// 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;
using System.Collections.Generic;
using System.Diagnostics;
using Microsoft.AspNetCore.Mvc.Abstractions;
using Microsoft.Extensions.Primitives;

namespace Microsoft.AspNetCore.Mvc.ModelBinding
{
    /// <summary>
    /// Represents the state of an attempt to bind values from an HTTP Request to an action method, which includes
    /// validation information.
    /// </summary>
    public class ModelStateDictionary : IReadOnlyDictionary<string, ModelStateEntry>
    {
        // Make sure to update the doc headers if this value is changed.
        /// <summary>
        /// The default value for <see cref="MaxAllowedErrors"/> of <c>200</c>.
        /// </summary>
        public static readonly int DefaultMaxAllowedErrors = 200;
        private static readonly char[] Delimiters = new char[] { '.', '[' };

        private readonly ModelStateNode _root;
        private int _maxAllowedErrors;

        /// <summary>
        /// Initializes a new instance of the <see cref="ModelStateDictionary"/> class.
        /// </summary>
        public ModelStateDictionary()
            : this(DefaultMaxAllowedErrors)
        {
        }

        /// <summary>
        /// Initializes a new instance of the <see cref="ModelStateDictionary"/> class.
        /// </summary>
        public ModelStateDictionary(int maxAllowedErrors)
        {
            MaxAllowedErrors = maxAllowedErrors;
            var emptySegment = new StringSegment(buffer: string.Empty);
            _root = new ModelStateNode(subKey: emptySegment)
            {
                Key = string.Empty
            };
        }

        /// <summary>
        /// Initializes a new instance of the <see cref="ModelStateDictionary"/> class by using values that are copied
        /// from the specified <paramref name="dictionary"/>.
        /// </summary>
        /// <param name="dictionary">The <see cref="ModelStateDictionary"/> to copy values from.</param>
        public ModelStateDictionary(ModelStateDictionary dictionary)
            : this(dictionary?.MaxAllowedErrors ?? DefaultMaxAllowedErrors)
        {
            if (dictionary == null)
            {
                throw new ArgumentNullException(nameof(dictionary));
            }

            Merge(dictionary);
        }

        /// <summary>
        /// Root entry for the <see cref="ModelStateDictionary"/>.
        /// </summary>
        public ModelStateEntry Root => _root;

        /// <summary>
        /// Gets or sets the maximum allowed model state errors in this instance of <see cref="ModelStateDictionary"/>.
        /// Defaults to <c>200</c>.
        /// </summary>
        /// <remarks>
        /// <para>
        /// <see cref="ModelStateDictionary"/> tracks the number of model errors added by calls to
        /// <see cref="AddModelError(string, Exception, ModelMetadata)"/> or
        /// <see cref="TryAddModelError(string, Exception, ModelMetadata)"/>.
        /// Once the value of <code>MaxAllowedErrors - 1</code> is reached, if another attempt is made to add an error,
        /// the error message will be ignored and a <see cref="TooManyModelErrorsException"/> will be added.
        /// </para>
        /// <para>
        /// Errors added via modifying <see cref="ModelStateEntry"/> directly do not count towards this limit.
        /// </para>
        /// </remarks>
        public int MaxAllowedErrors
        {
            get
            {
                return _maxAllowedErrors;
            }
            set
            {
                if (value < 0)
                {
                    throw new ArgumentOutOfRangeException(nameof(value));
                }

                _maxAllowedErrors = value;
            }
        }

        /// <summary>
        /// Gets a value indicating whether or not the maximum number of errors have been
        /// recorded.
        /// </summary>
        /// <remarks>
        /// Returns <c>true</c> if a <see cref="TooManyModelErrorsException"/> has been recorded;
        /// otherwise <c>false</c>.
        /// </remarks>
        public bool HasReachedMaxErrors
        {
            get { return ErrorCount >= MaxAllowedErrors; }
        }

        /// <summary>
        /// Gets the number of errors added to this instance of <see cref="ModelStateDictionary"/> via
        /// <see cref="M:AddModelError"/> or <see cref="M:TryAddModelError"/>.
        /// </summary>
        public int ErrorCount { get; private set; }

        /// <inheritdoc />
        public int Count { get; private set; }

        /// <summary>
        /// Gets the key sequence.
        /// </summary>
        public KeyEnumerable Keys => new KeyEnumerable(this);

        /// <inheritdoc />
        IEnumerable<string> IReadOnlyDictionary<string, ModelStateEntry>.Keys => Keys;

        /// <summary>
        /// Gets the value sequence.
        /// </summary>
        public ValueEnumerable Values => new ValueEnumerable(this);

        /// <inheritdoc />
        IEnumerable<ModelStateEntry> IReadOnlyDictionary<string, ModelStateEntry>.Values => Values;

        /// <summary>
        /// Gets a value that indicates whether any model state values in this model state dictionary is invalid or not validated.
        /// </summary>
        public bool IsValid
        {
            get
            {
                return ValidationState == ModelValidationState.Valid || ValidationState == ModelValidationState.Skipped;
            }
        }

        /// <inheritdoc />
        public ModelValidationState ValidationState => GetValidity(_root) ?? ModelValidationState.Valid;

        /// <inheritdoc />
        public ModelStateEntry this[string key]
        {
            get
            {
                if (key == null)
                {
                    throw new ArgumentNullException(nameof(key));
                }

                ModelStateEntry entry;
                TryGetValue(key, out entry);
                return entry;
            }
        }

        // Flag that indiciates if TooManyModelErrorException has already been added to this dictionary.
        private bool HasRecordedMaxModelError { get; set; }

        /// <summary>
        /// Adds the specified <paramref name="exception"/> to the <see cref="ModelStateEntry.Errors"/> instance
        /// that is associated with the specified <paramref name="key"/>.
        /// </summary>
        /// <param name="key">The key of the <see cref="ModelStateEntry"/> to add errors to.</param>
        /// <param name="exception">The <see cref="Exception"/> to add.</param>
        /// <param name="metadata">The <see cref="ModelMetadata"/> associated with the model.</param>
        public void AddModelError(string key, Exception exception, ModelMetadata metadata)
        {
            if (key == null)
            {
                throw new ArgumentNullException(nameof(key));
            }

            if (exception == null)
            {
                throw new ArgumentNullException(nameof(exception));
            }

            if (metadata == null)
            {
                throw new ArgumentNullException(nameof(metadata));
            }

            TryAddModelError(key, exception, metadata);
        }

        /// <summary>
        /// Attempts to add the specified <paramref name="exception"/> to the <see cref="ModelStateEntry.Errors"/>
        /// instance that is associated with the specified <paramref name="key"/>. If the maximum number of allowed
        /// errors has already been recorded, records a <see cref="TooManyModelErrorsException"/> exception instead.
        /// </summary>
        /// <param name="key">The key of the <see cref="ModelStateEntry"/> to add errors to.</param>
        /// <param name="exception">The <see cref="Exception"/> to add.</param>
        /// <param name="metadata">The <see cref="ModelMetadata"/> associated with the model.</param>
        /// <returns>
        /// <c>True</c> if the given error was added, <c>false</c> if the error was ignored.
        /// See <see cref="MaxAllowedErrors"/>.
        /// </returns>
        public bool TryAddModelError(string key, Exception exception, ModelMetadata metadata)
        {
            if (key == null)
            {
                throw new ArgumentNullException(nameof(key));
            }

            if (exception == null)
            {
                throw new ArgumentNullException(nameof(exception));
            }

            if (metadata == null)
            {
                throw new ArgumentNullException(nameof(metadata));
            }

            if (ErrorCount >= MaxAllowedErrors - 1)
            {
                EnsureMaxErrorsReachedRecorded();
                return false;
            }

            if (exception is FormatException || exception is OverflowException)
            {
                // Convert FormatExceptions and OverflowExceptions to Invalid value messages.
                ModelStateEntry entry;
                TryGetValue(key, out entry);

                var name = metadata.GetDisplayName();
                string errorMessage;
                if (entry == null)
                {
                    errorMessage = metadata.ModelBindingMessageProvider.UnknownValueIsInvalidAccessor(name);
                }
                else
                {
                    errorMessage = metadata.ModelBindingMessageProvider.AttemptedValueIsInvalidAccessor(
                        entry.AttemptedValue,
                        name);
                }

                return TryAddModelError(key, errorMessage);
            }

            ErrorCount++;
            AddModelErrorCore(key, exception);
            return true;
        }

        /// <summary>
        /// Adds the specified <paramref name="errorMessage"/> to the <see cref="ModelStateEntry.Errors"/> instance
        /// that is associated with the specified <paramref name="key"/>.
        /// </summary>
        /// <param name="key">The key of the <see cref="ModelStateEntry"/> to add errors to.</param>
        /// <param name="errorMessage">The error message to add.</param>
        public void AddModelError(string key, string errorMessage)
        {
            if (key == null)
            {
                throw new ArgumentNullException(nameof(key));
            }

            if (errorMessage == null)
            {
                throw new ArgumentNullException(nameof(errorMessage));
            }

            TryAddModelError(key, errorMessage);
        }

        /// <summary>
        /// Attempts to add the specified <paramref name="errorMessage"/> to the <see cref="ModelStateEntry.Errors"/>
        /// instance that is associated with the specified <paramref name="key"/>. If the maximum number of allowed
        /// errors has already been recorded, records a <see cref="TooManyModelErrorsException"/> exception instead.
        /// </summary>
        /// <param name="key">The key of the <see cref="ModelStateEntry"/> to add errors to.</param>
        /// <param name="errorMessage">The error message to add.</param>
        /// <returns>
        /// <c>True</c> if the given error was added, <c>false</c> if the error was ignored.
        /// See <see cref="MaxAllowedErrors"/>.
        /// </returns>
        public bool TryAddModelError(string key, string errorMessage)
        {
            if (key == null)
            {
                throw new ArgumentNullException(nameof(key));
            }

            if (errorMessage == null)
            {
                throw new ArgumentNullException(nameof(errorMessage));
            }

            if (ErrorCount >= MaxAllowedErrors - 1)
            {
                EnsureMaxErrorsReachedRecorded();
                return false;
            }

            ErrorCount++;
            var modelState = GetOrAddNode(key);
            Count += !modelState.IsContainerNode ? 0 : 1;
            modelState.ValidationState = ModelValidationState.Invalid;
            modelState.MarkNonContainerNode();
            modelState.Errors.Add(errorMessage);

            return true;
        }

        /// <summary>
        /// Returns the aggregate <see cref="ModelValidationState"/> for items starting with the
        /// specified <paramref name="key"/>.
        /// </summary>
        /// <param name="key">The key to look up model state errors for.</param>
        /// <returns>Returns <see cref="ModelValidationState.Unvalidated"/> if no entries are found for the specified
        /// key, <see cref="ModelValidationState.Invalid"/> if at least one instance is found with one or more model
        /// state errors; <see cref="ModelValidationState.Valid"/> otherwise.</returns>
        public ModelValidationState GetFieldValidationState(string key)
        {
            if (key == null)
            {
                throw new ArgumentNullException(nameof(key));
            }

            var item = GetNode(key);
            return GetValidity(item) ?? ModelValidationState.Unvalidated;
        }

        /// <summary>
        /// Returns <see cref="ModelValidationState"/> for the <paramref name="key"/>.
        /// </summary>
        /// <param name="key">The key to look up model state errors for.</param>
        /// <returns>Returns <see cref="ModelValidationState.Unvalidated"/> if no entry is found for the specified
        /// key, <see cref="ModelValidationState.Invalid"/> if an instance is found with one or more model
        /// state errors; <see cref="ModelValidationState.Valid"/> otherwise.</returns>
        public ModelValidationState GetValidationState(string key)
        {
            if (key == null)
            {
                throw new ArgumentNullException(nameof(key));
            }

            ModelStateEntry validationState;
            if (TryGetValue(key, out validationState))
            {
                return validationState.ValidationState;
            }

            return ModelValidationState.Unvalidated;
        }

        /// <summary>
        /// Marks the <see cref="ModelStateEntry.ValidationState"/> for the entry with the specified
        /// <paramref name="key"/> as <see cref="ModelValidationState.Valid"/>.
        /// </summary>
        /// <param name="key">The key of the <see cref="ModelStateEntry"/> to mark as valid.</param>
        public void MarkFieldValid(string key)
        {
            if (key == null)
            {
                throw new ArgumentNullException(nameof(key));
            }

            var modelState = GetOrAddNode(key);
            if (modelState.ValidationState == ModelValidationState.Invalid)
            {
                throw new InvalidOperationException(Resources.Validation_InvalidFieldCannotBeReset);
            }

            Count += !modelState.IsContainerNode ? 0 : 1;
            modelState.MarkNonContainerNode();
            modelState.ValidationState = ModelValidationState.Valid;
        }

        /// <summary>
        /// Marks the <see cref="ModelStateEntry.ValidationState"/> for the entry with the specified <paramref name="key"/>
        /// as <see cref="ModelValidationState.Skipped"/>.
        /// </summary>
        /// <param name="key">The key of the <see cref="ModelStateEntry"/> to mark as skipped.</param>
        public void MarkFieldSkipped(string key)
        {
            if (key == null)
            {
                throw new ArgumentNullException(nameof(key));
            }

            var modelState = GetOrAddNode(key);
            if (modelState.ValidationState == ModelValidationState.Invalid)
            {
                throw new InvalidOperationException(Resources.Validation_InvalidFieldCannotBeReset_ToSkipped);
            }

            Count += !modelState.IsContainerNode ? 0 : 1;
            modelState.MarkNonContainerNode();
            modelState.ValidationState = ModelValidationState.Skipped;
        }

        /// <summary>
        /// Copies the values from the specified <paramref name="dictionary"/> into this instance, overwriting
        /// existing values if keys are the same.
        /// </summary>
        /// <param name="dictionary">The <see cref="ModelStateDictionary"/> to copy values from.</param>
        public void Merge(ModelStateDictionary dictionary)
        {
            if (dictionary == null)
            {
                return;
            }

            foreach (var source in dictionary)
            {
                var target = GetOrAddNode(source.Key);
                Count += !target.IsContainerNode ? 0 : 1;
                ErrorCount += source.Value.Errors.Count - target.Errors.Count;
                target.Copy(source.Value);
                target.MarkNonContainerNode();
            }
        }

        /// <summary>
        /// Sets the of <see cref="ModelStateEntry.RawValue"/> and <see cref="ModelStateEntry.AttemptedValue"/> for
        /// the <see cref="ModelStateEntry"/> with the specified <paramref name="key"/>.
        /// </summary>
        /// <param name="key">The key for the <see cref="ModelStateEntry"/> entry.</param>
        /// <param name="rawValue">The raw value for the <see cref="ModelStateEntry"/> entry.</param>
        /// <param name="attemptedValue">
        /// The values of <paramref name="rawValue"/> in a comma-separated <see cref="string"/>.
        /// </param>
        public void SetModelValue(string key, object rawValue, string attemptedValue)
        {
            if (key == null)
            {
                throw new ArgumentNullException(nameof(key));
            }

            var modelState = GetOrAddNode(key);
            Count += !modelState.IsContainerNode ? 0 : 1;
            modelState.RawValue = rawValue;
            modelState.AttemptedValue = attemptedValue;
            modelState.MarkNonContainerNode();
        }

        /// <summary>
        /// Sets the value for the <see cref="ModelStateEntry"/> with the specified <paramref name="key"/>.
        /// </summary>
        /// <param name="key">The key for the <see cref="ModelStateEntry"/> entry</param>
        /// <param name="valueProviderResult">
        /// A <see cref="ValueProviderResult"/> with data for the <see cref="ModelStateEntry"/> entry.
        /// </param>
        public void SetModelValue(string key, ValueProviderResult valueProviderResult)
        {
            if (key == null)
            {
                throw new ArgumentNullException(nameof(key));
            }

            // Avoid creating a new array for rawValue if there's only one value.
            object rawValue;
            if (valueProviderResult == ValueProviderResult.None)
            {
                rawValue = null;
            }
            else if (valueProviderResult.Length == 1)
            {
                rawValue = valueProviderResult.Values[0];
            }
            else
            {
                rawValue = valueProviderResult.Values.ToArray();
            }

            SetModelValue(key, rawValue, valueProviderResult.ToString());
        }

        /// <summary>
        /// Clears <see cref="ModelStateDictionary"/> entries that match the key that is passed as parameter.
        /// </summary>
        /// <param name="key">The key of <see cref="ModelStateDictionary"/> to clear.</param>
        public void ClearValidationState(string key)
        {
            // If key is null or empty, clear all entries in the dictionary
            // else just clear the ones that have key as prefix
            var entries = FindKeysWithPrefix(key ?? string.Empty);
            foreach (var entry in entries)
            {
                entry.Value.Errors.Clear();
                entry.Value.ValidationState = ModelValidationState.Unvalidated;
            }
        }

        private ModelStateNode GetNode(string key) => GetNode(key, createIfNotExists: false);

        private ModelStateNode GetOrAddNode(string key) => GetNode(key, createIfNotExists: true);

        private ModelStateNode GetNode(string key, bool createIfNotExists)
        {
            Debug.Assert(key != null);
            if (key.Length == 0)
            {
                return _root;
            }

            // For a key of the format, foo.bar[0].baz[qux] we'll create the following nodes:
            // foo
            //  -> bar
            //   -> [0]
            //    -> baz
            //     -> [qux]

            var current = _root;
            var previousIndex = 0;
            int index;
            while ((index = key.IndexOfAny(Delimiters, previousIndex)) != -1)
            {
                var keyStart = previousIndex == 0 || key[previousIndex - 1] == '.'
                    ? previousIndex
                    : previousIndex - 1;
                var subKey = new StringSegment(key, keyStart, index - keyStart);
                current = current.GetNode(subKey, createIfNotExists);
                if (current == null)
                {
                    // createIfNotExists is set to false and a node wasn't found. Exit early.
                    return null;
                }

                previousIndex = index + 1;
            }

            if (previousIndex < key.Length)
            {
                var keyStart = previousIndex == 0 || key[previousIndex - 1] == '.'
                    ? previousIndex
                    : previousIndex - 1;
                var subKey = new StringSegment(key, keyStart, key.Length - keyStart);
                current = current.GetNode(subKey, createIfNotExists);
            }

            if (current != null && current.Key == null)
            {
                // Don't update the key if it's been previously assigned. This is to prevent change in key casing
                // e.g. modelState.SetModelValue("foo", .., ..);
                // var value = modelState["FOO"];
                current.Key = key;
            }

            return current;
        }

        private static ModelValidationState? GetValidity(ModelStateNode node)
        {
            if (node == null)
            {
                return null;
            }

            ModelValidationState? validationState = null;
            if (!node.IsContainerNode)
            {
                validationState = ModelValidationState.Valid;
                if (node.ValidationState == ModelValidationState.Unvalidated)
                {
                    // If any entries of a field is unvalidated, we'll treat the tree as unvalidated.
                    return ModelValidationState.Unvalidated;
                }

                if (node.ValidationState == ModelValidationState.Invalid)
                {
                    validationState = node.ValidationState;
                }
            }

            if (node.ChildNodes != null)
            {
                for (var i = 0; i < node.ChildNodes.Count; i++)
                {
                    var entryState = GetValidity(node.ChildNodes[i]);

                    if (entryState == ModelValidationState.Unvalidated)
                    {
                        return entryState;
                    }

                    if (validationState == null || entryState == ModelValidationState.Invalid)
                    {
                        validationState = entryState;
                    }
                }
            }

            return validationState;
        }

        private void EnsureMaxErrorsReachedRecorded()
        {
            if (!HasRecordedMaxModelError)
            {
                var exception = new TooManyModelErrorsException(Resources.ModelStateDictionary_MaxModelStateErrors);
                AddModelErrorCore(string.Empty, exception);
                HasRecordedMaxModelError = true;
                ErrorCount++;
            }
        }

        private void AddModelErrorCore(string key, Exception exception)
        {
            var modelState = GetOrAddNode(key);
            Count += !modelState.IsContainerNode ? 0 : 1;
            modelState.ValidationState = ModelValidationState.Invalid;
            modelState.MarkNonContainerNode();
            modelState.Errors.Add(exception);
        }

        /// <summary>
        /// Removes all keys and values from ths instance of <see cref="ModelStateDictionary"/>.
        /// </summary>
        public void Clear()
        {
            Count = 0;
            HasRecordedMaxModelError = false;
            ErrorCount = 0;
            _root.Reset();
            _root.ChildNodes.Clear();
        }

        /// <inheritdoc />
        public bool ContainsKey(string key)
        {
            if (key == null)
            {
                throw new ArgumentNullException(nameof(key));
            }

            return !GetNode(key)?.IsContainerNode ?? false;
        }

        /// <summary>
        /// Removes the <see cref="ModelStateEntry"/> with the specified <paramref name="key"/>.
        /// </summary>
        /// <param name="key">The key.</param>
        /// <returns><c>true</c> if the element is successfully removed; otherwise <c>false</c>. This method also
        /// returns <c>false</c> if key was not found.</returns>
        public bool Remove(string key)
        {
            if (key == null)
            {
                throw new ArgumentNullException(nameof(key));
            }

            var node = GetNode(key);
            if (node?.IsContainerNode == false)
            {
                Count--;
                ErrorCount -= node.Errors.Count;
                node.Reset();
                return true;
            }

            return false;
        }

        /// <inheritdoc />
        public bool TryGetValue(string key, out ModelStateEntry value)
        {
            if (key == null)
            {
                throw new ArgumentNullException(nameof(key));
            }

            var result = GetNode(key);
            if (result?.IsContainerNode == false)
            {
                value = result;
                return true;
            }

            value = null;
            return false;
        }

        /// <summary>
        /// Returns an enumerator that iterates through this instance of <see cref="ModelStateDictionary"/>.
        /// </summary>
        /// <returns>An <see cref="Enumerator"/>.</returns>
        public Enumerator GetEnumerator() => new Enumerator(this, prefix: string.Empty);

        /// <inheritdoc />
        IEnumerator<KeyValuePair<string, ModelStateEntry>>
            IEnumerable<KeyValuePair<string, ModelStateEntry>>.GetEnumerator() => GetEnumerator();

        /// <inheritdoc />
        IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();

        public static bool StartsWithPrefix(string prefix, string key)
        {
            if (prefix == null)
            {
                throw new ArgumentNullException(nameof(prefix));
            }

            if (key == null)
            {
                throw new ArgumentNullException(nameof(key));
            }

            if (prefix.Length == 0)
            {
                // Everything is prefixed by the empty string.
                return true;
            }

            if (prefix.Length > key.Length)
            {
                return false; // Not long enough.
            }

            if (!key.StartsWith(prefix, StringComparison.OrdinalIgnoreCase))
            {
                return false;
            }

            if (key.Length == prefix.Length)
            {
                // Exact match
                return true;
            }

            var charAfterPrefix = key[prefix.Length];
            if (charAfterPrefix == '.' || charAfterPrefix == '[')
            {
                return true;
            }

            return false;
        }

        public PrefixEnumerable FindKeysWithPrefix(string prefix)
        {
            if (prefix == null)
            {
                throw new ArgumentNullException(nameof(prefix));
            }

            return new PrefixEnumerable(this, prefix);
        }

        [DebuggerDisplay("SubKey={SubKey}, Key={Key}, ValidationState={ValidationState}")]
        private class ModelStateNode : ModelStateEntry
        {
            private bool _isContainerNode = true;

            public ModelStateNode(StringSegment subKey)
            {
                SubKey = subKey;
            }

            public List<ModelStateNode> ChildNodes { get; set; }

            public override IReadOnlyList<ModelStateEntry> Children => ChildNodes;

            public string Key { get; set; }

            public StringSegment SubKey { get; }

            public override bool IsContainerNode => _isContainerNode;

            public void MarkNonContainerNode()
            {
                _isContainerNode = false;
            }

            public void Copy(ModelStateEntry entry)
            {
                RawValue = entry.RawValue;
                AttemptedValue = entry.AttemptedValue;
                Errors.Clear();
                for (var i = 0; i < entry.Errors.Count; i++)
                {
                    Errors.Add(entry.Errors[i]);
                }

                ValidationState = entry.ValidationState;
            }

            public void Reset()
            {
                _isContainerNode = true;
                RawValue = null;
                AttemptedValue = null;
                ValidationState = ModelValidationState.Unvalidated;
                Errors.Clear();
            }

            public ModelStateNode GetNode(StringSegment subKey, bool createIfNotExists)
            {
                if (subKey.Length == 0)
                {
                    return this;
                }

                var index = BinarySearch(subKey);
                ModelStateNode modelStateNode = null;
                if (index >= 0)
                {
                    modelStateNode = ChildNodes[index];
                }
                else if (createIfNotExists)
                {
                    if (ChildNodes == null)
                    {
                        ChildNodes = new List<ModelStateNode>(1);
                    }

                    modelStateNode = new ModelStateNode(subKey);
                    ChildNodes.Insert(~index, modelStateNode);
                }

                return modelStateNode;
            }

            public override ModelStateEntry GetModelStateForProperty(string propertyName)
                => GetNode(new StringSegment(propertyName), createIfNotExists: false);

            private int BinarySearch(StringSegment searchKey)
            {
                if (ChildNodes == null)
                {
                    return -1;
                }

                var low = 0;
                var high = ChildNodes.Count - 1;
                while (low <= high)
                {
                    var mid = low + ((high - low) / 2);
                    var midKey = ChildNodes[mid].SubKey;
                    var result = midKey.Length - searchKey.Length;
                    if (result == 0)
                    {
                        result = string.Compare(
                            midKey.Buffer,
                            midKey.Offset,
                            searchKey.Buffer,
                            searchKey.Offset,
                            searchKey.Length,
                            StringComparison.OrdinalIgnoreCase);
                    }

                    if (result == 0)
                    {
                        return mid;
                    }
                    if (result < 0)
                    {
                        low = mid + 1;
                    }
                    else
                    {
                        high = mid - 1;
                    }
                }

                return ~low;
            }
        }

        public struct PrefixEnumerable : IEnumerable<KeyValuePair<string, ModelStateEntry>>
        {
            private readonly ModelStateDictionary _dictionary;
            private readonly string _prefix;

            public PrefixEnumerable(ModelStateDictionary dictionary, string prefix)
            {
                if (dictionary == null)
                {
                    throw new ArgumentNullException(nameof(dictionary));
                }

                if (prefix == null)
                {
                    throw new ArgumentNullException(nameof(prefix));
                }

                _dictionary = dictionary;
                _prefix = prefix;
            }

            public Enumerator GetEnumerator() => new Enumerator(_dictionary, _prefix);

            IEnumerator<KeyValuePair<string, ModelStateEntry>>
                IEnumerable<KeyValuePair<string, ModelStateEntry>>.GetEnumerator() => GetEnumerator();

            IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
        }

        public struct Enumerator : IEnumerator<KeyValuePair<string, ModelStateEntry>>
        {
            private readonly ModelStateNode _rootNode;
            private ModelStateNode _modelStateNode;
            private List<ModelStateNode> _nodes;
            private int _index;
            private bool _visitedRoot;

            public Enumerator(ModelStateDictionary dictionary, string prefix)
            {
                if (dictionary == null)
                {
                    throw new ArgumentNullException(nameof(dictionary));
                }

                if (prefix == null)
                {
                    throw new ArgumentNullException(nameof(prefix));
                }

                _index = -1;
                _rootNode = dictionary.GetNode(prefix);
                _modelStateNode = null;
                _nodes = null;
                _visitedRoot = false;
            }

            public KeyValuePair<string, ModelStateEntry> Current =>
                new KeyValuePair<string, ModelStateEntry>(_modelStateNode.Key, _modelStateNode);

            object IEnumerator.Current => Current;

            public void Dispose()
            {
            }

            public bool MoveNext()
            {
                if (_rootNode == null)
                {
                    return false;
                }

                if (!_visitedRoot)
                {
                    // Visit the root node
                    _visitedRoot = true;
                    if (_rootNode.ChildNodes?.Count > 0)
                    {
                        _nodes = new List<ModelStateNode> { _rootNode };
                    }

                    if (!_rootNode.IsContainerNode)
                    {
                        _modelStateNode = _rootNode;
                        return true;
                    }
                }

                if (_nodes == null)
                {
                    return false;
                }

                while (_nodes.Count > 0)
                {
                    var node = _nodes[0];
                    if (_index == node.ChildNodes.Count - 1)
                    {
                        // We've exhausted the current sublist.
                        _nodes.RemoveAt(0);
                        _index = -1;
                        continue;
                    }
                    else
                    {
                        _index++;
                    }

                    var currentChild = node.ChildNodes[_index];
                    if (currentChild.ChildNodes?.Count > 0)
                    {
                        _nodes.Add(currentChild);
                    }

                    if (!currentChild.IsContainerNode)
                    {
                        _modelStateNode = currentChild;
                        return true;
                    }
                }

                return false;
            }

            public void Reset()
            {
                _index = -1;
                _nodes.Clear();
                _visitedRoot = false;
                _modelStateNode = null;
            }
        }

        public struct KeyEnumerable : IEnumerable<string>
        {
            private readonly ModelStateDictionary _dictionary;

            public KeyEnumerable(ModelStateDictionary dictionary)
            {
                _dictionary = dictionary;
            }

            public KeyEnumerator GetEnumerator() => new KeyEnumerator(_dictionary, prefix: string.Empty);

            IEnumerator<string> IEnumerable<string>.GetEnumerator() => GetEnumerator();

            IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
        }

        public struct KeyEnumerator : IEnumerator<string>
        {
            private Enumerator _prefixEnumerator;

            public KeyEnumerator(ModelStateDictionary dictionary, string prefix)
            {
                _prefixEnumerator = new Enumerator(dictionary, prefix);
                Current = null;
            }

            public string Current { get; private set; }

            object IEnumerator.Current => Current;

            public void Dispose() => _prefixEnumerator.Dispose();

            public bool MoveNext()
            {
                var result = _prefixEnumerator.MoveNext();
                if (result)
                {
                    var current = _prefixEnumerator.Current;
                    Current = current.Key;
                }
                else
                {
                    Current = null;
                }

                return result;
            }

            public void Reset()
            {
                _prefixEnumerator.Reset();
                Current = null;
            }
        }

        public struct ValueEnumerable : IEnumerable<ModelStateEntry>
        {
            private readonly ModelStateDictionary _dictionary;

            public ValueEnumerable(ModelStateDictionary dictionary)
            {
                _dictionary = dictionary;
            }

            public ValueEnumerator GetEnumerator() => new ValueEnumerator(_dictionary, prefix: string.Empty);

            IEnumerator<ModelStateEntry> IEnumerable<ModelStateEntry>.GetEnumerator() => GetEnumerator();

            IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
        }

        public struct ValueEnumerator : IEnumerator<ModelStateEntry>
        {
            private Enumerator _prefixEnumerator;

            public ValueEnumerator(ModelStateDictionary dictionary, string prefix)
            {
                _prefixEnumerator = new Enumerator(dictionary, prefix);
                Current = null;
            }

            public ModelStateEntry Current { get; private set; }

            object IEnumerator.Current => Current;

            public void Dispose() => _prefixEnumerator.Dispose();

            public bool MoveNext()
            {
                var result = _prefixEnumerator.MoveNext();
                if (result)
                {
                    var current = _prefixEnumerator.Current;
                    Current = current.Value;
                }
                else
                {
                    Current = null;
                }

                return result;
            }

            public void Reset()
            {
                _prefixEnumerator.Reset();
                Current = null;
            }
        }
    }
}