// Always thread-safe

sclass FixedVarContext > VarContext {

  S[] names; // always sorted, may be null

  O[] values; // same length as names, may also be null

  *() {}

  *(S[] *names) { this(null, names); }

  *(VarContext parent, S[] *names) {

    super(parent);

    if (names != null) values = new O[names.length];

  }

  int indexOfVar(S name) {

    ret indexOfInSortedArray(names, name);

  }

  int indexOfVarMandatory(S name) {

    int idx = indexOfVar(name);

    if (idx < 0) fail("Variable not found: " + name + ", known: " + joinWithComma(names));

    ret idx;

  }

  O get(int idx) {

    ret values[idx];

  }

  O get(S name) {

    int idx = indexOfVar(name);

    if (idx >= 0) ret values[idx];

    if (parent != null)

      ret parent.get(name);

    null;

  }

  void set(int idx, O value) {

    values[idx] = value;

  }

  void set(S name, O value) {

    int idx = indexOfVar(name);

    if (idx >= 0)

      values[idx] = value;

    else

      fail("Variable " + name + " not defined in context");

  }

  AutoCloseable tempSet(S name, O value) {

    int idx = indexOfVar(name);

    if (idx >= 0) {

      O old = values[idx];

      values[idx] = value;

      ret -> values[idx] = old;

    } else

      fail("Variable " + name + " not defined in context");

  }

  void unset(S name) {

    set(name, null);

  }

  MapSO varMap() {

    new MapSO map;

    int n = l(names);

    for i to n:

      map.put(names[i], values[i]);

    ret map;

  }

}