stsys/lib/strpair.js

String rewriting system (semi-Thue system)

class StrPair
{
  constructor(left, right, parent1, parent2)
  {
    const regex = /^[a-zA-Z]*$/;
    this.left = left || '';
    this.right = right || '';
    if (!regex.test(this.left) || !regex.test(this.right))
      throw new Error('invalid string pair (illegal characters): ' + left + ' = ' + right);
    this.ordered = false;
    this.parent1 = parent1 || null;
    this.parent2 = parent2 || null;
    this.next = null;
  }

  toString(withParents)
  {
    const sb = [this.left, this.ordered ? '->' : '=', this.right];
    if (withParents)
    {
      sb.push('from');
      if (this.parent1)
        sb.push('#' + this.parent1.id, 'and', '#' + this.parent2.id);
      else
        sb.push('axiom');
    }
    return sb.join(' ');
  }

  copy()
  {
    return new StrPair(this.left, this.right);
  }

  isTrivial()
  {
    return this.left === this.right;
  }

  isObsolete()
  {
    return this.parent1 && !this.parent1.ordered || this.parent2 && !this.parent2.ordered;
  }

  getWeight()
  {
    return this.left.length + this.right.length;
  }

  reduce(reducer, rule)
  {
    let reduced = false;
    let s = reducer.reduce(this.left, rule);
    if (typeof s === 'string')
    {
      this.left = s;
      this.ordered = false;
      reduced = true;
    }
    s = reducer.reduce(this.right, rule);
    if (typeof s === 'string')
    {
      this.right = s;
      reduced = true;
    }
    return reduced;
  }

  order(comparer)
  {
    let cmp = comparer.compare(this.left, this.right);
    if (cmp < 0)
    {
      let s = this.left;
      this.left  = this.right;
      this.right = s;
    }
    else if (cmp === 0)
      throw new Error('invalid state: cannot order ' + this.left + ' and ' + this.right); 
    this.ordered = true;
  }

  overlapIntoAt(other, i)
  {
    return new StrPair(
      this.left.substring(0, i) + other.right,
      this.right + other.left.substring(this.left.length - i),
      this, other);
  }
}

module.exports.createStrPair = function(l, r, p1, p2)
{
  return new StrPair(l, r, p1, p2);
};
module.exports.parseStrPair = function(s)
{
  const i = s.indexOf('=');
  if (i < 0)
    throw new Error('invalid string pair (missing equality): ' + s);
  return new StrPair(s.substring(0, i).trim(), s.substring(i + 1).trim());
};