_kg"], 1455.0239028)
         self.assertNotIn("core_diameter_mm", result["inputs"])
 
+    def test_material_length_for_weight_is_material_only_and_round_trips(self):
+        inverse = calculate_material_length_for_weight(
+            target_material_weight_kg=700,
+            width_m=6,
+            area_weight_g_m2=495.395833,
+        )
+
+        self.assertEqual(inverse["status"], "success")
+        self.assertEqual(inverse["weight_scope"], "material_only")
+        self.assertNotIn("core_diameter_mm", inverse["inputs"])
+        round_trip = calculate_material_weight(
+            roll_length_m=inverse["material_length_m"],
+            width_m=6,
+            area_weight_g_m2=495.395833,
+        )
+        self.assertAlmostEqual(round_trip["material_weight_kg"], 700)
+
+    def test_material_length_for_weight_rejects_non_positive_inputs(self):
+        for changes in (
+            {"target_material_weight_kg": 0},
+            {"width_m": 0},
+            {"area_weight_g_m2": 0},
+        ):
+            with self.subTest(changes=changes):
+                values = {
+                    "target_material_weight_kg": 700,
+                    "width_m": 6,
+                    "area_weight_g_m2": 495,
+                }
+                values.update(changes)
+                self.assertEqual(
+                    calculate_material_length_for_weight(**values)["status"],
+                    "invalid_parameter",
+                )
+
     def test_roll_diameter_still_rejects_a_zero_core_diameter(self):
         result = calculate_roll(
             {