// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
 
using System;
using System.Collections;
using System.Collections.Generic;
using System.Diagnostics;
using System.Linq;
using System.Runtime.CompilerServices;
using Microsoft.Shared.Diagnostics;
 
namespace Microsoft.Extensions.AI.Evaluation.NLP.Common;
 
[DebuggerDisplay("{ToDebugString(),nq}")]
[CollectionBuilder(typeof(NGramExtensions), nameof(NGramExtensions.CreateNGram))]
internal readonly struct NGram<T> : IEquatable<NGram<T>>, IEnumerable<T>
    where T : IEquatable<T>
{
    public NGram(ReadOnlySpan<T> values)
        : this(values.ToArray())
    {
    }
 
    public NGram(params T[] values)
    {
        Values = Throw.IfNull(values, nameof(values));
        _ = Throw.IfLessThan(values.Length, 1, nameof(values));
    }
 
    public readonly T[] Values { get; }
 
    public int Length => Values.Length;
 
    public bool Equals(NGram<T> other)
        => Values.SequenceEqual(other.Values);
 
    public override bool Equals(object? obj) => obj is NGram<T> other && Equals(other);
 
    public override int GetHashCode()
    {
        int hashCode = 0;
        foreach (var value in Values)
        {
            hashCode = HashCode.Combine(hashCode, value.GetHashCode());
        }
 
        return hashCode;
    }
 
    public IEnumerator<T> GetEnumerator() => ((IEnumerable<T>)Values).GetEnumerator();
 
    IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
 
    public string ToDebugString() => $"[{string.Join(",", Values.Select(v => v.ToString()))}]";
}