request/src/request.validation/PropertyPath.cs

// Copyright (c) The Geekeey Authors
// SPDX-License-Identifier: EUPL-1.2

using System.Collections;
using System.Linq.Expressions;
using System.Text;
using System.Text.Json;
using System.Text.Json.Serialization;

namespace Geekeey.Request.Validation;

/// <summary>
/// Represents a property path used by validation problems.
/// </summary>
[JsonConverter(typeof(PropertyPathJsonConverter))]
public readonly struct PropertyPath : IEquatable<PropertyPath>, IEquatable<string>, IReadOnlyList<string>
{
	/// <summary>
	/// Creates a new property path.
	/// </summary>
	public PropertyPath(string value)
	{
		ArgumentNullException.ThrowIfNull(value);
		Value = value;
	}

	/// <summary>
	/// Creates a property path from a lambda expression.
	/// </summary>
	/// <typeparam name="T">The type containing the property.</typeparam>
	/// <param name="expression">The expression representing the property access.</param>
	/// <returns>A new <see cref="PropertyPath"/>.</returns>
	public static PropertyPath Of<T>(Expression<Func<T, object?>> expression)
	{
		ArgumentNullException.ThrowIfNull(expression);
		return FromExpression(expression);
	}

	internal static PropertyPath FromExpression(Expression expression)
	{
		var segments = new List<string>();

		var current = expression;
		while (current is not null)
		{
			switch (current)
			{
				case LambdaExpression expr:
					current = expr.Body;
					break;
				case MemberExpression expr:
					segments.Add(expr.Member.Name);
					current = expr.Expression;
					break;
				case MethodCallExpression { Method.Name: "get_Item", Arguments.Count: 1 } expr:
					segments.Add($"[{Evaluate(expr.Arguments[0])}]");
					current = expr.Object;
					break;
				case BinaryExpression { NodeType: ExpressionType.ArrayIndex } expr:
					segments.Add($"[{Evaluate(expr.Right)}]");
					current = expr.Left;
					break;
				case UnaryExpression { NodeType: ExpressionType.Convert or ExpressionType.ConvertChecked } expr:
					current = expr.Operand;
					break;
				case ParameterExpression:
					current = null;
					break;
				default:
					throw new InvalidOperationException("Only simple member access expressions are supported.");
			}
		}

		var builder = new StringBuilder();
		for (var i = segments.Count - 1; i >= 0; i--)
		{
			var segment = segments[i];
			if (builder.Length > 0 && !segment.StartsWith('['))
			{
				builder.Append('.');
			}

			builder.Append(segment);
		}

		return builder.ToString();
	}

	private static object? Evaluate(Expression expression)
	{
		if (expression is ConstantExpression constant)
		{
			return constant.Value;
		}

		if (expression is MemberExpression member && member.Expression is ConstantExpression closure)
		{
			var value = closure.Value;
			if (member.Member is System.Reflection.FieldInfo field)
			{
				return field.GetValue(value);
			}

			if (member.Member is System.Reflection.PropertyInfo prop)
			{
				return prop.GetValue(value);
			}
		}

		return Expression.Lambda(expression).Compile().DynamicInvoke();
	}

	/// <summary>
	/// Gets the raw property path.
	/// </summary>
	public string Value { get; } = string.Empty;

	/// <summary>
	/// Gets the number of path segments.
	/// </summary>
	public int Count
	{
		get
		{
			var count = 0;
			var enumerator = new Enumerator(Value);
			while (enumerator.MoveNext())
			{
				count++;
			}

			return count;
		}
	}

	/// <summary>
	/// Gets whether the property path contains no segments.
	/// </summary>
	public bool IsEmpty => Value.Length is 0;

	/// <summary>
	/// Gets the segment at the specified index.
	/// </summary>
	public ReadOnlySpan<char> this[int index]
	{
		get
		{
			ArgumentOutOfRangeException.ThrowIfNegative(index);

			var currentIndex = 0;
			var enumerator = new Enumerator(Value);
			while (enumerator.MoveNext())
			{
				if (currentIndex == index)
				{
					return enumerator.Current;
				}

				currentIndex++;
			}

			throw new ArgumentOutOfRangeException(nameof(index), index, "The segment index must refer to an existing property path segment.");
		}
	}

	/// <inheritdoc />
	public override string ToString()
	{
		return Value;
	}

	/// <summary>
	/// Converts a string into a property path.
	/// </summary>
	public static implicit operator PropertyPath(string value)
	{
		return new PropertyPath(value);
	}

	/// <summary>
	/// Converts a property path into its string representation.
	/// </summary>
	public static implicit operator string(PropertyPath propertyPath)
	{
		return propertyPath.Value;
	}

	/// <inheritdoc />
	public bool Equals(PropertyPath other)
	{
		return string.Equals(Value, other.Value, StringComparison.Ordinal);
	}

	/// <inheritdoc />
	public bool Equals(string? other)
	{
		return string.Equals(Value, other, StringComparison.Ordinal);
	}

	/// <inheritdoc />
	public override bool Equals(object? obj)
	{
		return (obj is PropertyPath other && Equals(other)) || (obj is string s && Equals(s));
	}

	/// <inheritdoc />
	public override int GetHashCode()
	{
		return StringComparer.Ordinal.GetHashCode(Value ?? string.Empty);
	}

	/// <summary>
	/// Compares two property paths for ordinal equality.
	/// </summary>
	public static bool operator ==(PropertyPath left, PropertyPath right)
	{
		return left.Equals(right);
	}

	/// <summary>
	/// Compares two property paths for ordinal inequality.
	/// </summary>
	public static bool operator !=(PropertyPath left, PropertyPath right)
	{
		return !left.Equals(right);
	}

	/// <summary>
	/// Combines two property paths using the same rules as nested validation paths.
	/// </summary>
	public static PropertyPath operator +(PropertyPath prefix, PropertyPath suffix)
	{
		if (prefix.Value is not { Length: > 0 })
		{
			return suffix;
		}

		if (suffix.Value is not { Length: > 0 })
		{
			return prefix;
		}

		return $"{prefix}.{suffix}";
	}

	/// <summary>
	/// Appends a collection index to a property path.
	/// </summary>
	public static PropertyPath operator +(PropertyPath propertyPath, int index)
	{
		return propertyPath.Value is { Length: > 0 } ? $"{propertyPath}[{index}]" : $"[{index}]";
	}

	/// <summary>
	/// Returns an allocation-free enumerator over the path segments.
	/// </summary>
	public Enumerator GetEnumerator()
	{
		return new Enumerator(Value);
	}

	/// <inheritdoc />
	string IReadOnlyList<string>.this[int index] => this[index].ToString();

	/// <inheritdoc />
	IEnumerator<string> IEnumerable<string>.GetEnumerator()
	{
		var enumerator = new Enumerator(Value);

		List<string> segments = [];
		while (enumerator.MoveNext())
		{
			segments.Add(enumerator.Current.ToString());
		}

		return segments.GetEnumerator();
	}

	/// <inheritdoc />
	IEnumerator IEnumerable.GetEnumerator()
	{
		var enumerator = new Enumerator(Value);

		List<string> segments = [];
		while (enumerator.MoveNext())
		{
			segments.Add(enumerator.Current.ToString());
		}

		return segments.GetEnumerator();
	}

	internal string ToJsonName(JsonNamingPolicy? namingPolicy)
	{
		var value = Value;
		if (namingPolicy is null || value.Length is 0)
		{
			return value;
		}

		var builder = new StringBuilder(value.Length);
		var first = true;
		var enumerator = new Enumerator(value);
		while (enumerator.MoveNext())
		{
			if (!first)
			{
				builder.Append('.');
			}

			if (enumerator.Current is { IsEmpty: false } segment)
			{
				if (segment.IndexOf('[') is >= 0 and var x)
				{
					builder.Append(namingPolicy.ConvertName(segment[..x].ToString()));
					builder.Append(segment[x..]);
				}
				else
				{
					builder.Append(namingPolicy.ConvertName(segment.ToString()));
				}
			}

			first = false;
		}

		return builder.ToString();
	}

	/// <summary>
	/// Enumerates path segments without allocations.
	/// </summary>
	public ref struct Enumerator
	{
		private readonly ReadOnlySpan<char> _value;
		private int _nextStart;

		internal Enumerator(ReadOnlySpan<char> value)
		{
			_value = value;
			_nextStart = value.Length is 0 ? -1 : 0;
			Current = default;
		}

		/// <summary>
		/// Gets the current path segment.
		/// </summary>
		public ReadOnlySpan<char> Current { get; private set; }

		/// <summary>
		/// Advances to the next path segment.
		/// </summary>
		public bool MoveNext()
		{
			if (_nextStart < 0)
			{
				return false;
			}

			var start = _nextStart;
			var bracketDepth = 0;

			for (var i = start; i < _value.Length; i++)
			{
				switch (_value[i])
				{
					case '[':
						bracketDepth++;
						break;
					case ']':
						bracketDepth = int.Max(bracketDepth - 1, 0);
						break;
					case '.' when bracketDepth is 0:
						Current = _value[start..i];
						_nextStart = i + 1;
						return true;
					default:
						break;
				}
			}

			Current = _value[start..];
			_nextStart = -1;
			return true;
		}
	}
}

internal sealed class PropertyPathJsonConverter : JsonConverter<PropertyPath>
{
	/// <inheritdoc />
	public override PropertyPath Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
	{
		if (reader.TokenType is JsonTokenType.String)
		{
			return new PropertyPath(reader.GetString()!);
		}

		throw new JsonException($"Expected {nameof(JsonTokenType.String)} but got {reader.TokenType}.");
	}

	/// <inheritdoc />
	public override void Write(Utf8JsonWriter writer, PropertyPath value, JsonSerializerOptions options)
	{
		writer.WriteStringValue(value.ToJsonName(options.PropertyNamingPolicy));
	}
}