refactor: improve knapsack selection with dynamic scaling, pre-filtering, and greedy budget filling

src/calculation_budget_constrains/service.py

@@ -1,6 +1,7 @@
 from collections import defaultdict
+import math
 import random
-from typing import Optional
+from typing import Optional, List, Dict, Tuple
 from uuid import UUID
 
 from sqlalchemy import Delete, Select
@@ -112,61 +113,88 @@ def greedy_selection(equipments: List[dict], budget: float) -> Tuple[List[dict],
     """Greedy fallback: select items by score until budget is used."""
     # Sort by priority_score descending
     equipments_sorted = sorted(equipments, key=lambda x: x["priority_score"], reverse=True)
-    total_cost = 0
+    current_cost = 0.0
     selected, excluded = [], []
     
     for eq in equipments_sorted:
-        if total_cost + eq["cost"] <= budget:
+        cost = eq.get("total_cost", 0.0)
+        if current_cost + cost <= budget:
             selected.append(eq)
-            total_cost += eq["cost"]
+            current_cost += cost
         else:
             excluded.append(eq)
     
     return selected, excluded
 
 
-def knapsack_selection(equipments: List[dict], budget: float, scale: int = 10_000_000) -> Tuple[List[dict], List[dict]]:
+def knapsack_selection(equipments: List[dict], budget: float, scale: Optional[float] = None) -> Tuple[List[dict], List[dict]]:
     """
     Select equipment optimally within budget using 0/1 knapsack DP.
-    Uses scaling + 1D DP optimization to avoid MemoryError.
-    Falls back to greedy if W is too large.
+    Uses dynamic scaling + 1D DP optimization to ensure accuracy and avoid MemoryError.
     """
-    n = len(equipments)
+    if not equipments:
+        return [], []
+
+    # Filter out items that are strictly above budget right away
+    # This helps significantly when budget is very low (e.g. 1)
+    eligible_items = []
+    strictly_excluded = []
+    for eq in equipments:
+        if eq["total_cost"] > budget:
+            strictly_excluded.append(eq)
+        else:
+            eligible_items.append(eq)
+
+    if not eligible_items:
+        return [], strictly_excluded
 
-    # Scale costs + budget
-    costs = [int(eq["total_cost"] // scale) for eq in equipments]
-    values = [eq["priority_score"] for eq in equipments]
+    n = len(eligible_items)
+
+    # Dynamic scaling: target W around 2000 for good precision/performance balance
+    if scale is None:
+        target_W = 2000
+        scale = max(1.0, budget / target_W)
+
+    costs = [int(math.ceil(eq["total_cost"] / scale)) for eq in eligible_items]
+    values = [eq["priority_score"] for eq in eligible_items]
     W = int(budget // scale)
 
-    # Fallback if W is still too large
+    # Fallback if W is still too large (memory safety)
     if W > 1_000_000:
-        print("too big")
         return greedy_selection(equipments, budget)
 
     # 1D DP array
     dp = [0.0] * (W + 1)
-    keep = [[False] * (W + 1) for _ in range(n)]  # track selection choices
+    keep = [[False] * (W + 1) for _ in range(n)]
 
     for i in range(n):
         cost, value = costs[i], values[i]
         for w in range(W, cost - 1, -1):
-            if dp[w - cost] + value >= dp[w]:  # <= FIXED HERE
+            if dp[w - cost] + value >= dp[w]:
                 dp[w] = dp[w - cost] + value
                 keep[i][w] = True
 
-    # Backtrack to find selected items
-    selected, excluded = [], []
+    # Backtrack
+    selected = []
+    backtrack_excluded = []
     w = W
     for i in range(n - 1, -1, -1):
         if keep[i][w]:
-            selected.append(equipments[i])
+            selected.append(eligible_items[i])
             w -= costs[i]
         else:
-            excluded.append(equipments[i])
+            backtrack_excluded.append(eligible_items[i])
+
+    excluded = backtrack_excluded + strictly_excluded
+
+    # Optional: greedy fill leftover budget with actual values
+    # This compensates for any conservative rounding from math.ceil
+    current_total_cost = sum(eq["total_cost"] for eq in selected)
+    remaining_budget = budget - current_total_cost
     
-    # Optional: fill leftover budget with zero-priority items
-    remaining_budget = budget - sum(eq["total_cost"] for eq in selected)
     if remaining_budget > 0:
+        # Sort excluded items by priority score for better greedy fill
+        excluded.sort(key=lambda x: x["priority_score"], reverse=True)
         for eq in excluded[:]:
             if eq["total_cost"] <= remaining_budget:
                 selected.append(eq)