// Splits by parentheses, white space, and special characters.
const splitRegex = new RegExp(/[ ()|&~!^]+/g);
const reservedKeywords = new Set([
  "and",
  "not",
  "or",
  "xor",
  "if",
  "then",
  "iff",
  "only",
  "true",
  "false",
  "equals",
  "imply",
  "implies",
  "nimply",
  "nimplies",
  "xnor",
  "nand",
  "nor"
]);

/**
 * Returns a list of all the unique variable names in the given expression.
 */
function extractVariables(expression: string): string[] {
  const words: string[] = expression.split(splitRegex);

  // Any word that isn't a reserved keyword is a variable.
  const variables: string[] = words
    .filter(word => word.length > 0)
    .filter(word => /^[A-Za-z0-9]+$/.test(word))
    .filter(word => !reservedKeywords.has(word.toLowerCase()));

  // Only count each variable once.
  const uniqueVariables = Array.from(new Set(variables));

  return uniqueVariables;
}

export default extractVariables;