using System.Diagnostics.CodeAnalysis;

namespace AvParser.Core.Parsing;

/// <summary>
/// Result of parsing a single record: either a value or a recoverable <see cref="ParseError"/>.
/// </summary>
/// <remarks>
/// A struct rather than a class hierarchy: parsers emit one of these per line, and on a
/// million-line input the allocation difference is the whole cost of the parse.
/// </remarks>
public readonly record struct ParseOutcome<T>
{
    private ParseOutcome(T? value, ParseError? error)
    {
        Value = value;
        Error = error;
    }

    /// <summary>The parsed value, or <see langword="null"/> when <see cref="IsSuccess"/> is false.</summary>
    public T? Value { get; }

    /// <summary>The failure, or <see langword="null"/> when <see cref="IsSuccess"/> is true.</summary>
    public ParseError? Error { get; }

    /// <summary><see langword="true"/> when a value was produced.</summary>
    [MemberNotNullWhen(false, nameof(Error))]
    public bool IsSuccess => Error is null;

    /// <summary>Creates a successful outcome.</summary>
    public static ParseOutcome<T> Success(T value) => new(value, null);

    /// <summary>Creates a failed outcome.</summary>
    public static ParseOutcome<T> Failure(ParseError error) =>
        new(default, error ?? throw new ArgumentNullException(nameof(error)));

    /// <summary>Creates a failed outcome from its parts.</summary>
    public static ParseOutcome<T> Failure(int lineNumber, string message) =>
        Failure(new ParseError(lineNumber, message));
}