fix: Changing way to sell with demand

public/js/engine/formulas.js

@@ -45,17 +45,26 @@ export function materialBatch(mm, totalProduced, rate, factor, discount) {
   const basis = Math.max(totalProduced * mm.batchShareOfTotal, rate * mm.batchSecondsOfProduction);
   const doublings = Math.max(0, Math.floor(Math.log2(Math.max(1, basis / mm.minBatch))));
   const size = mm.minBatch * 2 ** doublings;
-  const cost = Math.max(1, Math.ceil(size * materialUnitPrice(mm, totalProduced) * factor * discount));
-  return { size, cost };
+  const cents = size * materialUnitPrice(mm, totalProduced) * factor * discount;
+  return { size, cost: Math.max(0.01, Math.round(cents * 100) / 100) };
 }
 
 /**
- * Units customers want to buy per second at `price`.
- * Demand is maxed at or below the fair price and drops steeply above it, so
- * extra production is only worth selling at a higher price while supply is
- * short; to sell more at the fair price, the player needs more demand.
+ * Units customers want to buy per second at `price`: a smooth curve, with more
+ * demand the lower the price and less the higher it is (`referencePrice` is
+ * just where demand equals baseDemand x multiplier).
  */
 export function demandPerSecond(market, price, multiplier) {
-  const overpriced = Math.max(1, price / market.fairPrice);
-  return (market.baseDemand * multiplier) / overpriced ** market.elasticity;
+  return market.baseDemand * multiplier * (market.referencePrice / price) ** market.elasticity;
+}
+
+/**
+ * The price at which customers buy exactly what is produced. Selling for less
+ * gains no volume (everything is already sold); selling for more leaves stock
+ * unsold. It moves with output and every demand bonus, and the UI deliberately
+ * does not reveal it: the player finds it from the market feedback.
+ */
+export function balancedPrice(market, supplyPerSecond, multiplier) {
+  const demandAtReference = market.baseDemand * multiplier;
+  return market.referencePrice * (demandAtReference / supplyPerSecond) ** (1 / market.elasticity);
 }