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
// This file is part of ICU4X. For terms of use, please see the file
// called LICENSE at the top level of the ICU4X source tree
// (online at: https://github.com/unicode-org/icu4x/blob/main/LICENSE ).

//! Code for the [`DoublePlaceholder`] pattern backend.

use core::convert::Infallible;
use core::{cmp::Ordering, str::FromStr};
use either::Either;
use writeable::adapters::WriteableAsTryWriteableInfallible;
use writeable::Writeable;

use crate::common::*;
use crate::Error;

#[cfg(feature = "alloc")]
use alloc::{boxed::Box, string::String};

/// A two-value enum for the [`DoublePlaceholder`] pattern backend.
///
/// # Examples
///
/// ```
/// use core::cmp::Ordering;
/// use core::str::FromStr;
/// use icu_pattern::DoublePlaceholderKey;
/// use icu_pattern::DoublePlaceholderPattern;
/// use icu_pattern::PatternItem;
///
/// // Parse the string syntax
/// let pattern =
///     DoublePlaceholderPattern::try_from_str("Hello, {0} and {1}!", Default::default()).unwrap();
///
/// assert_eq!(
///     pattern.iter().cmp(
///         [
///             PatternItem::Literal("Hello, "),
///             PatternItem::Placeholder(DoublePlaceholderKey::Place0),
///             PatternItem::Literal(" and "),
///             PatternItem::Placeholder(DoublePlaceholderKey::Place1),
///             PatternItem::Literal("!")
///         ]
///         .into_iter()
///     ),
///     Ordering::Equal
/// );
/// ```
#[derive(Debug, Copy, Clone, PartialEq, Eq, PartialOrd, Ord)]
#[cfg_attr(feature = "serde", derive(serde::Deserialize, serde::Serialize))]
#[allow(clippy::exhaustive_enums)] // Defined to have two entries
pub enum DoublePlaceholderKey {
    /// The placeholder `{0}`.
    Place0,
    /// The placeholder `{1}`.
    Place1,
}

impl FromStr for DoublePlaceholderKey {
    type Err = Error;
    fn from_str(s: &str) -> Result<Self, Self::Err> {
        match s {
            "0" => Ok(Self::Place0),
            "1" => Ok(Self::Place1),
            _ => Err(Error::InvalidPlaceholder),
        }
    }
}

impl<W0, W1> PlaceholderValueProvider<DoublePlaceholderKey> for (W0, W1)
where
    W0: Writeable,
    W1: Writeable,
{
    type Error = Infallible;
    type W<'a>
        = WriteableAsTryWriteableInfallible<Either<&'a W0, &'a W1>>
    where
        W0: 'a,
        W1: 'a;
    const LITERAL_PART: writeable::Part = crate::PATTERN_LITERAL_PART;
    #[inline]
    fn value_for(&self, key: DoublePlaceholderKey) -> (Self::W<'_>, writeable::Part) {
        let writeable = match key {
            DoublePlaceholderKey::Place0 => Either::Left(&self.0),
            DoublePlaceholderKey::Place1 => Either::Right(&self.1),
        };
        (
            WriteableAsTryWriteableInfallible(writeable),
            crate::PATTERN_PLACEHOLDER_PART,
        )
    }
}

impl<W> PlaceholderValueProvider<DoublePlaceholderKey> for [W; 2]
where
    W: Writeable,
{
    type Error = Infallible;
    type W<'a>
        = WriteableAsTryWriteableInfallible<&'a W>
    where
        W: 'a;
    const LITERAL_PART: writeable::Part = crate::PATTERN_LITERAL_PART;
    #[inline]
    fn value_for(&self, key: DoublePlaceholderKey) -> (Self::W<'_>, writeable::Part) {
        let [item0, item1] = self;
        let writeable = match key {
            DoublePlaceholderKey::Place0 => item0,
            DoublePlaceholderKey::Place1 => item1,
        };
        (
            WriteableAsTryWriteableInfallible(writeable),
            crate::PATTERN_PLACEHOLDER_PART,
        )
    }
}

/// Internal representation of a placeholder
#[derive(Debug, Copy, Clone)]
struct DoublePlaceholderInfo {
    /// The placeholder key: 0 or 1
    pub key: DoublePlaceholderKey,
    /// An offset field. This can take one of two forms:
    /// - Encoded form: 1 + offset from start of literals
    /// - Decoded form: offset from start of store
    pub offset: usize,
}

impl DoublePlaceholderInfo {
    pub fn from_char(ch: char) -> Self {
        Self {
            key: if ((ch as usize) & 0x1) == 0 {
                DoublePlaceholderKey::Place0
            } else {
                DoublePlaceholderKey::Place1
            },
            offset: (ch as usize) >> 1,
        }
    }
    #[cfg(feature = "alloc")]
    pub fn try_to_char(self) -> Result<char, Error> {
        let usize_val = match self.key {
            DoublePlaceholderKey::Place0 => 0,
            DoublePlaceholderKey::Place1 => 1,
        } | (self.offset << 1);
        u32::try_from(usize_val)
            .ok()
            .and_then(|x| char::try_from(x).ok())
            .ok_or(Error::InvalidPattern)
    }
    /// Creates a PlaceholderInfo for an empty Place0
    pub fn no_place0() -> Self {
        Self {
            key: DoublePlaceholderKey::Place0,
            offset: 0,
        }
    }
    /// Changes Place0 to Place1 and vice-versa
    pub fn swap(self) -> Self {
        Self {
            key: match self.key {
                DoublePlaceholderKey::Place0 => DoublePlaceholderKey::Place1,
                DoublePlaceholderKey::Place1 => DoublePlaceholderKey::Place0,
            },
            offset: self.offset,
        }
    }
    /// Sets the offset to 0 (ignored placeholder), retaining the key
    #[cfg(feature = "alloc")]
    pub fn clear(self) -> Self {
        Self {
            key: self.key,
            offset: 0,
        }
    }
}

/// Backend for patterns containing zero, one, or two placeholders.
///
/// This empty type is not constructible.
///
/// # Placeholder Keys
///
/// The placeholder is either [`DoublePlaceholderKey::Place0`] or [`DoublePlaceholderKey::Place1`].
///
/// In [`Pattern::interpolate()`], pass a two-element array or tuple.
///
/// # Encoding Details
///
/// The first two code points contain the indices of the first and second placeholders with
/// the following encoding:
///
/// 1. First bit: 0 for [`DoublePlaceholderKey::Place0`], 1 for [`DoublePlaceholderKey::Place1`].
/// 2. Second and higher bits: 1 plus the byte offset of the placeholder counting from after
///    the placeholder index code points. If zero, skip this placeholder.
///
/// # Examples
///
/// Parsing a pattern into the encoding:
///
/// ```
/// use core::str::FromStr;
/// use icu_pattern::DoublePlaceholder;
/// use icu_pattern::Pattern;
///
/// // Parse the string syntax and check the resulting data store:
/// let pattern =
///     Pattern::<DoublePlaceholder>::try_from_str("Hello, {0} and {1}!", Default::default())
///         .unwrap();
///
/// assert_eq!("\x10\x1BHello,  and !", &pattern.store);
/// ```
///
/// Explanation of the lead code points:
///
/// 1. `\x10` is placeholder 0 at index 7: ((7 + 1) << 1) | 0
/// 2. `\x1B` is placeholder 1 at index 12: ((12 + 1) << 1) | 1
///
/// Example patterns supported by this backend:
///
/// ```
/// use core::str::FromStr;
/// use icu_pattern::DoublePlaceholder;
/// use icu_pattern::Pattern;
/// use icu_pattern::QuoteMode;
///
/// // Single numeric placeholder (note, "5" is used):
/// assert_eq!(
///     Pattern::<DoublePlaceholder>::try_from_str("{0} days ago", Default::default())
///         .unwrap()
///         .interpolate_to_string([5, 7]),
///     "5 days ago",
/// );
///
/// // No placeholder (note, the placeholder value is never accessed):
/// assert_eq!(
///     Pattern::<DoublePlaceholder>::try_from_str("yesterday", Default::default())
///         .unwrap()
///         .interpolate_to_string(["foo", "bar"]),
///     "yesterday",
/// );
///
/// // Escaped placeholder and a placeholder value 1 (note, "bar" is used):
/// assert_eq!(
///     Pattern::<DoublePlaceholder>::try_from_str("'{0}' {1}", QuoteMode::QuotingSupported.into())
///         .unwrap()
///         .interpolate_to_string(("foo", "bar")),
///     "{0} bar",
/// );
///
/// // Pattern with the placeholders in the opposite order:
/// assert_eq!(
///     Pattern::<DoublePlaceholder>::try_from_str("A {1} B {0} C", Default::default())
///         .unwrap()
///         .interpolate_to_string(("D", "E")),
///     "A E B D C",
/// );
///
/// // No literals, only placeholders:
/// assert_eq!(
///     Pattern::<DoublePlaceholder>::try_from_str("{1}{0}", Default::default())
///         .unwrap()
///         .interpolate_to_string((1, "A")),
///     "A1",
/// );
/// ```
///
/// [`Pattern::interpolate()`]: crate::Pattern::interpolate
#[derive(Debug, Copy, Clone, PartialEq, Eq)]
#[allow(clippy::exhaustive_enums)] // Empty Enum
pub enum DoublePlaceholder {}

impl crate::private::Sealed for DoublePlaceholder {}

impl PatternBackend for DoublePlaceholder {
    type PlaceholderKey<'a> = DoublePlaceholderKey;
    #[cfg(feature = "alloc")]
    type PlaceholderKeyCow<'a> = DoublePlaceholderKey;
    type Error<'a> = Infallible;
    type Store = str;
    type Iter<'a> = DoublePlaceholderPatternIterator<'a>;

    fn validate_store(store: &Self::Store) -> Result<(), Error> {
        let mut chars = store.chars();
        let ph_first_char = chars.next().ok_or(Error::InvalidPattern)?;
        let ph_second_char = chars.next().ok_or(Error::InvalidPattern)?;
        let initial_offset = ph_first_char.len_utf8() + ph_second_char.len_utf8();
        let ph_first = DoublePlaceholderInfo::from_char(ph_first_char);
        let ph_second = DoublePlaceholderInfo::from_char(ph_second_char);
        if ph_first.key == ph_second.key {
            return Err(Error::InvalidPattern);
        }
        if ph_first.offset > ph_second.offset && ph_second.offset > 0 {
            return Err(Error::InvalidPattern);
        }
        if ph_second.offset > store.len() - initial_offset + 1 {
            return Err(Error::InvalidPattern);
        }
        if (ph_second_char as usize) >= 0xD800 {
            return Err(Error::InvalidPattern);
        }
        Ok(())
    }

    fn iter_items(store: &Self::Store) -> Self::Iter<'_> {
        let mut chars = store.chars();
        let (mut ph_first, ph_first_len) = match chars.next() {
            Some(ch) => (DoublePlaceholderInfo::from_char(ch), ch.len_utf8()),
            None => {
                debug_assert!(false);
                (DoublePlaceholderInfo::no_place0(), 0)
            }
        };
        let (mut ph_second, ph_second_len) = match chars.next() {
            Some(ch) => (DoublePlaceholderInfo::from_char(ch), ch.len_utf8()),
            None => {
                debug_assert!(false);
                (ph_first.swap(), ph_first_len)
            }
        };
        let initial_offset = ph_first_len + ph_second_len;
        ph_first.offset += initial_offset - 1;
        ph_second.offset += initial_offset - 1;
        DoublePlaceholderPatternIterator {
            store,
            ph_first,
            ph_second,
            current_offset: initial_offset,
        }
    }

    #[cfg(feature = "alloc")]
    fn try_from_items<
        'cow,
        'ph,
        I: Iterator<Item = Result<PatternItemCow<'cow, Self::PlaceholderKey<'ph>>, Error>>,
    >(
        items: I,
    ) -> Result<Box<str>, Error> {
        let mut result = String::new();
        let mut first_ph = None;
        let mut second_ph = None;
        for item in items {
            match item? {
                PatternItemCow::Literal(s) => result.push_str(&s),
                PatternItemCow::Placeholder(ph_key) => {
                    if second_ph.is_some() {
                        return Err(Error::InvalidPattern);
                    }
                    let placeholder_offset = result.len() + 1;
                    if placeholder_offset >= 0xD800 {
                        return Err(Error::InvalidPattern);
                    }
                    let ph_info = DoublePlaceholderInfo {
                        key: ph_key,
                        offset: placeholder_offset,
                    };
                    if first_ph.is_none() {
                        first_ph.replace(ph_info);
                    } else {
                        second_ph.replace(ph_info);
                    }
                }
            }
        }
        let (first_ph, second_ph) = match (first_ph, second_ph) {
            (Some(a), Some(b)) => (a, b),
            (Some(a), None) => (a, a.swap().clear()),
            (None, None) => (
                DoublePlaceholderInfo::no_place0(),
                DoublePlaceholderInfo::no_place0().swap(),
            ),
            (None, Some(_)) => unreachable!("first_ph always populated before second_ph"),
        };
        if first_ph.key == second_ph.key {
            return Err(Error::InvalidPattern);
        }

        result.insert(0, second_ph.try_to_char()?);
        result.insert(0, first_ph.try_to_char()?);

        Ok(result.into_boxed_str())
    }

    fn empty() -> &'static Self::Store {
        "\0\u{1}"
    }
}

#[doc(hidden)] // TODO(#4467): Should be internal
#[derive(Debug)]
pub struct DoublePlaceholderPatternIterator<'a> {
    store: &'a str,
    ph_first: DoublePlaceholderInfo,
    ph_second: DoublePlaceholderInfo,
    current_offset: usize,
}

// Note: This impl is not exported via public bounds, but it might be in the
// future, and the compiler might be able to find it. The code is also
// reachable from `Iterator::size_hint`.
impl ExactSizeIterator for DoublePlaceholderPatternIterator<'_> {
    fn len(&self) -> usize {
        let mut chars = self.store.chars();
        let (mut ph_first, ph_first_len) = match chars.next() {
            Some(ch) => (DoublePlaceholderInfo::from_char(ch), ch.len_utf8()),
            None => {
                debug_assert!(false);
                (DoublePlaceholderInfo::no_place0(), 0)
            }
        };
        let (mut ph_second, ph_second_len) = match chars.next() {
            Some(ch) => (DoublePlaceholderInfo::from_char(ch), ch.len_utf8()),
            None => {
                debug_assert!(false);
                (ph_first.swap(), ph_first_len)
            }
        };
        let initial_offset = ph_first_len + ph_second_len;
        ph_first.offset += initial_offset - 1;
        ph_second.offset += initial_offset - 1;
        let store_len = self.store.len();

        #[allow(clippy::comparison_chain)]
        if ph_first.offset < initial_offset {
            // No placeholders
            if initial_offset < store_len {
                // No placeholder, non-empty literal
                1
            } else {
                // No placeholder, empty literal
                0
            }
        } else if ph_first.offset == initial_offset {
            // At least 1 placeholder, empty prefix
            if ph_second.offset < initial_offset {
                // Single placeholder
                if ph_first.offset < store_len {
                    // Single placeholder, empty prefix, non-empty suffix
                    2
                } else {
                    // Single placeholder, empty prefix, empty suffix
                    1
                }
            } else if ph_second.offset == ph_first.offset {
                // Two placeholders, empty infix
                if ph_first.offset < store_len {
                    // Two placeholders, empty prefix, empty infix, non-empty suffix
                    3
                } else {
                    // Two placeholders, empty prefix, empty infix, empty suffix
                    2
                }
            } else if ph_second.offset < store_len {
                // Two placeholders, empty prefix, non-empty infix, non-empty suffix
                4
            } else {
                // Two placeholders, empty prefix, non-empty infix, empty suffix
                3
            }
        } else {
            // At least 1 placeholder, non-empty prefix
            if ph_second.offset < initial_offset {
                // Single placeholder
                if ph_first.offset < store_len {
                    // Single placeholder, non-empty prefix, non-empty suffix
                    3
                } else {
                    // Single placeholder, non-empty prefix, empty suffix
                    2
                }
            } else if ph_second.offset == ph_first.offset {
                // Two placeholders, empty infix
                if ph_first.offset < store_len {
                    // Two placeholders, non-empty prefix, empty infix, non-empty suffix
                    4
                } else {
                    // Two placeholders, non-empty prefix, empty infix, empty suffix
                    3
                }
            } else if ph_second.offset < store_len {
                // Two placeholders, non-empty prefix, non-empty infix, non-empty suffix
                5
            } else {
                // Two placeholders, non-empty prefix, non-empty infix, empty suffix
                4
            }
        }
    }
}

impl<'a> Iterator for DoublePlaceholderPatternIterator<'a> {
    type Item = PatternItem<'a, DoublePlaceholderKey>;
    fn next(&mut self) -> Option<Self::Item> {
        match self.current_offset.cmp(&self.ph_first.offset) {
            Ordering::Less => {
                // Prefix
                let literal_str = match self.store.get(self.current_offset..self.ph_first.offset) {
                    Some(s) => s,
                    None => {
                        debug_assert!(false, "offsets are in range");
                        ""
                    }
                };
                self.current_offset = self.ph_first.offset;
                Some(PatternItem::Literal(literal_str))
            }
            Ordering::Equal => {
                // Placeholder
                self.ph_first.offset = 0;
                Some(PatternItem::Placeholder(self.ph_first.key))
            }
            Ordering::Greater => match self.current_offset.cmp(&self.ph_second.offset) {
                Ordering::Less => {
                    // Infix
                    let literal_str =
                        match self.store.get(self.current_offset..self.ph_second.offset) {
                            Some(s) => s,
                            None => {
                                debug_assert!(false, "offsets are in range");
                                ""
                            }
                        };
                    self.current_offset = self.ph_second.offset;
                    Some(PatternItem::Literal(literal_str))
                }
                Ordering::Equal => {
                    // Placeholder
                    self.ph_second.offset = 0;
                    Some(PatternItem::Placeholder(self.ph_second.key))
                }
                Ordering::Greater => {
                    // Suffix or end of string
                    let literal_str = match self.store.get(self.current_offset..) {
                        Some(s) => s,
                        None => {
                            debug_assert!(false, "offsets are in range");
                            ""
                        }
                    };
                    if literal_str.is_empty() {
                        // End of string
                        None
                    } else {
                        // Suffix
                        self.current_offset = self.store.len();
                        Some(PatternItem::Literal(literal_str))
                    }
                }
            },
        }
    }

    fn size_hint(&self) -> (usize, Option<usize>) {
        let len = self.len();
        (len, Some(len))
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::DoublePlaceholderPattern;

    #[test]
    fn test_permutations() {
        #[derive(Debug)]
        struct TestCase<'a> {
            pattern: &'a str,
            store: &'a str,
            /// Interpolation with 0=apple, 1=orange
            interpolated: &'a str,
        }
        let cases = [
            TestCase {
                pattern: "",
                store: "\x00\x01",
                interpolated: "",
            },
            TestCase {
                pattern: "{0}",
                store: "\x02\x01",
                interpolated: "apple",
            },
            TestCase {
                pattern: "X{0}",
                store: "\x04\x01X",
                interpolated: "Xapple",
            },
            TestCase {
                pattern: "{0}Y",
                store: "\x02\x01Y",
                interpolated: "appleY",
            },
            TestCase {
                pattern: "X{0}Y",
                store: "\x04\x01XY",
                interpolated: "XappleY",
            },
            TestCase {
                pattern: "{1}",
                store: "\x03\x00",
                interpolated: "orange",
            },
            TestCase {
                pattern: "X{1}",
                store: "\x05\x00X",
                interpolated: "Xorange",
            },
            TestCase {
                pattern: "{1}Y",
                store: "\x03\x00Y",
                interpolated: "orangeY",
            },
            TestCase {
                pattern: "X{1}Y",
                store: "\x05\x00XY",
                interpolated: "XorangeY",
            },
            TestCase {
                pattern: "{0}{1}",
                store: "\x02\x03",
                interpolated: "appleorange",
            },
            TestCase {
                pattern: "X{0}{1}",
                store: "\x04\x05X",
                interpolated: "Xappleorange",
            },
            TestCase {
                pattern: "{0}Y{1}",
                store: "\x02\x05Y",
                interpolated: "appleYorange",
            },
            TestCase {
                pattern: "{0}{1}Z",
                store: "\x02\x03Z",
                interpolated: "appleorangeZ",
            },
            TestCase {
                pattern: "X{0}Y{1}",
                store: "\x04\x07XY",
                interpolated: "XappleYorange",
            },
            TestCase {
                pattern: "X{0}{1}Z",
                store: "\x04\x05XZ",
                interpolated: "XappleorangeZ",
            },
            TestCase {
                pattern: "{0}Y{1}Z",
                store: "\x02\x05YZ",
                interpolated: "appleYorangeZ",
            },
            TestCase {
                pattern: "X{0}Y{1}Z",
                store: "\x04\x07XYZ",
                interpolated: "XappleYorangeZ",
            },
            TestCase {
                pattern: "{1}{0}",
                store: "\x03\x02",
                interpolated: "orangeapple",
            },
            TestCase {
                pattern: "X{1}{0}",
                store: "\x05\x04X",
                interpolated: "Xorangeapple",
            },
            TestCase {
                pattern: "{1}Y{0}",
                store: "\x03\x04Y",
                interpolated: "orangeYapple",
            },
            TestCase {
                pattern: "{1}{0}Z",
                store: "\x03\x02Z",
                interpolated: "orangeappleZ",
            },
            TestCase {
                pattern: "X{1}Y{0}",
                store: "\x05\x06XY",
                interpolated: "XorangeYapple",
            },
            TestCase {
                pattern: "X{1}{0}Z",
                store: "\x05\x04XZ",
                interpolated: "XorangeappleZ",
            },
            TestCase {
                pattern: "{1}Y{0}Z",
                store: "\x03\x04YZ",
                interpolated: "orangeYappleZ",
            },
            TestCase {
                pattern: "X{1}Y{0}Z",
                store: "\x05\x06XYZ",
                interpolated: "XorangeYappleZ",
            },
            TestCase {
                pattern: "01234567890123456789012345678901234567890123456789012345678901234567890123456789{0}Y{1}Z",
                store: "\u{A2}\u{A5}01234567890123456789012345678901234567890123456789012345678901234567890123456789YZ",
                interpolated: "01234567890123456789012345678901234567890123456789012345678901234567890123456789appleYorangeZ",
            },
        ];
        for cas in cases {
            let TestCase {
                pattern,
                store,
                interpolated,
            } = cas;
            let parsed =
                DoublePlaceholderPattern::try_from_str(pattern, Default::default()).unwrap();
            let actual = parsed
                .interpolate(["apple", "orange"])
                .write_to_string()
                .into_owned();
            assert_eq!(&parsed.store, store, "{cas:?}");
            assert_eq!(actual, interpolated, "{cas:?}");
        }
    }

    #[test]
    fn test_invalid() {
        let cases = [
            "",               // too short
            "\x00",           // too short
            "\x00\x00",       // duplicate placeholders
            "\x04\x03",       // first offset is after second offset
            "\x04\x05",       // second offset out of range (also first offset)
            "\x04\u{10001}@", // second offset too large for UTF-8
        ];
        let long_str = "0123456789".repeat(1000000);
        for cas in cases {
            let cas = cas.replace('@', &long_str);
            assert!(DoublePlaceholder::validate_store(&cas).is_err(), "{cas:?}");
        }
    }
}