package com.twitter.tweetypie.tweettext

import com.ibm.icu.text.BreakIterator

/**
 * Adapt the [[BreakIterator]] interface to a scala [[Iterator]]
 * over the offsets of user-perceived characters in a String.
 */
object GraphemeIndexIterator {

  /**
   * Produce an iterator over indices in the string that mark the end
   * of a user-perceived character (grapheme)
   */
  def ends(s: String): Iterator[Offset.CodeUnit] =
    // The start of every grapheme but the first is also a grapheme
    // end. The last grapheme ends at the end of the string.
    starts(s).drop(1) ++ Iterator(Offset.CodeUnit.length(s))

  /**
   * Produce an iterator over indices in the string that mark the start
   * of a user-perceived character (grapheme)
   */
  def starts(s: String): Iterator[Offset.CodeUnit] =
    new Iterator[Offset.CodeUnit] {
      private[this] val it = BreakIterator.getCharacterInstance()

      it.setText(s)

      override def hasNext: Boolean = it.current < s.length

      override def next: Offset.CodeUnit = {
        if (!hasNext) throw new IllegalArgumentException(s"${it.current()}, ${s.length}")

        // No matter what, we will be returning the value of `current`,
        // which is the index of the start of the next grapheme.
        val result = it.current()

        it.next()

        Offset.CodeUnit(result)
      }
    }
}