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);
 }

public/js/ui/view.js

@@ -6,7 +6,8 @@ import { t } from '../services/i18n.js';
 import { $, h, icon, refreshStaticIcons } from './dom.js';
 
 const SLIDER_STEPS = 1000;
-const TREND = { '-1': '▼', 1: '▲' }; // last move of the material price
+const MARKET_TOLERANCE = 1.15; // demand within ±15 % of output counts as balanced
+const TREND ={ '-1': '▼', 1: '▲' }; // last move of the material price
 
 function setText(node, text) {
   if (node.textContent !== text) node.textContent = text;
@@ -184,14 +185,28 @@ export function createView(actions) {
     );
   }
 
+  /**
+   * Feedback on the price: what customers want versus what is produced, and
+   * which side of the balance the player is on. It never gives the balanced
+   * price itself: it depends on output and every bonus, so it must be found.
+   */
+  function marketHint(d) {
+    const demand = fmt.number(d.demand);
+    if (d.rate <= 0) return t('ui.hintDemandOnly', { demand });
+    const ratio = d.demand / d.rate;
+    let verdict = 'ui.marketBalanced';
+    if (ratio > MARKET_TOLERANCE) verdict = 'ui.marketShort';
+    else if (ratio < 1 / MARKET_TOLERANCE) verdict = 'ui.marketLong';
+    return `${t('ui.hintDemand', { demand, production: fmt.number(d.rate) })} ${t(verdict)}`;
+  }
+
   function updateMarket(state, d) {
-    const { fairPrice, minPrice } = ctx.rules.market;
+    const { minPrice } = ctx.rules.market;
     if (document.activeElement !== el.priceInput) el.priceInput.value = state.price.toFixed(2);
     if (document.activeElement !== el.priceSlider) {
       el.priceSlider.value = Math.round(sliderFromPrice(Math.max(state.price, minPrice)));
     }
-    const hintKey = state.price > fairPrice ? 'ui.hintHigh' : 'ui.hintFair';
-    setText(el.priceHint, t(hintKey, { fair: money(fairPrice), demand: fmt.number(d.demand) }));
+    setText(el.priceHint, marketHint(d));
 
     const nextLevelMultiplier = ctx.rules.marketing.demandPerLevel ** (state.marketing + 1);
     setText(