src/lib/trie.c

/*
 * Copyright (C) 2016-2019 CZ.NIC, z.s.p.o. <knot-dns@labs.nic.cz>
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 *
 * 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/trie.h"

/*! \brief Error codes used in the library. */
enum knot_error {
    KNOT_EOK = 0,

    /* Directly mapped error codes. */
    KNOT_ENOMEM = -ENOMEM,
    KNOT_EINVAL = -EINVAL,
    KNOT_ENOENT = -ENOENT,
};

#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.
    uint8_t  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;
};

/* Included from other files */

/** Readability: avoid const-casts in code. */
static inline void free_const(const void* what)
{
    free((void*)what);
}

static inline void* mm_alloc(knot_mm_t* mm, size_t size)
{
    if (mm)
        return mm->alloc(mm->ctx, size);
    else
        return malloc(size);
}

static inline void mm_free(knot_mm_t* mm, const void* what)
{
    if (mm) {
        if (mm->free)
            mm->free((void*)what);
    } else
        free_const(what);
}

static void* mm_malloc(void* ctx, size_t n)
{
    (void)ctx;
    return malloc(n);
}

static void* mm_realloc(knot_mm_t* mm, void* what, size_t size, size_t prev_size)
{
    if (mm) {
        void* p = mm->alloc(mm->ctx, size);
        if (p == NULL) {
            return NULL;
        } else {
            if (what) {
                memcpy(p, what,
                    prev_size < size ? prev_size : size);
            }
            mm_free(mm, what);
            return p;
        }
    } else {
        return realloc(what, size);
    }
}

static inline void mm_ctx_init(knot_mm_t* mm)
{
    mm->ctx   = NULL;
    mm->alloc = mm_malloc;
    mm->free  = free;
}

/*! \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 uint8_t* 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 uint8_t* k1, uint32_t k1_len, const uint8_t* 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);
        int       i;
        for (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 uint8_t* 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 uint8_t* 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, uint8_t** 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;
}

/*!
 * \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 uint8_t* 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 uint8_t* 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 uint8_t* 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 uint8_t* 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);
    int i;
    for (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 uint8_t* 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;
}