Program = lines: (Line / (_/[\n]))* {
  return { instructions: lines.filter((line) => typeof line !== "string" || line.trim() != "") };
}

Line = _? instruction: (LabeledInstruction / Instruction) _? [\n]? {
  return instruction;
}

LabeledInstruction = label:Label _ instruction:Instruction {
  return { label, instruction };
}

Label = "[" _? label:LABEL_V _? "]" {
  return label;
}
  
Instruction = conditional: Conditional { return { conditional }; } 
  / assignment: Assignment { return { assignment }; }
  / goto: Goto { return { goto }; }

Goto = GOTO _ label: LABEL_V {
  return { label };
}

Conditional = "IF" _ variable: VAR  _? "!=" _? "0" _ goto: Goto {
  return { variable, goto };
}

Assignment = variable: VAR _ "<-" _ expr: Expression {
  if (expr.left != variable) {
    error("left hand variable must match right hand");
  }
  return { variable, expr };
}

Expression = left: VAR _ opr: OPERATION _ "1" {
  return { left, opr };
} / left: VAR  {
  return { left };
}

VAR = symbol:"Y" { return symbol } / symbol:("X" / "Z") ind:Integer+ {
  return symbol + ind;
}

GOTO = "GOTO"

OPERATION = "+" / "-"

LABEL_V = symbol:END_LABEL { return symbol } / symbol:[A-E] ind:Integer+ {
  return symbol + ind;
}

END_LABEL = "E"

Integer "integer"
  = [0-9]+ { return parseInt(text(), 10); }

_ "whitespace" = [ \t]+ {  }