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<!DOCTYPE html>
<html>
<head>
  <meta charset="utf-8">
  <title>Database.js</title>
</head>
<body>
  <script>
    'use strict';

    function isVar(x) {
      return typeof x === 'string' && x[0] === '$';
    }

    const EMPTY_BINDING = {facts: []};

    class Database {
      constructor(...facts) {
        this.facts = new Set(facts);
      }

      assert(fact) {
        this.facts.add(fact);
      }

      refute(fact) {
        this.facts.delete(fact);
      }

      query(qs, callback, bindings = [EMPTY_BINDING]) {
        if (qs.length === 0) {
          bindings.forEach(b => {
            const assert = (fact) => {
              fact.provenance = b.facts;
              this.assert(fact);
            }

            const refute = (fact) => this.refute(fact);

            callback(b, assert, refute)
          });
          return;
        }

        const newBindings = this.match(qs[0], this.facts, bindings);
        return this.query(qs.slice(1), callback, newBindings);
      }

      match(q, facts, bindings) {
        let filtered = [];

        for (let b of bindings) {
          for (let f of facts) {
            const newB = this.matchOne(q, f, b);
            if (newB) {
              filtered.push(newB);
            }
          }
        }

        return filtered;
      }

      matchOne(q, fact, binding) {
        if (q.length !== fact.length) {
          return null;
        }

        let newB = {...binding, facts: [...binding.facts, fact]};

        for (let i in q) {
          const qpart = q[i];

          if (isVar(qpart) && newB.hasOwnProperty(qpart) && newB[qpart] !== fact[i]) {
            return null;
          } else if (isVar(qpart)) {
            newB[qpart] = fact[i];
          } else if (qpart !== fact[i]) {
            return null;
          }
        }

        return newB;
      }
    }

    let db = new Database(
      ['C1', 'is a', 'circle', 'at', '(', 300, ',', 300, ')'],
      ['C1', '\'s', 'color', 'is', 'blue'],
      ['C1', '\'s', 'radius', 'is', 50],

      ['C2', 'is a', 'circle', 'at', '(', 200, ',', 500, ')'],
      ['C2', '\'s', 'color', 'is', 'red'],
      ['C2', '\'s', 'radius', 'is', 120],
    )

    db.query([
      ['$name', 'is a', 'circle', 'at', '(', '$x', ',', '$y', ')'],
      ['$name', '\'s', 'radius', 'is', '$r'],
      ['$name', '\'s', 'color', 'is', 'red'],
    ], m => console.log(m));

    db.query([
      ['$name', 'is a', 'circle', 'at', '(', '$x', ',', '$y', ')'],
      ['$name', '\'s', 'radius', 'is', '$r'],
    ], ({$name, $r}, assert, refute) => {
      console.log($name, $r);
      if ($r > 100) {
        assert([$name, 'is', 'large']);
      }
    })
  </script>
</body>
</html>