C＃字典快速搜索最接近键的值

Question

I have a really long string (thousands of lines). I'm running RegEx expressions against the string and trying to identify the line numbers of matches. However if I have a high match count (say, 10,000), to find the line numbers every time involves searching the html string again, which gets expensive.

What I want to do is search the string beforehand and build a hashtable of character positions of the line numbers. so I could use Dictionary and use the following code to find my line numbers.

//find line endings
int lineCount = 0;
for (int charCount = 0; charCount <= html.Length; charCount++)
{
     if (html[charCount] == '\n')
     {
         lineCount++;
         lineEndings.Add(charCount, lineCount);
     }
}

However, when I run my RegExes, how do I search this dictionary? the regex expression character position will need to be between two values in the lineEndings dictionary. What's the best / most efficient way to; given a Dictionary with a set of gapped keys, given a value that's not in the key list, to find the next closest key?

One thing I've tried, and I'm not sure how it would perform, is

lineEndings.First(n => n.Key >= match.Index).Value

Answer 1

Dictionaries don't work when your definition of "equal" is just "close".

It's important that items in a dictionary be transative. If A = B and B = C then A should equal C. If that's not the case (which it isn't, if equality is defined as just "close", things start breaking down.

To start with, there's no way that you can write an effective GetHashCode implementation here. The only way for it to ever be valid is for everything to just return the same value, which means you've just degraded your performance to a linear search anyway.

What you can do, given that you have a static set of strings, is put them all in a List or array, sort them, and then use a BinarySearch . Since the data appears to be static, the fact that adding items to the lookup table is expensive shouldn't be a problem. A binary search also is capable of telling you where the item you are searching for would belong if it should be added, this means you can go to the index at that position to find the "next" item, and subtract one to find the "previous" item.

Answer 2

You could use LINQ with your dictionary if you know what range you want the keys to be in. Something like this:

    Dictionary<int, string> Test1 = new Dictionary<int, string>();
    public Form1()
    {
        InitializeComponent();
        Test1.Add(1, "asdf");
        Test1.Add(2, "ghjh");
        Test1.Add(3, "jkl;");
        Test1.Add(4, "qwer");

        int max = 4;
        int min = 1;
        listBox1.DataSource = (from kvp in Test1
                               where (kvp.Key > min && kvp.Key < max)
                               select (kvp.Value)).ToList();

    }

This creates a collection of values from the dictionary where the keys are in a certain range.