summaryrefslogtreecommitdiffstats
path: root/lib/generic/trie.c
blob: 682b159ec7c71ed1fa520b356896b822e3976f38 (plain)
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
/*  Copyright (C) 2016-2018 CZ.NIC, z.s.p.o. <knot-dns@labs.nic.cz>
 *  SPDX-License-Identifier: GPL-3.0-or-later

    The code originated from https://github.com/fanf2/qp/blob/master/qp.c
    at revision 5f6d93753.
 */

#include <assert.h>
#include <stdlib.h>
#include <string.h>

#include "lib/generic/trie.h"
#include "lib/utils.h"
#include "contrib/ucw/lib.h"

#if defined(__i386) || defined(__x86_64) || defined(_M_IX86) \
	|| (defined(__BYTE_ORDER__) && defined(__ORDER_LITTLE_ENDIAN) \
		&& __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__)

	/*!
	 * \brief Use a pointer alignment hack to save memory.
	 *
	 * When on, isbranch() relies on the fact that in leaf_t the first pointer
	 * is aligned on multiple of 4 bytes and that the flags bitfield is
	 * overlaid over the lowest two bits of that pointer.
	 * Neither is really guaranteed by the C standards; the second part should
	 * be OK with x86_64 ABI and most likely any other little-endian platform.
	 * It would be possible to manipulate the right bits portably, but it would
	 * complicate the code nontrivially. C++ doesn't even guarantee type-punning.
	 * In debug mode we check this works OK when creating a new trie instance.
	 */
	#define FLAGS_HACK 1
#else
	#define FLAGS_HACK 0
#endif

typedef unsigned char byte;
#ifndef uint
typedef unsigned int uint;
#define uint uint
#endif
typedef uint bitmap_t; /*! Bit-maps, using the range of 1<<0 to 1<<16 (inclusive). */

typedef struct {
	uint32_t len; // 32 bits are enough for key lengths; probably even 16 bits would be.
	char chars[];
} tkey_t;

/*! \brief Leaf of trie. */
typedef struct {
	#if !FLAGS_HACK
		byte flags;
	#endif
	tkey_t *key; /*!< The pointer must be aligned to 4-byte multiples! */
	trie_val_t val;
} leaf_t;

/*! \brief A trie node is either leaf_t or branch_t. */
typedef union node node_t;

/*!
 * \brief Branch node of trie.
 *
 * - The flags distinguish whether the node is a leaf_t (0), or a branch
 *   testing the more-important nibble (1) or the less-important one (2).
 * - It stores the index of the byte that the node tests.  The combined
 *   value (index*4 + flags) increases in branch nodes as you go deeper
 *   into the trie.  All the keys below a branch are identical up to the
 *   nibble identified by the branch.  Indices have to be stored because
 *   we skip any branch nodes that would have a single child.
 *   (Consequently, the skipped parts of key have to be validated in a leaf.)
 * - The bitmap indicates which subtries are present.  The present child nodes
 *   are stored in the twigs array (with no holes between them).
 * - To simplify storing keys that are prefixes of each other, the end-of-string
 *   position is treated as another nibble value, ordered before all others.
 *   That affects the bitmap and twigs fields.
 *
 * \note The branch nodes are never allocated individually, but they are
 *   always part of either the root node or the twigs array of the parent.
 */
typedef struct {
	#if FLAGS_HACK
		uint32_t flags  : 2,
		         bitmap : 17; /*!< The first bitmap bit is for end-of-string child. */
	#else
		byte flags;
		uint32_t bitmap;
	#endif
	uint32_t index;
	node_t *twigs;
} branch_t;

union node {
	leaf_t leaf;
	branch_t branch;
};

struct trie {
	node_t root; // undefined when weight == 0, see empty_root()
	size_t weight;
	knot_mm_t mm;
};

/*! \brief Make the root node empty (debug-only). */
static inline void empty_root(node_t *root) {
#ifndef NDEBUG
	*root = (node_t){ .branch = {
		.flags = 3, // invalid value that fits
		.bitmap = 0,
		.index = -1,
		.twigs = NULL
	} };
#endif
}

/*! \brief Check that unportable code works OK (debug-only). */
static void assert_portability(void) {
#if FLAGS_HACK
	assert(((union node){ .leaf = {
			.key = (tkey_t *)(((uint8_t *)NULL) + 1),
			.val = NULL
		} }).branch.flags == 1);
#endif
}

/*! \brief Propagate error codes. */
#define ERR_RETURN(x) \
	do { \
		int err_code_ = x; \
		if (unlikely(err_code_ != KNOT_EOK)) \
			return err_code_; \
	} while (false)

/*!
 * \brief Count the number of set bits.
 *
 * \TODO This implementation may be relatively slow on some HW.
 */
static uint bitmap_weight(bitmap_t w)
{
	assert((w & ~((1 << 17) - 1)) == 0); // using the least-important 17 bits
	return __builtin_popcount(w);
}

/*! \brief Only keep the lowest bit in the bitmap (least significant -> twigs[0]). */
static bitmap_t bitmap_lowest_bit(bitmap_t w)
{
	assert((w & ~((1 << 17) - 1)) == 0); // using the least-important 17 bits
	return 1 << __builtin_ctz(w);
}

/*! \brief Test flags to determine type of this node. */
static bool isbranch(const node_t *t)
{
	uint f = t->branch.flags;
	assert(f <= 2);
	return f != 0;
}

/*! \brief Make a bitmask for testing a branch bitmap. */
static bitmap_t nibbit(byte k, uint flags)
{
	uint shift = (2 - flags) << 2;
	uint nibble = (k >> shift) & 0xf;
	return 1 << (nibble + 1/*because of prefix keys*/);
}

/*! \brief Extract a nibble from a key and turn it into a bitmask. */
static bitmap_t twigbit(const node_t *t, const char *key, uint32_t len)
{
	assert(isbranch(t));
	uint i = t->branch.index;

	if (i >= len)
		return 1 << 0; // leaf position

	return nibbit((byte)key[i], t->branch.flags);
}

/*! \brief Test if a branch node has a child indicated by a bitmask. */
static bool hastwig(const node_t *t, bitmap_t bit)
{
	assert(isbranch(t));
	return t->branch.bitmap & bit;
}

/*! \brief Compute offset of an existing child in a branch node. */
static uint twigoff(const node_t *t, bitmap_t b)
{
	assert(isbranch(t));
	return bitmap_weight(t->branch.bitmap & (b - 1));
}

/*! \brief Get pointer to a particular child of a branch node. */
static node_t* twig(node_t *t, uint i)
{
	assert(isbranch(t));
	return &t->branch.twigs[i];
}

/*!
 * \brief For a branch nod, compute offset of a child and child count.
 *
 * Having this separate might be meaningful for performance optimization.
 */
#define TWIGOFFMAX(off, max, t, b) do {			\
		(off) = twigoff((t), (b));		\
		(max) = bitmap_weight((t)->branch.bitmap);\
	} while(0)

/*! \brief Simple string comparator. */
static int key_cmp(const char *k1, uint32_t k1_len, const char *k2, uint32_t k2_len)
{
	int ret = memcmp(k1, k2, MIN(k1_len, k2_len));
	if (ret != 0) {
		return ret;
	}

	/* Key string is equal, compare lengths. */
	if (k1_len == k2_len) {
		return 0;
	} else if (k1_len < k2_len) {
		return -1;
	} else {
		return 1;
	}
}

trie_t* trie_create(knot_mm_t *mm)
{
	assert_portability();
	trie_t *trie = mm_alloc(mm, sizeof(trie_t));
	if (trie != NULL) {
		empty_root(&trie->root);
		trie->weight = 0;
		if (mm != NULL)
			trie->mm = *mm;
		else
			mm_ctx_init(&trie->mm);
	}
	return trie;
}

/*! \brief Free anything under the trie node, except for the passed pointer itself. */
static void clear_trie(node_t *trie, knot_mm_t *mm)
{
	if (!isbranch(trie)) {
		mm_free(mm, trie->leaf.key);
	} else {
		branch_t *b = &trie->branch;
		int len = bitmap_weight(b->bitmap);
		for (int i = 0; i < len; ++i)
			clear_trie(b->twigs + i, mm);
		mm_free(mm, b->twigs);
	}
}

void trie_free(trie_t *tbl)
{
	if (tbl == NULL)
		return;
	if (tbl->weight)
		clear_trie(&tbl->root, &tbl->mm);
	mm_free(&tbl->mm, tbl);
}

void trie_clear(trie_t *tbl)
{
	assert(tbl);
	if (!tbl->weight)
		return;
	clear_trie(&tbl->root, &tbl->mm);
	empty_root(&tbl->root);
	tbl->weight = 0;
}

size_t trie_weight(const trie_t *tbl)
{
	assert(tbl);
	return tbl->weight;
}

struct found {
	leaf_t *l;	/**< the found leaf (NULL if not found) */
	branch_t *p;	/**< the leaf's parent (if exists) */
	bitmap_t b;	/**< bit-mask with a single bit marking l under p */
};
/** Search trie for an item with the given key (equality only). */
static struct found find_equal(trie_t *tbl, const char *key, uint32_t len)
{
	assert(tbl);
	struct found ret0;
	memset(&ret0, 0, sizeof(ret0));
	if (!tbl->weight)
		return ret0;
	/* Current node and parent while descending (returned values basically). */
	node_t *t = &tbl->root;
	branch_t *p = NULL;
	bitmap_t b = 0;
	while (isbranch(t)) {
		__builtin_prefetch(t->branch.twigs);
		b = twigbit(t, key, len);
		if (!hastwig(t, b))
			return ret0;
		p = &t->branch;
		t = twig(t, twigoff(t, b));
	}
	if (key_cmp(key, len, t->leaf.key->chars, t->leaf.key->len) != 0)
		return ret0;
	return (struct found) {
		.l = &t->leaf,
		.p = p,
		.b = b,
	};
}
/** Find item with the first key (lexicographical order). */
static struct found find_first(trie_t *tbl)
{
	assert(tbl);
	if (!tbl->weight) {
		struct found ret0;
		memset(&ret0, 0, sizeof(ret0));
		return ret0;
	}
	/* Current node and parent while descending (returned values basically). */
	node_t *t = &tbl->root;
	branch_t *p = NULL;
	while (isbranch(t)) {
		p = &t->branch;
		t = &p->twigs[0];
	}
	return (struct found) {
		.l = &t->leaf,
		.p = p,
		.b = p ? bitmap_lowest_bit(p->bitmap) : 0,
	};
}

trie_val_t* trie_get_try(trie_t *tbl, const char *key, uint32_t len)
{
	struct found found = find_equal(tbl, key, len);
	return found.l ? &found.l->val : NULL;
}

trie_val_t* trie_get_first(trie_t *tbl, char **key, uint32_t *len)
{
	struct found found = find_first(tbl);
	if (!found.l)
		return NULL;
	if (key)
		*key = found.l->key->chars;
	if (len)
		*len = found.l->key->len;
	return &found.l->val;
}

/** Delete the found element (if any) and return value (unless NULL is passed) */
static int del_found(trie_t *tbl, struct found found, trie_val_t *val)
{
	if (!found.l)
		return KNOT_ENOENT;
	mm_free(&tbl->mm, found.l->key);
	if (val != NULL)
		*val = found.l->val; // we return trie_val_t directly when deleting
	--tbl->weight;
	branch_t * const p = found.p; // short-hand
	if (unlikely(!p)) { // whole trie was a single leaf
		assert(tbl->weight == 0);
		empty_root(&tbl->root);
		return KNOT_EOK;
	}
	// remove leaf t as child of p; get child index via pointer arithmetic
	int ci = ((union node *)found.l) - p->twigs,
	    cc = bitmap_weight(p->bitmap); // child count
	assert(ci >= 0 && ci < cc);

	if (cc == 2) { // collapse binary node p: move the other child to this node
		node_t *twigs = p->twigs;
		(*(union node *)p) = twigs[1 - ci]; // it might be a leaf or branch
		mm_free(&tbl->mm, twigs);
		return KNOT_EOK;
	}
	memmove(p->twigs + ci, p->twigs + ci + 1, sizeof(node_t) * (cc - ci - 1));
	p->bitmap &= ~found.b;
	node_t *twigs = mm_realloc(&tbl->mm, p->twigs, sizeof(node_t) * (cc - 1),
	                           sizeof(node_t) * cc);
	if (likely(twigs != NULL))
		p->twigs = twigs;
		/* We can ignore mm_realloc failure, only beware that next time
		 * the prev_size passed to it wouldn't be correct; TODO? */
	return KNOT_EOK;
}

int trie_del(trie_t *tbl, const char *key, uint32_t len, trie_val_t *val)
{
	struct found found = find_equal(tbl, key, len);
	return del_found(tbl, found, val);
}

int trie_del_first(trie_t *tbl, char *key, uint32_t *len, trie_val_t *val)
{
	struct found found = find_first(tbl);
	if (!found.l)
		return KNOT_ENOENT;
	if (key) {
		if (!len)
			return KNOT_EINVAL;
		if (*len < found.l->key->len)
			return kr_error(ENOSPC);
		memcpy(key, found.l->key->chars, found.l->key->len);
	}
	if (len) { // makes sense even with key == NULL
		*len = found.l->key->len;
	}
	return del_found(tbl, found, val);
}

/*!
 * \brief Stack of nodes, storing a path down a trie.
 *
 * The structure also serves directly as the public trie_it_t type,
 * in which case it always points to the current leaf, unless we've finished
 * (i.e. it->len == 0).
 */
typedef struct trie_it {
	node_t* *stack; /*!< The stack; malloc is used directly instead of mm. */
	uint32_t len;   /*!< Current length of the stack. */
	uint32_t alen;  /*!< Allocated/available length of the stack. */
	/*! \brief Initial storage for \a stack; it should fit in many use cases. */
	node_t* stack_init[60];
} nstack_t;

/*! \brief Create a node stack containing just the root (or empty). */
static void ns_init(nstack_t *ns, trie_t *tbl)
{
	assert(tbl);
	ns->stack = ns->stack_init;
	ns->alen = sizeof(ns->stack_init) / sizeof(ns->stack_init[0]);
	if (tbl->weight) {
		ns->len = 1;
		ns->stack[0] = &tbl->root;
	} else {
		ns->len = 0;
	}
}

/*! \brief Free inside of the stack, i.e. not the passed pointer itself. */
static void ns_cleanup(nstack_t *ns)
{
	assert(ns && ns->stack);
	if (likely(ns->stack == ns->stack_init))
		return;
	free(ns->stack);
	#ifndef NDEBUG
		ns->stack = NULL;
		ns->alen = 0;
	#endif
}

/*! \brief Allocate more space for the stack. */
static int ns_longer_alloc(nstack_t *ns)
{
	ns->alen *= 2;
	size_t new_size = sizeof(nstack_t) + ns->alen * sizeof(node_t *);
	node_t **st;
	if (ns->stack == ns->stack_init) {
		st = malloc(new_size);
		if (st != NULL)
			memcpy(st, ns->stack, ns->len * sizeof(node_t *));
	} else {
		st = realloc(ns->stack, new_size);
	}
	if (st == NULL)
		return KNOT_ENOMEM;
	ns->stack = st;
	return KNOT_EOK;
}

/*! \brief Ensure the node stack can be extended by one. */
static inline int ns_longer(nstack_t *ns)
{
	// get a longer stack if needed
	if (likely(ns->len < ns->alen))
		return KNOT_EOK;
	return ns_longer_alloc(ns); // hand-split the part suitable for inlining
}

/*!
 * \brief Find the "branching point" as if searching for a key.
 *
 *  The whole path to the point is kept on the passed stack;
 *  always at least the root will remain on the top of it.
 *  Beware: the precise semantics of this function is rather tricky.
 *  The top of the stack will contain: the corresponding leaf if exact match is found;
 *  or the immediate node below a branching-point-on-edge or the branching-point itself.
 *
 *  \param info   Set position of the point of first mismatch (in index and flags).
 *  \param first  Set the value of the first non-matching character (from trie),
 *                optionally; end-of-string character has value -256 (that's why it's int).
 *                Note: the character is converted to *unsigned* char (i.e. 0..255),
 *                as that's the ordering used in the trie.
 *
 *  \return KNOT_EOK or KNOT_ENOMEM.
 */
static int ns_find_branch(nstack_t *ns, const char *key, uint32_t len,
                          branch_t *info, int *first)
{
	assert(ns && ns->len && info);
	// First find some leaf with longest matching prefix.
	while (isbranch(ns->stack[ns->len - 1])) {
		ERR_RETURN(ns_longer(ns));
		node_t *t = ns->stack[ns->len - 1];
		__builtin_prefetch(t->branch.twigs);
		bitmap_t b = twigbit(t, key, len);
		// Even if our key is missing from this branch we need to
		// keep iterating down to a leaf. It doesn't matter which
		// twig we choose since the keys are all the same up to this
		// index. Note that blindly using twigoff(t, b) can cause
		// an out-of-bounds index if it equals twigmax(t).
		uint i = hastwig(t, b) ? twigoff(t, b) : 0;
		ns->stack[ns->len++] = twig(t, i);
	}
	tkey_t *lkey = ns->stack[ns->len-1]->leaf.key;
	// Find index of the first char that differs.
	uint32_t index = 0;
	while (index < MIN(len,lkey->len)) {
		if (key[index] != lkey->chars[index])
			break;
		else
			++index;
	}
	info->index = index;
	if (first)
		*first = lkey->len > index ? (unsigned char)lkey->chars[index] : -256;
	// Find flags: which half-byte has matched.
	uint flags;
	if (index == len && len == lkey->len) { // found equivalent key
		info->flags = flags = 0;
		goto success;
	}
	if (likely(index < MIN(len,lkey->len))) {
		byte k2 = (byte)lkey->chars[index];
		byte k1 = (byte)key[index];
		flags = ((k1 ^ k2) & 0xf0) ? 1 : 2;
	} else { // one is prefix of another
		flags = 1;
	}
	info->flags = flags;
	// now go up the trie from the current leaf
	branch_t *t;
	do {
		if (unlikely(ns->len == 1))
			goto success; // only the root stays on the stack
		t = (branch_t*)ns->stack[ns->len - 2];
		if (t->index < index || (t->index == index && t->flags < flags))
			goto success;
		--ns->len;
	} while (true);
success:
	#ifndef NDEBUG // invariants on successful return
		assert(ns->len);
		if (isbranch(ns->stack[ns->len - 1])) {
			t = &ns->stack[ns->len - 1]->branch;
			assert(t->index > index || (t->index == index && t->flags >= flags));
		}
		if (ns->len > 1) {
			t = &ns->stack[ns->len - 2]->branch;
			assert(t->index < index || (t->index == index
			       && (t->flags < flags || (t->flags == 1 && flags == 0))));
		}
	#endif
	return KNOT_EOK;
}

/*!
 * \brief Advance the node stack to the last leaf in the subtree.
 *
 * \return KNOT_EOK or KNOT_ENOMEM.
 */
static int ns_last_leaf(nstack_t *ns)
{
	assert(ns);
	do {
		ERR_RETURN(ns_longer(ns));
		node_t *t = ns->stack[ns->len - 1];
		if (!isbranch(t))
			return KNOT_EOK;
		int lasti = bitmap_weight(t->branch.bitmap) - 1;
		assert(lasti >= 0);
		ns->stack[ns->len++] = twig(t, lasti);
	} while (true);
}

/*!
 * \brief Advance the node stack to the first leaf in the subtree.
 *
 * \return KNOT_EOK or KNOT_ENOMEM.
 */
static int ns_first_leaf(nstack_t *ns)
{
	assert(ns && ns->len);
	do {
		ERR_RETURN(ns_longer(ns));
		node_t *t = ns->stack[ns->len - 1];
		if (!isbranch(t))
			return KNOT_EOK;
		ns->stack[ns->len++] = twig(t, 0);
	} while (true);
}

/*!
 * \brief Advance the node stack to the leaf that is previous to the current node.
 *
 * \note Prefix leaf under the current node DOES count (if present; perhaps questionable).
 * \return KNOT_EOK on success, KNOT_ENOENT on not-found, or possibly KNOT_ENOMEM.
 */
static int ns_prev_leaf(nstack_t *ns)
{
	assert(ns && ns->len > 0);

	node_t *t = ns->stack[ns->len - 1];
	if (hastwig(t, 1 << 0)) { // the prefix leaf
		t = twig(t, 0);
		ERR_RETURN(ns_longer(ns));
		ns->stack[ns->len++] = t;
		return KNOT_EOK;
	}

	do {
		if (ns->len < 2)
			return KNOT_ENOENT; // root without empty key has no previous leaf
		t = ns->stack[ns->len - 1];
		node_t *p = ns->stack[ns->len - 2];
		int pindex = t - p->branch.twigs; // index in parent via pointer arithmetic
		assert(pindex >= 0 && pindex <= 16);
		if (pindex > 0) { // t isn't the first child -> go down the previous one
			ns->stack[ns->len - 1] = twig(p, pindex - 1);
			return ns_last_leaf(ns);
		}
		// we've got to go up again
		--ns->len;
	} while (true);
}

/*!
 * \brief Advance the node stack to the leaf that is successor to the current node.
 *
 * \note Prefix leaf or anything else under the current node DOES count.
 * \return KNOT_EOK on success, KNOT_ENOENT on not-found, or possibly KNOT_ENOMEM.
 */
static int ns_next_leaf(nstack_t *ns)
{
	assert(ns && ns->len > 0);

	node_t *t = ns->stack[ns->len - 1];
	if (isbranch(t))
		return ns_first_leaf(ns);
	do {
		if (ns->len < 2)
			return KNOT_ENOENT; // not found, as no more parent is available
		t = ns->stack[ns->len - 1];
		node_t *p = ns->stack[ns->len - 2];
		int pindex = t - p->branch.twigs; // index in parent via pointer arithmetic
		assert(pindex >= 0 && pindex <= 16);
		int pcount = bitmap_weight(p->branch.bitmap);
		if (pindex + 1 < pcount) { // t isn't the last child -> go down the next one
			ns->stack[ns->len - 1] = twig(p, pindex + 1);
			return ns_first_leaf(ns);
		}
		// we've got to go up again
		--ns->len;
	} while (true);
}

int trie_get_leq(trie_t *tbl, const char *key, uint32_t len, trie_val_t **val)
{
	assert(tbl && val);
	*val = NULL; // so on failure we can just return;
	if (tbl->weight == 0)
		return KNOT_ENOENT;
	{ // Intentionally un-indented; until end of function, to bound cleanup attr.
	// First find a key with longest-matching prefix
	__attribute__((cleanup(ns_cleanup)))
		nstack_t ns_local;
	ns_init(&ns_local, tbl);
	nstack_t *ns = &ns_local;
	branch_t bp;
	int un_leaf; // first unmatched character in the leaf
	ERR_RETURN(ns_find_branch(ns, key, len, &bp, &un_leaf));
	int un_key = bp.index < len ? (unsigned char)key[bp.index] : -256;
	node_t *t = ns->stack[ns->len - 1];
	if (bp.flags == 0) { // found exact match
		*val = &t->leaf.val;
		return KNOT_EOK;
	}
	// Get t: the last node on matching path
	if (isbranch(t) && t->branch.index == bp.index && t->branch.flags == bp.flags) {
		// t is OK
	} else {
		// the top of the stack was the first unmatched node -> step up
		if (ns->len == 1) {
			// root was unmatched already
			if (un_key < un_leaf)
				return KNOT_ENOENT;
			ERR_RETURN(ns_last_leaf(ns));
			goto success;
		}
		--ns->len;
		t = ns->stack[ns->len - 1];
	}
	// Now we re-do the first "non-matching" step in the trie
	// but try the previous child if key was less (it may not exist)
	bitmap_t b = twigbit(t, key, len);
	int i = hastwig(t, b)
		? twigoff(t, b) - (un_key < un_leaf)
		: twigoff(t, b) - 1 /*twigoff returns successor when !hastwig*/;
	if (i >= 0) {
		ERR_RETURN(ns_longer(ns));
		ns->stack[ns->len++] = twig(t, i);
		ERR_RETURN(ns_last_leaf(ns));
	} else {
		ERR_RETURN(ns_prev_leaf(ns));
	}
success:
	assert(!isbranch(ns->stack[ns->len - 1]));
	*val = &ns->stack[ns->len - 1]->leaf.val;
	return 1;
	}
}

/*! \brief Initialize a new leaf, copying the key, and returning failure code. */
static int mk_leaf(node_t *leaf, const char *key, uint32_t len, knot_mm_t *mm)
{
	tkey_t *k = mm_alloc(mm, sizeof(tkey_t) + len);
	#if FLAGS_HACK
		assert(((uintptr_t)k) % 4 == 0); // we need an aligned pointer
	#endif
	if (unlikely(!k))
		return KNOT_ENOMEM;
	k->len = len;
	memcpy(k->chars, key, len);
	leaf->leaf = (leaf_t){
		#if !FLAGS_HACK
			.flags = 0,
		#endif
		.val = NULL,
		.key = k
	};
	return KNOT_EOK;
}

trie_val_t* trie_get_ins(trie_t *tbl, const char *key, uint32_t len)
{
	assert(tbl);
	// First leaf in an empty tbl?
	if (unlikely(!tbl->weight)) {
		if (unlikely(mk_leaf(&tbl->root, key, len, &tbl->mm)))
			return NULL;
		++tbl->weight;
		return &tbl->root.leaf.val;
	}
	{ // Intentionally un-indented; until end of function, to bound cleanup attr.
	// Find the branching-point
	__attribute__((cleanup(ns_cleanup)))
		nstack_t ns_local;
	ns_init(&ns_local, tbl);
	nstack_t *ns = &ns_local;
	branch_t bp; // branch-point: index and flags signifying the longest common prefix
	int k2; // the first unmatched character in the leaf
	if (unlikely(ns_find_branch(ns, key, len, &bp, &k2)))
		return NULL;
	node_t *t = ns->stack[ns->len - 1];
	if (bp.flags == 0) // the same key was already present
		return &t->leaf.val;
	node_t leaf;
	if (unlikely(mk_leaf(&leaf, key, len, &tbl->mm)))
		return NULL;

	if (isbranch(t) && bp.index == t->branch.index && bp.flags == t->branch.flags) {
		// The node t needs a new leaf child.
		bitmap_t b1 = twigbit(t, key, len);
		assert(!hastwig(t, b1));
		uint s, m; TWIGOFFMAX(s, m, t, b1); // new child position and original child count
		node_t *twigs = mm_realloc(&tbl->mm, t->branch.twigs,
				sizeof(node_t) * (m + 1), sizeof(node_t) * m);
		if (unlikely(!twigs))
			goto err_leaf;
		memmove(twigs + s + 1, twigs + s, sizeof(node_t) * (m - s));
		twigs[s] = leaf;
		t->branch.twigs = twigs;
		t->branch.bitmap |= b1;
		++tbl->weight;
		return &twigs[s].leaf.val;
	} else {
		// We need to insert a new binary branch with leaf at *t.
		// Note: it works the same for the case where we insert above root t.
		#ifndef NDEBUG
			if (ns->len > 1) {
				node_t *pt = ns->stack[ns->len - 2];
				assert(hastwig(pt, twigbit(pt, key, len)));
			}
		#endif
		node_t *twigs = mm_alloc(&tbl->mm, sizeof(node_t) * 2);
		if (unlikely(!twigs))
			goto err_leaf;
		node_t t2 = *t; // Save before overwriting t.
		t->branch.flags = bp.flags;
		t->branch.index = bp.index;
		t->branch.twigs = twigs;
		bitmap_t b1 = twigbit(t, key, len);
		bitmap_t b2 = unlikely(k2 == -256) ? (1 << 0) : nibbit(k2, bp.flags);
		t->branch.bitmap = b1 | b2;
		*twig(t, twigoff(t, b1)) = leaf;
		*twig(t, twigoff(t, b2)) = t2;
		++tbl->weight;
		return &twig(t, twigoff(t, b1))->leaf.val;
	};
err_leaf:
	mm_free(&tbl->mm, leaf.leaf.key);
	return NULL;
	}
}

/*! \brief Apply a function to every trie_val_t*, in order; a recursive solution. */
static int apply_trie(node_t *t, int (*f)(trie_val_t *, void *), void *d)
{
	assert(t);
	if (!isbranch(t))
		return f(&t->leaf.val, d);
	int child_count = bitmap_weight(t->branch.bitmap);
	for (int i = 0; i < child_count; ++i)
		ERR_RETURN(apply_trie(twig(t, i), f, d));
	return KNOT_EOK;
}

int trie_apply(trie_t *tbl, int (*f)(trie_val_t *, void *), void *d)
{
	assert(tbl && f);
	if (!tbl->weight)
		return KNOT_EOK;
	return apply_trie(&tbl->root, f, d);
}

/* These are all thin wrappers around static Tns* functions. */
trie_it_t* trie_it_begin(trie_t *tbl)
{
	assert(tbl);
	trie_it_t *it = malloc(sizeof(nstack_t));
	if (!it)
		return NULL;
	ns_init(it, tbl);
	if (it->len == 0) // empty tbl
		return it;
	if (ns_first_leaf(it)) {
		ns_cleanup(it);
		free(it);
		return NULL;
	}
	return it;
}

void trie_it_next(trie_it_t *it)
{
	assert(it && it->len);
	if (ns_next_leaf(it) != KNOT_EOK)
		it->len = 0;
}

bool trie_it_finished(trie_it_t *it)
{
	assert(it);
	return it->len == 0;
}

void trie_it_free(trie_it_t *it)
{
	if (!it)
		return;
	ns_cleanup(it);
	free(it);
}

const char* trie_it_key(trie_it_t *it, size_t *len)
{
	assert(it && it->len);
	node_t *t = it->stack[it->len - 1];
	assert(!isbranch(t));
	tkey_t *key = t->leaf.key;
	if (len)
		*len = key->len;
	return key->chars;
}

trie_val_t* trie_it_val(trie_it_t *it)
{
	assert(it && it->len);
	node_t *t = it->stack[it->len - 1];
	assert(!isbranch(t));
	return &t->leaf.val;
}