chore: update docstring and minor refactoring

Libplanet.Action.Tests/Mocks/MockWorldState.cs

@@ -22,7 +22,7 @@ public MockWorldState()
 
         IAccount? IWorldState.GetAccount(Address address)
         {
-            _accounts.TryGetValue(address, out IAccount? account);
+            _accounts.TryGetValue(address, out IAccount account);
             return account;
         }
     }

Libplanet.Action.Tests/State/WorldTest.cs

@@ -1,10 +1,11 @@
 using System.Collections.Immutable;
 using Bencodex.Types;
+using Libplanet.Action.State;
 using Libplanet.Action.Tests.Mocks;
 using Libplanet.Crypto;
 using Xunit;
 
-namespace Libplanet.Action.State.Tests
+namespace Libplanet.Action.Tests.State
 {
     public class WorldTest
     {
@@ -54,8 +55,8 @@ public void SetAccount()
 
             // TestGetAccount() will retrieve account from World.Delta
             world = world.SetAccount(accountAddress, mockAccount);
-            Assert.Single(world.Delta.UpdatedAccounts);
-            Assert.Equal(accountAddress, world.Delta.UpdatedAccounts.First());
+            Assert.Single(world.Delta.UpdatedAddresses);
+            Assert.Equal(accountAddress, world.Delta.UpdatedAddresses.First());
             Assert.Equal(mockAccount, world.GetAccount(accountAddress));
         }
 
@@ -69,12 +70,12 @@ public void Flush()
             IAccount emptyAccount = Account.Create(MockAccountState.Empty);
             Address accountAddress = RandomAddress;
             world = world.SetAccount(accountAddress, emptyAccount);
-            Assert.Single(world.Delta.UpdatedAccounts);
-            Assert.Equal(accountAddress, world.Delta.UpdatedAccounts.First());
+            Assert.Single(world.Delta.UpdatedAddresses);
+            Assert.Equal(accountAddress, world.Delta.UpdatedAddresses.First());
 
             // After flush, World.Delta moves to World._baseState.Delta
             world = World.Flush(world);
-            Assert.Empty(world.Delta.UpdatedAccounts);
+            Assert.Empty(world.Delta.UpdatedAddresses);
             Assert.NotNull(world.GetAccount(accountAddress));
         }
 

Libplanet.Action/State/IWorld.cs

@@ -1,13 +1,62 @@
+using System.Collections.Generic;
 using System.Diagnostics.Contracts;
 using Libplanet.Crypto;
 
 namespace Libplanet.Action.State
 {
+    /// <summary>
+    /// An interface to manipulate an world state with
+    /// maintaining <see cref="Delta"/>.
+    /// <para>It is like a map which is virtually initialized such
+    /// that every possible <see cref="Address"/> exists and
+    /// is mapped to <see langword="null"/>.  That means that:</para>
+    /// <list type="bullet">
+    /// <item>
+    /// <description>it does not have length,</description>
+    /// </item>
+    /// <item>
+    /// <description>its index getter never throws
+    /// <see cref="KeyNotFoundException"/>,
+    /// but returns <see langword="null"/> instead, and</description>
+    /// </item>
+    /// <item>
+    /// <description>filling an <see cref="Address"/> with
+    /// <see langword="null"/> account cannot be distinguished from
+    /// the <see cref="Address"/> having never been set to
+    /// any account.</description>
+    /// </item>
+    /// </list>
+    /// </summary>
+    /// <remarks>
+    /// This interface is immutable.  <see cref="SetAccount(Address, IAccount)"/>
+    /// method does not manipulate the instance, but returns a new
+    /// <see cref="IWorld"/> instance with updated states.
+    /// </remarks>
     internal interface IWorld : IWorldState
     {
+        /// <summary>
+        /// The <see cref="IWorld"/> representing the delta part of
+        /// this <see cref="IWorld"/>.
+        /// </summary>
         [Pure]
         IWorldDelta Delta { get; }
 
+        /// <summary>
+        /// Gets a new instance that the world state of the given
+        /// <paramref name="address"/> is set to the given
+        /// <paramref name="account"/>.
+        /// </summary>
+        /// <param name="address">The <see cref="Address"/> referring
+        /// the account to set its account.</param>
+        /// <param name="account">The new account to fill the account with.</param>
+        /// <returns>A new <see cref="IWorld"/> instance that
+        /// the account state of the given <paramref name="address"/>
+        /// is set to the given <paramref name="account"/>.</returns>
+        /// <remarks>
+        /// This method method does not manipulate the instance,
+        /// but returns a new <see cref="IWorld"/> instance
+        /// with updated world instead.
+        /// </remarks>
         [Pure]
         IWorld SetAccount(Address address, IAccount account);
     }

Libplanet.Action/State/IWorldDelta.cs

@@ -6,10 +6,19 @@ namespace Libplanet.Action.State
 {
     public interface IWorldDelta
     {
+        /// <summary>
+        /// A dictionary representing changed accounts for each <see cref="Address"/>.
+        /// </summary>
         [Pure]
         IImmutableDictionary<Address, IAccount> Accounts { get; }
 
+        /// <summary>
+        /// <para>
+        /// A set of <seealso cref="Address"/>es where each <see cref="Address"/> has
+        /// its account changed.
+        /// </para>
+        /// </summary>
         [Pure]
-        IImmutableSet<Address> UpdatedAccounts { get; }
+        IImmutableSet<Address> UpdatedAddresses { get; }
     }
 }

Libplanet.Action/State/IWorldState.cs

@@ -1,13 +1,46 @@
+using System.Collections.Generic;
 using System.Diagnostics.Contracts;
 using Libplanet.Crypto;
 
 namespace Libplanet.Action.State
 {
+    /// <summary>
+    /// An interface to fetch world states.
+    /// <para>It is like a readonly map which is virtually initialized such
+    /// that every possible <see cref="Address"/> exists and
+    /// is mapped to <see langword="null"/>.  That means that:</para>
+    /// <list type="bullet">
+    /// <item>
+    /// <description>it does not have length,</description>
+    /// </item>
+    /// <item>
+    /// <description>its index getter never throws
+    /// <see cref="KeyNotFoundException"/>,
+    /// but returns <see langword="null"/> instead, and</description>
+    /// </item>
+    /// <item>
+    /// <description>filling an <see cref="Address"/> with
+    /// <see langword="null"/> state cannot be distinguished from
+    /// the <see cref="Address"/> having never been set to
+    /// any state.</description>
+    /// </item>
+    /// </list>
+    /// </summary>
     public interface IWorldState
     {
-        [Pure]
+        /// <summary>
+        /// Whether <see cref="IWorldState"/> is in legacy state or not
+        /// </summary>
         bool Legacy { get; }
 
+        /// <summary>
+        /// Gets the world state of the given <paramref name="address"/>.
+        /// </summary>
+        /// <param name="address">The <see cref="Address"/> referring
+        /// the world to get its state.</param>
+        /// <returns>The world state of the given <paramref name="address"/>.
+        /// If it has never been set to any state it returns <see langword="null"/>
+        /// instead.</returns>
         [Pure]
         IAccount? GetAccount(Address address);
     }

Libplanet.Action/State/World.cs

@@ -4,6 +4,9 @@
 
 namespace Libplanet.Action.State
 {
+    /// <summary>
+    /// An internal implementation of <see cref="IWorld"/>.
+    /// </summary>
     [Pure]
     internal class World : IWorld
     {
@@ -21,12 +24,15 @@ private World(IWorldState baseState, IWorldDelta delta)
             Legacy = false;
         }
 
+        /// <inheritdoc/>
         [Pure]
         public IWorldDelta Delta { get; private set; }
 
+        /// <inheritdoc/>
         [Pure]
         public bool Legacy { get; private set; }
 
+        /// <inheritdoc/>
         [Pure]
         public IAccount? GetAccount(Address address)
         {
@@ -35,6 +41,7 @@ private World(IWorldState baseState, IWorldDelta delta)
                 : _baseState.GetAccount(address);
         }
 
+        /// <inheritdoc/>
         [Pure]
         public IWorld SetAccount(Address address, IAccount account) =>
             new World(this, new WorldDelta(Delta.Accounts.SetItem(address, account)))

Libplanet.Action/State/WorldDelta.cs

@@ -15,8 +15,10 @@ internal WorldDelta(IImmutableDictionary<Address, IAccount> accounts)
             Accounts = accounts;
         }
 
-        public IImmutableSet<Address> UpdatedAccounts => Accounts.Keys.ToImmutableHashSet();
+        /// <inheritdoc cref="IWorldDelta.UpdatedAddresses"/>
+        public IImmutableSet<Address> UpdatedAddresses => Accounts.Keys.ToImmutableHashSet();
 
+        /// <inheritdoc cref="IWorldDelta.Accounts"/>
         public IImmutableDictionary<Address, IAccount> Accounts { get; }
     }
 }