aboutsummaryrefslogtreecommitdiff
path: root/helix-core/src/transaction.rs
blob: 9d2a3e5c43a50c496ea82daa5708db64151e7e05 (plain) (blame)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
use smallvec::SmallVec;

use crate::{Range, Rope, Selection, Tendril};
use std::{borrow::Cow, iter::once};

/// (from, to, replacement)
pub type Change = (usize, usize, Option<Tendril>);
pub type Deletion = (usize, usize);

// TODO: pub(crate)
#[derive(Debug, Clone, PartialEq, Eq)]
pub enum Operation {
    /// Move cursor by n characters.
    Retain(usize),
    /// Delete n characters.
    Delete(usize),
    /// Insert text at position.
    Insert(Tendril),
}

#[derive(Debug, Copy, Clone, PartialEq, Eq)]
pub enum Assoc {
    Before,
    After,
}

#[derive(Debug, Default, Clone, PartialEq, Eq)]
pub struct ChangeSet {
    pub(crate) changes: Vec<Operation>,
    /// The required document length. Will refuse to apply changes unless it matches.
    len: usize,
    len_after: usize,
}

impl ChangeSet {
    pub fn with_capacity(capacity: usize) -> Self {
        Self {
            changes: Vec::with_capacity(capacity),
            len: 0,
            len_after: 0,
        }
    }

    #[must_use]
    pub fn new(doc: &Rope) -> Self {
        let len = doc.len_chars();
        Self {
            changes: Vec::new(),
            len,
            len_after: len,
        }
    }

    // TODO: from iter

    #[doc(hidden)] // used by lsp to convert to LSP changes
    pub fn changes(&self) -> &[Operation] {
        &self.changes
    }

    // Changeset builder operations: delete/insert/retain
    pub(crate) fn delete(&mut self, n: usize) {
        use Operation::*;
        if n == 0 {
            return;
        }

        self.len += n;

        if let Some(Delete(count)) = self.changes.last_mut() {
            *count += n;
        } else {
            self.changes.push(Delete(n));
        }
    }

    pub(crate) fn insert(&mut self, fragment: Tendril) {
        use Operation::*;

        if fragment.is_empty() {
            return;
        }

        // Avoiding std::str::len() to account for UTF-8 characters.
        self.len_after += fragment.chars().count();

        let new_last = match self.changes.as_mut_slice() {
            [.., Insert(prev)] | [.., Insert(prev), Delete(_)] => {
                prev.push_str(&fragment);
                return;
            }
            [.., last @ Delete(_)] => std::mem::replace(last, Insert(fragment)),
            _ => Insert(fragment),
        };

        self.changes.push(new_last);
    }

    pub(crate) fn retain(&mut self, n: usize) {
        use Operation::*;
        if n == 0 {
            return;
        }

        self.len += n;
        self.len_after += n;

        if let Some(Retain(count)) = self.changes.last_mut() {
            *count += n;
        } else {
            self.changes.push(Retain(n));
        }
    }

    /// Combine two changesets together.
    /// In other words,  If `this` goes `docA` → `docB` and `other` represents `docB` → `docC`, the
    /// returned value will represent the change `docA` → `docC`.
    pub fn compose(self, other: Self) -> Self {
        assert!(self.len_after == other.len);

        // composing fails in weird ways if one of the sets is empty
        // a: [] len: 0 len_after: 1 | b: [Insert(Tendril<UTF8>(inline: "\n")), Retain(1)] len 1
        if self.changes.is_empty() {
            return other;
        }
        if other.changes.is_empty() {
            return self;
        }

        let len = self.changes.len();

        let mut changes_a = self.changes.into_iter();
        let mut changes_b = other.changes.into_iter();

        let mut head_a = changes_a.next();
        let mut head_b = changes_b.next();

        let mut changes = Self::with_capacity(len); // TODO: max(a, b), shrink_to_fit() afterwards

        loop {
            use std::cmp::Ordering;
            use Operation::*;
            match (head_a, head_b) {
                // we are done
                (None, None) => {
                    break;
                }
                // deletion in A
                (Some(Delete(i)), b) => {
                    changes.delete(i);
                    head_a = changes_a.next();
                    head_b = b;
                }
                // insertion in B
                (a, Some(Insert(current))) => {
                    changes.insert(current);
                    head_a = a;
                    head_b = changes_b.next();
                }
                (None, val) | (val, None) => unreachable!("({:?})", val),
                (Some(Retain(i)), Some(Retain(j))) => match i.cmp(&j) {
                    Ordering::Less => {
                        changes.retain(i);
                        head_a = changes_a.next();
                        head_b = Some(Retain(j - i));
                    }
                    Ordering::Equal => {
                        changes.retain(i);
                        head_a = changes_a.next();
                        head_b = changes_b.next();
                    }
                    Ordering::Greater => {
                        changes.retain(j);
                        head_a = Some(Retain(i - j));
                        head_b = changes_b.next();
                    }
                },
                (Some(Insert(mut s)), Some(Delete(j))) => {
                    let len = s.chars().count();
                    match len.cmp(&j) {
                        Ordering::Less => {
                            head_a = changes_a.next();
                            head_b = Some(Delete(j - len));
                        }
                        Ordering::Equal => {
                            head_a = changes_a.next();
                            head_b = changes_b.next();
                        }
                        Ordering::Greater => {
                            // TODO: cover this with a test
                            // figure out the byte index of the truncated string end
                            let (pos, _) = s.char_indices().nth(j).unwrap();
                            s.replace_range(0..pos, "");
                            head_a = Some(Insert(s));
                            head_b = changes_b.next();
                        }
                    }
                }
                (Some(Insert(s)), Some(Retain(j))) => {
                    let len = s.chars().count();
                    match len.cmp(&j) {
                        Ordering::Less => {
                            changes.insert(s);
                            head_a = changes_a.next();
                            head_b = Some(Retain(j - len));
                        }
                        Ordering::Equal => {
                            changes.insert(s);
                            head_a = changes_a.next();
                            head_b = changes_b.next();
                        }
                        Ordering::Greater => {
                            // figure out the byte index of the truncated string end
                            let (pos, _) = s.char_indices().nth(j).unwrap();
                            let mut before = s;
                            let after = before.split_off(pos);

                            changes.insert(before);
                            head_a = Some(Insert(after));
                            head_b = changes_b.next();
                        }
                    }
                }
                (Some(Retain(i)), Some(Delete(j))) => match i.cmp(&j) {
                    Ordering::Less => {
                        changes.delete(i);
                        head_a = changes_a.next();
                        head_b = Some(Delete(j - i));
                    }
                    Ordering::Equal => {
                        changes.delete(j);
                        head_a = changes_a.next();
                        head_b = changes_b.next();
                    }
                    Ordering::Greater => {
                        changes.delete(j);
                        head_a = Some(Retain(i - j));
                        head_b = changes_b.next();
                    }
                },
            };
        }

        // starting len should still equal original starting len
        debug_assert!(changes.len == self.len);

        changes
    }

    /// Given another change set starting in the same document, maps this
    /// change set over the other, producing a new change set that can be
    /// applied to the document produced by applying `other`. When
    /// `before` is `true`, order changes as if `this` comes before
    /// `other`, otherwise (the default) treat `other` as coming first.
    ///
    /// Given two changes `A` and `B`, `A.compose(B.map(A))` and
    /// `B.compose(A.map(B, true))` will produce the same document. This
    /// provides a basic form of [operational
    /// transformation](https://en.wikipedia.org/wiki/Operational_transformation),
    /// and can be used for collaborative editing.
    pub fn map(self, _other: Self) -> Self {
        unimplemented!()
    }

    /// Returns a new changeset that reverts this one. Useful for `undo` implementation.
    /// The document parameter expects the original document before this change was applied.
    pub fn invert(&self, original_doc: &Rope) -> Self {
        assert!(original_doc.len_chars() == self.len);

        let mut changes = Self::with_capacity(self.changes.len());

        let mut pos = 0;

        for change in &self.changes {
            use Operation::*;
            match change {
                Retain(n) => {
                    changes.retain(*n);
                    pos += n;
                }
                Delete(n) => {
                    let text = Cow::from(original_doc.slice(pos..pos + *n));
                    changes.insert(Tendril::from(text.as_ref()));
                    pos += n;
                }
                Insert(s) => {
                    let chars = s.chars().count();
                    changes.delete(chars);
                }
            }
        }

        changes
    }

    /// Returns true if applied successfully.
    pub fn apply(&self, text: &mut Rope) -> bool {
        if text.len_chars() != self.len {
            return false;
        }

        let mut pos = 0;

        for change in &self.changes {
            use Operation::*;
            match change {
                Retain(n) => {
                    pos += n;
                }
                Delete(n) => {
                    text.remove(pos..pos + *n);
                    // pos += n;
                }
                Insert(s) => {
                    text.insert(pos, s);
                    pos += s.chars().count();
                }
            }
        }
        true
    }

    /// `true` when the set is empty.
    #[inline]
    pub fn is_empty(&self) -> bool {
        self.changes.is_empty() || self.changes == [Operation::Retain(self.len)]
    }

    /// Map a (mostly) *sorted* list of positions through the changes.
    ///
    /// This is equivalent to updating each position with `map_pos`:
    ///
    /// ``` no-compile
    /// for (pos, assoc) in positions {
    ///     *pos = changes.map_pos(*pos, assoc);
    /// }
    /// ```
    /// However this function is significantly faster for sorted lists running
    /// in `O(N+M)` instead of `O(NM)`. This function also handles unsorted/
    /// partially sorted lists. However, in that case worst case complexity is
    /// again `O(MN)`.  For lists that are often/mostly sorted (like the end of diagnostic ranges)
    /// performance is usally close to `O(N + M)`
    pub fn update_positions<'a>(&self, positions: impl Iterator<Item = (&'a mut usize, Assoc)>) {
        use Operation::*;

        let mut positions = positions.peekable();

        let mut old_pos = 0;
        let mut new_pos = 0;
        let mut iter = self.changes.iter().enumerate().peekable();

        'outer: loop {
            macro_rules! map {
                ($map: expr, $i: expr) => {
                    loop {
                        let Some((pos, assoc)) = positions.peek_mut() else { return; };
                        if **pos < old_pos {
                            // Positions are not sorted, revert to the last Operation that
                            // contains this position and continue iterating from there.
                            // We can unwrap here since `pos` can not be negative
                            // (unsigned integer) and iterating backwards to the start
                            // should always move us back to the start
                            for (i, change) in self.changes[..$i].iter().enumerate().rev() {
                                match change {
                                    Retain(i) => {
                                        old_pos -= i;
                                        new_pos -= i;
                                    }
                                    Delete(i) => {
                                        old_pos -= i;
                                    }
                                    Insert(ins) => {
                                        new_pos -= ins.chars().count();
                                    }
                                }
                                if old_pos <= **pos {
                                    iter = self.changes[i..].iter().enumerate().peekable();
                                }
                            }
                            debug_assert!(old_pos <= **pos, "Reverse Iter across changeset works");
                            continue 'outer;
                        }
                        let Some(new_pos) = $map(**pos, *assoc) else { break; };
                        **pos = new_pos;
                        positions.next();
                    }
                };
            }

            let Some((i, change)) = iter.next() else {
                map!(|pos, _| (old_pos == pos).then_some(new_pos), self.changes.len());
                break;
            };

            let len = match change {
                Delete(i) | Retain(i) => *i,
                Insert(_) => 0,
            };
            let mut old_end = old_pos + len;

            match change {
                Retain(_) => {
                    map!(
                        |pos, _| (old_end > pos).then_some(new_pos + (pos - old_pos)),
                        i
                    );
                    new_pos += len;
                }
                Delete(_) => {
                    // in range
                    map!(|pos, _| (old_end > pos).then_some(new_pos), i);
                }
                Insert(s) => {
                    let ins = s.chars().count();

                    // a subsequent delete means a replace, consume it
                    if let Some((_, Delete(len))) = iter.peek() {
                        iter.next();

                        old_end = old_pos + len;
                        // in range of replaced text
                        map!(
                            |pos, assoc| (old_end > pos).then(|| {
                                // at point or tracking before
                                if pos == old_pos || assoc == Assoc::Before {
                                    new_pos
                                } else {
                                    // place to end of insert
                                    new_pos + ins
                                }
                            }),
                            i
                        );
                    } else {
                        // at insert point
                        map!(
                            |pos, assoc| (old_pos == pos).then(|| {
                                // return position before inserted text
                                if assoc == Assoc::Before {
                                    new_pos
                                } else {
                                    // after text
                                    new_pos + ins
                                }
                            }),
                            i
                        );
                    }

                    new_pos += ins;
                }
            }
            old_pos = old_end;
        }
        let out_of_bounds: Vec<_> = positions.collect();

        panic!("Positions {out_of_bounds:?} are out of range for changeset len {old_pos}!",)
    }

    /// Map a position through the changes.
    ///
    /// `assoc` indicates which side to associate the position with. `Before` will keep the
    /// position close to the character before, and will place it before insertions over that
    /// range, or at that point. `After` will move it forward, placing it at the end of such
    /// insertions.
    pub fn map_pos(&self, mut pos: usize, assoc: Assoc) -> usize {
        self.update_positions(once((&mut pos, assoc)));
        pos
    }

    pub fn changes_iter(&self) -> ChangeIterator {
        ChangeIterator::new(self)
    }
}

/// Transaction represents a single undoable unit of changes. Several changes can be grouped into
/// a single transaction.
#[derive(Debug, Default, Clone, PartialEq, Eq)]
pub struct Transaction {
    changes: ChangeSet,
    selection: Option<Selection>,
}

impl Transaction {
    /// Create a new, empty transaction.
    pub fn new(doc: &Rope) -> Self {
        Self {
            changes: ChangeSet::new(doc),
            selection: None,
        }
    }

    /// Changes made to the buffer.
    pub fn changes(&self) -> &ChangeSet {
        &self.changes
    }

    /// When set, explicitly updates the selection.
    pub fn selection(&self) -> Option<&Selection> {
        self.selection.as_ref()
    }

    /// Returns true if applied successfully.
    pub fn apply(&self, doc: &mut Rope) -> bool {
        if self.changes.is_empty() {
            return true;
        }

        // apply changes to the document
        self.changes.apply(doc)
    }

    /// Generate a transaction that reverts this one.
    pub fn invert(&self, original: &Rope) -> Self {
        let changes = self.changes.invert(original);

        Self {
            changes,
            selection: None,
        }
    }

    pub fn compose(mut self, other: Self) -> Self {
        self.changes = self.changes.compose(other.changes);
        // Other selection takes precedence
        self.selection = other.selection;
        self
    }

    pub fn with_selection(mut self, selection: Selection) -> Self {
        self.selection = Some(selection);
        self
    }

    /// Generate a transaction from a set of potentially overlapping changes. The `change_ranges`
    /// iterator yield the range (of removed text) in the old document for each edit. If any change
    /// overlaps with a range overlaps with a previous range then that range is ignored.
    ///
    /// The `process_change` callback is called for each edit that is not ignored (in the order
    /// yielded by `changes`) and should return the new text that the associated range will be
    /// replaced with.
    ///
    /// To make this function more flexible the iterator can yield additional data for each change
    /// that is passed to `process_change`
    pub fn change_ignore_overlapping<T>(
        doc: &Rope,
        change_ranges: impl Iterator<Item = (usize, usize, T)>,
        mut process_change: impl FnMut(usize, usize, T) -> Option<Tendril>,
    ) -> Self {
        let mut last = 0;
        let changes = change_ranges.filter_map(|(from, to, data)| {
            if from < last {
                return None;
            }
            let tendril = process_change(from, to, data);
            last = to;
            Some((from, to, tendril))
        });
        Self::change(doc, changes)
    }

    /// Generate a transaction from a set of changes.
    pub fn change<I>(doc: &Rope, changes: I) -> Self
    where
        I: Iterator<Item = Change>,
    {
        let len = doc.len_chars();

        let (lower, upper) = changes.size_hint();
        let size = upper.unwrap_or(lower);
        let mut changeset = ChangeSet::with_capacity(2 * size + 1); // rough estimate

        let mut last = 0;
        for (from, to, tendril) in changes {
            // Verify ranges are ordered and not overlapping
            debug_assert!(last <= from);
            // Verify ranges are correct
            debug_assert!(
                from <= to,
                "Edit end must end before it starts (should {from} <= {to})"
            );

            // Retain from last "to" to current "from"
            changeset.retain(from - last);
            let span = to - from;
            match tendril {
                Some(text) => {
                    changeset.insert(text);
                    changeset.delete(span);
                }
                None => changeset.delete(span),
            }
            last = to;
        }

        changeset.retain(len - last);

        Self::from(changeset)
    }

    /// Generate a transaction from a set of potentially overlapping deletions
    /// by merging overlapping deletions together.
    pub fn delete<I>(doc: &Rope, deletions: I) -> Self
    where
        I: Iterator<Item = Deletion>,
    {
        let len = doc.len_chars();

        let (lower, upper) = deletions.size_hint();
        let size = upper.unwrap_or(lower);
        let mut changeset = ChangeSet::with_capacity(2 * size + 1); // rough estimate

        let mut last = 0;
        for (mut from, to) in deletions {
            if last > to {
                continue;
            }
            if last > from {
                from = last
            }
            debug_assert!(
                from <= to,
                "Edit end must end before it starts (should {from} <= {to})"
            );
            // Retain from last "to" to current "from"
            changeset.retain(from - last);
            changeset.delete(to - from);
            last = to;
        }

        changeset.retain(len - last);

        Self::from(changeset)
    }

    pub fn insert_at_eof(mut self, text: Tendril) -> Transaction {
        self.changes.insert(text);
        self
    }

    /// Generate a transaction with a change per selection range.
    pub fn change_by_selection<F>(doc: &Rope, selection: &Selection, f: F) -> Self
    where
        F: FnMut(&Range) -> Change,
    {
        Self::change(doc, selection.iter().map(f))
    }

    pub fn change_by_selection_ignore_overlapping(
        doc: &Rope,
        selection: &Selection,
        mut change_range: impl FnMut(&Range) -> (usize, usize),
        mut create_tendril: impl FnMut(usize, usize) -> Option<Tendril>,
    ) -> (Transaction, Selection) {
        let mut last_selection_idx = None;
        let mut new_primary_idx = None;
        let mut ranges: SmallVec<[Range; 1]> = SmallVec::new();
        let process_change = |change_start, change_end, (idx, range): (usize, &Range)| {
            // update the primary idx
            if idx == selection.primary_index() {
                new_primary_idx = Some(idx);
            } else if new_primary_idx.is_none() {
                if idx > selection.primary_index() {
                    new_primary_idx = last_selection_idx;
                } else {
                    last_selection_idx = Some(idx);
                }
            }
            ranges.push(*range);
            create_tendril(change_start, change_end)
        };
        let transaction = Self::change_ignore_overlapping(
            doc,
            selection.iter().enumerate().map(|range| {
                let (change_start, change_end) = change_range(range.1);
                (change_start, change_end, range)
            }),
            process_change,
        );

        (
            transaction,
            Selection::new(ranges, new_primary_idx.unwrap_or(0)),
        )
    }

    /// Generate a transaction with a deletion per selection range.
    /// Compared to using `change_by_selection` directly these ranges may overlap.
    /// In that case they are merged
    pub fn delete_by_selection<F>(doc: &Rope, selection: &Selection, f: F) -> Self
    where
        F: FnMut(&Range) -> Deletion,
    {
        Self::delete(doc, selection.iter().map(f))
    }

    /// Insert text at each selection head.
    pub fn insert(doc: &Rope, selection: &Selection, text: Tendril) -> Self {
        Self::change_by_selection(doc, selection, |range| {
            (range.head, range.head, Some(text.clone()))
        })
    }

    pub fn changes_iter(&self) -> ChangeIterator {
        self.changes.changes_iter()
    }
}

impl From<ChangeSet> for Transaction {
    fn from(changes: ChangeSet) -> Self {
        Self {
            changes,
            selection: None,
        }
    }
}

pub struct ChangeIterator<'a> {
    iter: std::iter::Peekable<std::slice::Iter<'a, Operation>>,
    pos: usize,
}

impl<'a> ChangeIterator<'a> {
    fn new(changeset: &'a ChangeSet) -> Self {
        let iter = changeset.changes.iter().peekable();
        Self { iter, pos: 0 }
    }
}

impl<'a> Iterator for ChangeIterator<'a> {
    type Item = Change;

    fn next(&mut self) -> Option<Self::Item> {
        use Operation::*;

        loop {
            match self.iter.next()? {
                Retain(len) => {
                    self.pos += len;
                }
                Delete(len) => {
                    let start = self.pos;
                    self.pos += len;
                    return Some((start, self.pos, None));
                }
                Insert(s) => {
                    let start = self.pos;
                    // a subsequent delete means a replace, consume it
                    if let Some(Delete(len)) = self.iter.peek() {
                        self.iter.next();

                        self.pos += len;
                        return Some((start, self.pos, Some(s.clone())));
                    } else {
                        return Some((start, start, Some(s.clone())));
                    }
                }
            }
        }
    }
}

#[cfg(test)]
mod test {
    use super::*;
    use crate::history::State;

    #[test]
    fn composition() {
        use Operation::*;

        let a = ChangeSet {
            changes: vec![
                Retain(5),
                Insert(" test!".into()),
                Retain(1),
                Delete(2),
                Insert("abc".into()),
            ],
            len: 8,
            len_after: 15,
        };

        let b = ChangeSet {
            changes: vec![Delete(10), Insert("世orld".into()), Retain(5)],
            len: 15,
            len_after: 10,
        };

        let mut text = Rope::from("hello xz");

        // should probably return cloned text
        let composed = a.compose(b);
        assert_eq!(composed.len, 8);
        assert!(composed.apply(&mut text));
        assert_eq!(text, "世orld! abc");
    }

    #[test]
    fn invert() {
        use Operation::*;

        let changes = ChangeSet {
            changes: vec![Retain(4), Insert("test".into()), Delete(5), Retain(3)],
            len: 12,
            len_after: 11,
        };

        let doc = Rope::from("世界3 hello xz");
        let revert = changes.invert(&doc);

        let mut doc2 = doc.clone();
        changes.apply(&mut doc2);

        // a revert is different
        assert_ne!(changes, revert);
        assert_ne!(doc, doc2);

        // but inverting a revert will give us the original
        assert_eq!(changes, revert.invert(&doc2));

        // applying a revert gives us back the original
        revert.apply(&mut doc2);
        assert_eq!(doc, doc2);
    }

    #[test]
    fn map_pos() {
        use Operation::*;

        // maps inserts
        let cs = ChangeSet {
            changes: vec![Retain(4), Insert("!!".into()), Retain(4)],
            len: 8,
            len_after: 10,
        };

        assert_eq!(cs.map_pos(0, Assoc::Before), 0); // before insert region
        assert_eq!(cs.map_pos(4, Assoc::Before), 4); // at insert, track before
        assert_eq!(cs.map_pos(4, Assoc::After), 6); // at insert, track after
        assert_eq!(cs.map_pos(5, Assoc::Before), 7); // after insert region

        // maps deletes
        let cs = ChangeSet {
            changes: vec![Retain(4), Delete(4), Retain(4)],
            len: 12,
            len_after: 8,
        };
        assert_eq!(cs.map_pos(0, Assoc::Before), 0); // at start
        assert_eq!(cs.map_pos(4, Assoc::Before), 4); // before a delete
        assert_eq!(cs.map_pos(5, Assoc::Before), 4); // inside a delete
        assert_eq!(cs.map_pos(5, Assoc::After), 4); // inside a delete

        // TODO: delete tracking

        // stays inbetween replacements
        let cs = ChangeSet {
            changes: vec![
                Insert("ab".into()),
                Delete(2),
                Insert("cd".into()),
                Delete(2),
            ],
            len: 4,
            len_after: 4,
        };
        assert_eq!(cs.map_pos(2, Assoc::Before), 2);
        assert_eq!(cs.map_pos(2, Assoc::After), 2);
        // unsorted selection
        let cs = ChangeSet {
            changes: vec![
                Insert("ab".into()),
                Delete(2),
                Insert("cd".into()),
                Delete(2),
            ],
            len: 4,
            len_after: 4,
        };
        let mut positions = [4, 2];
        cs.update_positions(positions.iter_mut().map(|pos| (pos, Assoc::After)));
        assert_eq!(positions, [4, 2]);
    }

    #[test]
    fn transaction_change() {
        let mut doc = Rope::from("hello world!\ntest 123");
        let transaction = Transaction::change(
            &doc,
            // (1, 1, None) is a useless 0-width delete that gets factored out
            vec![(1, 1, None), (6, 11, Some("void".into())), (12, 17, None)].into_iter(),
        );
        transaction.apply(&mut doc);
        assert_eq!(doc, Rope::from_str("hello void! 123"));
    }

    #[test]
    fn changes_iter() {
        let doc = Rope::from("hello world!\ntest 123");
        let changes = vec![(6, 11, Some("void".into())), (12, 17, None)];
        let transaction = Transaction::change(&doc, changes.clone().into_iter());
        assert_eq!(transaction.changes_iter().collect::<Vec<_>>(), changes);
    }

    #[test]
    fn optimized_composition() {
        let mut state = State {
            doc: "".into(),
            selection: Selection::point(0),
        };
        let t1 = Transaction::insert(&state.doc, &state.selection, Tendril::from("h"));
        t1.apply(&mut state.doc);
        state.selection = state.selection.clone().map(t1.changes());
        let t2 = Transaction::insert(&state.doc, &state.selection, Tendril::from("e"));
        t2.apply(&mut state.doc);
        state.selection = state.selection.clone().map(t2.changes());
        let t3 = Transaction::insert(&state.doc, &state.selection, Tendril::from("l"));
        t3.apply(&mut state.doc);
        state.selection = state.selection.clone().map(t3.changes());
        let t4 = Transaction::insert(&state.doc, &state.selection, Tendril::from("l"));
        t4.apply(&mut state.doc);
        state.selection = state.selection.clone().map(t4.changes());
        let t5 = Transaction::insert(&state.doc, &state.selection, Tendril::from("o"));
        t5.apply(&mut state.doc);
        state.selection = state.selection.clone().map(t5.changes());

        assert_eq!(state.doc, Rope::from_str("hello"));

        // changesets as follows:
        // h
        // retain 1, e
        // retain 2, l

        let changes = t1
            .changes
            .compose(t2.changes)
            .compose(t3.changes)
            .compose(t4.changes)
            .compose(t5.changes);

        use Operation::*;
        assert_eq!(changes.changes, &[Insert("hello".into())]);
        // instead of insert h, insert e, insert l, insert l, insert o
    }

    #[test]
    fn combine_with_empty() {
        let empty = Rope::from("");
        let a = ChangeSet::new(&empty);

        let mut b = ChangeSet::new(&empty);
        b.insert("a".into());

        let changes = a.compose(b);

        use Operation::*;
        assert_eq!(changes.changes, &[Insert("a".into())]);
    }

    #[test]
    fn combine_with_utf8() {
        const TEST_CASE: &str = "Hello, これはヘリックスエディターです!";

        let empty = Rope::from("");
        let a = ChangeSet::new(&empty);

        let mut b = ChangeSet::new(&empty);
        b.insert(TEST_CASE.into());

        let changes = a.compose(b);

        use Operation::*;
        assert_eq!(changes.changes, &[Insert(TEST_CASE.into())]);
        assert_eq!(changes.len_after, TEST_CASE.chars().count());
    }
}