1 // Copyright 2014 The Rust Project Developers. See the COPYRIGHT
2 // file at the top-level directory of this distribution and at
3 // http://rust-lang.org/COPYRIGHT.
5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
8 // option. This file may not be copied, modified, or distributed
9 // except according to those terms.
11 // This is pretty much entirely stolen from TreeSet, since BTreeMap has an identical interface
14 use core::cmp::Ordering::{self, Less, Greater, Equal};
17 use core::iter::{Peekable, Map, FromIterator};
18 use core::ops::{BitOr, BitAnd, BitXor, Sub};
21 use btree_map::{BTreeMap, Keys};
25 // FIXME(conventions): implement bounded iterators
27 /// A set based on a B-Tree.
29 /// See BTreeMap's documentation for a detailed discussion of this collection's performance
30 /// benefits and drawbacks.
32 /// It is a logic error for an item to be modified in such a way that the item's ordering relative
33 /// to any other item, as determined by the `Ord` trait, changes while it is in the set. This is
34 /// normally only possible through `Cell`, `RefCell`, global state, I/O, or unsafe code.
35 #[derive(Clone, Hash, PartialEq, Eq, Ord, PartialOrd)]
36 #[stable(feature = "rust1", since = "1.0.0")]
37 pub struct BTreeSet<T>{
41 /// An iterator over a BTreeSet's items.
42 #[stable(feature = "rust1", since = "1.0.0")]
43 pub struct Iter<'a, T: 'a> {
47 /// An owning iterator over a BTreeSet's items.
48 #[stable(feature = "rust1", since = "1.0.0")]
49 pub struct IntoIter<T> {
50 iter: Map<::btree_map::IntoIter<T, ()>, fn((T, ())) -> T>
53 /// An iterator over a sub-range of BTreeSet's items.
54 pub struct Range<'a, T: 'a> {
55 iter: Map<::btree_map::Range<'a, T, ()>, fn((&'a T, &'a ())) -> &'a T>
58 /// A lazy iterator producing elements in the set difference (in-order).
59 #[stable(feature = "rust1", since = "1.0.0")]
60 pub struct Difference<'a, T:'a> {
61 a: Peekable<Iter<'a, T>>,
62 b: Peekable<Iter<'a, T>>,
65 /// A lazy iterator producing elements in the set symmetric difference (in-order).
66 #[stable(feature = "rust1", since = "1.0.0")]
67 pub struct SymmetricDifference<'a, T:'a> {
68 a: Peekable<Iter<'a, T>>,
69 b: Peekable<Iter<'a, T>>,
72 /// A lazy iterator producing elements in the set intersection (in-order).
73 #[stable(feature = "rust1", since = "1.0.0")]
74 pub struct Intersection<'a, T:'a> {
75 a: Peekable<Iter<'a, T>>,
76 b: Peekable<Iter<'a, T>>,
79 /// A lazy iterator producing elements in the set union (in-order).
80 #[stable(feature = "rust1", since = "1.0.0")]
81 pub struct Union<'a, T:'a> {
82 a: Peekable<Iter<'a, T>>,
83 b: Peekable<Iter<'a, T>>,
86 impl<T: Ord> BTreeSet<T> {
87 /// Makes a new BTreeSet with a reasonable choice of B.
92 /// # #![allow(unused_mut)]
93 /// use std::collections::BTreeSet;
95 /// let mut set: BTreeSet<i32> = BTreeSet::new();
97 #[stable(feature = "rust1", since = "1.0.0")]
98 pub fn new() -> BTreeSet<T> {
99 BTreeSet { map: BTreeMap::new() }
102 /// Makes a new BTreeSet with the given B.
104 /// B cannot be less than 2.
105 #[unstable(feature = "btree_b",
106 reason = "probably want this to be on the type, eventually",
108 #[deprecated(since = "1.4.0", reason = "niche API")]
110 pub fn with_b(b: usize) -> BTreeSet<T> {
111 BTreeSet { map: BTreeMap::with_b(b) }
115 impl<T> BTreeSet<T> {
116 /// Gets an iterator over the BTreeSet's contents.
121 /// use std::collections::BTreeSet;
123 /// let set: BTreeSet<usize> = [1, 2, 3, 4].iter().cloned().collect();
125 /// for x in set.iter() {
126 /// println!("{}", x);
129 /// let v: Vec<_> = set.iter().cloned().collect();
130 /// assert_eq!(v, [1, 2, 3, 4]);
132 #[stable(feature = "rust1", since = "1.0.0")]
133 pub fn iter(&self) -> Iter<T> {
134 Iter { iter: self.map.keys() }
138 impl<T: Ord> BTreeSet<T> {
139 /// Constructs a double-ended iterator over a sub-range of elements in the set, starting
140 /// at min, and ending at max. If min is `Unbounded`, then it will be treated as "negative
141 /// infinity", and if max is `Unbounded`, then it will be treated as "positive infinity".
142 /// Thus range(Unbounded, Unbounded) will yield the whole collection.
147 /// #![feature(btree_range, collections_bound)]
149 /// use std::collections::BTreeSet;
150 /// use std::collections::Bound::{Included, Unbounded};
152 /// let mut set = BTreeSet::new();
156 /// for &elem in set.range(Included(&4), Included(&8)) {
157 /// println!("{}", elem);
159 /// assert_eq!(Some(&5), set.range(Included(&4), Unbounded).next());
161 #[unstable(feature = "btree_range",
162 reason = "matches collection reform specification, waiting for dust to settle",
164 pub fn range<'a, Min: ?Sized + Ord = T, Max: ?Sized + Ord = T>(&'a self, min: Bound<&Min>,
166 -> Range<'a, T> where
167 T: Borrow<Min> + Borrow<Max>,
169 fn first<A, B>((a, _): (A, B)) -> A { a }
170 let first: fn((&'a T, &'a ())) -> &'a T = first; // coerce to fn pointer
172 Range { iter: self.map.range(min, max).map(first) }
176 impl<T: Ord> BTreeSet<T> {
177 /// Visits the values representing the difference, in ascending order.
182 /// use std::collections::BTreeSet;
184 /// let mut a = BTreeSet::new();
188 /// let mut b = BTreeSet::new();
192 /// let diff: Vec<_> = a.difference(&b).cloned().collect();
193 /// assert_eq!(diff, [1]);
195 #[stable(feature = "rust1", since = "1.0.0")]
196 pub fn difference<'a>(&'a self, other: &'a BTreeSet<T>) -> Difference<'a, T> {
197 Difference{a: self.iter().peekable(), b: other.iter().peekable()}
200 /// Visits the values representing the symmetric difference, in ascending order.
205 /// use std::collections::BTreeSet;
207 /// let mut a = BTreeSet::new();
211 /// let mut b = BTreeSet::new();
215 /// let sym_diff: Vec<_> = a.symmetric_difference(&b).cloned().collect();
216 /// assert_eq!(sym_diff, [1, 3]);
218 #[stable(feature = "rust1", since = "1.0.0")]
219 pub fn symmetric_difference<'a>(&'a self, other: &'a BTreeSet<T>)
220 -> SymmetricDifference<'a, T> {
221 SymmetricDifference{a: self.iter().peekable(), b: other.iter().peekable()}
224 /// Visits the values representing the intersection, in ascending order.
229 /// use std::collections::BTreeSet;
231 /// let mut a = BTreeSet::new();
235 /// let mut b = BTreeSet::new();
239 /// let intersection: Vec<_> = a.intersection(&b).cloned().collect();
240 /// assert_eq!(intersection, [2]);
242 #[stable(feature = "rust1", since = "1.0.0")]
243 pub fn intersection<'a>(&'a self, other: &'a BTreeSet<T>)
244 -> Intersection<'a, T> {
245 Intersection{a: self.iter().peekable(), b: other.iter().peekable()}
248 /// Visits the values representing the union, in ascending order.
253 /// use std::collections::BTreeSet;
255 /// let mut a = BTreeSet::new();
258 /// let mut b = BTreeSet::new();
261 /// let union: Vec<_> = a.union(&b).cloned().collect();
262 /// assert_eq!(union, [1, 2]);
264 #[stable(feature = "rust1", since = "1.0.0")]
265 pub fn union<'a>(&'a self, other: &'a BTreeSet<T>) -> Union<'a, T> {
266 Union{a: self.iter().peekable(), b: other.iter().peekable()}
269 /// Returns the number of elements in the set.
274 /// use std::collections::BTreeSet;
276 /// let mut v = BTreeSet::new();
277 /// assert_eq!(v.len(), 0);
279 /// assert_eq!(v.len(), 1);
281 #[stable(feature = "rust1", since = "1.0.0")]
282 pub fn len(&self) -> usize { self.map.len() }
284 /// Returns true if the set contains no elements.
289 /// use std::collections::BTreeSet;
291 /// let mut v = BTreeSet::new();
292 /// assert!(v.is_empty());
294 /// assert!(!v.is_empty());
296 #[stable(feature = "rust1", since = "1.0.0")]
297 pub fn is_empty(&self) -> bool { self.len() == 0 }
299 /// Clears the set, removing all values.
304 /// use std::collections::BTreeSet;
306 /// let mut v = BTreeSet::new();
309 /// assert!(v.is_empty());
311 #[stable(feature = "rust1", since = "1.0.0")]
312 pub fn clear(&mut self) {
316 /// Returns `true` if the set contains a value.
318 /// The value may be any borrowed form of the set's value type,
319 /// but the ordering on the borrowed form *must* match the
320 /// ordering on the value type.
325 /// use std::collections::BTreeSet;
327 /// let set: BTreeSet<_> = [1, 2, 3].iter().cloned().collect();
328 /// assert_eq!(set.contains(&1), true);
329 /// assert_eq!(set.contains(&4), false);
331 #[stable(feature = "rust1", since = "1.0.0")]
332 pub fn contains<Q: ?Sized>(&self, value: &Q) -> bool where T: Borrow<Q>, Q: Ord {
333 self.map.contains_key(value)
336 /// Returns a reference to the value in the set, if any, that is equal to the given value.
338 /// The value may be any borrowed form of the set's value type,
339 /// but the ordering on the borrowed form *must* match the
340 /// ordering on the value type.
341 #[unstable(feature = "set_recovery", issue = "28050")]
342 pub fn get<Q: ?Sized>(&self, value: &Q) -> Option<&T> where T: Borrow<Q>, Q: Ord {
343 Recover::get(&self.map, value)
346 /// Returns `true` if the set has no elements in common with `other`.
347 /// This is equivalent to checking for an empty intersection.
352 /// use std::collections::BTreeSet;
354 /// let a: BTreeSet<_> = [1, 2, 3].iter().cloned().collect();
355 /// let mut b = BTreeSet::new();
357 /// assert_eq!(a.is_disjoint(&b), true);
359 /// assert_eq!(a.is_disjoint(&b), true);
361 /// assert_eq!(a.is_disjoint(&b), false);
363 #[stable(feature = "rust1", since = "1.0.0")]
364 pub fn is_disjoint(&self, other: &BTreeSet<T>) -> bool {
365 self.intersection(other).next().is_none()
368 /// Returns `true` if the set is a subset of another.
373 /// use std::collections::BTreeSet;
375 /// let sup: BTreeSet<_> = [1, 2, 3].iter().cloned().collect();
376 /// let mut set = BTreeSet::new();
378 /// assert_eq!(set.is_subset(&sup), true);
380 /// assert_eq!(set.is_subset(&sup), true);
382 /// assert_eq!(set.is_subset(&sup), false);
384 #[stable(feature = "rust1", since = "1.0.0")]
385 pub fn is_subset(&self, other: &BTreeSet<T>) -> bool {
386 // Stolen from TreeMap
387 let mut x = self.iter();
388 let mut y = other.iter();
389 let mut a = x.next();
390 let mut b = y.next();
401 Greater => return false,
402 Equal => a = x.next(),
410 /// Returns `true` if the set is a superset of another.
415 /// use std::collections::BTreeSet;
417 /// let sub: BTreeSet<_> = [1, 2].iter().cloned().collect();
418 /// let mut set = BTreeSet::new();
420 /// assert_eq!(set.is_superset(&sub), false);
424 /// assert_eq!(set.is_superset(&sub), false);
427 /// assert_eq!(set.is_superset(&sub), true);
429 #[stable(feature = "rust1", since = "1.0.0")]
430 pub fn is_superset(&self, other: &BTreeSet<T>) -> bool {
431 other.is_subset(self)
434 /// Adds a value to the set.
436 /// If the set did not have a value present, `true` is returned.
438 /// If the set did have this key present, that value is returned, and the
439 /// entry is not updated. See the [module-level documentation] for more.
441 /// [module-level documentation]: index.html#insert-and-complex-keys
446 /// use std::collections::BTreeSet;
448 /// let mut set = BTreeSet::new();
450 /// assert_eq!(set.insert(2), true);
451 /// assert_eq!(set.insert(2), false);
452 /// assert_eq!(set.len(), 1);
454 #[stable(feature = "rust1", since = "1.0.0")]
455 pub fn insert(&mut self, value: T) -> bool {
456 self.map.insert(value, ()).is_none()
459 /// Adds a value to the set, replacing the existing value, if any, that is equal to the given
460 /// one. Returns the replaced value.
461 #[unstable(feature = "set_recovery", issue = "28050")]
462 pub fn replace(&mut self, value: T) -> Option<T> {
463 Recover::replace(&mut self.map, value)
466 /// Removes a value from the set. Returns `true` if the value was
467 /// present in the set.
469 /// The value may be any borrowed form of the set's value type,
470 /// but the ordering on the borrowed form *must* match the
471 /// ordering on the value type.
476 /// use std::collections::BTreeSet;
478 /// let mut set = BTreeSet::new();
481 /// assert_eq!(set.remove(&2), true);
482 /// assert_eq!(set.remove(&2), false);
484 #[stable(feature = "rust1", since = "1.0.0")]
485 pub fn remove<Q: ?Sized>(&mut self, value: &Q) -> bool where T: Borrow<Q>, Q: Ord {
486 self.map.remove(value).is_some()
489 /// Removes and returns the value in the set, if any, that is equal to the given one.
491 /// The value may be any borrowed form of the set's value type,
492 /// but the ordering on the borrowed form *must* match the
493 /// ordering on the value type.
494 #[unstable(feature = "set_recovery", issue = "28050")]
495 pub fn take<Q: ?Sized>(&mut self, value: &Q) -> Option<T> where T: Borrow<Q>, Q: Ord {
496 Recover::take(&mut self.map, value)
500 #[stable(feature = "rust1", since = "1.0.0")]
501 impl<T: Ord> FromIterator<T> for BTreeSet<T> {
502 fn from_iter<I: IntoIterator<Item=T>>(iter: I) -> BTreeSet<T> {
503 let mut set = BTreeSet::new();
509 #[stable(feature = "rust1", since = "1.0.0")]
510 impl<T> IntoIterator for BTreeSet<T> {
512 type IntoIter = IntoIter<T>;
514 /// Gets an iterator for moving out the BtreeSet's contents.
519 /// use std::collections::BTreeSet;
521 /// let set: BTreeSet<usize> = [1, 2, 3, 4].iter().cloned().collect();
523 /// let v: Vec<_> = set.into_iter().collect();
524 /// assert_eq!(v, [1, 2, 3, 4]);
526 fn into_iter(self) -> IntoIter<T> {
527 fn first<A, B>((a, _): (A, B)) -> A { a }
528 let first: fn((T, ())) -> T = first; // coerce to fn pointer
530 IntoIter { iter: self.map.into_iter().map(first) }
534 #[stable(feature = "rust1", since = "1.0.0")]
535 impl<'a, T> IntoIterator for &'a BTreeSet<T> {
537 type IntoIter = Iter<'a, T>;
539 fn into_iter(self) -> Iter<'a, T> {
544 #[stable(feature = "rust1", since = "1.0.0")]
545 impl<T: Ord> Extend<T> for BTreeSet<T> {
547 fn extend<Iter: IntoIterator<Item=T>>(&mut self, iter: Iter) {
554 #[stable(feature = "extend_ref", since = "1.2.0")]
555 impl<'a, T: 'a + Ord + Copy> Extend<&'a T> for BTreeSet<T> {
556 fn extend<I: IntoIterator<Item=&'a T>>(&mut self, iter: I) {
557 self.extend(iter.into_iter().cloned());
561 #[stable(feature = "rust1", since = "1.0.0")]
562 impl<T: Ord> Default for BTreeSet<T> {
563 fn default() -> BTreeSet<T> {
568 #[stable(feature = "rust1", since = "1.0.0")]
569 impl<'a, 'b, T: Ord + Clone> Sub<&'b BTreeSet<T>> for &'a BTreeSet<T> {
570 type Output = BTreeSet<T>;
572 /// Returns the difference of `self` and `rhs` as a new `BTreeSet<T>`.
577 /// use std::collections::BTreeSet;
579 /// let a: BTreeSet<_> = vec![1, 2, 3].into_iter().collect();
580 /// let b: BTreeSet<_> = vec![3, 4, 5].into_iter().collect();
582 /// let result = &a - &b;
583 /// let result_vec: Vec<_> = result.into_iter().collect();
584 /// assert_eq!(result_vec, [1, 2]);
586 fn sub(self, rhs: &BTreeSet<T>) -> BTreeSet<T> {
587 self.difference(rhs).cloned().collect()
591 #[stable(feature = "rust1", since = "1.0.0")]
592 impl<'a, 'b, T: Ord + Clone> BitXor<&'b BTreeSet<T>> for &'a BTreeSet<T> {
593 type Output = BTreeSet<T>;
595 /// Returns the symmetric difference of `self` and `rhs` as a new `BTreeSet<T>`.
600 /// use std::collections::BTreeSet;
602 /// let a: BTreeSet<_> = vec![1, 2, 3].into_iter().collect();
603 /// let b: BTreeSet<_> = vec![2, 3, 4].into_iter().collect();
605 /// let result = &a ^ &b;
606 /// let result_vec: Vec<_> = result.into_iter().collect();
607 /// assert_eq!(result_vec, [1, 4]);
609 fn bitxor(self, rhs: &BTreeSet<T>) -> BTreeSet<T> {
610 self.symmetric_difference(rhs).cloned().collect()
614 #[stable(feature = "rust1", since = "1.0.0")]
615 impl<'a, 'b, T: Ord + Clone> BitAnd<&'b BTreeSet<T>> for &'a BTreeSet<T> {
616 type Output = BTreeSet<T>;
618 /// Returns the intersection of `self` and `rhs` as a new `BTreeSet<T>`.
623 /// use std::collections::BTreeSet;
625 /// let a: BTreeSet<_> = vec![1, 2, 3].into_iter().collect();
626 /// let b: BTreeSet<_> = vec![2, 3, 4].into_iter().collect();
628 /// let result = &a & &b;
629 /// let result_vec: Vec<_> = result.into_iter().collect();
630 /// assert_eq!(result_vec, [2, 3]);
632 fn bitand(self, rhs: &BTreeSet<T>) -> BTreeSet<T> {
633 self.intersection(rhs).cloned().collect()
637 #[stable(feature = "rust1", since = "1.0.0")]
638 impl<'a, 'b, T: Ord + Clone> BitOr<&'b BTreeSet<T>> for &'a BTreeSet<T> {
639 type Output = BTreeSet<T>;
641 /// Returns the union of `self` and `rhs` as a new `BTreeSet<T>`.
646 /// use std::collections::BTreeSet;
648 /// let a: BTreeSet<_> = vec![1, 2, 3].into_iter().collect();
649 /// let b: BTreeSet<_> = vec![3, 4, 5].into_iter().collect();
651 /// let result = &a | &b;
652 /// let result_vec: Vec<_> = result.into_iter().collect();
653 /// assert_eq!(result_vec, [1, 2, 3, 4, 5]);
655 fn bitor(self, rhs: &BTreeSet<T>) -> BTreeSet<T> {
656 self.union(rhs).cloned().collect()
660 #[stable(feature = "rust1", since = "1.0.0")]
661 impl<T: Debug> Debug for BTreeSet<T> {
662 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
663 f.debug_set().entries(self.iter()).finish()
667 impl<'a, T> Clone for Iter<'a, T> {
668 fn clone(&self) -> Iter<'a, T> { Iter { iter: self.iter.clone() } }
670 #[stable(feature = "rust1", since = "1.0.0")]
671 impl<'a, T> Iterator for Iter<'a, T> {
674 fn next(&mut self) -> Option<&'a T> { self.iter.next() }
675 fn size_hint(&self) -> (usize, Option<usize>) { self.iter.size_hint() }
677 #[stable(feature = "rust1", since = "1.0.0")]
678 impl<'a, T> DoubleEndedIterator for Iter<'a, T> {
679 fn next_back(&mut self) -> Option<&'a T> { self.iter.next_back() }
681 #[stable(feature = "rust1", since = "1.0.0")]
682 impl<'a, T> ExactSizeIterator for Iter<'a, T> {}
685 #[stable(feature = "rust1", since = "1.0.0")]
686 impl<T> Iterator for IntoIter<T> {
689 fn next(&mut self) -> Option<T> { self.iter.next() }
690 fn size_hint(&self) -> (usize, Option<usize>) { self.iter.size_hint() }
692 #[stable(feature = "rust1", since = "1.0.0")]
693 impl<T> DoubleEndedIterator for IntoIter<T> {
694 fn next_back(&mut self) -> Option<T> { self.iter.next_back() }
696 #[stable(feature = "rust1", since = "1.0.0")]
697 impl<T> ExactSizeIterator for IntoIter<T> {}
700 impl<'a, T> Clone for Range<'a, T> {
701 fn clone(&self) -> Range<'a, T> { Range { iter: self.iter.clone() } }
703 impl<'a, T> Iterator for Range<'a, T> {
706 fn next(&mut self) -> Option<&'a T> { self.iter.next() }
708 impl<'a, T> DoubleEndedIterator for Range<'a, T> {
709 fn next_back(&mut self) -> Option<&'a T> { self.iter.next_back() }
712 /// Compare `x` and `y`, but return `short` if x is None and `long` if y is None
713 fn cmp_opt<T: Ord>(x: Option<&T>, y: Option<&T>,
714 short: Ordering, long: Ordering) -> Ordering {
716 (None , _ ) => short,
718 (Some(x1), Some(y1)) => x1.cmp(y1),
722 impl<'a, T> Clone for Difference<'a, T> {
723 fn clone(&self) -> Difference<'a, T> {
724 Difference { a: self.a.clone(), b: self.b.clone() }
727 #[stable(feature = "rust1", since = "1.0.0")]
728 impl<'a, T: Ord> Iterator for Difference<'a, T> {
731 fn next(&mut self) -> Option<&'a T> {
733 match cmp_opt(self.a.peek(), self.b.peek(), Less, Less) {
734 Less => return self.a.next(),
735 Equal => { self.a.next(); self.b.next(); }
736 Greater => { self.b.next(); }
742 impl<'a, T> Clone for SymmetricDifference<'a, T> {
743 fn clone(&self) -> SymmetricDifference<'a, T> {
744 SymmetricDifference { a: self.a.clone(), b: self.b.clone() }
747 #[stable(feature = "rust1", since = "1.0.0")]
748 impl<'a, T: Ord> Iterator for SymmetricDifference<'a, T> {
751 fn next(&mut self) -> Option<&'a T> {
753 match cmp_opt(self.a.peek(), self.b.peek(), Greater, Less) {
754 Less => return self.a.next(),
755 Equal => { self.a.next(); self.b.next(); }
756 Greater => return self.b.next(),
762 impl<'a, T> Clone for Intersection<'a, T> {
763 fn clone(&self) -> Intersection<'a, T> {
764 Intersection { a: self.a.clone(), b: self.b.clone() }
767 #[stable(feature = "rust1", since = "1.0.0")]
768 impl<'a, T: Ord> Iterator for Intersection<'a, T> {
771 fn next(&mut self) -> Option<&'a T> {
773 let o_cmp = match (self.a.peek(), self.b.peek()) {
776 (Some(a1), Some(b1)) => Some(a1.cmp(b1)),
780 Some(Less) => { self.a.next(); }
781 Some(Equal) => { self.b.next(); return self.a.next() }
782 Some(Greater) => { self.b.next(); }
788 impl<'a, T> Clone for Union<'a, T> {
789 fn clone(&self) -> Union<'a, T> {
790 Union { a: self.a.clone(), b: self.b.clone() }
793 #[stable(feature = "rust1", since = "1.0.0")]
794 impl<'a, T: Ord> Iterator for Union<'a, T> {
797 fn next(&mut self) -> Option<&'a T> {
799 match cmp_opt(self.a.peek(), self.b.peek(), Greater, Less) {
800 Less => return self.a.next(),
801 Equal => { self.b.next(); return self.a.next() }
802 Greater => return self.b.next(),