How to write parser for unified diff syntax

Question

Should I use RegexParsers, StandardTokenParsers or are these suitable at all for parsing this kind of syntax? Example of the syntax can be found from here.

Answer 1

This format was designed to be easy to parse, you can do it without any regular expressions and without tokenizing your input. Just go line by line and look at the first couple of characters. The file header and chunks headers will require a little more attention, but it's nothing you can't do with split.

Of course, if you want to learn how to use some parsing libraries, then go for it.

Answer 2

I'd use regex. It simplifies a few things, and makes the rest standard.

def process(src: scala.io.Source) {
  import scala.util.matching.Regex

  val FilePattern = """(.*) ''(.*)''"""
  val OriginalFile = new Regex("--- "+FilePattern, "path", "timestamp")
  val NewFile = new Regex("+++ "+FilePattern, "path", "timestamp")
  val Chunk = new Regex("""@@ -(\d+),(\d+) +(\d+),(\d+) @@""", "orgStarting", "orgSize", "newStarting", "newSize")
  val AddedLine = """+(.*)""".r
  val RemovedLine = """-(.*)""".r
  val UnchangedLine = """ (.*)""".r

  src.getLines() foreach {
    case OriginalFile(path, timestamp) => println("Original file: "+path)
    case NewFile(path, timestamp) => println("New file: "+path)
    case Chunk(l1, s1, l2, s2) => println("Modifying %d lines at line %d, to %d lines at %d" format (s1, l1, s2, l2))
    case AddedLine(line) => println("Adding line "+line)
    case RemovedLine(line) => println("Removing line "+line)
    case UnchangedLine(line) => println("Keeping line "+line)
  }
}

Answer 3

Here is a solution using RegexParsers.

import scala.util.parsing.combinator.RegexParsers

object UnifiedDiffParser extends RegexParsers {

  // case classes representing the data of the diff
  case class UnifiedDiff(oldFile: File, newFile: File, changeChunks: List[ChangeChunk])
  case class File(name: String, timeStamp: String)
  case class ChangeChunk(rangeInformation: RangeInformation, changeLines: List[String])
  case class RangeInformation(oldOffset: Int, oldLength: Int, newOffset: Int, newLength: Int)

  override def skipWhitespace = false

  def unifiedDiff: Parser[UnifiedDiff] = oldFile ~ newFile ~ rep1(changeChunk) ^^ {
    case of ~ nf ~ l => UnifiedDiff(of, nf, l)
  }   

  def oldFile: Parser[File] = ("--- " ~> filename) ~ ("""\s+""".r ~> timestamp <~ newline) ^^ {
    case f~t => File(f, t)
  }   
  def newFile: Parser[File] = ("+++ " ~> filename) ~ ("""\s+""".r ~> timestamp <~ newline) ^^ {
    case f~t => File(f, t)
  }   
  def filename: Parser[String] = """[\S]+""".r
  def timestamp: Parser[String] = """.*""".r

  def changeChunk: Parser[ChangeChunk] = rangeInformation ~ (newline ~> rep1(lineChange)) ^^ {
    case ri ~ l => ChangeChunk(ri, l)
  }   
  def rangeInformation: Parser[RangeInformation] = ("@@ " ~> "-" ~> number) ~ ("," ~> number) ~ (" +" ~> number) ~ ("," ~> number) <~ " @@" ^^ {
    case a ~ b ~ c ~ d => RangeInformation(a, b, c, d)
  }   

  def lineChange: Parser[String] = contextLine | addedLine | deletedLine
  def contextLine: Parser[String] = """ .*""".r <~ newline
  def addedLine: Parser[String] = """\+.*""".r <~ newline
  def deletedLine: Parser[String] = """-.*""".r <~ newline

  def newline: Parser[String] = """\n""".r
  def number: Parser[Int] = """\d+""".r ^^ {_.toInt}

  def main(args: Array[String]) {
    val reader = { 
      if (args.length == 0) {
        // read from stdin
        Console.in
      } else {
        new java.io.FileReader(args(0))
      }   
    }   
    println(parseAll(unifiedDiff, reader))
  }   
}