init

market/market.go

@@ -0,0 +1,48 @@
+// Package market prices ore and settles deliveries. Prices are integers
+// (credits per kg) that fall as supply accumulates and recover over time.
+package market
+
+import "wh/world"
+
+var basePrice = [world.NumOre]int64{world.Iron: 2, world.Nickel: 6, world.Ice: 3, world.Platinum: 300}
+
+// saturation is the supply (kg) at which an ore's price has halved.
+var saturation = [world.NumOre]int64{world.Iron: 400_000, world.Nickel: 200_000, world.Ice: 300_000, world.Platinum: 5_000}
+
+type Market struct {
+	Supply [world.NumOre]int64 // decayed running total of kg sold
+	Prices [world.NumOre]int64 // credits per kg, fixed for the day
+}
+
+func New() Market {
+	m := Market{}
+	m.reprice()
+	return m
+}
+
+func (m *Market) reprice() {
+	for o := range m.Prices {
+		p := basePrice[o] * saturation[o] / (saturation[o] + m.Supply[o])
+		if p < 1 {
+			p = 1
+		}
+		m.Prices[o] = p
+	}
+}
+
+// Value returns the payout for a cargo at today's prices.
+func (m *Market) Value(cargo [world.NumOre]int64) int64 {
+	var v int64
+	for o, kg := range cargo {
+		v += kg * m.Prices[o]
+	}
+	return v
+}
+
+// EndOfDay folds the day's sales into supply (with 10% decay) and reprices.
+func (m *Market) EndOfDay(sold [world.NumOre]int64) {
+	for o := range m.Supply {
+		m.Supply[o] = m.Supply[o]*9/10 + sold[o]
+	}
+	m.reprice()
+}