// Copyright (c) .NET Foundation. All rights reserved.
// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.

using System;
using System.Collections.Generic;
using Microsoft.Extensions.Primitives;

namespace Microsoft.AspNetCore.WebUtilities
{
    public struct KeyValueAccumulator
    {
        private Dictionary<string, StringValues> _accumulator;
        private Dictionary<string, List<string>> _expandingAccumulator;

        public void Append(string key, string value)
        {
            if (_accumulator == null)
            {
                _accumulator = new Dictionary<string, StringValues>(StringComparer.OrdinalIgnoreCase);
            }

            StringValues values;
            if (_accumulator.TryGetValue(key, out values))
            {
                if (values.Count == 0)
                {
                    // Marker entry for this key to indicate entry already in expanding list dictionary
                    _expandingAccumulator[key].Add(value);
                }
                else if (values.Count == 1)
                {
                    // Second value for this key
                    _accumulator[key] = new string[] { values[0], value };
                }
                else
                {
                    // Third value for this key
                    // Add zero count entry and move to data to expanding list dictionary
                    _accumulator[key] = default(StringValues);

                    if (_expandingAccumulator == null)
                    {
                        _expandingAccumulator = new Dictionary<string, List<string>>(StringComparer.OrdinalIgnoreCase);
                    }

                    // Already 3 entries so use starting allocated as 8; then use List's expansion mechanism for more
                    var list = new List<string>(8);
                    var array = values.ToArray();

                    list.Add(array[0]);
                    list.Add(array[1]);
                    list.Add(value);

                    _expandingAccumulator[key] = list;
                }
            }
            else
            {
                // First value for this key
                _accumulator[key] = new StringValues(value);
            }
        }

        public bool HasValues => _accumulator != null;

        public int Count => _accumulator?.Count ?? 0;

        public Dictionary<string, StringValues> GetResults()
        {
            if (_expandingAccumulator != null)
            {
                // Coalesce count 3+ multi-value entries into _accumulator dictionary
                foreach (var entry in _expandingAccumulator)
                {
                    _accumulator[entry.Key] = new StringValues(entry.Value.ToArray());
                }
            }

            return _accumulator ?? new Dictionary<string, StringValues>(0, StringComparer.OrdinalIgnoreCase);
        }
    }
}