From 71dad18e8de23f693166759b2bcce27091795fde Mon Sep 17 00:00:00 2001
From: Adrien Jaguenet <adrien@jague.net>
Date: Tue, 22 Sep 2026 15:28:11 +0200
Subject: [PATCH] Let central banks work their reserves and cleared the
 landlocked famines

Central banks now redeem the national money foreign banks hold, sell reserves to defend a weak rate, and cover part of strategic imports from reserves. Five repeatable technologies raise each commodity's output per unit of energy. Hostile troops on a road or sea lane sever trade, power and unit supply. Middlemen buy food to resell to neighbours at a markup, and starving regions buy before those topping up their buffers, which clears the landlocked famines. The economic map's trade graph gains a colour legend.
---
 client/css/style.css              |  33 ++++++
 client/index.html                 |   4 +
 client/js/map_view.js             |   2 +
 client/js/map_view/modes.js       |  30 +++++
 client/js/modals/format.js        |  10 ++
 shared/data/economy.js            |  18 +++
 shared/data/effects.js            |   9 ++
 shared/data/resources.js          |  26 +++++
 shared/data/technologies.js       |  64 +++++++++++
 shared/game_state/industry.js     |  18 +++
 shared/game_state/money.js        | 185 ++++++++++++++++++++++++++++--
 shared/game_state/resources.js    | 141 +++++++++++++++++++++--
 tests/central_bank_fx_test.js     |  92 +++++++++++++++
 tests/food_security_test.js       |  43 +++++++
 tests/map_modes_test.js           |  34 ++++++
 tests/resource_efficiency_test.js | 114 ++++++++++++++++++
 tests/trade_route_test.js         |  55 +++++++++
 17 files changed, 857 insertions(+), 21 deletions(-)
 create mode 100644 tests/central_bank_fx_test.js
 create mode 100644 tests/food_security_test.js
 create mode 100644 tests/resource_efficiency_test.js
 create mode 100644 tests/trade_route_test.js

diff --git a/client/css/style.css b/client/css/style.css
index 440ffb1..c0e2bba 100644
--- a/client/css/style.css
+++ b/client/css/style.css
@@ -754,6 +754,39 @@ input:focus, select:focus { border-color: var(--accent); }
   color: var(--text);
   font-variant-numeric: tabular-nums;
 }
+/* The trade-route key, shown under the colour scale in the economic modes. */
+.map-legend-graph {
+  margin-top: 10px;
+  border-top: 1px solid var(--panel-border);
+  padding-top: 8px;
+}
+.map-legend-resources {
+  display: flex;
+  flex-direction: column;
+  gap: 4px;
+  font-size: 11px;
+  color: var(--text);
+}
+.map-legend-resource {
+  display: flex;
+  align-items: center;
+  gap: 6px;
+}
+.map-legend-swatch {
+  width: 16px;
+  height: 3px;
+  border-radius: 2px;
+  flex: none;
+}
+.map-legend-node {
+  width: 8px;
+  height: 8px;
+  margin: 0 4px;
+  border-radius: 50%;
+  background: #ffffff;
+  border: 1px solid rgba(0, 0, 0, 0.6);
+  flex: none;
+}
 
 /* The inspected tile / city panel is anchored to the bottom right. */
 #tile-panel {
diff --git a/client/index.html b/client/index.html
index a9bf023..a2eb631 100644
--- a/client/index.html
+++ b/client/index.html
@@ -136,6 +136,10 @@
             <span id="map-legend-mid"></span>
             <span id="map-legend-max"></span>
           </div>
+          <div id="map-legend-graph" class="map-legend-graph hidden">
+            <div class="map-legend-title">Trade routes</div>
+            <div id="map-legend-resources" class="map-legend-resources"></div>
+          </div>
         </div>
 
         <div id="tile-panel" class="panel hidden">
diff --git a/client/js/map_view.js b/client/js/map_view.js
index 5f56061..914b72c 100644
--- a/client/js/map_view.js
+++ b/client/js/map_view.js
@@ -147,6 +147,8 @@ export class MapView {
     this._fogChanged = null;
     // The colour scale shown while an economic mode is active.
     this.$legend = $("#map-legend");
+    this.$legendGraph = $("#map-legend-graph");
+    this.$legendResources = $("#map-legend-resources");
     this._labelSvg = null;
     this._labelEntries = [];
     this._countryLabels = [];
diff --git a/client/js/map_view/modes.js b/client/js/map_view/modes.js
index 7363580..95036bc 100644
--- a/client/js/map_view/modes.js
+++ b/client/js/map_view/modes.js
@@ -67,6 +67,7 @@ export const modeMethods = {
     const graph = this.resourceGraph || { nodes: [], links: [] };
     if (!this.economic || graph.links.length === 0) {
       if (this.$resources[0].firstChild) this.$resources.empty();
+      this._renderTradeLegend();
       return;
     }
     const colours = Object.fromEntries(RESOURCES.map((r) => [r.id, r.colour]));
@@ -86,6 +87,35 @@ export const modeMethods = {
       fragment.appendChild(dot);
     }
     this.$resources.empty()[0].appendChild(fragment);
+    this._renderTradeLegend();
+  },
+
+  // The key for the trade graph: one swatch per commodity whose deliveries are
+  // on screen, plus the storage-node marker. Shown only with the graph.
+  _renderTradeLegend() {
+    const $el = this.$legendGraph;
+    if (!$el || !$el.length) return;
+    const graph = this.resourceGraph || { nodes: [], links: [] };
+    if (!this.economic || graph.links.length === 0) {
+      $el.addClass("hidden");
+      return;
+    }
+    $el.removeClass("hidden");
+    const present = new Set(graph.links.map((link) => link[4]));
+    const $list = this.$legendResources.empty();
+    for (const resource of RESOURCES) {
+      if (!present.has(resource.id)) continue;
+      $list.append(
+        $("<div class='map-legend-resource'></div>")
+          .append($("<span class='map-legend-swatch'></span>").css("background", resource.colour))
+          .append($("<span></span>").text(resource.name))
+      );
+    }
+    $list.append(
+      $("<div class='map-legend-resource'></div>")
+        .append($("<span class='map-legend-node'></span>"))
+        .append($("<span></span>").text("Storage node"))
+    );
   },
 
   // Political shows when the user asked for it, or while terrain mode is
diff --git a/client/js/modals/format.js b/client/js/modals/format.js
index cb69995..68566f9 100644
--- a/client/js/modals/format.js
+++ b/client/js/modals/format.js
@@ -46,6 +46,16 @@ export function formatEffect(effect, level) {
       return `${value >= 0 ? "+" : ""}${(value * 100).toFixed(1)}% construction speed`;
     case "rail_speed":
       return `${value >= 0 ? "+" : ""}${(value * 100).toFixed(1)}% railway speed`;
+    case "energy_efficiency":
+      return `${value >= 0 ? "+" : ""}${(value * 100).toFixed(1)}% electricity per fuel`;
+    case "steel_efficiency":
+      return `${value >= 0 ? "+" : ""}${(value * 100).toFixed(1)}% steel per energy`;
+    case "food_efficiency":
+      return `${value >= 0 ? "+" : ""}${(value * 100).toFixed(1)}% food per energy`;
+    case "luxury_efficiency":
+      return `${value >= 0 ? "+" : ""}${(value * 100).toFixed(1)}% luxury per energy`;
+    case "hightech_efficiency":
+      return `${value >= 0 ? "+" : ""}${(value * 100).toFixed(1)}% high-tech per energy`;
     default:
       return `${effect.stat} ${value.toFixed(2)}`;
   }
diff --git a/shared/data/economy.js b/shared/data/economy.js
index 535e4cc..c0b525f 100644
--- a/shared/data/economy.js
+++ b/shared/data/economy.js
@@ -60,6 +60,24 @@ export const MONEY = {
   // raises the price it pays by this share, capped at `maxBidMultiplier`.
   bidPremium: 0.3,
   maxBidMultiplier: 2,
+  // The central bank's foreign-exchange desk. It works the reserves the nation
+  // earned abroad: it retires the national money foreign banks hold, sells more
+  // reserves to defend a weak rate, and pays part of the nation's foreign
+  // purchases out of reserves to keep food and energy imports cheap.
+  fx: {
+    // Share of the bank's foreign reserves spent redeeming the national money
+    // held abroad on an ordinary day.
+    redemptionPerDay: 0.004,
+    // Extra share sold on a day the national currency sits below `weakBelow`.
+    interventionPerDay: 0.05,
+    // The value against par the bank treats as a weak rate to defend.
+    weakBelow: 0.98,
+    // The share of a foreign purchase of each resource the bank covers from its
+    // reserves, and the most of its reserves it will spend subsidising imports
+    // in one day.
+    importSubsidy: { food: 0.5, energy: 0.5, steel: 0.25, luxury: 0, hightech: 0.25 },
+    importSubsidyPerDay: 0.02,
+  },
 };
 
 export const ECONOMY = {
diff --git a/shared/data/effects.js b/shared/data/effects.js
index faf3b40..92d976d 100644
--- a/shared/data/effects.js
+++ b/shared/data/effects.js
@@ -14,6 +14,15 @@ export const EFFECT_ARTILLERY_ATTACK = "artillery_attack";
 export const EFFECT_CONSTRUCTION_SPEED = "construction_speed";
 export const EFFECT_RAIL_SPEED = "rail_speed";
 
+// Resource efficiency: the national bonus to a commodity's output per unit of
+// energy input. One technology per commodity raises its own stat, and the
+// simulation divides that commodity's energy draw by `1 + bonus`.
+export const EFFECT_ENERGY_EFFICIENCY = "energy_efficiency";
+export const EFFECT_STEEL_EFFICIENCY = "steel_efficiency";
+export const EFFECT_FOOD_EFFICIENCY = "food_efficiency";
+export const EFFECT_LUXURY_EFFICIENCY = "luxury_efficiency";
+export const EFFECT_HIGHTECH_EFFICIENCY = "hightech_efficiency";
+
 export function effect(stat, baseValue, exponent = 1.0) {
   return { stat, baseValue, exponent };
 }
diff --git a/shared/data/resources.js b/shared/data/resources.js
index 91526a5..7092ec0 100644
--- a/shared/data/resources.js
+++ b/shared/data/resources.js
@@ -6,6 +6,14 @@
 // here are the main tuning knobs for the whole resource economy; the per-person
 // needs and the energy-per-material figures come straight from the roadmap.
 
+import {
+  EFFECT_ENERGY_EFFICIENCY,
+  EFFECT_STEEL_EFFICIENCY,
+  EFFECT_FOOD_EFFICIENCY,
+  EFFECT_LUXURY_EFFICIENCY,
+  EFFECT_HIGHTECH_EFFICIENCY,
+} from "./effects.js";
+
 // Energy is stored and moved as kWh. These keep the catalogue readable.
 export const KWH_PER_MWH = 1_000;
 export const KWH_PER_GWH = 1_000_000;
@@ -73,6 +81,17 @@ export const RESOURCE_INDEX = Object.fromEntries(RESOURCES.map((r, i) => [r.id,
 // The four stockable resources, in catalogue order. Energy is handled as a flow.
 export const STORABLE_RESOURCE_IDS = RESOURCES.filter((r) => r.storable).map((r) => r.id);
 
+// The modifier each resource's efficiency technology writes. The simulation
+// divides that resource's energy draw by `1 + modifier`, so a higher figure is
+// more of the commodity produced for the same power.
+export const RESOURCE_EFFICIENCY_EFFECT = {
+  energy: EFFECT_ENERGY_EFFICIENCY,
+  steel: EFFECT_STEEL_EFFICIENCY,
+  food: EFFECT_FOOD_EFFICIENCY,
+  luxury: EFFECT_LUXURY_EFFICIENCY,
+  hightech: EFFECT_HIGHTECH_EFFICIENCY,
+};
+
 export function resourceById(id) {
   return RESOURCES[RESOURCE_INDEX[id]] || null;
 }
@@ -240,6 +259,13 @@ export const RESOURCE_RULES = {
 // global market.
 export const RESOURCE_TRADE_RADIUS = 8;
 
+// A region that can reach a food surplus it does not need buys extra to resell
+// to nearby regions that cannot, acting as a middleman. The freight a buyer pays
+// on the longer outbound leg is its margin, so it only relays toward a market
+// farther than its own source, and stocks this many days of the neighbours'
+// shortfall.
+export const MIDDLEMAN = { coverDays: 3, markup: 0.25 };
+
 // Resource-producing buildings. Built only outside cities, they occupy a land
 // tile like a military improvement and are managed through the same tile-works
 // queue. A power plant produces energy; every other building consumes energy
diff --git a/shared/data/technologies.js b/shared/data/technologies.js
index fd6568f..749a12d 100644
--- a/shared/data/technologies.js
+++ b/shared/data/technologies.js
@@ -16,6 +16,11 @@ import {
   EFFECT_ARTILLERY_ATTACK,
   EFFECT_CONSTRUCTION_SPEED,
   EFFECT_RAIL_SPEED,
+  EFFECT_ENERGY_EFFICIENCY,
+  EFFECT_STEEL_EFFICIENCY,
+  EFFECT_FOOD_EFFICIENCY,
+  EFFECT_LUXURY_EFFICIENCY,
+  EFFECT_HIGHTECH_EFFICIENCY,
   effect,
 } from "./effects.js";
 
@@ -141,6 +146,65 @@ export const TECHNOLOGIES = [
     prerequisites: ["scientific_method"],
     effects: [effect(EFFECT_RAIL_SPEED, 0.01, 1.0)],
   },
+
+  // Efficiency research: one repeatable technology per commodity that raises the
+  // output the nation gets from each unit of energy. Each level adds 5% to that
+  // commodity's output per input, so the power drawn to make it falls.
+  {
+    id: "efficient_generation",
+    name: "Efficient Generation",
+    description: "Better turbines and heat recovery wring more electricity from every tonne of fuel.",
+    theme: "economy",
+    cost: 50,
+    repeatable: true,
+    repeatCostGrowth: 0.03,
+    prerequisites: ["renewable_energy"],
+    effects: [effect(EFFECT_ENERGY_EFFICIENCY, 0.05, 1.0)],
+  },
+  {
+    id: "advanced_metallurgy",
+    name: "Advanced Metallurgy",
+    description: "Continuous casting and heat recycling smelt more steel for less power.",
+    theme: "economy",
+    cost: 50,
+    repeatable: true,
+    repeatCostGrowth: 0.03,
+    prerequisites: ["industrial_automation"],
+    effects: [effect(EFFECT_STEEL_EFFICIENCY, 0.05, 1.0)],
+  },
+  {
+    id: "precision_agriculture",
+    name: "Precision Agriculture",
+    description: "Climate-controlled growing and tailored feed raise the food won from each unit of power, on the land and in synthesis.",
+    theme: "economy",
+    cost: 50,
+    repeatable: true,
+    repeatCostGrowth: 0.03,
+    prerequisites: ["scientific_method"],
+    effects: [effect(EFFECT_FOOD_EFFICIENCY, 0.05, 1.0)],
+  },
+  {
+    id: "automated_extraction",
+    name: "Automated Extraction",
+    description: "Automated mines and sorting win more gold and diamonds for the same power.",
+    theme: "economy",
+    cost: 50,
+    repeatable: true,
+    repeatCostGrowth: 0.03,
+    prerequisites: ["industrial_automation"],
+    effects: [effect(EFFECT_LUXURY_EFFICIENCY, 0.05, 1.0)],
+  },
+  {
+    id: "advanced_lithography",
+    name: "Advanced Lithography",
+    description: "Finer process nodes pack more chips onto every wafer, cutting the power each component costs.",
+    theme: "science",
+    cost: 80,
+    repeatable: true,
+    repeatCostGrowth: 0.03,
+    prerequisites: ["quantum_computing"],
+    effects: [effect(EFFECT_HIGHTECH_EFFICIENCY, 0.05, 1.0)],
+  },
 ];
 
 export function technologyById(id) {
diff --git a/shared/game_state/industry.js b/shared/game_state/industry.js
index 08a83a6..f826a2c 100644
--- a/shared/game_state/industry.js
+++ b/shared/game_state/industry.js
@@ -18,6 +18,7 @@ import { ECONOMY } from "../data/economy.js";
 import { RESOURCE_RULES } from "../data/resources.js";
 import {
   RESOURCE_BUILDING_UPGRADE,
+  RESOURCE_EFFICIENCY_EFFECT,
   resourceBuildingOutputAt,
   resourceBuildingInputAt,
   resourceBuildingUpgradeMoneyCost,
@@ -75,6 +76,23 @@ export const industryMethods = {
     return resourceBuildingInputAt(proto, this.resourceBuildingLevel(k), field);
   },
 
+  // The nation's output-per-input bonus for one commodity, from its efficiency
+  // research. Energy drawn to make that commodity is divided by `1 + bonus`.
+  // Read from the technology modifiers alone, so seeding never primes the
+  // cached building-modifier totals with a half-built world.
+  resourceEfficiency(civ, resource) {
+    if (civ < 0) return 0;
+    const stat = RESOURCE_EFFICIENCY_EFFECT[resource];
+    if (!stat) return 0;
+    let cached = this._resourceEfficiencyCache ? this._resourceEfficiencyCache.get(civ) : null;
+    if (!cached || cached.version !== this._modifiersVersion) {
+      if (!this._resourceEfficiencyCache) this._resourceEfficiencyCache = new Map();
+      cached = { version: this._modifiersVersion, values: this.getCivTechnologyModifiers(civ) };
+      this._resourceEfficiencyCache.set(civ, cached);
+    }
+    return Math.max(0, cached.values[stat] || 0);
+  },
+
   // ------------------------------------------------------------ money --
 
   // A buyer's payment reaches the producing building, into its own cash, in the
diff --git a/shared/game_state/money.js b/shared/game_state/money.js
index 9257d37..e29f66a 100644
--- a/shared/game_state/money.js
+++ b/shared/game_state/money.js
@@ -33,6 +33,8 @@ export const moneyMethods = {
     this.currencyValues = new Map();
     this._currencyDemand = new Map();
     this._currencyCivMap = null;
+    // What each central bank has spent subsidising imports in the current day.
+    this._fxSubsidySpent = new Map();
     for (let civ = 0; civ < this.civilisations.length; civ++) {
       this.centralBankRate.set(civ, MONEY.defaultInterestRate);
       this.centralBankReserves.set(civ, emptyBasket());
@@ -234,6 +236,136 @@ export const moneyMethods = {
     return total;
   },
 
+  // ------------------------------------------------------ fx desk --
+
+  // Lower a central bank's holding of one currency, never below zero.
+  _reduceReserve(civ, code, amount) {
+    const basket = this.centralBankReserves ? this.centralBankReserves.get(civ) : null;
+    if (!basket || !(amount > 0)) return;
+    basket.set(code, Math.max(0, (basket.get(code) || 0) - amount));
+  },
+
+  // Retire part of a central bank's issued-currency liability.
+  _reduceCentralBankDebt(civ, code, amount) {
+    const debts = this.centralBankDebt ? this.centralBankDebt.get(civ) : null;
+    if (!debts || !(amount > 0)) return;
+    debts.set(code, Math.max(0, (debts.get(code) || 0) - amount));
+  },
+
+  // Which reserve to spend redeeming our own currency: prefer the holder's own
+  // currency, so both banks cancel a reserve, else our largest foreign balance.
+  _fxPaymentCurrency(civ, other, ownCode) {
+    const reserves = this.centralBankReserves.get(civ);
+    if (!reserves) return null;
+    const otherCode = this.currencyOf(other).code;
+    if (otherCode !== ownCode && (reserves.get(otherCode) || 0) > 0) return otherCode;
+    let best = null;
+    let bestAmount = 0;
+    for (const [code, amount] of reserves) {
+      if (code === ownCode || !(amount > 0)) continue;
+      if (amount > bestAmount) {
+        bestAmount = amount;
+        best = code;
+      }
+    }
+    return best;
+  },
+
+  // Spend up to `budget` (in the bank's own currency) of foreign reserves buying
+  // back the national money foreign banks hold, then retire it. The reserves
+  // change hands and the money leaves circulation, so the currency is bid for
+  // without anything being minted. Returns the own-currency amount redeemed.
+  _redeemOwnCurrency(civ, budget) {
+    const code = this.currencyOf(civ).code;
+    const ownValue = this.currencyValue(civ);
+    const reserves = this.centralBankReserves ? this.centralBankReserves.get(civ) : null;
+    if (!reserves || !(budget > 0)) return 0;
+    let remaining = budget;
+    let redeemed = 0;
+    for (let other = 0; other < this.civilisations.length && remaining > 0; other++) {
+      if (other === civ) continue;
+      const holder = this.centralBankReserves.get(other);
+      if (!holder) continue;
+      const held = holder.get(code) || 0;
+      if (!(held > 0)) continue;
+      const payCode = this._fxPaymentCurrency(civ, other, code);
+      if (!payCode) continue;
+      const payValue = this.currencyValueOf(payCode);
+      const havePay = reserves.get(payCode) || 0;
+      if (!(havePay > 0) || !(payValue > 0)) continue;
+      const take = Math.min(remaining, held);
+      let payQty = (take * ownValue) / payValue;
+      if (payQty > havePay) payQty = havePay;
+      if (!(payQty > 0)) continue;
+      const actual = (payQty * payValue) / ownValue;
+      reserves.set(payCode, havePay - payQty);
+      if (payCode === this.currencyOf(other).code) {
+        // Handing a bank its own money back retires its liability.
+        this._reduceCentralBankDebt(other, payCode, payQty);
+      } else {
+        this._addReserve(other, payCode, payQty);
+      }
+      holder.set(code, held - actual);
+      this._reduceCentralBankDebt(civ, code, actual);
+      // The bid for our money and the offer of the reserve move the day's rates.
+      this._recordCurrencyDemand(civ, actual * ownValue);
+      this._recordCurrencyDemand(other, -(payQty * payValue));
+      remaining -= actual;
+      redeemed += actual;
+    }
+    return redeemed;
+  },
+
+  // The day's reserve policy for one nation: retire a slice of the national
+  // money held abroad, and sell more reserves when the rate is weak.
+  _settleCentralBankFx(civ) {
+    if (!this.centralBankReserves) return 0;
+    const code = this.currencyOf(civ).code;
+    const reserves = this.centralBankReserves.get(civ);
+    if (!reserves) return 0;
+    const reserveValue = this._basketValue(reserves, civ);
+    if (!(reserveValue > 0)) return 0;
+    let heldAbroad = 0;
+    for (let other = 0; other < this.civilisations.length; other++) {
+      if (other === civ) continue;
+      const basket = this.centralBankReserves.get(other);
+      if (basket) heldAbroad += basket.get(code) || 0;
+    }
+    if (!(heldAbroad > 0)) return 0;
+    let budget = reserveValue * MONEY.fx.redemptionPerDay;
+    if (this.currencyValue(civ) < MONEY.fx.weakBelow) {
+      budget += reserveValue * MONEY.fx.interventionPerDay;
+    }
+    budget = Math.min(budget, heldAbroad);
+    if (!(budget > 0)) return 0;
+    return this._redeemOwnCurrency(civ, budget);
+  },
+
+  // What the central bank will cover of one foreign purchase, in global value,
+  // for the day's import subsidy. It is bounded by the day's budget; the bill
+  // is only actually paid when the seller's currency is in reserve.
+  _centralBankImportSubsidy(civ, resource, globalValue) {
+    if (!(globalValue > 0) || !this.centralBankReserves) return 0;
+    const rate = (MONEY.fx.importSubsidy && MONEY.fx.importSubsidy[resource]) || 0;
+    if (!(rate > 0)) return 0;
+    const reserves = this.centralBankReserves.get(civ);
+    if (!reserves) return 0;
+    const reserveValue = this._basketValue(reserves, civ);
+    if (!(reserveValue > 0)) return 0;
+    const spent = this._fxSubsidySpent ? (this._fxSubsidySpent.get(civ) || 0) : 0;
+    const budget = reserveValue * MONEY.fx.importSubsidyPerDay * this.currencyValue(civ) - spent;
+    if (!(budget > 0)) return 0;
+    return Math.min(globalValue * rate, budget);
+  },
+
+  // Record what a purchase actually drew from the subsidy budget, so the day's
+  // cap holds across every buyer.
+  _recordImportSubsidy(civ, globalValue) {
+    if (!(globalValue > 0)) return;
+    if (!this._fxSubsidySpent) this._fxSubsidySpent = new Map();
+    this._fxSubsidySpent.set(civ, (this._fxSubsidySpent.get(civ) || 0) + globalValue);
+  },
+
   // A buyer acquires `globalValue` (in global units) worth of `targetCiv`'s
   // currency: first from what it already holds, then from that nation's central
   // bank, paying with its own national currency (borrowing from its own bank if
@@ -243,10 +375,16 @@ export const moneyMethods = {
   //
   // `charge` overrides who pays the national-currency cost: a region does by
   // default, but a government buys with the treasury. `useHeld` is false for the
-  // treasury, which holds no basket of its own.
-  _regionAcquireCurrency(buyerCity, targetCiv, globalValue, charge = null, useHeld = true) {
-    if (!(globalValue > 0)) return { code: this.currencyOf(targetCiv).code, amount: 0 };
+  // treasury, which holds no basket of its own. `subsidize` is a global value the
+  // buyer's central bank covers from its reserves of the seller's currency, so
+  // the buyer pays less for the same goods; the seller is still paid in full.
+  _regionAcquireCurrency(
+    buyerCity, targetCiv, globalValue, charge = null, useHeld = true, subsidize = 0
+  ) {
     const target = this.currencyOf(targetCiv);
+    if (!(globalValue > 0)) {
+      return { code: target.code, amount: 0, spent: 0, subsidised: 0 };
+    }
     const buyer = this.currencyOf(buyerCity.civ);
     const vTarget = this.currencyValue(targetCiv);
     const vBuyer = this.currencyValue(buyerCity.civ);
@@ -254,10 +392,23 @@ export const moneyMethods = {
     const basket = useHeld ? this.regionCash.get(buyerCity.id) : null;
     const heldTarget = basket ? basket.get(target.code) || 0 : 0;
     const fromHeld = Math.min(heldTarget, needTarget);
-    const remaining = needTarget - fromHeld;
+    let remaining = needTarget - fromHeld;
+    // The buyer's central bank may cover part of the bill with the reserves of
+    // the seller's currency it holds.
+    let subsidised = 0;
+    if (subsidize > 0 && remaining > 0 && buyerCity.civ !== targetCiv) {
+      const bank = this.centralBankReserves ? this.centralBankReserves.get(buyerCity.civ) : null;
+      const bankHeld = bank ? (bank.get(target.code) || 0) : 0;
+      const cover = Math.min(remaining, bankHeld, subsidize / vTarget);
+      if (cover > 0) {
+        bank.set(target.code, bankHeld - cover);
+        subsidised = cover;
+        remaining -= cover;
+      }
+    }
     // The buyer currency actually spent acquiring the foreign currency. The
-    // held portion costs nothing, so an import tariff can fall only on what was
-    // really exchanged.
+    // held and subsidised portions cost the buyer nothing, so an import tariff
+    // can fall only on what was really exchanged.
     let spent = 0;
     if (remaining > 0) {
       const costBuyer = remaining * vTarget / vBuyer;
@@ -272,7 +423,12 @@ export const moneyMethods = {
       this._addCentralBankDebt(targetCiv, target.code, remaining);
     }
     if (fromHeld > 0) basket.set(target.code, heldTarget - fromHeld);
-    return { code: target.code, amount: fromHeld + remaining, spent };
+    return {
+      code: target.code,
+      amount: fromHeld + subsidised + remaining,
+      spent,
+      subsidised: subsidised * vTarget,
+    };
   },
 
   // --------------------------------------------------- demand/rates --
@@ -431,11 +587,13 @@ export const moneyMethods = {
   // --------------------------------------------------- daily tick --
 
   // The day's monetary work: debts to the central bank accrue interest, regions
-  // repatriate any foreign money they still hold and pay down what they owe, and
-  // the day's trading leaves each currency's value drifting with net demand. The
-  // interest rate is the borrowing cost, not a direct inflation lever.
+  // repatriate any foreign money they still hold and pay down what they owe,
+  // the central banks work their reserves, and the day's trading leaves each
+  // currency's value drifting with net demand. The interest rate is the
+  // borrowing cost, not a direct inflation lever.
   _tickMoney() {
     if (!this.regionCash) return;
+    this._fxSubsidySpent = new Map();
     for (const city of this.cities) {
       const basket = this.regionCash.get(city.id);
       const rate = this.getCentralBankRate(city.civ) / DAYS_PER_YEAR;
@@ -453,6 +611,13 @@ export const moneyMethods = {
         const rate = this.getCentralBankRate(civ) / DAYS_PER_YEAR;
         this.budgets.set(civ, budget * (1 + rate));
       }
+    }
+    // The central banks spend their reserves before the day's rates are set, so
+    // an intervention shows up in the demand it records.
+    for (let civ = 0; civ < this.civilisations.length; civ++) {
+      this._settleCentralBankFx(civ);
+    }
+    for (let civ = 0; civ < this.civilisations.length; civ++) {
       const demand = this._currencyDemand.get(civ) || 0;
       const mass = this._currencyMass(civ);
       const value = this.currencyValue(civ) * (1 + MONEY.demandSensitivity * (demand / mass));
diff --git a/shared/game_state/resources.js b/shared/game_state/resources.js
index 3e46629..c743ed0 100644
--- a/shared/game_state/resources.js
+++ b/shared/game_state/resources.js
@@ -16,6 +16,7 @@ import {
   RESOURCE_RULES,
   RESOURCE_MARKET_BASE,
   RESOURCE_TRADE_RADIUS,
+  MIDDLEMAN,
   FAMINE,
   tileImprovementById,
   isResourceTileBuilding,
@@ -85,6 +86,9 @@ export const resourceMethods = {
     }
     this.resourceLinks = [];
     this._resourceShortages = new Map();
+    // The food each region has bought to relay on to its neighbours, so only
+    // that stock carries the middleman margin when it is resold.
+    this._relayStock = new Map();
     // The regions whose food store has run dry and cannot feed their people.
     this._famine = new Set();
     this._resourceVersion = 0;
@@ -1189,6 +1193,13 @@ export const resourceMethods = {
     return false;
   },
 
+  // The grid power a region's land harvest draws, after its food-efficiency
+  // research has cut the energy each tonne costs.
+  _foodHarvestEnergy(civ, produced) {
+    return producerEnergyPerDay("food", produced) /
+      (1 + this.resourceEfficiency(civ, "food"));
+  },
+
   // ------------------------------------------------------------ daily tick --
 
   _tickResources() {
@@ -1240,7 +1251,7 @@ export const resourceMethods = {
       });
       const component = ledger.get(components.get(k));
       if (component) {
-        const foodPower = producerEnergyPerDay("food", produced);
+        const foodPower = this._foodHarvestEnergy(component.civ, produced);
         component.foodEnergy += foodPower;
         component.foodConsumers.push({ city, amount: foodPower });
       }
@@ -1268,12 +1279,17 @@ export const resourceMethods = {
       for (const supplier of this._nearbySuppliers(city, "food", nodes, allowSea)) {
         buyable += this._nodeSurplus(supplier, "food");
       }
-      const extra = synthesisedFoodFor(foodPrice, energyPrice, shortfall, buyable);
+      // Food efficiency cheapens synthesis, so it is worth growing more before
+      // the market wins; the energy it actually spends falls by the same share.
+      const foodEfficiency = 1 + this.resourceEfficiency(city.civ, "food");
+      const extra = synthesisedFoodFor(
+        foodPrice, energyPrice / foodEfficiency, shortfall, buyable
+      );
       if (!(extra > 0)) continue;
       const componentId = components.get(key(city.coords.x, city.coords.y));
       const component = ledger.get(componentId);
       if (!component) continue;
-      const energy = foodSynthesisEnergy(extra);
+      const energy = foodSynthesisEnergy(extra) / foodEfficiency;
       component.foodEnergy += energy;
       component.foodConsumers.push({ city, amount: energy });
       pendingFood.push({
@@ -1320,6 +1336,7 @@ export const resourceMethods = {
     // Food lands in the region's store, scaled by the power the grid could give
     // it. Food is served before the converters, so the harvest survives a
     // power squeeze that idles the factories.
+    const grownFood = new Map();
     for (const entry of pendingFood) {
       const component = ledger.get(entry.component);
       const scale = component ? component.foodScale : 1;
@@ -1327,8 +1344,13 @@ export const resourceMethods = {
       if (produced <= 0) continue;
       this._ensureResourceStock(key(entry.coords.x, entry.coords.y)).food += produced;
       market.get("food").supply += produced;
+      grownFood.set(entry.cityId, (grownFood.get(entry.cityId) || 0) + produced);
     }
 
+    // Middlemen top up for neighbours whose own power-limited harvest fell short
+    // before anyone buys, so the relay stock is on hand in the same day.
+    this._relayFood(grownFood, nodes);
+
     // Converters turn the power they were allocated into their material, held
     // at the building's own tile.
     for (const [k, id] of this.tileImprovements) {
@@ -1470,6 +1492,9 @@ export const resourceMethods = {
       const component = ledger.get(components.get(k));
       if (!component) continue;
       if (isResourceTileBuilding(proto)) {
+        // Efficiency research cuts the power each commodity costs: the draw is
+        // divided by `1 + bonus`, so the same output needs less energy.
+        const efficiency = 1 + this.resourceEfficiency(component.civ, proto.resource);
         if (proto.resource === "energy") {
           const output = this.resourceBuildingOutputAt(k, proto);
           component.supply += output;
@@ -1478,12 +1503,12 @@ export const resourceMethods = {
           const agent = this.buildingAgents ? this.buildingAgents.get(k) : null;
           const scale = this._converterProductionScale(k, proto, agent);
           this._converterScales.set(k, scale);
-          const amount = this.resourceBuildingInputAt(k, proto, "energyPerDay") * scale;
+          const amount = this.resourceBuildingInputAt(k, proto, "energyPerDay") * scale / efficiency;
           component.converterEnergy += amount;
           component.converters.push({ k, amount });
         }
         if (proto.fuelEnergyPerDay) {
-          const fuel = this.resourceBuildingInputAt(k, proto, "fuelEnergyPerDay");
+          const fuel = this.resourceBuildingInputAt(k, proto, "fuelEnergyPerDay") / efficiency;
           component.fuelEnergy += fuel;
           component.cityEnergy += fuel;
         }
@@ -1594,6 +1619,21 @@ export const resourceMethods = {
     return null;
   },
 
+  // The tiles held by hostile troops: an army camped on a road or a fleet in a
+  // sea lane severs the route while it stands there. Trade, grid power and unit
+  // supply searches all route around these tiles, so moving a unit in forces
+  // the next recalculation to find another way -- or none.
+  _hostileOccupied(civ) {
+    const blocked = new Set();
+    for (const unit of this.units) {
+      if (unit.civ === civ) continue;
+      if (!this.isAtWar(civ, unit.civ)) continue;
+      if (!this._isMilitary(unit) || this._isAirUnit(unit)) continue;
+      blocked.add(key(unit.coords.x, unit.coords.y));
+    }
+    return blocked;
+  },
+
   // The grid components reachable from a city over the transport network, mapped
   // to the least energy it takes to carry a unit there. A sea leg needs a
   // working port; a land leg needs road or railway; goods never cross the land
@@ -1602,13 +1642,14 @@ export const resourceMethods = {
     const reach = new Map();
     const origin = key(city.coords.x, city.coords.y);
     const originComponent = components.get(origin);
+    const blocked = this._hostileOccupied(city.civ);
     const visited = new Set([origin]);
     const queue = [{ coords: city.coords, energy: 0 }];
     while (queue.length > 0) {
       const current = queue.shift();
       for (const neighbour of this._neighbours(current.coords)) {
         const nk = key(neighbour.x, neighbour.y);
-        if (visited.has(nk)) continue;
+        if (visited.has(nk) || blocked.has(nk)) continue;
         const tile = this.tiles[nk];
         if (!tile || !(tile.terrainClass === "Land" || tile.terrainClass === "Sea" ||
             tile.terrainClass === "Ice")) continue;
@@ -1640,12 +1681,68 @@ export const resourceMethods = {
     return reach;
   },
 
+  // Middlemen: a region that can reach a food surplus it does not itself need
+  // buys extra to resell to nearby regions that have no reach of their own. It
+  // charges a margin on that resale, which covers its transport and turns a
+  // profit, so food hops past a region that is itself a net importer.
+  _relayFood(grownFood, nodes) {
+    const need = new Map();
+    const stock = new Map();
+    for (const city of this.cities) {
+      need.set(city.id, foodNeedPerDay(this.getCityEconomy(city).population));
+      stock.set(city.id, this.getCityResourceStock(city).food || 0);
+    }
+    // What each region actually grew today, and whether it can reach any food
+    // surplus on its own.
+    const sources = new Map();
+    const allowSea = new Map();
+    for (const city of this.cities) {
+      const sea = this._cityMarketAccess(city) && !this._isEncircled(city.coords, city.civ);
+      allowSea.set(city.id, sea);
+      sources.set(city.id, this._nearbySuppliers(city, "food", nodes, sea));
+    }
+    for (const city of this.cities) {
+      const suppliers = sources.get(city.id);
+      if (!suppliers || suppliers.length === 0) continue;
+      let relayDemand = 0;
+      for (const other of this.cities) {
+        if (other.id === city.id) continue;
+        if ((sources.get(other.id) || []).length > 0) continue;
+        const shortfall = Math.max(0, need.get(other.id) - (grownFood.get(other.id) || 0));
+        if (shortfall <= 0) continue;
+        if (stock.get(other.id) >= need.get(other.id) * 2) continue;
+        // The buyer must be able to reach this region's store.
+        if (this.topology.tileDistance(city.coords, other.coords) > RESOURCE_TRADE_RADIUS) {
+          continue;
+        }
+        relayDemand += shortfall;
+      }
+      if (!(relayDemand > 0)) continue;
+      const wanted = relayDemand * MIDDLEMAN.coverDays;
+      const store = this.getCityResourceStock(city);
+      const bought = this._procureFromNeighbours(city, "food", wanted, store, nodes, {
+        allowSea: allowSea.get(city.id),
+      });
+      if (bought > 0) {
+        this._relayStock.set(city.id, (this._relayStock.get(city.id) || 0) + bought);
+      }
+    }
+  },
+
   _consumeCityResources(components, market, nodes = null) {
     nodes = nodes || this._resourceNodes();
     // How much of each supplier's surplus earlier buyers have already claimed
     // today, so a contested surplus goes to whoever bids highest.
     this._nodeClaims = new Map();
-    for (const city of this.cities) {
+    // Starving regions buy first: a city topping up its month-long buffer must
+    // not strip a neighbour whose store is empty of the last reachable surplus.
+    const urgency = (city) => {
+      const store = this.getCityResourceStock(city);
+      const need = Math.max(1e-9, foodNeedPerDay(this.getCityEconomy(city).population));
+      return (store.food || 0) / need;
+    };
+    const order = this.cities.slice().sort((a, b) => urgency(a) - urgency(b));
+    for (const city of order) {
       const population = this.getCityEconomy(city).population;
       const needs = {
         food: foodNeedPerDay(population),
@@ -1775,7 +1872,10 @@ export const resourceMethods = {
       const { supplier, foreign, goods, freight } = quote;
       const bought = Math.min(quote.available, remaining);
       if (!(bought > 0)) continue;
-      const value = (goods + freight) * bought;
+      // A region selling food it bought to relay charges its middleman margin on
+      // that stock, so the resale covers its transport and turns a profit.
+      const markup = this._middlemanMarkup(supplier, id, bought, price);
+      const value = (goods + freight) * bought + markup;
       const vBuyer = this.currencyValue(city.civ);
       const charge = payer
         ? (cost) => payer.charge(cost)
@@ -1787,10 +1887,14 @@ export const resourceMethods = {
         // The seller's government charges the foreign buyer an export tariff,
         // in the seller's currency, on top of the goods.
         const exportTax = value * this.getTaxRate(supplier.civ, "export", id);
+        // The central bank pays part of the bill from its reserves, so a region
+        // or converter imports food and energy more cheaply than the market.
+        const subsidy = this._centralBankImportSubsidy(city.civ, id, value + exportTax);
         const acquired = this._regionAcquireCurrency(
-          city, supplier.civ, value + exportTax, charge, !payer
+          city, supplier.civ, value + exportTax, charge, !payer, subsidy
         );
         if (!acquired) continue;
+        if (acquired.subsidised > 0) this._recordImportSubsidy(city.civ, acquired.subsidised);
         const total = value + exportTax;
         const sellerShare = total > 0 ? value / total : 1;
         const sellerAmount = acquired.amount * sellerShare;
@@ -1928,6 +2032,7 @@ export const resourceMethods = {
     const results = [];
     const byKey = new Map(nodes.map((node) => [key(node.coords.x, node.coords.y), node]));
     const origin = key(city.coords.x, city.coords.y);
+    const blocked = this._hostileOccupied(city.civ);
     const visited = new Set([origin]);
     // Every delivery runs over the transport network: railways and roads on
     // land, shipping lanes across water (which need a working port).
@@ -1948,7 +2053,7 @@ export const resourceMethods = {
       if (current.tiles >= RESOURCE_TRADE_RADIUS) continue;
       for (const neighbour of this._neighbours(current.coords)) {
         const nk = key(neighbour.x, neighbour.y);
-        if (visited.has(nk)) continue;
+        if (visited.has(nk) || blocked.has(nk)) continue;
         const tile = this.tiles[nk];
         if (!tile || !(tile.terrainClass === "Land" || tile.terrainClass === "Sea" ||
             tile.terrainClass === "Ice")) continue;
@@ -1981,6 +2086,19 @@ export const resourceMethods = {
     return results;
   },
 
+  // The middleman margin a region charges when it sells food it bought to relay.
+  // Only the stock it actually bought for resale carries the margin, and the
+  // stock is drawn down as it sells.
+  _middlemanMarkup(supplier, id, bought, price) {
+    if (id !== "food" || !this._relayStock || supplier.kind !== "city") return 0;
+    const cityId = supplier.city.id;
+    const held = this._relayStock.get(cityId) || 0;
+    if (!(held > 0)) return 0;
+    const sold = Math.min(held, bought);
+    this._relayStock.set(cityId, held - sold);
+    return sold * price * MIDDLEMAN.markup;
+  },
+
   _nodeSurplus(node, id) {
     if (id === "hightech") return Math.max(0, node.stock.hightech || 0);
     const population = node.city ? this.getCityEconomy(node.city).population : 0;
@@ -2165,13 +2283,14 @@ export const resourceMethods = {
     const here = this.cityAt(unit.coords);
     if (here && here.civ === civ) return here;
     if (this._hasSeaSupply(unit.coords, civ)) return this._nearestAlliedCity(civ, unit.coords);
+    const blocked = this._hostileOccupied(civ);
     const visited = new Set([key(unit.coords.x, unit.coords.y)]);
     const queue = [unit.coords];
     while (queue.length > 0) {
       const coords = queue.shift();
       for (const neighbour of this._neighbours(coords)) {
         const nk = key(neighbour.x, neighbour.y);
-        if (visited.has(nk)) continue;
+        if (visited.has(nk) || blocked.has(nk)) continue;
         const tile = this.tiles[nk];
         if (!tile) continue;
         const sea = tile.terrainClass === "Sea";
diff --git a/tests/central_bank_fx_test.js b/tests/central_bank_fx_test.js
new file mode 100644
index 0000000..f80fd56
--- /dev/null
+++ b/tests/central_bank_fx_test.js
@@ -0,0 +1,92 @@
+import { TestCase } from "./framework/test_case.js";
+import { smallState, cityOf } from "./framework/helpers.js";
+import { MONEY } from "../shared/data.js";
+
+// Gives both central banks a symmetric reserve of each other's currency and the
+// matching issued-currency debt, so the FX desk has something to work.
+function bookReserves(state, amount = 1000) {
+  const a = state.currencyOf(0).code;
+  const b = state.currencyOf(1).code;
+  state._addReserve(1, a, amount);
+  state._addReserve(0, b, amount);
+  state._addCentralBankDebt(0, a, amount);
+  state._addCentralBankDebt(1, b, amount);
+  return { a, b };
+}
+
+export class CentralBankFxTest extends TestCase {
+  test_redeeming_own_currency_retires_it_from_the_foreign_bank() {
+    const state = smallState();
+    const { a, b } = bookReserves(state, 1000);
+    const redeemed = state._redeemOwnCurrency(0, 500);
+    this.assertApprox(redeemed, 500);
+    // Britain hands back the francs it held; France hands back the pounds.
+    this.assertApprox(state.getCentralBankReserves(1).get(a), 500);
+    this.assertApprox(state.getCentralBankReserves(0).get(b), 500);
+    // Both issued-currency liabilities shrink with the money retired.
+    this.assertApprox(state.getCentralBankDebt(0).get(a), 500);
+    this.assertApprox(state.getCentralBankDebt(1).get(b), 500);
+    // The bid for francs and the offer of pounds register as currency demand.
+    this.assertGreater(state._currencyDemand.get(0) || 0, 0);
+    this.assertLess(state._currencyDemand.get(1) || 0, 0);
+  }
+
+  test_a_weak_rate_sells_more_reserves_than_a_strong_one() {
+    const strong = smallState();
+    bookReserves(strong, 1_000_000);
+    strong.setCurrencyValue(0, 1);
+    const ordinary = strong._settleCentralBankFx(0);
+
+    const weak = smallState();
+    bookReserves(weak, 1_000_000);
+    weak.setCurrencyValue(0, 0.9);
+    const defended = weak._settleCentralBankFx(0);
+
+    this.assertGreater(ordinary, 0, "the bank redeems even when the rate is calm");
+    this.assertGreater(defended, ordinary, "a weak rate is defended harder");
+  }
+
+  test_the_bank_leaves_its_reserves_alone_without_money_held_abroad() {
+    const state = smallState();
+    const b = state.currencyOf(1).code;
+    state._addReserve(0, b, 1_000_000);
+    this.assertApprox(state._settleCentralBankFx(0), 0);
+  }
+
+  test_the_import_subsidy_draws_on_its_own_currency_reserves() {
+    const state = smallState();
+    const buyer = cityOf(state, 0);
+    const a = state.currencyOf(0).code;
+    const b = state.currencyOf(1).code;
+    state._addReserve(0, b, 1_000_000);
+    const before = state.getRegionCash(buyer).get(a) || 0;
+    const acquired = state._regionAcquireCurrency(buyer, 1, 1000, null, true, 400);
+    this.assertApprox(acquired.subsidised, 400, 1e-6, "the bank covered its share");
+    this.assertApprox(acquired.amount, 1000, 1e-6, "the seller is paid in full");
+    this.assertApprox(
+      state.getCentralBankReserves(0).get(b), 999_600, 1e-6,
+      "the subsidy came out of reserves"
+    );
+    const after = state.getRegionCash(buyer).get(a) || 0;
+    this.assertApprox(before - after, 600, 1e-6, "the buyer paid only the rest");
+  }
+
+  test_the_import_subsidy_budget_is_capped_for_the_day() {
+    const state = smallState();
+    const b = state.currencyOf(1).code;
+    state._addReserve(0, b, 1_000_000);
+    state._fxSubsidySpent = new Map();
+    const budget = 1_000_000 * MONEY.fx.importSubsidyPerDay;
+    const first = state._centralBankImportSubsidy(0, "food", 1_000_000);
+    this.assertApprox(first, budget, 1e-6);
+    state._recordImportSubsidy(0, first);
+    this.assertApprox(state._centralBankImportSubsidy(0, "food", 1_000_000), 0);
+  }
+
+  test_no_subsidy_for_a_resource_the_bank_does_not_back() {
+    const state = smallState();
+    const b = state.currencyOf(1).code;
+    state._addReserve(0, b, 1_000_000);
+    this.assertEqual(state._centralBankImportSubsidy(0, "luxury", 1_000_000), 0);
+  }
+}
diff --git a/tests/food_security_test.js b/tests/food_security_test.js
new file mode 100644
index 0000000..ad1ad68
--- /dev/null
+++ b/tests/food_security_test.js
@@ -0,0 +1,43 @@
+import { TestCase } from "./framework/test_case.js";
+import { smallState, cityOf } from "./framework/helpers.js";
+import { RESOURCE_IDS, MIDDLEMAN } from "../shared/data.js";
+import { foodNeedPerDay } from "../shared/resources.js";
+
+export class FoodSecurityTest extends TestCase {
+  test_starving_regions_buy_before_stocked_ones() {
+    const state = smallState();
+    const cities = state.cities.filter((city) => city.civ === 0);
+    this.assertGreater(cities.length, 1, "the fixture has two regions");
+    const starving = cities[0];
+    const stocked = cities[1];
+    state.getCityResourceStock(starving).food = 0;
+    state.getCityResourceStock(stocked).food =
+      foodNeedPerDay(state.getCityEconomy(stocked).population) * 5;
+    const order = [];
+    const orig = state._procureFromNeighbours.bind(state);
+    state._procureFromNeighbours = function (city, ...rest) {
+      order.push(city.id);
+      return orig(city, ...rest);
+    };
+    const market = new Map(RESOURCE_IDS.map((id) => [
+      id, { supply: 0, demand: 0, price: state.getResourcePrice(id) },
+    ]));
+    state._consumeCityResources(new Map(), market);
+    this.assertEqual(order[0], starving.id, "the empty region buys first");
+  }
+
+  test_a_middleman_charges_a_margin_on_relayed_food() {
+    const state = smallState();
+    const city = cityOf(state, 0);
+    state._relayStock.set(city.id, 100);
+    const price = state.getResourcePrice("food");
+    const supplier = { kind: "city", city };
+    const markup = state._middlemanMarkup(supplier, "food", 60, price);
+    this.assertApprox(markup, 60 * price * MIDDLEMAN.markup);
+    this.assertApprox(state._relayStock.get(city.id), 40, 1e-9, "only the sold stock is drawn");
+    this.assertEqual(
+      state._middlemanMarkup(supplier, "steel", 10, price), 0,
+      "only food carries the margin"
+    );
+  }
+}
diff --git a/tests/map_modes_test.js b/tests/map_modes_test.js
index e15dca5..fb496db 100644
--- a/tests/map_modes_test.js
+++ b/tests/map_modes_test.js
@@ -159,6 +159,40 @@ export class MapModesTest extends TestCase {
     }
   }
 
+  async test_the_delivery_graph_has_a_colour_legend() {
+    const state = smallState();
+    const env = await setupDom();
+    const map = this.buildMap(env);
+    try {
+      map.applySnapshot(state.snapshot(0));
+      map.setMapMode("population");
+      map.setResourceGraph({
+        nodes: [[1, 1, 1]],
+        links: [[0, 0, 1, 1, "steel", 10], [2, 2, 3, 3, "food", 5]],
+      });
+      this.assertFalse(
+        env.$("#map-legend-graph").hasClass("hidden"), "the trade-route key is shown"
+      );
+      const labels = env.$("#map-legend-resources .map-legend-resource")
+        .map((_, el) => env.$(el).text()).get();
+      this.assertTrue(labels.some((t) => t.includes("Steel")), "steel is keyed");
+      this.assertTrue(labels.some((t) => t.includes("Food")), "food is keyed");
+      this.assertTrue(labels.some((t) => t.includes("Storage node")), "the node marker is keyed");
+      this.assertSize(
+        env.$("#map-legend-resources .map-legend-swatch"), 2,
+        "one swatch per commodity on the map"
+      );
+      map.setMapMode("terrain");
+      map.setResourceGraph({ nodes: [[1, 1, 1]], links: [[0, 0, 1, 1, "steel", 10]] });
+      this.assertTrue(
+        env.$("#map-legend-graph").hasClass("hidden"),
+        "the key hides off the economic map"
+      );
+    } finally {
+      teardownDom(env);
+    }
+  }
+
   async test_fog_rebuilds_are_coalesced_between_frames() {
     const env = await setupDom();
     const map = this.buildMap(env);
diff --git a/tests/resource_efficiency_test.js b/tests/resource_efficiency_test.js
new file mode 100644
index 0000000..e168bb8
--- /dev/null
+++ b/tests/resource_efficiency_test.js
@@ -0,0 +1,114 @@
+import { TestCase } from "./framework/test_case.js";
+import { smallState } from "./framework/helpers.js";
+import { key } from "../shared/hex.js";
+import {
+  RESOURCE_IDS,
+  RESOURCE_EFFICIENCY_EFFECT,
+  TECHNOLOGIES,
+} from "../shared/data.js";
+import { producerEnergyPerDay } from "../shared/resources.js";
+
+// Puts a resource building on the first free land tile of `civ`.
+function placeProducer(state, id, civ = 0) {
+  for (const coords of state._territoryByCiv.get(civ) || []) {
+    if (state.cityAt(coords)) continue;
+    if (!state._isLand(coords)) continue;
+    const k = key(coords.x, coords.y);
+    if (state.tileImprovements.has(k)) continue;
+    state.tileImprovements.set(k, id);
+    state.tileImprovementOwner.set(k, civ);
+    state.tileImprovementHp.set(k, 1000);
+    state._ensureResourceStock(k);
+    state._ensureBuildingAgent(k);
+    return { coords, k };
+  }
+  return null;
+}
+
+// Studies one level of a repeatable technology for a nation.
+function research(state, civ, id) {
+  const index = state.technologies.findIndex((tech) => tech.id === id);
+  state.researched.get(civ).add(index);
+  const counts = state.repeatCounts.get(civ);
+  counts.set(id, (counts.get(id) || 0) + 1);
+  state._touchModifiers();
+}
+
+export class ResourceEfficiencyDataTest extends TestCase {
+  test_there_is_exactly_one_efficiency_technology_per_resource() {
+    for (const resource of RESOURCE_IDS) {
+      const stat = RESOURCE_EFFICIENCY_EFFECT[resource];
+      const matching = TECHNOLOGIES.filter(
+        (tech) => (tech.effects || []).some((e) => e.stat === stat)
+      );
+      this.assertSize(matching, 1, `one technology raises ${resource} output per input`);
+      this.assertTrue(matching[0].repeatable, `${resource} efficiency is repeatable`);
+    }
+  }
+}
+
+export class ResourceEfficiencyModelTest extends TestCase {
+  test_efficiency_is_zero_until_the_technology_is_researched() {
+    const state = smallState();
+    this.assertApprox(state.resourceEfficiency(0, "steel"), 0);
+    research(state, 0, "advanced_metallurgy");
+    this.assertApprox(state.resourceEfficiency(0, "steel"), 0.05);
+    research(state, 0, "advanced_metallurgy");
+    this.assertApprox(state.resourceEfficiency(0, "steel"), 0.10, 1e-9);
+    this.assertApprox(state.resourceEfficiency(1, "steel"), 0, 1e-9, "research is national");
+  }
+
+  test_steel_efficiency_cuts_the_mill_s_energy_draw() {
+    const state = smallState();
+    const placed = placeProducer(state, "steel_mill");
+    const components = state._resourceLandComponents();
+    const componentId = components.get(placed.k);
+    const draw = () => state._buildEnergyLedger(components).get(componentId).converters
+      .find((entry) => entry.k === placed.k).amount;
+    const before = draw();
+    this.assertGreater(before, 0);
+    research(state, 0, "advanced_metallurgy");
+    const after = draw();
+    this.assertApprox(before / after, 1.05, 1e-6, "the same steel costs 5% less power");
+  }
+
+  test_energy_efficiency_cuts_the_plant_s_fuel() {
+    const state = smallState();
+    const placed = placeProducer(state, "coal_power_plant");
+    const components = state._resourceLandComponents();
+    const componentId = components.get(placed.k);
+    const fuel = () => state._buildEnergyLedger(components).get(componentId).fuelEnergy;
+    const before = fuel();
+    this.assertGreater(before, 0);
+    research(state, 0, "efficient_generation");
+    const after = fuel();
+    this.assertApprox(before / after, 1.05, 1e-6, "the same power burns 5% less fuel");
+  }
+
+  test_every_efficiency_technology_reads_back_on_its_own_resource() {
+    const state = smallState();
+    const byResource = {
+      energy: "efficient_generation",
+      steel: "advanced_metallurgy",
+      food: "precision_agriculture",
+      luxury: "automated_extraction",
+      hightech: "advanced_lithography",
+    };
+    for (const [resource, tech] of Object.entries(byResource)) {
+      research(state, 0, tech);
+      this.assertApprox(state.resourceEfficiency(0, resource), 0.05, 1e-9, resource);
+    }
+  }
+
+  test_food_efficiency_cuts_the_harvest_s_energy() {
+    const state = smallState();
+    this.assertApprox(
+      state._foodHarvestEnergy(0, 1000), producerEnergyPerDay("food", 1000)
+    );
+    research(state, 0, "precision_agriculture");
+    this.assertApprox(
+      state._foodHarvestEnergy(0, 1000), producerEnergyPerDay("food", 1000) / 1.05, 1e-6,
+      "better farming grows the same food for less power"
+    );
+  }
+}
diff --git a/tests/trade_route_test.js b/tests/trade_route_test.js
new file mode 100644
index 0000000..e5c2650
--- /dev/null
+++ b/tests/trade_route_test.js
@@ -0,0 +1,55 @@
+import { TestCase } from "./framework/test_case.js";
+import { smallState, cityOf } from "./framework/helpers.js";
+import { key } from "../shared/hex.js";
+
+// Puts a food-holding producer on an adjacent owned land tile and wires the road,
+// so it is a reachable supplier for the city.
+function placeSupplier(state, city) {
+  const neighbour = state.topology.neighbours(city.coords.x, city.coords.y)
+    .find((n) => state._isLand(n) && state.civAt(n) === city.civ);
+  const k = key(neighbour.x, neighbour.y);
+  if (!state.roads.has(k) && !state.railways.has(k)) state.roads.add(k);
+  state.tileImprovements.set(k, "steel_mill");
+  state.tileImprovementOwner.set(k, city.civ);
+  state.tileImprovementHp.set(k, 1000);
+  state._ensureResourceStock(k).food = 1_000_000;
+  state._ensureBuildingAgent(k);
+  return { coords: neighbour, k };
+}
+
+function hasSupplierAt(result, coords) {
+  return result.some((entry) => entry.coords.x === coords.x && entry.coords.y === coords.y);
+}
+
+export class TradeRouteTest extends TestCase {
+  test_a_hostile_unit_cuts_a_trade_route() {
+    const state = smallState();
+    const city = cityOf(state, 0);
+    const supplier = placeSupplier(state, city);
+    const nodes = state._resourceNodes();
+    this.assertTrue(
+      hasSupplierAt(state._nearbySuppliers(city, "food", nodes), supplier.coords),
+      "the supplier is reachable while the road is clear"
+    );
+    // A hostile army standing on the road closes it.
+    state._declareWar(0, 1);
+    state._spawnUnit(supplier.coords, 1, state.protoUnits[0]);
+    this.assertFalse(
+      hasSupplierAt(state._nearbySuppliers(city, "food", nodes), supplier.coords),
+      "the hostile unit cut the route"
+    );
+  }
+
+  test_a_neutral_unit_leaves_the_route_open() {
+    const state = smallState();
+    const city = cityOf(state, 0);
+    const supplier = placeSupplier(state, city);
+    // No war: the same troops are neutral traffic.
+    state._spawnUnit(supplier.coords, 1, state.protoUnits[0]);
+    const nodes = state._resourceNodes();
+    this.assertTrue(
+      hasSupplierAt(state._nearbySuppliers(city, "food", nodes), supplier.coords),
+      "neutral troops do not cut the route"
+    );
+  }
+}