Expeditions rework (#18960)

Content.Shared/Random/RandomSystem.cs

@@ -0,0 +1,58 @@
+using Robust.Shared.Random;
+using Robust.Shared.Utility;
+
+namespace Content.Shared.Random;
+
+public sealed class RandomSystem : EntitySystem
+{
+    public IBudgetEntry? GetBudgetEntry(ref float budget, ref float probSum, IList<IBudgetEntry> entries, System.Random random)
+    {
+        DebugTools.Assert(budget > 0f);
+
+        if (entries.Count == 0)
+            return null;
+
+        // - Pick an entry
+        // - Remove the cost from budget
+        // - If our remaining budget is under maxCost then start pruning unavailable entries.
+        random.Shuffle(entries);
+        var budgetEntry = (IBudgetEntry) GetProbEntry(entries, probSum, random);
+
+        budget -= budgetEntry.Cost;
+
+        // Prune invalid entries.
+        for (var i = 0; i < entries.Count; i++)
+        {
+            var entry = entries[i];
+
+            if (entry.Cost < budget)
+                continue;
+
+            entries.RemoveSwap(i);
+            i--;
+            probSum -= entry.Prob;
+        }
+
+        return budgetEntry;
+    }
+
+    /// <summary>
+    /// Gets a random entry based on each entry having a different probability.
+    /// </summary>
+    public IProbEntry GetProbEntry(IEnumerable<IProbEntry> entries, float probSum, System.Random random)
+    {
+        var value = random.NextFloat() * probSum;
+
+        foreach (var entry in entries)
+        {
+            value -= entry.Prob;
+
+            if (value < 0f)
+            {
+                return entry;
+            }
+        }
+
+        throw new InvalidOperationException();
+    }
+}