shouldBreak function

int shouldBreak(int start, List<int> mid, int end)

shouldBreak returns whether a break is allowed between the two given grapheme breaking classes

Implementation

int shouldBreak(int start, List<int> mid, int end) {
  final all = [start];
  all.addAll(mid);
  all.addAll([end]);
  final previous = all[all.length - 2];
  final next = end;

  // Lookahead termintor for:
  // GB10. (E_Base | EBG) Extend* ?	E_Modifier
  final eModifierIndex = all.lastIndexOf(E_Modifier);
  if (eModifierIndex > 1 &&
      all.sublist(1, eModifierIndex).every((c) => c == Extend) &&
      [Extend, E_Base, E_Base_GAZ].indexOf(start) == -1) {
    return Break;
  }

  // Lookahead termintor for:
  // GB12. ^ (RI RI)* RI	?	RI
  // GB13. [^RI] (RI RI)* RI	?	RI
  var rIIndex = all.lastIndexOf(Regional_Indicator);
  if (rIIndex > 0 &&
      all.sublist(1, rIIndex).every((c) => c == Regional_Indicator) &&
      [Prepend, Regional_Indicator].indexOf(previous) == -1) {
    if (all.where((c) => c == Regional_Indicator).length % 2 == 1) {
      return BreakLastRegional;
    } else {
      return BreakPenultimateRegional;
    }
  }

  // GB3. CR X LF
  if (previous == CR && next == LF) {
    return NotBreak;
  }
  // GB4. (Control|CR|LF) ÷
  else if (previous == Control || previous == CR || previous == LF) {
    if (next == E_Modifier && mid.every((c) => c == Extend)) {
      return Break;
    } else {
      return BreakStart;
    }
  }
  // GB5. ÷ (Control|CR|LF)
  else if (next == Control || next == CR || next == LF) {
    return BreakStart;
  }
  // GB6. L X (L|V|LV|LVT)
  else if (previous == L &&
      (next == L || next == V || next == LV || next == LVT)) {
    return NotBreak;
  }
  // GB7. (LV|V) X (V|T)
  else if ((previous == LV || previous == V) && (next == V || next == T)) {
    return NotBreak;
  }
  // GB8. (LVT|T) X (T)
  else if ((previous == LVT || previous == T) && next == T) {
    return NotBreak;
  }
  // GB9. X (Extend|ZWJ)
  else if (next == Extend || next == ZWJ) {
    return NotBreak;
  }
  // GB9a. X SpacingMark
  else if (next == SpacingMark) {
    return NotBreak;
  }
  // GB9b. Prepend X
  else if (previous == Prepend) {
    return NotBreak;
  }

  // GB10. (E_Base | EBG) Extend* ?	E_Modifier
  final previousNonExtendIndex =
      all.indexOf(Extend) != -1 ? all.lastIndexOf(Extend) - 1 : all.length - 2;
  if (previousNonExtendIndex != -1 &&
      [E_Base, E_Base_GAZ].indexOf(all[previousNonExtendIndex]) != -1 &&
      all.length > previousNonExtendIndex + 1 &&
      sliceFromEnd(all, previousNonExtendIndex + 1).every((c) => c == Extend) &&
      next == E_Modifier) {
    return NotBreak;
  }

  // GB11. ZWJ ? (Glue_After_Zwj | EBG)
  if (previous == ZWJ && [Glue_After_Zwj, E_Base_GAZ].indexOf(next) != -1) {
    return NotBreak;
  }

  // GB12. ^ (RI RI)* RI ? RI
  // GB13. [^RI] (RI RI)* RI ? RI
  if (mid.indexOf(Regional_Indicator) != -1) {
    return Break;
  }
  if (previous == Regional_Indicator && next == Regional_Indicator) {
    return NotBreak;
  }

  // GB999. Any ? Any
  return BreakStart;
}