feat: More bonus (exponential) and method to list icons needed

public/js/engine/endless.js

@@ -0,0 +1,32 @@
+// Endless upgrades: once a family of upgrades is complete (its `requires`
+// milestone is owned), it continues forever as a repeatable line. Every level
+// multiplies the effect once more and costs exponentially more, up to
+// `maxLevel` levels, which nobody will ever reach.
+
+import { bulkCost, maxAffordable } from './formulas.js';
+
+export const lineLevel = (state, id) => state.endless[id] ?? 0;
+
+export function lineAvailable(state, line) {
+  return !line.requires || state.upgrades.includes(line.requires);
+}
+
+/** How many levels `amount` ("max" or a number) buys right now, and what they cost. */
+export function endlessQuote(state, ctx, id, amount) {
+  const line = ctx.endlessById.get(id);
+  const level = lineLevel(state, id);
+  const wanted =
+    amount === 'max' ? maxAffordable(line.baseCost, line.costGrowth, level, state.funds) : amount;
+  const count = Math.max(0, Math.min(wanted, line.maxLevel - level));
+  return { count, cost: bulkCost(line.baseCost, line.costGrowth, level, count) };
+}
+
+export function buyEndless(state, ctx, id, amount = 1) {
+  const line = ctx.endlessById.get(id);
+  if (!line || !lineAvailable(state, line)) return false;
+  const { count, cost } = endlessQuote(state, ctx, id, amount);
+  if (count <= 0 || !(state.funds >= cost)) return false; // an infinite cost is never affordable
+  state.funds -= cost;
+  state.endless[id] = lineLevel(state, id) + count;
+  return true;
+}