Rename Option/Account to Market/Ledger, add Result-based error handling

Split GuessMarketContext into a separate CatalogContext for read-only
event listing, wrap fallible operations in Result<T> instead of throwing,
and wire load/save of events and users through a JSON Wrapper.

18 files changed, 295 insertions, 133 deletions

diff --git a/service/src/main/java/market/guess/service/LocalCatalogContext.java b/service/src/main/java/market/guess/service/LocalCatalogContext.java
new file mode 100644
index 0000000..15a5890
--- /dev/null
+++ b/service/src/main/java/market/guess/service/LocalCatalogContext.java
@@ -0,0 +1,45 @@
+package market.guess.service;
+
+import java.nio.file.Path;
+import java.util.List;
+import market.guess.api.CatalogContext;
+import market.guess.api.EventDetailDTO;
+import market.guess.api.EventSummaryDTO;
+import market.guess.api.LoadResultDTO;
+import market.guess.api.Result;
+import market.guess.service.domain.Event;
+import market.guess.service.infrastructure.MarketContext;
+import market.guess.service.infrastructure.provider.Loader;
+
+public final class LocalCatalogContext implements CatalogContext {
+
+  private final MarketContext context;
+  private final Loader provider;
+
+  public LocalCatalogContext(Loader provider, MarketContext context) {
+    super();
+    this.provider = provider;
+    this.context = context;
+  }
+
+  @Override
+  public Result<LoadResultDTO> loadEvents(Path path) {
+    return Result.ok(provider.load(path));
+  }
+
+  @Override
+  public Result<List<EventSummaryDTO>> getAllEvents() {
+    return Result.ok(context.getEvents().getAll().stream().map(Event::toEventSummary).toList());
+  }
+
+  @Override
+  public Result<EventDetailDTO> getEvent(String eventKey) {
+    var optional = context.getEvents().get(eventKey);
+
+    if (optional.isEmpty()) return Result.error("Unable to find event.");
+    var event = optional.get();
+
+    return Result.ok(
+        new EventDetailDTO(event.toEventSummary(), event.toMarketState(), event.getHistory(), ""));
+  }
+}
diff --git a/service/src/main/java/market/guess/service/LocalGuessMarketContext.java b/service/src/main/java/market/guess/service/LocalGuessMarketContext.java
index b405329..8f97432 100644
--- a/service/src/main/java/market/guess/service/LocalGuessMarketContext.java
+++ b/service/src/main/java/market/guess/service/LocalGuessMarketContext.java
@@ -4,83 +4,66 @@ import java.nio.file.Path;
 import java.util.List;
 import market.guess.api.AccountDTO;
 import market.guess.api.EventDetailDTO;
-import market.guess.api.EventSummaryDTO;
 import market.guess.api.GuessMarketContext;
 import market.guess.api.LoadResultDTO;
+import market.guess.api.ProblemDTO;
 import market.guess.api.PurchaseReceiptDTO;
-import market.guess.exception.GuessMarketException;
-import market.guess.service.domain.Event;
+import market.guess.api.Result;
 import market.guess.service.infrastructure.MarketContext;
-import market.guess.service.infrastructure.provider.Loader;
 
 public final class LocalGuessMarketContext implements GuessMarketContext {
 
   private final TradingOperation tradingOperation;
   private final MarketContext context;
-  private final Loader provider;
 
-  public LocalGuessMarketContext(
-      Loader provider, MarketContext context, TradingOperation tradingOperation) {
+  public LocalGuessMarketContext(MarketContext context, TradingOperation tradingOperation) {
     super();
-    this.provider = provider;
     this.context = context;
     this.tradingOperation = tradingOperation;
   }
 
   @Override
-  public LoadResultDTO loadEvents(Path path) {
-    return provider.load(path);
-  }
-
-  @Override
-  public List<EventSummaryDTO> listEvents() {
-    return context.getEvents().getAll().stream().map(Event::toEventSummary).toList();
-  }
-
-  @Override
-  public boolean isLoaded() {
-    throw new UnsupportedOperationException("Unimplemented method 'isLoaded'");
-  }
-
-  @Override
-  public EventDetailDTO getEvent(String eventKey) {
-    throw new UnsupportedOperationException("Unimplemented method 'getEvent'");
-  }
-
-  @Override
-  public PurchaseReceiptDTO buyShares(
-      String userName, String eventKey, String optionKey, int quantity)
-      throws GuessMarketException {
+  public Result<PurchaseReceiptDTO> buyShares(
+      String userName, String eventKey, String optionKey, int quantity) {
     var optional = context.getEvents().get(eventKey);
 
     var userOptional = context.getUsers().get(userName);
-    if (optional.isEmpty() || userOptional.isEmpty()) throw new GuessMarketException();
+    if (optional.isEmpty() || userOptional.isEmpty()) Result.error("Unable to find user/event.");
 
     var event = optional.get();
     var user = userOptional.get();
     var option = event.getOption(optionKey);
     var trade = tradingOperation.buy(event, user, option, quantity);
-    return new PurchaseReceiptDTO();
+    return Result.ok(new PurchaseReceiptDTO());
   }
 
   @Override
-  public EventDetailDTO closeEvent(String userName, String eventKey, String winningOptionKey)
-      throws GuessMarketException {
+  public Result<EventDetailDTO> closeEvent(
+      String userName, String eventKey, String winningOptionKey) {
     throw new UnsupportedOperationException("Unimplemented method 'account'");
   }
 
   @Override
-  public AccountDTO account(String userName) {
+  public Result<AccountDTO> account(String userName) {
     throw new UnsupportedOperationException("Unimplemented method 'account'");
   }
 
   @Override
-  public void saveState(Path path) throws GuessMarketException {
+  public Result<Void> saveState(Path path) {
     context.save();
+    return Result.ok();
   }
 
   @Override
-  public LoadResultDTO restoreState(Path path) {
-    throw new UnsupportedOperationException("Unimplemented method 'restoreState'");
+  public Result<LoadResultDTO> restoreState(Path path) {
+
+    try {
+      context.load();
+    } catch (Exception e) {
+      return Result.error(
+          "Failed to load.",
+          LoadResultDTO.failure(path.toString(), List.of(new ProblemDTO("", e.getMessage(), ""))));
+    }
+    return Result.ok(LoadResultDTO.success(path.toString(), context.getEvents().getAll().size()));
   }
 }
diff --git a/service/src/main/java/market/guess/service/TradingOperation.java b/service/src/main/java/market/guess/service/TradingOperation.java
index c8153c2..63572c0 100644
--- a/service/src/main/java/market/guess/service/TradingOperation.java
+++ b/service/src/main/java/market/guess/service/TradingOperation.java
@@ -1,13 +1,13 @@
 package market.guess.service;
 
 import market.guess.service.domain.Event;
-import market.guess.service.domain.Option;
+import market.guess.service.domain.Market;
 import market.guess.service.domain.Settlement;
 import market.guess.service.domain.Trade;
 import market.guess.service.domain.User;
 
 public interface TradingOperation {
-  Trade buy(Event event, User user, Option Option, int quantity);
+  Trade buy(Event event, User user, Market Option, int quantity);
 
   Settlement settle(Event event, String winningOption);
 }
diff --git a/service/src/main/java/market/guess/service/TradingOperationImpl.java b/service/src/main/java/market/guess/service/TradingOperationImpl.java
index f6c9aa1..641a3a9 100644
--- a/service/src/main/java/market/guess/service/TradingOperationImpl.java
+++ b/service/src/main/java/market/guess/service/TradingOperationImpl.java
@@ -1,7 +1,7 @@
 package market.guess.service;
 
 import market.guess.service.domain.Event;
-import market.guess.service.domain.Option;
+import market.guess.service.domain.Market;
 import market.guess.service.domain.Settlement;
 import market.guess.service.domain.Trade;
 import market.guess.service.domain.User;
@@ -9,7 +9,7 @@ import market.guess.service.domain.User;
 public final class TradingOperationImpl implements TradingOperation {
 
   @Override
-  public Trade buy(Event event, User user, Option Option, int quantity) {
+  public Trade buy(Event event, User user, Market Option, int quantity) {
     // TODO Auto-generated method stub
     throw new UnsupportedOperationException("Unimplemented method 'buy'");
   }
diff --git a/service/src/main/java/market/guess/service/domain/Account.java b/service/src/main/java/market/guess/service/domain/Account.java
deleted file mode 100644
index 48d5054..0000000
--- a/service/src/main/java/market/guess/service/domain/Account.java
+++ /dev/null
@@ -1,49 +0,0 @@
-package market.guess.service.domain;
-
-import java.math.BigDecimal;
-import java.time.Clock;
-import java.util.ArrayList;
-import java.util.List;
-import market.guess.api.LedgerType;
-
-public final class Account {
-  private final String owner;
-  private final List<LedgerEntry> entries = new ArrayList<>();
-  private BigDecimal balance;
-  private int runningId;
-
-  private final Clock clock;
-
-  public Account(String owner, BigDecimal initialBalance) {
-    this(owner, initialBalance, Clock.systemUTC());
-  }
-
-  public Account(String owner, BigDecimal initialBalance, Clock clock) {
-    super();
-    this.owner = owner;
-    this.balance = BigDecimalOptions.toMoney(initialBalance);
-    this.clock = clock;
-  }
-
-  public String getOwner() {
-    return owner;
-  }
-
-  public BigDecimal getBalance() {
-    return balance;
-  }
-
-  public boolean credit(LedgerType type, BigDecimal amount, String note) {
-    record(type, amount, note);
-    return true;
-  }
-
-  public boolean debit(LedgerType type, BigDecimal amount, String note) {
-    record(type, amount.negate(), note);
-    return true;
-  }
-
-  private void record(LedgerType type, BigDecimal amount, String note) {
-    entries.add(new LedgerEntry(runningId++, clock.instant(), type, amount, balance, note));
-  }
-}
diff --git a/service/src/main/java/market/guess/service/domain/Event.java b/service/src/main/java/market/guess/service/domain/Event.java
index ce35e82..5c98908 100644
--- a/service/src/main/java/market/guess/service/domain/Event.java
+++ b/service/src/main/java/market/guess/service/domain/Event.java
@@ -7,6 +7,9 @@ import java.util.List;
 import market.guess.api.CommissionTiming;
 import market.guess.api.EventStatus;
 import market.guess.api.EventSummaryDTO;
+import market.guess.api.MarketStateDTO;
+import market.guess.api.TradeRowDTO;
+import market.guess.service.ledger.Ledger;
 import market.guess.service.mechanism.TradingMechanism;
 
 public final class Event {
@@ -17,13 +20,13 @@ public final class Event {
   private final int commissionPercent;
   private final CommissionTiming commissionTiming;
   private final TradingMechanism mechanism;
-  private final List<Option> options;
-  private final Account account;
+  private final List<Market> markets;
+  private final Ledger ledger;
   private final String marketMaker;
 
   private int runningId;
   private final List<Trade> trades = new ArrayList<>();
-  private EventStatus status;
+  private EventStatus state;
   private String winningOptionKey;
   private BigDecimal settlementCommission = BigDecimalOptions.ZERO_MONEY;
 
@@ -36,7 +39,7 @@ public final class Event {
       CommissionTiming commissionTiming,
       TradingMechanism mechanism,
       EventStatus status,
-      List<Option> options,
+      List<Market> markets,
       String marketMaker) {
     this.eventKey = eventKey;
     this.displayId = displayId;
@@ -45,10 +48,10 @@ public final class Event {
     this.commissionPercent = commissionPercent;
     this.commissionTiming = commissionTiming;
     this.mechanism = mechanism;
-    this.options = List.copyOf(options);
+    this.markets = markets;
     this.marketMaker = marketMaker;
-    this.account = new Account(eventKey, BigDecimalOptions.ZERO_MONEY);
-    this.status = status;
+    this.ledger = new Ledger(eventKey, BigDecimalOptions.ZERO_MONEY);
+    this.state = status;
   }
 
   public String getEventKey() {
@@ -79,16 +82,16 @@ public final class Event {
     return mechanism;
   }
 
-  public List<Option> getOptions() {
-    return options;
+  public List<Market> getMarkets() {
+    return markets;
   }
 
   public List<String> getOptionsNames() {
-    return options.stream().map(Option::name).toList();
+    return markets.stream().map(Market::getKey).toList();
   }
 
-  public Account getAccount() {
-    return account;
+  public Ledger getLedger() {
+    return ledger;
   }
 
   public String getMarketMaker() {
@@ -99,12 +102,12 @@ public final class Event {
     return trades;
   }
 
-  public EventStatus getStatus() {
-    return status;
+  public EventStatus getState() {
+    return state;
   }
 
-  public void setStatus(EventStatus status) {
-    this.status = status;
+  public void setState(EventStatus status) {
+    this.state = status;
   }
 
   public String getWinningOptionKey() {
@@ -118,12 +121,13 @@ public final class Event {
   public Trade recordTrade(
       Instant time,
       String user,
-      Option option,
+      Market option,
       int quantity,
       BigDecimal cost,
       BigDecimal commission) {
     var trade =
-        new Trade(runningId++, time, user, option.key(), option.name(), quantity, cost, commission);
+        new Trade(
+            runningId++, time, user, option.getKey(), option.getName(), quantity, cost, commission);
     trades.add(trade);
 
     return trade;
@@ -143,6 +147,20 @@ public final class Event {
         .add(settlementCommission);
   }
 
+  public List<TradeRowDTO> getHistory() {
+    var history = new ArrayList<TradeRowDTO>();
+    for (var trade : trades.reversed()) {
+      history.add(
+          new TradeRowDTO(
+              trade.at().toString(),
+              trade.userName(),
+              trade.optionName(),
+              String.valueOf(trade.quantity()),
+              trade.sharesCost().toPlainString()));
+    }
+    return history;
+  }
+
   public EventSummaryDTO toEventSummary() {
     return new EventSummaryDTO(
         eventKey,
@@ -152,14 +170,23 @@ public final class Event {
         commissionPercent,
         commissionTiming,
         mechanism.getType(),
-        status,
+        state,
         getOptionsNames(),
-        account.getBalance().toPlainString());
+        ledger.getBalance().toPlainString());
+  }
+
+  public MarketStateDTO toMarketState() {
+    return new MarketStateDTO(
+        eventKey,
+        mechanism.getType(),
+        markets.stream().map(Market::toMarket).toList(),
+        String.valueOf(ledger.getBalance()),
+        "");
   }
 
-  public Option getOption(String optionKey) {
-    return options.stream()
-        .filter(option -> option.key().equalsIgnoreCase(optionKey))
+  public Market getOption(String optionKey) {
+    return markets.stream()
+        .filter(option -> option.getKey().equalsIgnoreCase(optionKey))
         .findFirst()
         .orElseThrow(() -> new IllegalArgumentException("No such option."));
   }
diff --git a/service/src/main/java/market/guess/service/domain/Market.java b/service/src/main/java/market/guess/service/domain/Market.java
new file mode 100644
index 0000000..ce95bae
--- /dev/null
+++ b/service/src/main/java/market/guess/service/domain/Market.java
@@ -0,0 +1,32 @@
+package market.guess.service.domain;
+
+import market.guess.api.MarketDTO;
+
+public final class Market {
+  private final String key;
+  private final String name;
+
+  private int volume;
+
+  public Market(String key, String name) {
+    super();
+    this.key = key;
+    this.name = name;
+  }
+
+  public String getName() {
+    return name;
+  }
+
+  public String getKey() {
+    return key;
+  }
+
+  public int getVolume() {
+    return volume;
+  }
+
+  public MarketDTO toMarket() {
+    return new MarketDTO(key, name, "", String.valueOf(volume));
+  }
+}
diff --git a/service/src/main/java/market/guess/service/domain/Option.java b/service/src/main/java/market/guess/service/domain/Option.java
deleted file mode 100644
index 7f2ff86..0000000
--- a/service/src/main/java/market/guess/service/domain/Option.java
+++ /dev/null
@@ -1,3 +0,0 @@
-package market.guess.service.domain;
-
-public final record Option(String key, String name) {}
diff --git a/service/src/main/java/market/guess/service/domain/User.java b/service/src/main/java/market/guess/service/domain/User.java
index b2084b4..3536c4e 100644
--- a/service/src/main/java/market/guess/service/domain/User.java
+++ b/service/src/main/java/market/guess/service/domain/User.java
@@ -1,10 +1,11 @@
 package market.guess.service.domain;
 
 import java.math.BigDecimal;
+import market.guess.service.ledger.Ledger;
 
 public final class User {
   private final String name;
-  private final Account account;
+  private final Ledger account;
 
   public User(String name, int initialBalance) {
     this(name, BigDecimalOptions.toMoney(initialBalance));
@@ -12,14 +13,14 @@ public final class User {
 
   public User(String name, BigDecimal initialBalance) {
     this.name = name;
-    this.account = new Account(name, initialBalance);
+    this.account = new Ledger(name, initialBalance);
   }
 
   public String getName() {
     return name;
   }
 
-  public Account getAccount() {
+  public Ledger getLedger() {
     return account;
   }
 }
diff --git a/service/src/main/java/market/guess/service/infrastructure/MarketContext.java b/service/src/main/java/market/guess/service/infrastructure/MarketContext.java
index e769384..d9fe3f8 100644
--- a/service/src/main/java/market/guess/service/infrastructure/MarketContext.java
+++ b/service/src/main/java/market/guess/service/infrastructure/MarketContext.java
@@ -2,16 +2,23 @@ package market.guess.service.infrastructure;
 
 import com.google.gson.Gson;
 import com.google.gson.GsonBuilder;
+import com.google.gson.JsonSyntaxException;
 import java.io.IOException;
 import java.nio.charset.StandardCharsets;
 import java.nio.file.Files;
+import java.nio.file.Path;
 import java.nio.file.Paths;
 import java.time.Instant;
+import java.util.List;
+import market.guess.service.domain.Event;
+import market.guess.service.domain.User;
 import market.guess.service.infrastructure.adapter.InstantTypeAdapter;
 import market.guess.service.infrastructure.repository.EventRepository;
 import market.guess.service.infrastructure.repository.UserRepository;
 
 public final class MarketContext {
+  record Wrapper(List<Event> events, List<User> users) {}
+
   private final EventRepository events;
   private final UserRepository users;
 
@@ -39,11 +46,34 @@ public final class MarketContext {
 
     try {
       if (path.getParent() != null) Files.createDirectories(path.getParent());
-      Files.writeString(path, GSON.toJson(events), StandardCharsets.UTF_8);
+      Files.writeString(
+          path,
+          GSON.toJson(new Wrapper(events.getAll(), users.getAll())),
+          StandardCharsets.UTF_8);
     } catch (IOException e) {
 
     }
   }
 
-  public void load() {}
+  public void load() {
+    try {
+      var wrapper =
+          GSON.fromJson(
+              Files.readString(Path.of("test-file.json"), StandardCharsets.UTF_8), Wrapper.class);
+      events.clear();
+      users.clear();
+
+      for (var event : wrapper.events()) {
+        events.add(event);
+      }
+
+      for (var user : wrapper.users()) {
+        users.add(user);
+      }
+
+    } catch (JsonSyntaxException | IOException e) {
+      // TODO Auto-generated catch block
+      e.printStackTrace();
+    }
+  }
 }
diff --git a/service/src/main/java/market/guess/service/infrastructure/mapper/v1/EventMapperV1.java b/service/src/main/java/market/guess/service/infrastructure/mapper/v1/EventMapperV1.java
index 8754555..7984c41 100644
--- a/service/src/main/java/market/guess/service/infrastructure/mapper/v1/EventMapperV1.java
+++ b/service/src/main/java/market/guess/service/infrastructure/mapper/v1/EventMapperV1.java
@@ -5,7 +5,7 @@ import java.util.List;
 import market.guess.api.CommissionTiming;
 import market.guess.api.EventStatus;
 import market.guess.service.domain.Event;
-import market.guess.service.domain.Option;
+import market.guess.service.domain.Market;
 import market.guess.service.infrastructure.mapper.Mapper;
 import market.guess.service.mechanism.LmsrTradingMechanism;
 import market.guess.service.model.v1.GMEvent;
@@ -45,10 +45,10 @@ public final class EventMapperV1 implements Mapper<GMEvent, Event> {
     };
   }
 
-  private static List<Option> getOptions(GMEvent x) {
-    var ret = new ArrayList<Option>(x.getGMOptions().getGMOption().size());
+  private static List<Market> getOptions(GMEvent x) {
+    var ret = new ArrayList<Market>(x.getGMOptions().getGMOption().size());
     for (var i = 0; i < x.getGMOptions().getGMOption().size(); i++) {
-      ret.add(new Option(getEventKey(x) + ":" + i, x.getGMOptions().getGMOption().get(i)));
+      ret.add(new Market(getEventKey(x) + ":" + i, x.getGMOptions().getGMOption().get(i)));
     }
     return ret;
   }
diff --git a/service/src/main/java/market/guess/service/infrastructure/mapper/v2/EventMapperV2.java b/service/src/main/java/market/guess/service/infrastructure/mapper/v2/EventMapperV2.java
index a4fc4e7..0dd8e43 100644
--- a/service/src/main/java/market/guess/service/infrastructure/mapper/v2/EventMapperV2.java
+++ b/service/src/main/java/market/guess/service/infrastructure/mapper/v2/EventMapperV2.java
@@ -5,7 +5,7 @@ import java.util.List;
 import market.guess.api.CommissionTiming;
 import market.guess.api.EventStatus;
 import market.guess.service.domain.Event;
-import market.guess.service.domain.Option;
+import market.guess.service.domain.Market;
 import market.guess.service.infrastructure.mapper.Mapper;
 import market.guess.service.mechanism.LmsrTradingMechanism;
 import market.guess.service.mechanism.OrderBookTradingMechanism;
@@ -28,7 +28,7 @@ public final class EventMapperV2 implements Mapper<GMEvent, Event> {
         source.getCommission().getValue(),
         getTiming(source.getCommission().getType()),
         getMechanism(source, options.size()),
-        EventStatus.NOT_STARTED,
+        EventStatus.DRAFT,
         options,
         "");
   }
@@ -59,10 +59,10 @@ public final class EventMapperV2 implements Mapper<GMEvent, Event> {
     throw new IllegalArgumentException();
   }
 
-  private static List<Option> getOptions(GMEvent x) {
-    var ret = new ArrayList<Option>(x.getGMOptions().getGMOption().size());
+  private static List<Market> getOptions(GMEvent x) {
+    var ret = new ArrayList<Market>(x.getGMOptions().getGMOption().size());
     for (var i = 0; i < x.getGMOptions().getGMOption().size(); i++) {
-      ret.add(new Option(getEventKey(x) + ":" + i, x.getGMOptions().getGMOption().get(i)));
+      ret.add(new Market(getEventKey(x) + ":" + i, x.getGMOptions().getGMOption().get(i)));
     }
     return ret;
   }
diff --git a/service/src/main/java/market/guess/service/infrastructure/repository/InMemoryEventRepository.java b/service/src/main/java/market/guess/service/infrastructure/repository/InMemoryEventRepository.java
index 0ed5b20..8b8eeb2 100644
--- a/service/src/main/java/market/guess/service/infrastructure/repository/InMemoryEventRepository.java
+++ b/service/src/main/java/market/guess/service/infrastructure/repository/InMemoryEventRepository.java
@@ -24,4 +24,9 @@ public final class InMemoryEventRepository implements EventRepository {
   public List<Event> getAll() {
     return Collections.unmodifiableList(events);
   }
+
+  @Override
+  public void clear() {
+    events.clear();
+  }
 }
diff --git a/service/src/main/java/market/guess/service/infrastructure/repository/InMemoryUserRepository.java b/service/src/main/java/market/guess/service/infrastructure/repository/InMemoryUserRepository.java
index b4014e5..b611a77 100644
--- a/service/src/main/java/market/guess/service/infrastructure/repository/InMemoryUserRepository.java
+++ b/service/src/main/java/market/guess/service/infrastructure/repository/InMemoryUserRepository.java
@@ -24,4 +24,9 @@ public final class InMemoryUserRepository implements UserRepository {
   public List<User> getAll() {
     return Collections.unmodifiableList(users);
   }
+
+  @Override
+  public void clear() {
+    users.clear();
+  }
 }
diff --git a/service/src/main/java/market/guess/service/infrastructure/repository/Repository.java b/service/src/main/java/market/guess/service/infrastructure/repository/Repository.java
index 13f3901..41cb578 100644
--- a/service/src/main/java/market/guess/service/infrastructure/repository/Repository.java
+++ b/service/src/main/java/market/guess/service/infrastructure/repository/Repository.java
@@ -9,4 +9,6 @@ public interface Repository<T> {
   Optional<T> get(String id);
 
   List<T> getAll();
+
+  void clear();
 }
diff --git a/service/src/main/java/market/guess/service/ledger/Ledger.java b/service/src/main/java/market/guess/service/ledger/Ledger.java
new file mode 100644
index 0000000..744fde7
--- /dev/null
+++ b/service/src/main/java/market/guess/service/ledger/Ledger.java
@@ -0,0 +1,35 @@
+package market.guess.service.ledger;
+
+import java.math.BigDecimal;
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.List;
+import market.guess.service.domain.BigDecimalOptions;
+
+public final class Ledger {
+  private final String owner;
+  private BigDecimal balance;
+  private final List<LedgerEntry> entries = new ArrayList<>();
+
+  public Ledger(String owner, BigDecimal initialBalance) {
+    super();
+    this.owner = owner;
+    this.balance = BigDecimalOptions.toMoney(initialBalance);
+  }
+
+  public String getOwner() {
+    return owner;
+  }
+
+  public BigDecimal getBalance() {
+    return balance;
+  }
+
+  public List<LedgerEntry> getEntries() {
+    return Collections.unmodifiableList(entries);
+  }
+
+  public void record(LedgerEntry entry) {
+    entries.add(entry);
+  }
+}
diff --git a/service/src/main/java/market/guess/service/ledger/LedgerContext.java b/service/src/main/java/market/guess/service/ledger/LedgerContext.java
new file mode 100644
index 0000000..caecf55
--- /dev/null
+++ b/service/src/main/java/market/guess/service/ledger/LedgerContext.java
@@ -0,0 +1,49 @@
+package market.guess.service.ledger;
+
+import java.math.BigDecimal;
+import java.time.Clock;
+import market.guess.api.LedgerType;
+import market.guess.service.infrastructure.repository.UserRepository;
+
+public final class LedgerContext {
+  private final Clock clock;
+  private final UserRepository users;
+
+  private int runningId;
+
+  public LedgerContext(UserRepository users) {
+    this(users, Clock.systemUTC());
+  }
+
+  public LedgerContext(UserRepository users, Clock clock) {
+    super();
+    this.clock = clock;
+    this.users = users;
+  }
+
+  public boolean credit(String name, LedgerType type, BigDecimal amount, String note) {
+    record(name, type, amount, note);
+    return true;
+  }
+
+  public boolean debit(String name, LedgerType type, BigDecimal amount, String note) {
+    record(name, type, amount.negate(), note);
+    return true;
+  }
+
+  private void record(String name, LedgerType type, BigDecimal amount, String note) {
+    users
+        .get(name)
+        .ifPresent(
+            u ->
+                u.getLedger()
+                    .record(
+                        new LedgerEntry(
+                            runningId++,
+                            clock.instant(),
+                            type,
+                            amount,
+                            u.getLedger().getBalance().add(amount),
+                            note)));
+  }
+}
diff --git a/service/src/main/java/market/guess/service/domain/LedgerEntry.java b/service/src/main/java/market/guess/service/ledger/LedgerEntry.java
index 7e6ff83..8008f20 100644
--- a/service/src/main/java/market/guess/service/domain/LedgerEntry.java
+++ b/service/src/main/java/market/guess/service/ledger/LedgerEntry.java
@@ -1,4 +1,4 @@
-package market.guess.service.domain;
+package market.guess.service.ledger;
 
 import java.math.BigDecimal;
 import java.time.Instant;