Beam/Beam/ImmutableState.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Text.Json.Serialization;
using System.Threading.Tasks;

namespace Beam {
    public readonly struct ImmutableState {
        readonly string[] state;

        [JsonConstructor]
        public ImmutableState(string[] state) {
            this.state = state ?? [];
        }

        public string[] State => state ?? [];

        public readonly Span<string> AsSpan() => state ?? [];

        public readonly State Copy()
            => new((string[])(state ?? []).Clone());

        public readonly string this[Index i] {
            get => state[i];
        }

        public static implicit operator State(ImmutableState state)
            => state.Copy();
    }
}