laika.parse.text.WhitespacePreprocessor.scala Maven / Gradle / Ivy

/*
 * Copyright 2012-2020 the original author or authors.
 *
 * 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 laika.parse.text

/** Processes whitespace, removing or replacing most whitespace characters except
  * for newline and space.
  *
  * It modifies string input in the following ways:
  *
  * - Replaces all occurrences of tabs with the corresponding number of spaces,
  *   depending on the column the tab is placed in and the configured `tabStops` value.
  *
  * - Removes any return character.
  *
  * - Replaces form feed and vertical tab with spaces.
  *
  * The processor should run on text input before it is passed to the actual
  * parsers as they would not be able to deal with tabs properly.
  *
  * @author Jens Halm
  */
class WhitespacePreprocessor private[text] extends (String => String) {

  /** The number of columns between tab stops.
    */
  val tabStops = 4

  /** Processes whitespace, removing or replacing most whitespace characters except
    *  for newline and space.
    */
  def apply(input: String): String = {

    val end = input.length
    val buf = new StringBuilder

    var i   = 0
    var col = 0
    var cur = 0

    def cut(add: String) = {
      buf ++= input.substring(cur, i) ++= add
      cur = i + 1
    }

    while (i < end) {
      input.charAt(i) match {
        case '\f' | '\u000b' =>
          cut(" ")
          col += 1
        case '\r'            =>
          cut("")
        case '\n'            =>
          col = 0
        case '\t'            =>
          val spaces = tabStops - (col % tabStops)
          cut(" " * spaces)
          col += spaces
        case _               =>
          col += 1
      }
      i += 1
    }

    buf ++= input.substring(cur)
    buf.toString
  }

}

/** Companion for creating instances of WhitespacePreprocessor.
  */
object WhitespacePreprocessor {

  /** Processes all whitespace in the specified string.
    */
  val forString: String => String = new WhitespacePreprocessor

}