feat: teach the trader that prices can move

Previously we assuemed the buy/sell price was fixed
which is wrong (and really wrong for low trade volumes)
This does not yet fix the low trade
volume problem, but it does make
it possible to fix it.

packages/cli/lib/behavior/central_command.dart

@@ -657,7 +657,7 @@ class CentralCommand {
           good.tradeSymbolObject,
         );
         if (unitsNeeded > 0) {
-          yield SellOpp(
+          yield SellOpp.fromContract(
             marketSymbol: good.destination,
             tradeSymbol: good.tradeSymbolObject,
             contractId: contract.id,

packages/cli/lib/behavior/trader.dart

@@ -121,6 +121,7 @@ Future<DateTime?> _handleAtSourceWithDeal(
   final good = currentMarket.marketTradeGood(dealTradeSymbol)!;
 
   final maxPerUnitPrice = costedDeal.maxPurchaseUnitPrice;
+  final nextExpectedPrice = costedDeal.predictNextPurchasePrice;
 
   // Could this get confused by having other cargo in our hold?
   final units = unitsToPurchase(good, ship, costedDeal.maxUnitsToBuy);
@@ -153,7 +154,7 @@ Future<DateTime?> _handleAtSourceWithDeal(
         ship,
         'Purchased ${transaction.quantity} ${transaction.tradeSymbol} '
         '@ ${transaction.perUnitPrice} (expected '
-        '${costedDeal.deal.purchasePrice}) = '
+        '${creditsString(nextExpectedPrice)}) = '
         '${creditsString(transaction.creditChange)}',
       );
     }

packages/cli/lib/cache/market_prices.dart

@@ -99,6 +99,48 @@ class MarketPrice {
   /// The timestamp of the price record.
   final DateTime timestamp;
 
+  /// Predict the price of buying the Nth unit of this good.
+  /// Unit is a 0-based index of the unit being purchased.
+  int predictPurchasePriceForUnit(int unit) {
+    // TODO(eseidel): movementPerBatch is wrong.
+    // For large trade volumes, it's probably 0, for small trade volumes
+    // it can be very large.
+    const movementPerBatch = 1;
+    final batchCount = unit ~/ tradeVolume;
+    final movement = movementPerBatch * batchCount;
+    return purchasePrice + movement;
+  }
+
+  /// Predict the price of buying the Nth unit of this good.
+  /// Unit is a 0-based index of the unit being purchased.
+  int predictSellPriceForUnit(int unit) {
+    // TODO(eseidel): movementPerBatch is wrong.
+    // For large trade volumes, it's probably 0, for small trade volumes
+    // it can be very large.
+    const movementPerBatch = -1;
+    final batchCount = unit ~/ tradeVolume;
+    final movement = movementPerBatch * batchCount;
+    return sellPrice + movement;
+  }
+
+  /// Predict the total price of buying [units] of this good.
+  int totalPurchasePriceFor(int units) {
+    var totalPrice = 0;
+    for (var i = 0; i < units; i++) {
+      totalPrice += predictPurchasePriceForUnit(i);
+    }
+    return totalPrice;
+  }
+
+  /// Predict the total price of buying [units] of this good.
+  int totalSellPriceFor(int units) {
+    var totalPrice = 0;
+    for (var i = 0; i < units; i++) {
+      totalPrice += predictSellPriceForUnit(i);
+    }
+    return totalPrice;
+  }
+
   @override
   bool operator ==(Object other) =>
       identical(this, other) ||

packages/cli/lib/market_scan.dart

@@ -7,38 +7,48 @@ import 'package:meta/meta.dart';
 @immutable
 class BuyOpp {
   /// Create a new BuyOpp.
-  const BuyOpp({
-    required this.marketSymbol,
-    required this.tradeSymbol,
-    required this.price,
-  });
+  const BuyOpp(this.marketPrice);
+
+  /// State of the market where this buy opportunity was found.
+  final MarketPrice marketPrice;
 
   /// The symbol of the market where the good can be purchased.
-  final WaypointSymbol marketSymbol;
+  WaypointSymbol get marketSymbol => marketPrice.waypointSymbol;
 
   /// The symbol of the good offered for purchase.
-  final TradeSymbol tradeSymbol;
+  TradeSymbol get tradeSymbol => marketPrice.tradeSymbol;
 
   /// The price of the good.
-  final int price;
+  int get price => marketPrice.purchasePrice;
 }
 
 /// A potential sale opportunity.  Only public for testing.
 @immutable
 class SellOpp {
-  /// Create a new SellOpp.
-  const SellOpp({
+  /// Create a new SellOpp from a MarketPrice.
+  SellOpp.fromMarketPrice(MarketPrice this.marketPrice)
+      : marketSymbol = marketPrice.waypointSymbol,
+        tradeSymbol = marketPrice.tradeSymbol,
+        price = marketPrice.sellPrice,
+        contractId = null,
+        maxUnits = null;
+
+  /// Create a new SellOpp from a contract.
+  const SellOpp.fromContract({
     required this.marketSymbol,
     required this.tradeSymbol,
     required this.price,
-    this.contractId,
-    this.maxUnits,
-  });
+    required this.contractId,
+    required this.maxUnits,
+  }) : marketPrice = null;
+
+  /// State of the market where this sell opportunity was found.
+  final MarketPrice? marketPrice;
 
   /// The symbol of the market where the good can be sold.
   final WaypointSymbol marketSymbol;
 
-  /// The symbol of the good.
+  /// The symbol of the good offered for sold.
   final TradeSymbol tradeSymbol;
 
   /// The price of the good.
@@ -85,22 +95,8 @@ class _MarketScanBuilder {
 
   /// Record potential deals from the given historical market price.
   void visitMarketPrice(MarketPrice marketPrice) {
-    final marketSymbol = marketPrice.waypointSymbol;
-    final tradeSymbol = marketPrice.tradeSymbol;
-    _addBuyOpp(
-      BuyOpp(
-        marketSymbol: marketSymbol,
-        tradeSymbol: tradeSymbol,
-        price: marketPrice.purchasePrice,
-      ),
-    );
-    _addSellOpp(
-      SellOpp(
-        marketSymbol: marketSymbol,
-        tradeSymbol: tradeSymbol,
-        price: marketPrice.sellPrice,
-      ),
-    );
+    _addBuyOpp(BuyOpp(marketPrice));
+    _addSellOpp(SellOpp.fromMarketPrice(marketPrice));
   }
 }