src/com/google/common/base/CaseFormat.java

/*
 * Copyright (C) 2006 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.base;

import java.util.Locale;
import java.util.regex.Matcher;
import java.util.regex.Pattern;

/**
 * Utility class for converting between various case formats.
 *
 * @author Mike Bostock
 * @since 2009.09.15 <b>tentative</b>
 */
public enum CaseFormat {

  /**
   * Hyphenated variable naming convention, e.g., "lower-hyphen".
   */
  LOWER_HYPHEN(Pattern.compile("[-]"), "-"),

  /**
   * C++ variable naming convention, e.g., "lower_underscore".
   */
  LOWER_UNDERSCORE(Pattern.compile("[_]"), "_"),

  /**
   * Java variable naming convention, e.g., "lowerCamel".
   */
  LOWER_CAMEL(Pattern.compile("[A-Z]"), ""),

  /**
   * Java and C++ class naming convention, e.g., "UpperCamel".
   */
  UPPER_CAMEL(Pattern.compile("[A-Z]"), ""),

  /**
   * Java and C++ constant naming convention, e.g., "UPPER_UNDERSCORE".
   */
  UPPER_UNDERSCORE(Pattern.compile("[_]"), "_");

  private final Pattern wordBoundary;
  private final String wordSeparator;

  private CaseFormat(Pattern wordBoundary, String wordSeparator) {
    this.wordBoundary = wordBoundary;
    this.wordSeparator = wordSeparator;
  }

  /**
   * Converts the specified {@code String s} from this format to the specified
   * {@code format}. A "best effort" approach is taken; if {@code s} does not
   * conform to the assumed format, then the behavior of this method is
   * undefined but we make a reasonable effort at converting anyway.
   */
  public String to(CaseFormat format, String s) {
    if (format == null) {
      throw new NullPointerException();
    }
    if (s == null) {
      throw new NullPointerException();
    }

    /* optimize case where no conversion is required */
    if (format == this) {
      return s;
    }

    /* optimize cases where no camel conversion is required */
    switch (this) {
      case LOWER_HYPHEN:
        switch (format) {
          case LOWER_UNDERSCORE: return s.replace("-", "_");
          case UPPER_UNDERSCORE: return s.replace("-", "_").toUpperCase(Locale.US);
        }
        break;
      case LOWER_UNDERSCORE:
        switch (format) {
          case LOWER_HYPHEN: return s.replace("_", "-");
          case UPPER_UNDERSCORE: return s.toUpperCase(Locale.US);
        }
        break;
      case UPPER_UNDERSCORE:
        switch (format) {
          case LOWER_HYPHEN: return s.replace("_", "-").toLowerCase(Locale.US);
          case LOWER_UNDERSCORE: return s.toLowerCase(Locale.US);
        }
        break;
    }

    /* otherwise, deal with camel conversion */
    StringBuilder out = null;
    int i = 0;
    for (Matcher matcher = wordBoundary.matcher(s); matcher.find();) {
      int j = matcher.start();
      if (i == 0) {
        /* include some extra space for separators */
        out = new StringBuilder(s.length() + 4 * wordSeparator.length());
        out.append(format.normalizeFirstWord(s.substring(i, j)));
      } else {
        out.append(format.normalizeWord(s.substring(i, j)));
      }
      out.append(format.wordSeparator);
      i = j + wordSeparator.length();
    }
    if (i == 0) {
      return format.normalizeFirstWord(s);
    }
    out.append(format.normalizeWord(s.substring(i)));
    return out.toString();
  }

  private String normalizeFirstWord(String word) {
    switch (this) {
      case LOWER_CAMEL: return word.toLowerCase(Locale.US);
      default: return normalizeWord(word);
    }
  }

  private String normalizeWord(String word) {
    switch (this) {
      case LOWER_HYPHEN: return word.toLowerCase(Locale.US);
      case LOWER_UNDERSCORE: return word.toLowerCase(Locale.US);
      case LOWER_CAMEL: return toTitleCase(word);
      case UPPER_CAMEL: return toTitleCase(word);
      case UPPER_UNDERSCORE: return word.toUpperCase(Locale.US);
    }
    throw new RuntimeException("unknown case: " + this);
  }

  private static String toTitleCase(String word) {
    return (word.length() < 2) ? word.toUpperCase(Locale.US)
        : (Character.toTitleCase(word.charAt(0))
           + word.substring(1).toLowerCase(Locale.US));
  }

}