使用区分文化的比较从字符串中获取子字符串

Question

有没有办法使用文化敏感的相等比较从字符串中获取匹配的子字符串？例如,EN-US区域性下,æ和ae被视为相等."Encyclopædia".IndexOf("aed")评估8,表示匹配; 但是,有没有办法提取匹配的子字符串æd,这不涉及迭代源字符串？注意,所寻找和匹配的子串的长度可以相差几个字符.

Answer 1

我最终通过首先调用IndexOf获取匹配的起始位置，然后迭代地尝试识别其长度来解决这个问题。我针对与指定子串长度相同的匹配的热路径进行了优化；在这种情况下，仅执行一次比较。

public static class StringExtensions
{
    public static void Find(this string source, string substring, StringComparison comparisonType, out int matchIndex, out int matchLength)
    {
        Find(source, substring, 0, source.Length, comparisonType, out matchIndex, out matchLength);
    }

    public static void Find(this string source, string substring, int searchIndex, StringComparison comparisonType, out int matchIndex, out int matchLength)
    {
        Find(source, substring, searchIndex, source.Length - searchIndex, comparisonType, out matchIndex, out matchLength);
    }

    public static void Find(this string source, string substring, int searchIndex, int searchLength, StringComparison comparisonType, out int matchIndex, out int matchLength)
    {
        matchIndex = source.IndexOf(substring, searchIndex, searchLength, comparisonType);
        if (matchIndex == -1)
        {
            matchLength = -1;
            return;
        }

        matchLength = FindMatchLength(source, substring, searchIndex, searchLength, comparisonType, matchIndex);

        // Defensive programming, but should never happen
        if (matchLength == -1)
            matchIndex = -1;
    }

    private static int FindMatchLength(string source, string substring, int searchIndex, int searchLength, StringComparison comparisonType, int matchIndex)
    {
        int matchLengthMaximum = searchLength - (matchIndex - searchIndex);
        int matchLengthInitial = Math.Min(substring.Length, matchLengthMaximum);

        // Hot path: match length is same as substring length.
        if (Compare(source, matchIndex, matchLengthInitial, substring, 0, substring.Length, comparisonType) == 0)
            return matchLengthInitial;

        int matchLengthDecrementing = matchLengthInitial - 1;
        int matchLengthIncrementing = matchLengthInitial + 1;

        while (matchLengthDecrementing >= 0 || matchLengthIncrementing <= matchLengthMaximum)
        {
            if (matchLengthDecrementing >= 0)
            {
                if (Compare(source, matchIndex, matchLengthDecrementing, substring, 0, substring.Length, comparisonType) == 0)
                    return matchLengthDecrementing;

                matchLengthDecrementing--;
            }

            if (matchLengthIncrementing <= matchLengthMaximum)
            {
                if (Compare(source, matchIndex, matchLengthIncrementing, substring, 0, substring.Length, comparisonType) == 0)
                    return matchLengthIncrementing;

                matchLengthIncrementing++;
            }
        }

        // Should never happen
        return -1;
    }

    private static int Compare(string strA, int indexA, int lengthA, string strB, int indexB, int lengthB, StringComparison comparisonType)
    {
        switch (comparisonType)
        {
            case StringComparison.CurrentCulture:
                return CultureInfo.CurrentCulture.CompareInfo.Compare(strA, indexA, lengthA, strB, indexB, lengthB, CompareOptions.None);

            case StringComparison.CurrentCultureIgnoreCase:
                return CultureInfo.CurrentCulture.CompareInfo.Compare(strA, indexA, lengthA, strB, indexB, lengthB, CompareOptions.IgnoreCase);

            case StringComparison.InvariantCulture:
                return CultureInfo.InvariantCulture.CompareInfo.Compare(strA, indexA, lengthA, strB, indexB, lengthB, CompareOptions.None);

            case StringComparison.InvariantCultureIgnoreCase:
                return CultureInfo.InvariantCulture.CompareInfo.Compare(strA, indexA, lengthA, strB, indexB, lengthB, CompareOptions.IgnoreCase);

            case StringComparison.Ordinal:
                return CultureInfo.InvariantCulture.CompareInfo.Compare(strA, indexA, lengthA, strB, indexB, lengthB, CompareOptions.Ordinal);

            case StringComparison.OrdinalIgnoreCase:
                return CultureInfo.InvariantCulture.CompareInfo.Compare(strA, indexA, lengthA, strB, indexB, lengthB, CompareOptions.OrdinalIgnoreCase);

            default:
                throw new ArgumentException("The string comparison type passed in is currently not supported.", nameof(comparisonType));
        }
    }
}

使用示例：

int index, length;
source.Find(remove, StringComparison.CurrentCulture, out index, out length);
string clean = index < 0 ? source : source.Remove(index, length);