src/linked_list.c

/*
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS HEADER.
 *
 * Copyright 2021 Mike Becker, Olaf Wintermann All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions are met:
 *
 *   1. Redistributions of source code must retain the above copyright
 *      notice, this list of conditions and the following disclaimer.
 *
 *   2. Redistributions in binary form must reproduce the above copyright
 *      notice, this list of conditions and the following disclaimer in the
 *      documentation and/or other materials provided with the distribution.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
 * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE
 * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
 * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
 * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
 * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
 * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
 * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
 * POSSIBILITY OF SUCH DAMAGE.
 */

#include "cx/linked_list.h"
#include "cx/utils.h"
#include "cx/compare.h"
#include <string.h>
#include <assert.h>

// LOW LEVEL LINKED LIST FUNCTIONS

#define CX_LL_PTR(cur, off) (*(void**)(((char*)(cur))+(off)))
#define ll_prev(node) CX_LL_PTR(node, loc_prev)
#define ll_next(node) CX_LL_PTR(node, loc_next)
#define ll_advance(node) CX_LL_PTR(node, loc_advance)
#define ll_data(node) (((char*)(node))+loc_data)

void *cx_linked_list_at(
        const void *start,
        size_t start_index,
        ptrdiff_t loc_advance,
        size_t index
) {
    assert(start != NULL);
    assert(loc_advance >= 0);
    size_t i = start_index;
    const void *cur = start;
    while (i != index && cur != NULL) {
        cur = ll_advance(cur);
        i < index ? i++ : i--;
    }
    return (void *) cur;
}

ssize_t cx_linked_list_find(
        const void *start,
        ptrdiff_t loc_advance,
        ptrdiff_t loc_data,
        cx_compare_func cmp_func,
        const void *elem
) {
    void *dummy;
    return cx_linked_list_find_node(
            &dummy, start,
            loc_advance, loc_data,
            cmp_func, elem
    );
}

ssize_t cx_linked_list_find_node(
        void **result,
        const void *start,
        ptrdiff_t loc_advance,
        ptrdiff_t loc_data,
        cx_compare_func cmp_func,
        const void *elem
) {
    assert(result != NULL);
    assert(start != NULL);
    assert(loc_advance >= 0);
    assert(loc_data >= 0);
    assert(cmp_func);

    const void *node = start;
    ssize_t index = 0;
    do {
        void *current = ll_data(node);
        if (cmp_func(current, elem) == 0) {
            *result = (void *) node;
            return index;
        }
        node = ll_advance(node);
        index++;
    } while (node != NULL);
    *result = NULL;
    return -1;
}

void *cx_linked_list_first(
        const void *node,
        ptrdiff_t loc_prev
) {
    return cx_linked_list_last(node, loc_prev);
}

void *cx_linked_list_last(
        const void *node,
        ptrdiff_t loc_next
) {
    assert(node != NULL);
    assert(loc_next >= 0);

    const void *cur = node;
    const void *last;
    do {
        last = cur;
    } while ((cur = ll_next(cur)) != NULL);

    return (void *) last;
}

void *cx_linked_list_prev(
        const void *begin,
        ptrdiff_t loc_next,
        const void *node
) {
    assert(begin != NULL);
    assert(node != NULL);
    assert(loc_next >= 0);
    if (begin == node) return NULL;
    const void *cur = begin;
    const void *next;
    while (1) {
        next = ll_next(cur);
        if (next == node) return (void *) cur;
        cur = next;
    }
}

void cx_linked_list_link(
        void *left,
        void *right,
        ptrdiff_t loc_prev,
        ptrdiff_t loc_next
) {
    assert(loc_next >= 0);
    ll_next(left) = right;
    if (loc_prev >= 0) {
        ll_prev(right) = left;
    }
}

void cx_linked_list_unlink(
        void *left,
        void *right,
        ptrdiff_t loc_prev,
        ptrdiff_t loc_next
) {
    assert (loc_next >= 0);
    assert(ll_next(left) == right);
    ll_next(left) = NULL;
    if (loc_prev >= 0) {
        assert(ll_prev(right) == left);
        ll_prev(right) = NULL;
    }
}

void cx_linked_list_add(
        void **begin,
        void **end,
        ptrdiff_t loc_prev,
        ptrdiff_t loc_next,
        void *new_node
) {
    void *last;
    if (end == NULL) {
        assert(begin != NULL);
        last = *begin == NULL ? NULL : cx_linked_list_last(*begin, loc_next);
    } else {
        last = *end;
    }
    cx_linked_list_insert_chain(begin, end, loc_prev, loc_next, last, new_node, new_node);
}

void cx_linked_list_prepend(
        void **begin,
        void **end,
        ptrdiff_t loc_prev,
        ptrdiff_t loc_next,
        void *new_node
) {
    cx_linked_list_insert_chain(begin, end, loc_prev, loc_next, NULL, new_node, new_node);
}

void cx_linked_list_insert(
        void **begin,
        void **end,
        ptrdiff_t loc_prev,
        ptrdiff_t loc_next,
        void *node,
        void *new_node
) {
    cx_linked_list_insert_chain(begin, end, loc_prev, loc_next, node, new_node, new_node);
}

void cx_linked_list_insert_chain(
        void **begin,
        void **end,
        ptrdiff_t loc_prev,
        ptrdiff_t loc_next,
        void *node,
        void *insert_begin,
        void *insert_end
) {
    // find the end of the chain, if not specified
    if (insert_end == NULL) {
        insert_end = cx_linked_list_last(insert_begin, loc_next);
    }

    // determine the successor
    void *successor;
    if (node == NULL) {
        assert(begin != NULL || (end != NULL && loc_prev >= 0));
        if (begin != NULL) {
            successor = *begin;
            *begin = insert_begin;
        } else {
            successor = *end == NULL ? NULL : cx_linked_list_first(*end, loc_prev);
        }
    } else {
        successor = ll_next(node);
        cx_linked_list_link(node, insert_begin, loc_prev, loc_next);
    }

    if (successor == NULL) {
        // the list ends with the new chain
        if (end != NULL) {
            *end = insert_end;
        }
    } else {
        cx_linked_list_link(insert_end, successor, loc_prev, loc_next);
    }
}

void cx_linked_list_insert_sorted(
        void **begin,
        void **end,
        ptrdiff_t loc_prev,
        ptrdiff_t loc_next,
        void *new_node,
        cx_compare_func cmp_func
) {
    assert(ll_next(new_node) == NULL);
    cx_linked_list_insert_sorted_chain(
            begin, end, loc_prev, loc_next, new_node, cmp_func);
}

void cx_linked_list_insert_sorted_chain(
        void **begin,
        void **end,
        ptrdiff_t loc_prev,
        ptrdiff_t loc_next,
        void *insert_begin,
        cx_compare_func cmp_func
) {
    assert(begin != NULL);
    assert(loc_next >= 0);
    assert(insert_begin != NULL);

    // track currently observed nodes
    void *dest_prev = NULL;
    void *dest = *begin;
    void *src = insert_begin;

    // special case: list is empty
    if (dest == NULL) {
        *begin = src;
        if (end != NULL) {
            *end = cx_linked_list_last(src, loc_next);
        }
        return;
    }

    // search the list for insertion points
    while (dest != NULL && src != NULL) {
        // compare current list node with source node
        // if less or equal, skip
        if (cmp_func(dest, src) <= 0) {
            dest_prev = dest;
            dest = ll_next(dest);
            continue;
        }

        // determine chain of elements that can be inserted
        void *end_of_chain = src;
        void *next_in_chain = ll_next(src);
        while (next_in_chain != NULL) {
            // once we become larger than the list elem, break
            if (cmp_func(dest, next_in_chain) <= 0) {
                break;
            }
            // otherwise, we can insert one more
            end_of_chain = next_in_chain;
            next_in_chain = ll_next(next_in_chain);
        }

        // insert the elements
        if (dest_prev == NULL) {
            // new begin
            *begin = src;
        } else {
            cx_linked_list_link(dest_prev, src, loc_prev, loc_next);
        }
        cx_linked_list_link(end_of_chain, dest, loc_prev, loc_next);

        // continue with next
        src = next_in_chain;
        dest_prev = dest;
        dest = ll_next(dest);
    }

    // insert remaining items
    if (src != NULL) {
        cx_linked_list_link(dest_prev, src, loc_prev, loc_next);
    }

    // determine new end of list, if requested
    if (end != NULL) {
        *end = cx_linked_list_last(
                dest != NULL ? dest : dest_prev, loc_next);
    }
}

size_t cx_linked_list_remove_chain(
        void **begin,
        void **end,
        ptrdiff_t loc_prev,
        ptrdiff_t loc_next,
        void *node,
        size_t num
) {
    assert(node != NULL);
    assert(loc_next >= 0);
    assert(loc_prev >= 0 || begin != NULL);

    // easy exit
    if (num == 0) return 0;

    // find adjacent nodes
    void *prev;
    if (loc_prev >= 0) {
        prev = ll_prev(node);
    } else {
        prev = cx_linked_list_prev(*begin, loc_next, node);
    }

    void *next = ll_next(node);
    size_t removed = 1;
    for (; removed < num && next != NULL ; removed++) {
        next = ll_next(next);
    }

    // update next pointer of prev node, or set begin
    if (prev == NULL) {
        if (begin != NULL) {
            *begin = next;
        }
    } else {
        ll_next(prev) = next;
    }

    // update prev pointer of next node, or set end
    if (next == NULL) {
        if (end != NULL) {
            *end = prev;
        }
    } else if (loc_prev >= 0) {
        ll_prev(next) = prev;
    }

    return removed;
}

size_t cx_linked_list_size(
        const void *node,
        ptrdiff_t loc_next
) {
    assert(loc_next >= 0);
    size_t size = 0;
    while (node != NULL) {
        node = ll_next(node);
        size++;
    }
    return size;
}

#ifndef CX_LINKED_LIST_SORT_SBO_SIZE
#define CX_LINKED_LIST_SORT_SBO_SIZE 1024
#endif

static void cx_linked_list_sort_merge(
        ptrdiff_t loc_prev,
        ptrdiff_t loc_next,
        ptrdiff_t loc_data,
        size_t length,
        void *ls,
        void *le,
        void *re,
        cx_compare_func cmp_func,
        void **begin,
        void **end
) {
    void *sbo[CX_LINKED_LIST_SORT_SBO_SIZE];
    void **sorted = length >= CX_LINKED_LIST_SORT_SBO_SIZE ?
                    malloc(sizeof(void *) * length) : sbo;
    if (sorted == NULL) abort();
    void *rc, *lc;

    lc = ls;
    rc = le;
    size_t n = 0;
    while (lc && lc != le && rc != re) {
        if (cmp_func(ll_data(lc), ll_data(rc)) <= 0) {
            sorted[n] = lc;
            lc = ll_next(lc);
        } else {
            sorted[n] = rc;
            rc = ll_next(rc);
        }
        n++;
    }
    while (lc && lc != le) {
        sorted[n] = lc;
        lc = ll_next(lc);
        n++;
    }
    while (rc && rc != re) {
        sorted[n] = rc;
        rc = ll_next(rc);
        n++;
    }

    // Update pointer
    if (loc_prev >= 0) ll_prev(sorted[0]) = NULL;
    cx_for_n (i, length - 1) {
        cx_linked_list_link(sorted[i], sorted[i + 1], loc_prev, loc_next);
    }
    ll_next(sorted[length - 1]) = NULL;

    *begin = sorted[0];
    *end = sorted[length - 1];
    if (sorted != sbo) {
        free(sorted);
    }
}

void cx_linked_list_sort( // NOLINT(misc-no-recursion) - purposely recursive function
        void **begin,
        void **end,
        ptrdiff_t loc_prev,
        ptrdiff_t loc_next,
        ptrdiff_t loc_data,
        cx_compare_func cmp_func
) {
    assert(begin != NULL);
    assert(loc_next >= 0);
    assert(loc_data >= 0);
    assert(cmp_func);

    void *lc, *ls, *le, *re;

    // set start node
    ls = *begin;

    // early exit when this list is empty
    if (ls == NULL) return;

    // check how many elements are already sorted
    lc = ls;
    size_t ln = 1;
    while (ll_next(lc) != NULL && cmp_func(ll_data(ll_next(lc)), ll_data(lc)) > 0) {
        lc = ll_next(lc);
        ln++;
    }
    le = ll_next(lc);

    // if first unsorted node is NULL, the list is already completely sorted
    if (le != NULL) {
        void *rc;
        size_t rn = 1;
        rc = le;
        // skip already sorted elements
        while (ll_next(rc) != NULL && cmp_func(ll_data(ll_next(rc)), ll_data(rc)) > 0) {
            rc = ll_next(rc);
            rn++;
        }
        re = ll_next(rc);

        // {ls,...,le->prev} and {rs,...,re->prev} are sorted - merge them
        void *sorted_begin, *sorted_end;
        cx_linked_list_sort_merge(loc_prev, loc_next, loc_data,
                                  ln + rn, ls, le, re, cmp_func,
                                  &sorted_begin, &sorted_end);

        // Something left? Sort it!
        size_t remainder_length = cx_linked_list_size(re, loc_next);
        if (remainder_length > 0) {
            void *remainder = re;
            cx_linked_list_sort(&remainder, NULL, loc_prev, loc_next, loc_data, cmp_func);

            // merge sorted list with (also sorted) remainder
            cx_linked_list_sort_merge(loc_prev, loc_next, loc_data,
                                      ln + rn + remainder_length,
                                      sorted_begin, remainder, NULL, cmp_func,
                                      &sorted_begin, &sorted_end);
        }
        *begin = sorted_begin;
        if (end) *end = sorted_end;
    }
}

int cx_linked_list_compare(
        const void *begin_left,
        const void *begin_right,
        ptrdiff_t loc_advance,
        ptrdiff_t loc_data,
        cx_compare_func cmp_func
) {
    const void *left = begin_left, *right = begin_right;

    while (left != NULL && right != NULL) {
        const void *left_data = ll_data(left);
        const void *right_data = ll_data(right);
        int result = cmp_func(left_data, right_data);
        if (result != 0) return result;
        left = ll_advance(left);
        right = ll_advance(right);
    }

    if (left != NULL) { return 1; }
    else if (right != NULL) { return -1; }
    else { return 0; }
}

void cx_linked_list_reverse(
        void **begin,
        void **end,
        ptrdiff_t loc_prev,
        ptrdiff_t loc_next
) {
    assert(begin != NULL);
    assert(loc_next >= 0);

    // swap all links
    void *prev = NULL;
    void *cur = *begin;
    while (cur != NULL) {
        void *next = ll_next(cur);

        ll_next(cur) = prev;
        if (loc_prev >= 0) {
            ll_prev(cur) = next;
        }

        prev = cur;
        cur = next;
    }

    // update begin and end
    if (end != NULL) {
        *end = *begin;
    }
    *begin = prev;
}

// HIGH LEVEL LINKED LIST IMPLEMENTATION

typedef struct cx_linked_list_node cx_linked_list_node;
struct cx_linked_list_node {
    cx_linked_list_node *prev;
    cx_linked_list_node *next;
    char payload[];
};

#define CX_LL_LOC_PREV offsetof(cx_linked_list_node, prev)
#define CX_LL_LOC_NEXT offsetof(cx_linked_list_node, next)
#define CX_LL_LOC_DATA offsetof(cx_linked_list_node, payload)

typedef struct {
    struct cx_list_s base;
    cx_linked_list_node *begin;
    cx_linked_list_node *end;
} cx_linked_list;

static cx_linked_list_node *cx_ll_node_at(
        const cx_linked_list *list,
        size_t index
) {
    if (index >= list->base.collection.size) {
        return NULL;
    } else if (index > list->base.collection.size / 2) {
        return cx_linked_list_at(list->end, list->base.collection.size - 1, CX_LL_LOC_PREV, index);
    } else {
        return cx_linked_list_at(list->begin, 0, CX_LL_LOC_NEXT, index);
    }
}

static cx_linked_list_node *cx_ll_malloc_node(const struct cx_list_s *list) {
    return cxMalloc(list->collection.allocator,
                    sizeof(cx_linked_list_node) + list->collection.elem_size);
}

static int cx_ll_insert_at(
        struct cx_list_s *list,
        cx_linked_list_node *node,
        const void *elem
) {

    // create the new new_node
    cx_linked_list_node *new_node = cx_ll_malloc_node(list);

    // sortir if failed
    if (new_node == NULL) return 1;

    // initialize new new_node
    new_node->prev = new_node->next = NULL;
    memcpy(new_node->payload, elem, list->collection.elem_size);

    // insert
    cx_linked_list *ll = (cx_linked_list *) list;
    cx_linked_list_insert_chain(
            (void **) &ll->begin, (void **) &ll->end,
            CX_LL_LOC_PREV, CX_LL_LOC_NEXT,
            node, new_node, new_node
    );

    // increase the size and return
    list->collection.size++;
    return 0;
}

static size_t cx_ll_insert_array(
        struct cx_list_s *list,
        size_t index,
        const void *array,
        size_t n
) {
    // out-of bounds and corner case check
    if (index > list->collection.size || n == 0) return 0;

    // find position efficiently
    cx_linked_list_node *node = index == 0 ? NULL : cx_ll_node_at((cx_linked_list *) list, index - 1);

    // perform first insert
    if (0 != cx_ll_insert_at(list, node, array)) {
        return 1;
    }

    // is there more?
    if (n == 1) return 1;

    // we now know exactly where we are
    node = node == NULL ? ((cx_linked_list *) list)->begin : node->next;

    // we can add the remaining nodes and immediately advance to the inserted node
    const char *source = array;
    for (size_t i = 1; i < n; i++) {
        source += list->collection.elem_size;
        if (0 != cx_ll_insert_at(list, node, source)) {
            return i;
        }
        node = node->next;
    }
    return n;
}

static int cx_ll_insert_element(
        struct cx_list_s *list,
        size_t index,
        const void *element
) {
    return 1 != cx_ll_insert_array(list, index, element, 1);
}

static _Thread_local cx_compare_func cx_ll_insert_sorted_cmp_func;

static int cx_ll_insert_sorted_cmp_helper(const void *l, const void *r) {
    const cx_linked_list_node *left = l;
    const cx_linked_list_node *right = r;
    return cx_ll_insert_sorted_cmp_func(left->payload, right->payload);
}

static size_t cx_ll_insert_sorted(
        struct cx_list_s *list,
        const void *array,
        size_t n
) {
    // special case
    if (n == 0) return 0;

    // create a new chain of nodes
    cx_linked_list_node *chain = cx_ll_malloc_node(list);
    if (chain == NULL) return 0;

    memcpy(chain->payload, array, list->collection.elem_size);
    chain->prev = NULL;
    chain->next = NULL;

    // add all elements from the array to that chain
    cx_linked_list_node *prev = chain;
    const char *src = array;
    size_t inserted = 1;
    for (; inserted < n; inserted++) {
        cx_linked_list_node *next = cx_ll_malloc_node(list);
        if (next == NULL) break;
        src += list->collection.elem_size;
        memcpy(next->payload, src, list->collection.elem_size);
        prev->next = next;
        next->prev = prev;
        prev = next;
    }
    prev->next = NULL;

    // invoke the low level function
    cx_linked_list *ll = (cx_linked_list *) list;
    cx_ll_insert_sorted_cmp_func = list->collection.cmpfunc;
    cx_linked_list_insert_sorted_chain(
            (void **) &ll->begin,
            (void **) &ll->end,
            CX_LL_LOC_PREV,
            CX_LL_LOC_NEXT,
            chain,
            cx_ll_insert_sorted_cmp_helper
    );

    // adjust the list metadata
    list->collection.size += inserted;

    return inserted;
}

static size_t cx_ll_remove(
        struct cx_list_s *list,
        size_t index,
        size_t num,
        void *targetbuf
) {
    cx_linked_list *ll = (cx_linked_list *) list;
    cx_linked_list_node *node = cx_ll_node_at(ll, index);

    // out-of-bounds check
    if (node == NULL) return 0;

    // remove
    size_t removed = cx_linked_list_remove_chain(
            (void **) &ll->begin,
            (void **) &ll->end,
            CX_LL_LOC_PREV,
            CX_LL_LOC_NEXT,
            node,
            num
    );

    // adjust size
    list->collection.size -= removed;

    // copy or destroy the removed chain
    if (targetbuf == NULL) {
        cx_linked_list_node *n = node;
        cx_for_n(i, removed) {
            // element destruction
            cx_invoke_destructor(list, n->payload);

            // free the node
            cxFree(list->collection.allocator, n);

            // next
            n = n->next;
        }
    } else {
        char *dest = targetbuf;
        cx_linked_list_node *n = node;
        cx_for_n(i, removed) {
            // copy payload
            memcpy(dest, n->payload, list->collection.elem_size);

            // advance target buffer
            dest += list->collection.elem_size;

            // free the node
            cxFree(list->collection.allocator, n);

            // next
            n = n->next;
        }
    }

    return removed;
}

static void cx_ll_clear(struct cx_list_s *list) {
    if (list->collection.size == 0) return;

    cx_linked_list *ll = (cx_linked_list *) list;
    cx_linked_list_node *node = ll->begin;
    while (node != NULL) {
        cx_invoke_destructor(list, node->payload);
        cx_linked_list_node *next = node->next;
        cxFree(list->collection.allocator, node);
        node = next;
    }
    ll->begin = ll->end = NULL;
    list->collection.size = 0;
}

#ifndef CX_LINKED_LIST_SWAP_SBO_SIZE
#define CX_LINKED_LIST_SWAP_SBO_SIZE 128
#endif
unsigned cx_linked_list_swap_sbo_size = CX_LINKED_LIST_SWAP_SBO_SIZE;

static int cx_ll_swap(
        struct cx_list_s *list,
        size_t i,
        size_t j
) {
    if (i >= list->collection.size || j >= list->collection.size) return 1;
    if (i == j) return 0;

    // perform an optimized search that finds both elements in one run
    cx_linked_list *ll = (cx_linked_list *) list;
    size_t mid = list->collection.size / 2;
    size_t left, right;
    if (i < j) {
        left = i;
        right = j;
    } else {
        left = j;
        right = i;
    }
    cx_linked_list_node *nleft, *nright;
    if (left < mid && right < mid) {
        // case 1: both items left from mid
        nleft = cx_ll_node_at(ll, left);
        assert(nleft != NULL);
        nright = nleft;
        for (size_t c = left; c < right; c++) {
            nright = nright->next;
        }
    } else if (left >= mid && right >= mid) {
        // case 2: both items right from mid
        nright = cx_ll_node_at(ll, right);
        assert(nright != NULL);
        nleft = nright;
        for (size_t c = right; c > left; c--) {
            nleft = nleft->prev;
        }
    } else {
        // case 3: one item left, one item right

        // chose the closest to begin / end
        size_t closest;
        size_t other;
        size_t diff2boundary = list->collection.size - right - 1;
        if (left <= diff2boundary) {
            closest = left;
            other = right;
            nleft = cx_ll_node_at(ll, left);
        } else {
            closest = right;
            other = left;
            diff2boundary = left;
            nright = cx_ll_node_at(ll, right);
        }

        // is other element closer to us or closer to boundary?
        if (right - left <= diff2boundary) {
            // search other element starting from already found element
            if (closest == left) {
                nright = nleft;
                for (size_t c = left; c < right; c++) {
                    nright = nright->next;
                }
            } else {
                nleft = nright;
                for (size_t c = right; c > left; c--) {
                    nleft = nleft->prev;
                }
            }
        } else {
            // search other element starting at the boundary
            if (closest == left) {
                nright = cx_ll_node_at(ll, other);
            } else {
                nleft = cx_ll_node_at(ll, other);
            }
        }
    }

    if (list->collection.elem_size > CX_LINKED_LIST_SWAP_SBO_SIZE) {
        cx_linked_list_node *prev = nleft->prev;
        cx_linked_list_node *next = nright->next;
        cx_linked_list_node *midstart = nleft->next;
        cx_linked_list_node *midend = nright->prev;

        if (prev == NULL) {
            ll->begin = nright;
        } else {
            prev->next = nright;
        }
        nright->prev = prev;
        if (midstart == nright) {
            // special case: both nodes are adjacent
            nright->next = nleft;
            nleft->prev = nright;
        } else {
            // likely case: a chain is between the two nodes
            nright->next = midstart;
            midstart->prev = nright;
            midend->next = nleft;
            nleft->prev = midend;
        }
        nleft->next = next;
        if (next == NULL) {
            ll->end = nleft;
        } else {
            next->prev = nleft;
        }
    } else {
        // swap payloads to avoid relinking
        char buf[CX_LINKED_LIST_SWAP_SBO_SIZE];
        memcpy(buf, nleft->payload, list->collection.elem_size);
        memcpy(nleft->payload, nright->payload, list->collection.elem_size);
        memcpy(nright->payload, buf, list->collection.elem_size);
    }

    return 0;
}

static void *cx_ll_at(
        const struct cx_list_s *list,
        size_t index
) {
    cx_linked_list *ll = (cx_linked_list *) list;
    cx_linked_list_node *node = cx_ll_node_at(ll, index);
    return node == NULL ? NULL : node->payload;
}

static ssize_t cx_ll_find_remove(
        struct cx_list_s *list,
        const void *elem,
        bool remove
) {
    if (remove) {
        cx_linked_list *ll = ((cx_linked_list *) list);
        cx_linked_list_node *node;
        ssize_t index = cx_linked_list_find_node(
                (void **) &node,
                ll->begin,
                CX_LL_LOC_NEXT, CX_LL_LOC_DATA,
                list->collection.cmpfunc, elem
        );
        if (node != NULL) {
            cx_invoke_destructor(list, node->payload);
            cx_linked_list_remove((void **) &ll->begin, (void **) &ll->end,
                                  CX_LL_LOC_PREV, CX_LL_LOC_NEXT, node);
            list->collection.size--;
            cxFree(list->collection.allocator, node);
        }
        return index;
    } else {
        return cx_linked_list_find(
                ((cx_linked_list *) list)->begin,
                CX_LL_LOC_NEXT, CX_LL_LOC_DATA,
                list->collection.cmpfunc, elem
        );
    }
}

static void cx_ll_sort(struct cx_list_s *list) {
    cx_linked_list *ll = (cx_linked_list *) list;
    cx_linked_list_sort((void **) &ll->begin, (void **) &ll->end,
                        CX_LL_LOC_PREV, CX_LL_LOC_NEXT, CX_LL_LOC_DATA,
                        list->collection.cmpfunc);
}

static void cx_ll_reverse(struct cx_list_s *list) {
    cx_linked_list *ll = (cx_linked_list *) list;
    cx_linked_list_reverse((void **) &ll->begin, (void **) &ll->end, CX_LL_LOC_PREV, CX_LL_LOC_NEXT);
}

static int cx_ll_compare(
        const struct cx_list_s *list,
        const struct cx_list_s *other
) {
    cx_linked_list *left = (cx_linked_list *) list;
    cx_linked_list *right = (cx_linked_list *) other;
    return cx_linked_list_compare(left->begin, right->begin,
                                  CX_LL_LOC_NEXT, CX_LL_LOC_DATA,
                                  list->collection.cmpfunc);
}

static bool cx_ll_iter_valid(const void *it) {
    const struct cx_iterator_s *iter = it;
    return iter->elem_handle != NULL;
}

static void cx_ll_iter_next(void *it) {
    struct cx_iterator_s *iter = it;
    if (iter->base.remove) {
        iter->base.remove = false;
        struct cx_list_s *list = iter->src_handle.m;
        cx_linked_list *ll = iter->src_handle.m;
        cx_linked_list_node *node = iter->elem_handle;
        iter->elem_handle = node->next;
        cx_invoke_destructor(list, node->payload);
        cx_linked_list_remove((void **) &ll->begin, (void **) &ll->end,
                              CX_LL_LOC_PREV, CX_LL_LOC_NEXT, node);
        list->collection.size--;
        cxFree(list->collection.allocator, node);
    } else {
        iter->index++;
        cx_linked_list_node *node = iter->elem_handle;
        iter->elem_handle = node->next;
    }
}

static void cx_ll_iter_prev(void *it) {
    struct cx_iterator_s *iter = it;
    if (iter->base.remove) {
        iter->base.remove = false;
        struct cx_list_s *list = iter->src_handle.m;
        cx_linked_list *ll = iter->src_handle.m;
        cx_linked_list_node *node = iter->elem_handle;
        iter->elem_handle = node->prev;
        iter->index--;
        cx_invoke_destructor(list, node->payload);
        cx_linked_list_remove((void **) &ll->begin, (void **) &ll->end,
                              CX_LL_LOC_PREV, CX_LL_LOC_NEXT, node);
        list->collection.size--;
        cxFree(list->collection.allocator, node);
    } else {
        iter->index--;
        cx_linked_list_node *node = iter->elem_handle;
        iter->elem_handle = node->prev;
    }
}

static void *cx_ll_iter_current(const void *it) {
    const struct cx_iterator_s *iter = it;
    cx_linked_list_node *node = iter->elem_handle;
    return node->payload;
}

static CxIterator cx_ll_iterator(
        const struct cx_list_s *list,
        size_t index,
        bool backwards
) {
    CxIterator iter;
    iter.index = index;
    iter.src_handle.c = list;
    iter.elem_handle = cx_ll_node_at((const cx_linked_list *) list, index);
    iter.elem_size = list->collection.elem_size;
    iter.elem_count = list->collection.size;
    iter.base.valid = cx_ll_iter_valid;
    iter.base.current = cx_ll_iter_current;
    iter.base.next = backwards ? cx_ll_iter_prev : cx_ll_iter_next;
    iter.base.mutating = false;
    iter.base.remove = false;
    return iter;
}

static int cx_ll_insert_iter(
        CxIterator *iter,
        const void *elem,
        int prepend
) {
    struct cx_list_s *list = iter->src_handle.m;
    cx_linked_list_node *node = iter->elem_handle;
    if (node != NULL) {
        assert(prepend >= 0 && prepend <= 1);
        cx_linked_list_node *choice[2] = {node, node->prev};
        int result = cx_ll_insert_at(list, choice[prepend], elem);
        if (result == 0) {
            iter->elem_count++;
            if (prepend) {
                iter->index++;
            }
        }
        return result;
    } else {
        int result = cx_ll_insert_element(list, list->collection.size, elem);
        if (result == 0) {
            iter->elem_count++;
            iter->index = list->collection.size;
        }
        return result;
    }
}

static void cx_ll_destructor(CxList *list) {
    cx_linked_list *ll = (cx_linked_list *) list;

    cx_linked_list_node *node = ll->begin;
    while (node) {
        cx_invoke_destructor(list, node->payload);
        void *next = node->next;
        cxFree(list->collection.allocator, node);
        node = next;
    }

    cxFree(list->collection.allocator, list);
}

static cx_list_class cx_linked_list_class = {
        cx_ll_destructor,
        cx_ll_insert_element,
        cx_ll_insert_array,
        cx_ll_insert_sorted,
        cx_ll_insert_iter,
        cx_ll_remove,
        cx_ll_clear,
        cx_ll_swap,
        cx_ll_at,
        cx_ll_find_remove,
        cx_ll_sort,
        cx_ll_compare,
        cx_ll_reverse,
        cx_ll_iterator,
};

CxList *cxLinkedListCreate(
        const CxAllocator *allocator,
        cx_compare_func comparator,
        size_t elem_size
) {
    if (allocator == NULL) {
        allocator = cxDefaultAllocator;
    }

    cx_linked_list *list = cxCalloc(allocator, 1, sizeof(cx_linked_list));
    if (list == NULL) return NULL;

    list->base.cl = &cx_linked_list_class;
    list->base.collection.allocator = allocator;

    if (elem_size > 0) {
        list->base.collection.elem_size = elem_size;
        list->base.collection.cmpfunc = comparator;
    } else {
        list->base.collection.cmpfunc = comparator == NULL ? cx_cmp_ptr : comparator;
        cxListStorePointers((CxList *) list);
    }

    return (CxList *) list;
}