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Jimfs is an in-memory implementation of Java 7's java.nio.file abstract file system API.
There is a newer version: 1.3.0
/*
 * Copyright 2013 Google Inc.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.google.common.jimfs;

import com.google.common.base.Ascii;
import com.google.common.base.Function;

import com.ibm.icu.lang.UCharacter;

import java.text.Normalizer;
import java.util.regex.Pattern;

/**
 * Normalizations that can be applied to names in paths. Includes Unicode normalizations and
 * normalizations for case insensitive paths. These normalizations can be set in
 * {@code Configuration.Builder} when creating a Jimfs file system instance and are automatically
 * applied to paths in the file system.
 *
 * @author Colin Decker
 */
public enum PathNormalization implements Function {

  /**
   * No normalization.
   */
  NONE(0) {
    @Override
    public String apply(String string) {
      return string;
    }
  },

  /**
   * Unicode composed normalization (form {@linkplain java.text.Normalizer.Form#NFC NFC}).
   */
  NFC(Pattern.CANON_EQ) {
    @Override
    public String apply(String string) {
      return Normalizer.normalize(string, Normalizer.Form.NFC);
    }
  },

  /**
   * Unicode decomposed normalization (form {@linkplain java.text.Normalizer.Form#NFD NFD}).
   */
  NFD(Pattern.CANON_EQ) {
    @Override
    public String apply(String string) {
      return Normalizer.normalize(string, Normalizer.Form.NFD);
    }
  },
    
  /*
   * Some notes on case folding/case insensitivity of file systems:
   *
   * In general (I don't have any counterexamples) case-insensitive file systems handle
   * their case insensitivity in a locale-independent way. NTFS, for example, writes a
   * special case mapping file ($UpCase) to the file system when it's first initialized,
   * and this is not affected by the locale of either the user or the copy of Windows
   * being used. This means that it will NOT handle i/I-variants in filenames as you'd
   * expect for Turkic languages, even for a Turkish user who has installed a Turkish
   * copy of Windows.
   */

  /**
   * Unicode case folding for case insensitive paths. Requires ICU4J on the classpath.
   */
  CASE_FOLD_UNICODE(Pattern.CASE_INSENSITIVE | Pattern.UNICODE_CASE) {
    @Override
    public String apply(String string) {
      try {
        return UCharacter.foldCase(string, true);
      } catch (NoClassDefFoundError e) {
        NoClassDefFoundError error =
            new NoClassDefFoundError(
                "PathNormalization.CASE_FOLD_UNICODE requires ICU4J. "
                    + "Did you forget to include it on your classpath?");
        error.initCause(e);
        throw error;
      }
    }
  },

  /**
   * ASCII case folding for simple case insensitive paths.
   */
  CASE_FOLD_ASCII(Pattern.CASE_INSENSITIVE) {
    @Override
    public String apply(String string) {
      return Ascii.toLowerCase(string);
    }
  };

  private final int patternFlags;

  private PathNormalization(int patternFlags) {
    this.patternFlags = patternFlags;
  }

  /**
   * Applies this normalization to the given string, returning the normalized result.
   */
  @Override
  public abstract String apply(String string);

  /**
   * Returns the flags that should be used when creating a regex {@link Pattern} in order to
   * approximate this normalization.
   */
  public int patternFlags() {
    return patternFlags;
  }

  /**
   * Applies the given normalizations to the given string in order, returning the normalized
   * result.
   */
  public static String normalize(String string, Iterable normalizations) {
    String result = string;
    for (PathNormalization normalization : normalizations) {
      result = normalization.apply(result);
    }
    return result;
  }

  /**
   * Compiles a regex pattern using flags based on the given normalizations.
   */
  public static Pattern compilePattern(String regex, Iterable normalizations) {
    int flags = 0;
    for (PathNormalization normalization : normalizations) {
      flags |= normalization.patternFlags();
    }
    return Pattern.compile(regex, flags);
  }
}