MLEM/MLEM.Data/Json/StaticJsonConverter.cs

using System;
using System.Collections.Generic;
using System.Reflection;
using Newtonsoft.Json;

#if NET6_0_OR_GREATER
using System.Diagnostics.CodeAnalysis;
#endif

namespace MLEM.Data.Json {
    /// <summary>
    /// A <see cref="JsonConverter{T}"/> that doesn't actually serialize the object, but instead serializes the name given to it by the underlying <see cref="Dictionary{T,T}"/>.
    /// Optionally, the name of a <see cref="Dictionary{TKey,TValue}"/> can be passed to this converter when used in the <see cref="JsonConverterAttribute"/> by passing the arguments for the <see cref="StaticJsonConverter{T}(Type,string)"/> constructor as <see cref="JsonConverterAttribute.ConverterParameters"/>.
    /// </summary>
    /// <typeparam name="T">The type of the object to convert</typeparam>
    public class StaticJsonConverter<T> : JsonConverter<T> {

        /// <summary>
        /// The entries, or registry, that this static json converter uses.
        /// </summary>
        protected readonly Dictionary<string, T> Entries;
        /// <summary>
        /// The inverse of this static json converter's <see cref="Entries"/>, mapping the values to their respective keys.
        /// Note that this dictionary is only created once and is not automatically updated should the <see cref="Entries"/> be modified.
        /// </summary>
        protected readonly Dictionary<T, string> Inverse;
        /// <summary>
        /// Whether to throw a <see cref="KeyNotFoundException"/> in <see cref="ReadJson"/> if a key is missing, or throw a <see cref="JsonSerializationException"/> if a stored json value is not a string.
        /// If this is <see langword="false"/>, <see cref="ReadJson"/> returns <see langword="default"/> instead.
        /// </summary>
        protected readonly bool ThrowOnRead;

        /// <summary>
        /// Creates a new static json converter using the given underlying <see cref="Dictionary{T,T}"/>.
        /// </summary>
        /// <param name="entries">The entries, or registry, that this static json converter uses.</param>
        /// <param name="throwOnRead">Whether to throw a <see cref="KeyNotFoundException"/> in <see cref="ReadJson"/> if a key is missing, or throw a <see cref="JsonSerializationException"/> if a stored json value is not a string. If this is <see langword="false"/>, <see cref="ReadJson"/> returns <see langword="default"/> instead.</param>
        public StaticJsonConverter(Dictionary<string, T> entries, bool throwOnRead = false) {
            this.Entries = entries;
            this.Inverse = StaticJsonConverter<T>.CreateInverse(entries);
            this.ThrowOnRead = throwOnRead;
        }

        /// <summary>
        /// Creates a new static json converter by finding the underlying <see cref="Dictionary{TKey,TValue}"/> from the given type and member name
        /// </summary>
        /// <param name="type">The type that the dictionary is declared in</param>
        /// <param name="memberName">The name of the dictionary itself</param>
        public StaticJsonConverter(
#if NET6_0_OR_GREATER
            [DynamicallyAccessedMembers(DynamicallyAccessedMemberTypes.PublicProperties | DynamicallyAccessedMemberTypes.NonPublicProperties | DynamicallyAccessedMemberTypes.PublicFields | DynamicallyAccessedMemberTypes.NonPublicFields)]
#endif
            Type type, string memberName) : this(StaticJsonConverter<T>.GetEntries(type, memberName)) {}

        /// <summary>
        /// Creates a new static json converter by finding the underlying <see cref="Dictionary{TKey,TValue}"/> from the given type and member name
        /// </summary>
        /// <param name="type">The type that the dictionary is declared in</param>
        /// <param name="memberName">The name of the dictionary itself</param>
        /// <param name="throwOnRead">Whether to throw a <see cref="KeyNotFoundException"/> in <see cref="ReadJson"/> if a key is missing, or throw a <see cref="JsonSerializationException"/> if a stored json value is not a string. If this is <see langword="false"/>, <see cref="ReadJson"/> returns <see langword="default"/> instead.</param>
        public StaticJsonConverter(
#if NET6_0_OR_GREATER
            [DynamicallyAccessedMembers(DynamicallyAccessedMemberTypes.PublicProperties | DynamicallyAccessedMemberTypes.NonPublicProperties | DynamicallyAccessedMemberTypes.PublicFields | DynamicallyAccessedMemberTypes.NonPublicFields)]
#endif
            Type type, string memberName, bool throwOnRead) : this(StaticJsonConverter<T>.GetEntries(type, memberName), throwOnRead) {}

        /// <summary>Writes the JSON representation of the object.</summary>
        /// <param name="writer">The <see cref="T:Newtonsoft.Json.JsonWriter" /> to write to.</param>
        /// <param name="value">The value.</param>
        /// <param name="serializer">The calling serializer.</param>
        public override void WriteJson(JsonWriter writer, T value, JsonSerializer serializer) {
            if (value == null) {
                writer.WriteNull();
                return;
            }
            if (!this.Inverse.TryGetValue(value, out var key))
                throw new KeyNotFoundException($"Cannot write {value} that is not a registered entry");
            writer.WriteValue(key);
        }

        /// <summary>Reads the JSON representation of the object.</summary>
        /// <param name="reader">The <see cref="T:Newtonsoft.Json.JsonReader" /> to read from.</param>
        /// <param name="objectType">Type of the object.</param>
        /// <param name="existingValue">The existing value of object being read. If there is no existing value then <c>null</c> will be used.</param>
        /// <param name="hasExistingValue">The existing value has a value.</param>
        /// <param name="serializer">The calling serializer.</param>
        /// <returns>The object value.</returns>
        public override T ReadJson(JsonReader reader, Type objectType, T existingValue, bool hasExistingValue, JsonSerializer serializer) {
            if (reader.Value == null)
                return default;
            if (reader.TokenType != JsonToken.String) {
                if (this.ThrowOnRead)
                    throw new JsonSerializationException($"Expected a string value for {reader.Value}, got a {reader.TokenType}");
                return default;
            }
            if (!this.Entries.TryGetValue((string) reader.Value, out var ret) && this.ThrowOnRead)
                throw new KeyNotFoundException($"Could not find registered entry for {reader.Value}");
            return ret;
        }

        /// <summary>
        /// Returns the entries, or registry, that this static json converter should use, using reflection to find the passed <paramref name="memberName"/> in the passed <paramref name="type"/>.
        /// </summary>
        /// <param name="type">The type to find the entries in.</param>
        /// <param name="memberName">The name of the field or property that the entries are in within the given <paramref name="type"/>.</param>
        /// <returns>The found property or field's value.</returns>
        /// <exception cref="ArgumentException">Thrown if there is no property or field with the <paramref name="memberName"/> in the given <paramref name="type"/>.</exception>
        /// <exception cref="InvalidCastException">Thrown if the value of the <paramref name="memberName"/> is not of the expected type.</exception>
        protected static Dictionary<string, T> GetEntries(
#if NET6_0_OR_GREATER
            [DynamicallyAccessedMembers(DynamicallyAccessedMemberTypes.PublicProperties | DynamicallyAccessedMemberTypes.NonPublicProperties | DynamicallyAccessedMemberTypes.PublicFields | DynamicallyAccessedMemberTypes.NonPublicFields)]
#endif
            Type type, string memberName) {
            const BindingFlags flags = BindingFlags.Instance | BindingFlags.Static | BindingFlags.Public | BindingFlags.NonPublic;
            var value = type.GetProperty(memberName, flags)?.GetValue(null) ?? type.GetField(memberName, flags)?.GetValue(null);
            if (value == null)
                throw new ArgumentException($"There is no property or field value for name {memberName}", nameof(memberName));
            return value as Dictionary<string, T> ?? throw new InvalidCastException($"{value} is not of expected type {typeof(T)}");
        }

        /// <summary>
        /// Creates the inverse version of the passed <paramref name="entries"/>, mapping the values to their keys, and throws an exception if there are any duplicate values.
        /// </summary>
        /// <param name="entries">The entries to create an inverse of.</param>
        /// <returns>The inverse of the given <paramref name="entries"/>, mapping the values to their keys.</returns>
        /// <exception cref="ArgumentException">Thrown if the <paramref name="entries"/> contains duplicate values, making it impossible to create a valid inverse.</exception>
        protected static Dictionary<T, string> CreateInverse(Dictionary<string, T> entries) {
            var ret = new Dictionary<T, string>();
            foreach (var entry in entries) {
                if (ret.ContainsKey(entry.Value))
                    throw new ArgumentException($"Cannot create a static json converter with duplicate value {entry.Value}");
                ret.Add(entry.Value, entry.Key);
            }
            return ret;
        }

    }
}