algorithm fix

code/modules/reagents/chemistry/chemical_reaction.dm

@@ -38,6 +38,8 @@
 	/// required reagents as ratios. path or id is supported, prefer paths for compile time checking.
 	/// all of these will then make 1 unit of the reagent.
 	var/list/required_reagents = list()
+	/// cached: total volume of all required reagents for a single unit-multiplier (every 1 multiplier basically) reacted
+	var/tmp/required_reagents_unit_volume
 	/// require whole numbers of all reagents taken
 	///
 	/// * has no effect on ticked (non-instant) reactions
@@ -194,6 +196,10 @@
 		if(isnull(display_description) && !isnull(resolved))
 			display_description = resolved.display_description
 
+	required_reagents_unit_volume = 0
+	for(var/id in required_reagents)
+		required_reagents_unit_volume += required_reagents[id]
+
 //obtains any special data that will be provided to the reaction products
 //this is called just before reactants are removed.
 // todo: rework data system

code/modules/reagents/chemistry/reagent_holder-reactions.dm

@@ -171,9 +171,9 @@
 			maximum_multiplier = min(maximum_multiplier, amount / reaction.required_reagents[id])
 			total_reactant_volume += amount
 
-		// results
+		// results; take into account ingredients consumed if we were to run to completion
 		if(reaction.result_amount > 0)
-			maximum_multiplier = min(maximum_multiplier, ((maximum_volume - total_volume) + total_reactant_volume) / reaction.result_amount)
+			maximum_multiplier = min(maximum_multiplier, ((maximum_volume - total_volume) + maximum_multiplier * required_reagents_unit_volume) / reaction.result_amount)
 
 		// equilibrium (this relies on ingredients and must be #2 after ingredients which is #1)
 		if(reaction.equilibrium != INFINITY)
@@ -275,15 +275,19 @@
 		var/maximum_multiplier = INFINITY
 
 		if(reaction.require_whole_numbers)
-			if(reaction.result && reaction.result_amount > 0)
-				maximum_multiplier = min(maximum_multiplier, floor((maximum_volume - total_volume) / reaction.result_amount))
+			// tally up required reagents
 			for(var/id in reaction.required_reagents)
 				maximum_multiplier = min(maximum_multiplier, floor(legacy_direct_access_reagent_amount(id) / reaction.required_reagents[id]))
-		else
+			// tally up remaining space, taking into account the reagents we would consume if we reacted to completion
 			if(reaction.result && reaction.result_amount > 0)
-				maximum_multiplier = min(maximum_multiplier, (maximum_volume - total_volume) / reaction.result_amount)
+				maximum_multiplier = min(maximum_multiplier, floor(((maximum_volume - total_volume) + (maximum_multiplier * required_reagents_unit_volume)) / reaction.result_amount))
+		else
+			// tally up required reagents
 			for(var/id in reaction.required_reagents)
 				maximum_multiplier = min(maximum_multiplier, legacy_direct_access_reagent_amount(id) / reaction.required_reagents[id])
+			// tally up remaining space, taking into account the reagents we would consume if we reacted to completion
+			if(reaction.result && reaction.result_amount > 0)
+				maximum_multiplier = min(maximum_multiplier, ((maximum_volume - total_volume) + (maximum_multiplier * required_reagents_unit_volume)) / reaction.result_amount)
 
 		if(maximum_multiplier <= 0)
 			continue