How to create a simple prefix index in Java?

Question

I have big set of urls and I want to implement an autocompletion. I don\'t like the complexity of the naive approach as it is linear with the set size:

for(S         


        

Answer 1

The Regexp implementation java.util.regex.Pattern can efficiently handle prefixes:

StringBuilder buffer = new StringBuilder();
for (String prefix : prefixes) {
    if (buffer.length() > 0)
        buffer.append("|");
    buffer.append(prefix);
}
Pattern prefixPattern = Pattern.compile("^(" + buffer + ")");

You can test all prefixes:

boolean containsPrefix = prefixPattern.matcher(stringToTest).find();

Note: for simplicity, prefix strings are not escaped. Regexp characters [, ], \, *, ?, $, ^, (, ), {, } and | have to be prefixed by \.