namespace Canon.Core.GrammarParser;

/// <summary>
/// LR语法中的一个项目集规范族
/// 也就是自动机中的一个状态
/// </summary>
public class LrState : IEquatable<LrState>
{
    /// <summary>
    /// 项目集规范族
    /// </summary>
    public required HashSet<Expression> Expressions { get; init; }

    /// <summary>
    /// 自动机的迁移规则
    /// </summary>
    public Dictionary<TerminatorBase, LrState> Transformer { get; } = [];

    /// <summary>
    /// 向状态中添加一个迁移规则
    /// </summary>
    /// <param name="terminator">迁移的条件</param>
    /// <param name="next">迁移到达的状态</param>
    /// <exception cref="InvalidOperationException">如果在状态中已经存在该迁移规则且迁移到的状态和欲设置的状态不同
    /// 抛出无效操作异常</exception>
    public void AddTransform(TerminatorBase terminator, LrState next)
    {
        if (Transformer.TryGetValue(terminator, out LrState? state))
        {
            if (state != next)
            {
                throw new InvalidOperationException("A terminator transform to two different states");
            }
        }
        else
        {
            Transformer.Add(terminator, next);
        }
    }

    public bool Equals(LrState? other)
    {
        if (other is null)
        {
            return false;
        }

        if (Expressions.Count != other.Expressions.Count)
        {
            return false;
        }

        // 如果两个集合的大小相等，且一个是另一个的子集，那么两个集合相等。
        return Expressions.IsSubsetOf(other.Expressions);
    }

    public override bool Equals(object? obj)
    {
        if (obj is not LrState other)
        {
            return false;
        }

        return Equals(other);
    }

    public override int GetHashCode()
    {
        int hash = 0;

        foreach (Expression expression in Expressions)
        {
            hash ^= expression.GetHashCode();
        }

        return hash;
    }

    public static bool operator ==(LrState a, LrState b)
    {
        return a.Equals(b);
    }

    public static bool operator !=(LrState a, LrState b)
    {
        return !a.Equals(b);
    }
}