aspnetcore/src/Microsoft.AspNetCore.Mvc.Core/Internal/PrefixContainer.cs

// 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 System.Diagnostics;

namespace Microsoft.AspNetCore.Mvc.Internal
{
    /// <summary>
    /// This is a container for prefix values. It normalizes all the values into dotted-form and then stores
    /// them in a sorted array. All queries for prefixes are also normalized to dotted-form, and searches
    /// for ContainsPrefix are done with a binary search.
    /// </summary>
    public class PrefixContainer
    {
        private static readonly char[] Delimiters = new char[] { '[', '.' };

        private readonly ICollection<string> _originalValues;
        private readonly string[] _sortedValues;

        public PrefixContainer(ICollection<string> values)
        {
            if (values == null)
            {
                throw new ArgumentNullException(nameof(values));
            }

            _originalValues = values;

            if (_originalValues.Count == 0)
            {
                _sortedValues = Array.Empty<string>();
            }
            else
            {
                _sortedValues = new string[_originalValues.Count];
                _originalValues.CopyTo(_sortedValues, 0);
                Array.Sort(_sortedValues, StringComparer.OrdinalIgnoreCase);
            }
        }

        public bool ContainsPrefix(string prefix)
        {
            if (prefix == null)
            {
                throw new ArgumentNullException(nameof(prefix));
            }

            if (_sortedValues.Length == 0)
            {
                return false;
            }

            if (prefix.Length == 0)
            {
                return true; // Empty prefix matches all elements.
            }

            return BinarySearch(prefix) > -1;
        }

        // Given "foo.bar", "foo.hello", "something.other", foo[abc].baz and asking for prefix "foo" will return:
        // - "bar"/"foo.bar"
        // - "hello"/"foo.hello"
        // - "abc"/"foo[abc]"
        public IDictionary<string, string> GetKeysFromPrefix(string prefix)
        {
            var result = new Dictionary<string, string>(StringComparer.OrdinalIgnoreCase);

            foreach (var entry in _originalValues)
            {
                if (entry != null)
                {
                    if (entry.Length == prefix.Length)
                    {
                        // No key in this entry
                        continue;
                    }

                    if (prefix.Length == 0)
                    {
                        GetKeyFromEmptyPrefix(entry, result);
                    }
                    else if (entry.StartsWith(prefix, StringComparison.OrdinalIgnoreCase))
                    {
                        GetKeyFromNonEmptyPrefix(prefix, entry, result);
                    }
                }
            }

            return result;
        }

        private static void GetKeyFromEmptyPrefix(string entry, IDictionary<string, string> results)
        {
            string key;
            string fullName;
            var delimiterPosition = entry.IndexOfAny(Delimiters, 0);

            if (delimiterPosition == 0 && entry[0] == '[')
            {
                // Handle an entry such as "[key]".
                var bracketPosition = entry.IndexOf(']', 1);
                if (bracketPosition == -1)
                {
                    // Malformed for dictionary.
                    return;
                }

                key = entry.Substring(1, bracketPosition - 1);
                fullName = entry.Substring(0, bracketPosition + 1);
            }
            else
            {
                // Handle an entry such as "key", "key.property" and "key[index]".
                key = delimiterPosition == -1 ? entry : entry.Substring(0, delimiterPosition);
                fullName = key;
            }

            if (!results.ContainsKey(key))
            {
                results.Add(key, fullName);
            }
        }

        private static void GetKeyFromNonEmptyPrefix(string prefix, string entry, IDictionary<string, string> results)
        {
            string key;
            string fullName;
            var keyPosition = prefix.Length + 1;

            switch (entry[prefix.Length])
            {
                case '.':
                    // Handle an entry such as "prefix.key", "prefix.key.property" and "prefix.key[index]".
                    var delimiterPosition = entry.IndexOfAny(Delimiters, keyPosition);
                    if (delimiterPosition == -1)
                    {
                        // Neither '.' nor '[' found later in the name. Use rest of the string.
                        key = entry.Substring(keyPosition);
                        fullName = entry;
                    }
                    else
                    {
                        key = entry.Substring(keyPosition, delimiterPosition - keyPosition);
                        fullName = entry.Substring(0, delimiterPosition);
                    }
                    break;

                case '[':
                    // Handle an entry such as "prefix[key]".
                    var bracketPosition = entry.IndexOf(']', keyPosition);
                    if (bracketPosition == -1)
                    {
                        // Malformed for dictionary
                        return;
                    }

                    key = entry.Substring(keyPosition, bracketPosition - keyPosition);
                    fullName = entry.Substring(0, bracketPosition + 1);
                    break;

                default:
                    // Ignore an entry such as "prefixA".
                    return;
            }

            if (!results.ContainsKey(key))
            {
                results.Add(key, fullName);
            }
        }

        // This is tightly coupled to the definition at ModelStateDictionary.StartsWithPrefix
        private int BinarySearch(string prefix)
        {
            var start = 0;
            var end = _sortedValues.Length - 1;

            while (start <= end)
            {
                var pivot = start + ((end - start) / 2);
                var candidate = _sortedValues[pivot];
                var compare = string.Compare(
                    prefix,
                    0,
                    candidate,
                    0,
                    prefix.Length,
                    StringComparison.OrdinalIgnoreCase);
                if (compare == 0)
                {
                    Debug.Assert(candidate.StartsWith(prefix, StringComparison.OrdinalIgnoreCase));

                    // Okay, now we have a candidate that starts with the prefix. If the candidate is longer than
                    // the prefix, we need to look at the next character and see if it's a delimiter.
                    if (candidate.Length == prefix.Length)
                    {
                        // Exact match
                        return pivot;
                    }

                    var c = candidate[prefix.Length];
                    if (c == '.' || c == '[')
                    {
                        // Match, followed by delimiter
                        return pivot;
                    }

                    // Okay, so the candidate has some extra text. We need to keep searching.
                    //
                    // Can often assume the candidate string is greater than the prefix e.g. that works for
                    //  prefix: product
                    //  candidate: productId
                    // most of the time because "product", "product.id", etc. will sort earlier than "productId". But,
                    // the assumption isn't correct if "product[0]" is also in _sortedValues because that value will
                    // sort later than "productId".
                    //
                    // Fall back to brute force and cover all the cases.
                    return LinearSearch(prefix, start, end);
                }

                if (compare > 0)
                {
                    start = pivot + 1;
                }
                else
                {
                    end = pivot - 1;
                }
            }

            return ~start;
        }

        private int LinearSearch(string prefix, int start, int end)
        {
            for (; start <= end; start++)
            {
                var candidate = _sortedValues[start];
                var compare = string.Compare(
                    prefix,
                    0,
                    candidate,
                    0,
                    prefix.Length,
                    StringComparison.OrdinalIgnoreCase);
                if (compare == 0)
                {
                    Debug.Assert(candidate.StartsWith(prefix, StringComparison.OrdinalIgnoreCase));

                    // Okay, now we have a candidate that starts with the prefix. If the candidate is longer than
                    // the prefix, we need to look at the next character and see if it's a delimiter.
                    if (candidate.Length == prefix.Length)
                    {
                        // Exact match
                        return start;
                    }

                    var c = candidate[prefix.Length];
                    if (c == '.' || c == '[')
                    {
                        // Match, followed by delimiter
                        return start;
                    }

                    // Keep checking until we've passed all StartsWith() matches.
                }

                if (compare < 0)
                {
                    // Prefix is less than the candidate. No potential matches left.
                    break;
                }
            }

            return ~start;
        }
    }
}