putty/tree234.c

/*
 * tree234.c: reasonably generic 2-3-4 tree routines. Currently
 * supports insert, delete, find and iterate operations.
 */

#include <stdio.h>
#include <stdlib.h>

#include "tree234.h"

#define mknew(typ) ( (typ *) malloc (sizeof (typ)) )
#define sfree free

#ifdef TEST
#define LOG(x) (printf x)
#else
#define LOG(x)
#endif

struct tree234_Tag {
    node234 *root;
    cmpfn234 cmp;
};

struct node234_Tag {
    node234 *parent;
    node234 *kids[4];
    void *elems[3];
};

/*
 * Create a 2-3-4 tree.
 */
tree234 *newtree234(cmpfn234 cmp) {
    tree234 *ret = mknew(tree234);
    LOG(("created tree %p\n", ret));
    ret->root = NULL;
    ret->cmp = cmp;
    return ret;
}

/*
 * Free a 2-3-4 tree (not including freeing the elements).
 */
static void freenode234(node234 *n) {
    if (!n)
	return;
    freenode234(n->kids[0]);
    freenode234(n->kids[1]);
    freenode234(n->kids[2]);
    freenode234(n->kids[3]);
    sfree(n);
}
void freetree234(tree234 *t) {
    freenode234(t->root);
    sfree(t);
}

/*
 * Add an element e to a 2-3-4 tree t. Returns e on success, or if
 * an existing element compares equal, returns that.
 */
void *add234(tree234 *t, void *e) {
    node234 *n, **np, *left, *right;
    void *orig_e = e;
    int c;

    LOG(("adding node %p to tree %p\n", e, t));
    if (t->root == NULL) {
	t->root = mknew(node234);
	t->root->elems[1] = t->root->elems[2] = NULL;
	t->root->kids[0] = t->root->kids[1] = NULL;
	t->root->kids[2] = t->root->kids[3] = NULL;
	t->root->parent = NULL;
	t->root->elems[0] = e;
	LOG(("  created root %p\n", t->root));
	return orig_e;
    }

    np = &t->root;
    while (*np) {
	n = *np;
	LOG(("  node %p: %p [%p] %p [%p] %p [%p] %p\n",
	     n, n->kids[0], n->elems[0], n->kids[1], n->elems[1],
	     n->kids[2], n->elems[2], n->kids[3]));
	if ((c = t->cmp(e, n->elems[0])) < 0)
	    np = &n->kids[0];
	else if (c == 0)
	    return n->elems[0];	       /* already exists */
	else if (n->elems[1] == NULL || (c = t->cmp(e, n->elems[1])) < 0)
	    np = &n->kids[1];
	else if (c == 0)
	    return n->elems[1];	       /* already exists */
	else if (n->elems[2] == NULL || (c = t->cmp(e, n->elems[2])) < 0)
	    np = &n->kids[2];
	else if (c == 0)
	    return n->elems[2];	       /* already exists */
	else
	    np = &n->kids[3];
	LOG(("  moving to child %d (%p)\n", np - n->kids, *np));
    }

    /*
     * We need to insert the new element in n at position np.
     */
    left = NULL;
    right = NULL;
    while (n) {
	LOG(("  at %p: %p [%p] %p [%p] %p [%p] %p\n",
	     n, n->kids[0], n->elems[0], n->kids[1], n->elems[1],
	     n->kids[2], n->elems[2], n->kids[3]));
	LOG(("  need to insert %p [%p] %p at position %d\n",
	     left, e, right, np - n->kids));
	if (n->elems[1] == NULL) {
	    /*
	     * Insert in a 2-node; simple.
	     */
	    if (np == &n->kids[0]) {
		LOG(("  inserting on left of 2-node\n"));
		n->kids[2] = n->kids[1];
		n->elems[1] = n->elems[0];
		n->kids[1] = right;
		n->elems[0] = e;
		n->kids[0] = left;
	    } else { /* np == &n->kids[1] */
		LOG(("  inserting on right of 2-node\n"));
		n->kids[2] = right;
		n->elems[1] = e;
		n->kids[1] = left;
	    }
	    if (n->kids[0]) n->kids[0]->parent = n;
	    if (n->kids[1]) n->kids[1]->parent = n;
	    if (n->kids[2]) n->kids[2]->parent = n;
	    LOG(("  done\n"));
	    break;
	} else if (n->elems[2] == NULL) {
	    /*
	     * Insert in a 3-node; simple.
	     */
	    if (np == &n->kids[0]) {
		LOG(("  inserting on left of 3-node\n"));
		n->kids[3] = n->kids[2];
		n->elems[2] = n->elems[1];
		n->kids[2] = n->kids[1];
		n->elems[1] = n->elems[0];
		n->kids[1] = right;
		n->elems[0] = e;
		n->kids[0] = left;
	    } else if (np == &n->kids[1]) {
		LOG(("  inserting in middle of 3-node\n"));
		n->kids[3] = n->kids[2];
		n->elems[2] = n->elems[1];
		n->kids[2] = right;
		n->elems[1] = e;
		n->kids[1] = left;
	    } else { /* np == &n->kids[2] */
		LOG(("  inserting on right of 3-node\n"));
		n->kids[3] = right;
		n->elems[2] = e;
		n->kids[2] = left;
	    }
	    if (n->kids[0]) n->kids[0]->parent = n;
	    if (n->kids[1]) n->kids[1]->parent = n;
	    if (n->kids[2]) n->kids[2]->parent = n;
	    if (n->kids[3]) n->kids[3]->parent = n;
	    LOG(("  done\n"));
	    break;
	} else {
	    node234 *m = mknew(node234);
	    m->parent = n->parent;
	    LOG(("  splitting a 4-node; created new node %p\n", m));
	    /*
	     * Insert in a 4-node; split into a 2-node and a
	     * 3-node, and move focus up a level.
	     * 
	     * I don't think it matters which way round we put the
	     * 2 and the 3. For simplicity, we'll put the 3 first
	     * always.
	     */
	    if (np == &n->kids[0]) {
		m->kids[0] = left;
		m->elems[0] = e;
		m->kids[1] = right;
		m->elems[1] = n->elems[0];
		m->kids[2] = n->kids[1];
		e = n->elems[1];
		n->kids[0] = n->kids[2];
		n->elems[0] = n->elems[2];
		n->kids[1] = n->kids[3];
	    } else if (np == &n->kids[1]) {
		m->kids[0] = n->kids[0];
		m->elems[0] = n->elems[0];
		m->kids[1] = left;
		m->elems[1] = e;
		m->kids[2] = right;
		e = n->elems[1];
		n->kids[0] = n->kids[2];
		n->elems[0] = n->elems[2];
		n->kids[1] = n->kids[3];
	    } else if (np == &n->kids[2]) {
		m->kids[0] = n->kids[0];
		m->elems[0] = n->elems[0];
		m->kids[1] = n->kids[1];
		m->elems[1] = n->elems[1];
		m->kids[2] = left;
		/* e = e; */
		n->kids[0] = right;
		n->elems[0] = n->elems[2];
		n->kids[1] = n->kids[3];
	    } else { /* np == &n->kids[3] */
		m->kids[0] = n->kids[0];
		m->elems[0] = n->elems[0];
		m->kids[1] = n->kids[1];
		m->elems[1] = n->elems[1];
		m->kids[2] = n->kids[2];
		n->kids[0] = left;
		n->elems[0] = e;
		n->kids[1] = right;
		e = n->elems[2];
	    }
	    m->kids[3] = n->kids[3] = n->kids[2] = NULL;
	    m->elems[2] = n->elems[2] = n->elems[1] = NULL;
	    if (m->kids[0]) m->kids[0]->parent = m;
	    if (m->kids[1]) m->kids[1]->parent = m;
	    if (m->kids[2]) m->kids[2]->parent = m;
	    if (n->kids[0]) n->kids[0]->parent = n;
	    if (n->kids[1]) n->kids[1]->parent = n;
	    LOG(("  left (%p): %p [%p] %p [%p] %p\n", m,
		 m->kids[0], m->elems[0],
		 m->kids[1], m->elems[1],
		 m->kids[2]));
	    LOG(("  right (%p): %p [%p] %p\n", n,
		 n->kids[0], n->elems[0],
		 n->kids[1]));
	    left = m;
	    right = n;
	}
	if (n->parent)
	    np = (n->parent->kids[0] == n ? &n->parent->kids[0] :
		  n->parent->kids[1] == n ? &n->parent->kids[1] :
		  n->parent->kids[2] == n ? &n->parent->kids[2] :
		  &n->parent->kids[3]);
	n = n->parent;
    }

    /*
     * If we've come out of here by `break', n will still be
     * non-NULL and we've finished. If we've come here because n is
     * NULL, we need to create a new root for the tree because the
     * old one has just split into two.
     */
    if (!n) {
	LOG(("  root is overloaded, split into two\n"));
	t->root = mknew(node234);
	t->root->kids[0] = left;
	t->root->elems[0] = e;
	t->root->kids[1] = right;
	t->root->elems[1] = NULL;
	t->root->kids[2] = NULL;
	t->root->elems[2] = NULL;
	t->root->kids[3] = NULL;
	t->root->parent = NULL;
	if (t->root->kids[0]) t->root->kids[0]->parent = t->root;
	if (t->root->kids[1]) t->root->kids[1]->parent = t->root;
	LOG(("  new root is %p [%p] %p\n",
	     t->root->kids[0], t->root->elems[0], t->root->kids[1]));
    }

    return orig_e;
}

/*
 * Find an element e in a 2-3-4 tree t. Returns NULL if not found.
 * e is always passed as the first argument to cmp, so cmp can be
 * an asymmetric function if desired. cmp can also be passed as
 * NULL, in which case the compare function from the tree proper
 * will be used.
 */
void *find234(tree234 *t, void *e, cmpfn234 cmp) {
    node234 *n;
    int c;

    if (t->root == NULL)
	return NULL;

    if (cmp == NULL)
	cmp = t->cmp;

    n = t->root;
    while (n) {
	if ( (c = cmp(e, n->elems[0])) < 0)
	    n = n->kids[0];
	else if (c == 0)
	    return n->elems[0];
	else if (n->elems[1] == NULL || (c = cmp(e, n->elems[1])) < 0)
	    n = n->kids[1];
	else if (c == 0)
	    return n->elems[1];
	else if (n->elems[2] == NULL || (c = cmp(e, n->elems[2])) < 0)
	    n = n->kids[2];
	else if (c == 0)
	    return n->elems[2];
	else
	    n = n->kids[3];
    }

    /*
     * We've found our way to the bottom of the tree and we know
     * where we would insert this node if we wanted to. But it
     * isn't there.
     */
    return NULL;
}

/*
 * Delete an element e in a 2-3-4 tree. Does not free the element,
 * merely removes all links to it from the tree nodes.
 */
void del234(tree234 *t, void *e) {
    node234 *n;
    int ei = -1;

    n = t->root;
    LOG(("deleting %p from tree %p\n", e, t));
    while (1) {
	while (n) {
	    int c;
	    int ki;
	    node234 *sub;

	    LOG(("  node %p: %p [%p] %p [%p] %p [%p] %p\n",
		 n, n->kids[0], n->elems[0], n->kids[1], n->elems[1],
		 n->kids[2], n->elems[2], n->kids[3]));	
	    if ((c = t->cmp(e, n->elems[0])) < 0) {
		ki = 0;
	    } else if (c == 0) {
		ei = 0; break;
	    } else if (n->elems[1] == NULL || (c = t->cmp(e, n->elems[1])) < 0) {
		ki = 1;
	    } else if (c == 0) {
		ei = 1; break;
	    } else if (n->elems[2] == NULL || (c = t->cmp(e, n->elems[2])) < 0) {
		ki = 2;
	    } else if (c == 0) {
		ei = 2; break;
	    } else {
		ki = 3;
	    }
	    /*
	     * Recurse down to subtree ki. If it has only one element,
	     * we have to do some transformation to start with.
	     */
	    LOG(("  moving to subtree %d\n", ki));
	    sub = n->kids[ki];
	    if (!sub->elems[1]) {
		LOG(("  subtree has only one element!\n", ki));
		if (ki > 0 && n->kids[ki-1]->elems[1]) {
		    /*
		     * Case 3a, left-handed variant. Child ki has
		     * only one element, but child ki-1 has two or
		     * more. So we need to move a subtree from ki-1
		     * to ki.
		     * 
		     *                . C .                     . B .
		     *               /     \     ->            /     \
		     * [more] a A b B c   d D e      [more] a A b   c C d D e
		     */
		    node234 *sib = n->kids[ki-1];
		    int lastelem = (sib->elems[2] ? 2 :
				    sib->elems[1] ? 1 : 0);
		    sub->kids[2] = sub->kids[1];
		    sub->elems[1] = sub->elems[0];
		    sub->kids[1] = sub->kids[0];
		    sub->elems[0] = n->elems[ki-1];
		    sub->kids[0] = sib->kids[lastelem+1];
		    if (sub->kids[0]) sub->kids[0]->parent = sub;
		    n->elems[ki-1] = sib->elems[lastelem];
		    sib->kids[lastelem+1] = NULL;
		    sib->elems[lastelem] = NULL;
		    LOG(("  case 3a left\n"));
		} else if (ki < 3 && n->kids[ki+1] &&
			   n->kids[ki+1]->elems[1]) {
		    /*
		     * Case 3a, right-handed variant. ki has only
		     * one element but ki+1 has two or more. Move a
		     * subtree from ki+1 to ki.
		     * 
		     *      . B .                             . C .
		     *     /     \                ->         /     \
		     *  a A b   c C d D e [more]      a A b B c   d D e [more]
		     */
		    node234 *sib = n->kids[ki+1];
		    int j;
		    sub->elems[1] = n->elems[ki];
		    sub->kids[2] = sib->kids[0];
		    if (sub->kids[2]) sub->kids[2]->parent = sub;
		    n->elems[ki] = sib->elems[0];
		    sib->kids[0] = sib->kids[1];
		    for (j = 0; j < 2 && sib->elems[j+1]; j++) {
			sib->kids[j+1] = sib->kids[j+2];
			sib->elems[j] = sib->elems[j+1];
		    }
		    sib->kids[j+1] = NULL;
		    sib->elems[j] = NULL;
		    LOG(("  case 3a right\n"));
		} else {
		    /*
		     * Case 3b. ki has only one element, and has no
		     * neighbour with more than one. So pick a
		     * neighbour and merge it with ki, taking an
		     * element down from n to go in the middle.
		     *
		     *      . B .                .
		     *     /     \     ->        |
		     *  a A b   c C d      a A b B c C d
		     * 
		     * (Since at all points we have avoided
		     * descending to a node with only one element,
		     * we can be sure that n is not reduced to
		     * nothingness by this move, _unless_ it was
		     * the very first node, ie the root of the
		     * tree. In that case we remove the now-empty
		     * root and replace it with its single large
		     * child as shown.)
		     */
		    node234 *sib;
		    int j;

		    if (ki > 0)
			ki--;
		    sib = n->kids[ki];
		    sub = n->kids[ki+1];

		    sub->kids[3] = sub->kids[1];
		    sub->elems[2] = sub->elems[0];
		    sub->kids[2] = sub->kids[0];
		    sub->elems[1] = n->elems[ki];
		    sub->kids[1] = sib->kids[1];
		    if (sub->kids[1]) sub->kids[1]->parent = sub;
		    sub->elems[0] = sib->elems[0];
		    sub->kids[0] = sib->kids[0];
		    if (sub->kids[0]) sub->kids[0]->parent = sub;

		    sfree(sib);

		    /*
		     * That's built the big node in sub. Now we
		     * need to remove the reference to sib in n.
		     */
		    for (j = ki; j < 3 && n->kids[j+1]; j++) {
			n->kids[j] = n->kids[j+1];
			n->elems[j] = j<2 ? n->elems[j+1] : NULL;
		    }
		    n->kids[j] = NULL;
		    if (j < 3) n->elems[j] = NULL;
		    LOG(("  case 3b ki=%d\n", ki));

		    if (!n->elems[0]) {
			/*
			 * The root is empty and needs to be
			 * removed.
			 */
			LOG(("  shifting root!\n"));
			t->root = sub;
			sub->parent = NULL;
			sfree(n);
		    }
		}
	    }
	    n = sub;
	}
	if (ei==-1)
	    return;		       /* nothing to do; `already removed' */

	/*
	 * Treat special case: this is the one remaining item in
	 * the tree. n is the tree root (no parent), has one
	 * element (no elems[1]), and has no kids (no kids[0]).
	 */
	if (!n->parent && !n->elems[1] && !n->kids[0]) {
	    LOG(("  removed last element in tree\n"));
	    sfree(n);
	    t->root = NULL;
	    return;
	}

	/*
	 * Now we have the element we want, as n->elems[ei], and we
	 * have also arranged for that element not to be the only
	 * one in its node. So...
	 */

	if (!n->kids[0] && n->elems[1]) {
	    /*
	     * Case 1. n is a leaf node with more than one element,
	     * so it's _really easy_. Just delete the thing and
	     * we're done.
	     */
	    int i;
	    LOG(("  case 1\n"));
	    for (i = ei; i < 2 && n->elems[i+1]; i++)
		n->elems[i] = n->elems[i+1];
	    n->elems[i] = NULL;
	    return;		       /* finished! */
	} else if (n->kids[ei]->elems[1]) {
	    /*
	     * Case 2a. n is an internal node, and the root of the
	     * subtree to the left of e has more than one element.
	     * So find the predecessor p to e (ie the largest node
	     * in that subtree), place it where e currently is, and
	     * then start the deletion process over again on the
	     * subtree with p as target.
	     */
	    node234 *m = n->kids[ei];
	    void *target;
	    LOG(("  case 2a\n"));
	    while (m->kids[0]) {
		m = (m->kids[3] ? m->kids[3] :
		     m->kids[2] ? m->kids[2] :
		     m->kids[1] ? m->kids[1] : m->kids[0]);		     
	    }
	    target = (m->elems[2] ? m->elems[2] :
		      m->elems[1] ? m->elems[1] : m->elems[0]);
	    n->elems[ei] = target;
	    n = n->kids[ei];
	    e = target;
	} else if (n->kids[ei+1]->elems[1]) {
	    /*
	     * Case 2b, symmetric to 2a but s/left/right/ and
	     * s/predecessor/successor/. (And s/largest/smallest/).
	     */
	    node234 *m = n->kids[ei+1];
	    void *target;
	    LOG(("  case 2b\n"));
	    while (m->kids[0]) {
		m = m->kids[0];
	    }
	    target = m->elems[0];
	    n->elems[ei] = target;
	    n = n->kids[ei+1];
	    e = target;
	} else {
	    /*
	     * Case 2c. n is an internal node, and the subtrees to
	     * the left and right of e both have only one element.
	     * So combine the two subnodes into a single big node
	     * with their own elements on the left and right and e
	     * in the middle, then restart the deletion process on
	     * that subtree, with e still as target.
	     */
	    node234 *a = n->kids[ei], *b = n->kids[ei+1];
	    int j;

	    LOG(("  case 2c\n"));
	    a->elems[1] = n->elems[ei];
	    a->kids[2] = b->kids[0];
	    if (a->kids[2]) a->kids[2]->parent = a;
	    a->elems[2] = b->elems[0];
	    a->kids[3] = b->kids[1];
	    if (a->kids[3]) a->kids[3]->parent = a;
	    sfree(b);
	    /*
	     * That's built the big node in a, and destroyed b. Now
	     * remove the reference to b (and e) in n.
	     */
	    for (j = ei; j < 2 && n->elems[j+1]; j++) {
		n->elems[j] = n->elems[j+1];
		n->kids[j+1] = n->kids[j+2];
	    }
	    n->elems[j] = NULL;
	    n->kids[j+1] = NULL;
            /*
             * It's possible, in this case, that we've just removed
             * the only element in the root of the tree. If so,
             * shift the root.
             */
            if (n->elems[0] == NULL) {
                LOG(("  shifting root!\n"));
                t->root = a;
                a->parent = NULL;
                sfree(n);
            }
	    /*
	     * Now go round the deletion process again, with n
	     * pointing at the new big node and e still the same.
	     */
	    n = a;
	}
    }
}

/*
 * Iterate over the elements of a tree234, in order.
 */
void *first234(tree234 *t, enum234 *e) {
    node234 *n = t->root;
    if (!n)
	return NULL;
    while (n->kids[0])
	n = n->kids[0];
    e->node = n;
    e->posn = 0;
    return n->elems[0];
}

void *next234(enum234 *e) {
    node234 *n = e->node;
    int pos = e->posn;

    if (n->kids[pos+1]) {
	n = n->kids[pos+1];
	while (n->kids[0])
	    n = n->kids[0];
	e->node = n;
	e->posn = 0;
	return n->elems[0];
    }

    if (pos < 2 && n->elems[pos+1]) {
	e->posn = pos+1;
	return n->elems[e->posn];
    }

    do {
	node234 *nn = n->parent;
	if (nn == NULL)
	    return NULL;	       /* end of tree */
	pos = (nn->kids[0] == n ? 0 :
	       nn->kids[1] == n ? 1 :
	       nn->kids[2] == n ? 2 : 3);
	n = nn;
    } while (pos == 3 || n->kids[pos+1] == NULL);

    e->node = n;
    e->posn = pos;
    return n->elems[pos];
}

#ifdef TEST

/*
 * Test code for the 2-3-4 tree. This code maintains an alternative
 * representation of the data in the tree, in an array (using the
 * obvious and slow insert and delete functions). After each tree
 * operation, the verify() function is called, which ensures all
 * the tree properties are preserved (node->child->parent always
 * equals node; number of kids == 0 or number of elements + 1;
 * ordering property between elements of a node and elements of its
 * children is preserved; tree has the same depth everywhere; every
 * node has at least one element) and also ensures the list
 * represented by the tree is the same list it should be. (This
 * last check also verifies the ordering properties, because the
 * `same list it should be' is by definition correctly ordered. It
 * also ensures all nodes are distinct, because the enum functions
 * would get caught in a loop if not.)
 */

#include <stdarg.h>

/*
 * Error reporting function.
 */
void error(char *fmt, ...) {
    va_list ap;
    printf("ERROR: ");
    va_start(ap, fmt);
    vfprintf(stdout, fmt, ap);
    va_end(ap);
    printf("\n");
}

/* The array representation of the data. */
void **array;
int arraylen, arraysize;
cmpfn234 cmp;

/* The tree representation of the same data. */
tree234 *tree;

typedef struct {
    int treedepth;
    int elemcount;
} chkctx;

void chknode(chkctx *ctx, int level, node234 *node,
             void *lowbound, void *highbound) {
    int nkids, nelems;
    int i;

    /* Count the non-NULL kids. */
    for (nkids = 0; nkids < 4 && node->kids[nkids]; nkids++);
    /* Ensure no kids beyond the first NULL are non-NULL. */
    for (i = nkids; i < 4; i++)
        if (node->kids[i]) {
            error("node %p: nkids=%d but kids[%d] non-NULL",
                   node, nkids, i);
        }

    /* Count the non-NULL elements. */
    for (nelems = 0; nelems < 3 && node->elems[nelems]; nelems++);
    /* Ensure no elements beyond the first NULL are non-NULL. */
    for (i = nelems; i < 3; i++)
        if (node->elems[i]) {
            error("node %p: nelems=%d but elems[%d] non-NULL",
                   node, nelems, i);
        }

    if (nkids == 0) {
        /*
         * If nkids==0, this is a leaf node; verify that the tree
         * depth is the same everywhere.
         */
        if (ctx->treedepth < 0)
            ctx->treedepth = level;    /* we didn't know the depth yet */
        else if (ctx->treedepth != level)
            error("node %p: leaf at depth %d, previously seen depth %d",
                   node, level, ctx->treedepth);
    } else {
        /*
         * If nkids != 0, then it should be nelems+1, unless nelems
         * is 0 in which case nkids should also be 0 (and so we
         * shouldn't be in this condition at all).
         */
        int shouldkids = (nelems ? nelems+1 : 0);
        if (nkids != shouldkids) {
            error("node %p: %d elems should mean %d kids but has %d",
                   node, nelems, shouldkids, nkids);
        }
    }

    /*
     * nelems should be at least 1.
     */
    if (nelems == 0) {
        error("node %p: no elems", node, nkids);
    }

    /*
     * Add nelems to the running element count of the whole tree
     * (to ensure the enum234 routines see them all).
     */
    ctx->elemcount += nelems;

    /*
     * Check ordering property: all elements should be strictly >
     * lowbound, strictly < highbound, and strictly < each other in
     * sequence. (lowbound and highbound are NULL at edges of tree
     * - both NULL at root node - and NULL is considered to be <
     * everything and > everything. IYSWIM.)
     */
    for (i = -1; i < nelems; i++) {
        void *lower = (i == -1 ? lowbound : node->elems[i]);
        void *higher = (i+1 == nelems ? highbound : node->elems[i+1]);
        if (lower && higher && cmp(lower, higher) >= 0) {
            error("node %p: kid comparison [%d=%s,%d=%s] failed",
                   node, i, lower, i+1, higher);
        }
    }

    /*
     * Check parent pointers: all non-NULL kids should have a
     * parent pointer coming back to this node.
     */
    for (i = 0; i < nkids; i++)
        if (node->kids[i]->parent != node) {
            error("node %p kid %d: parent ptr is %p not %p",
                   node, i, node->kids[i]->parent, node);
        }


    /*
     * Now (finally!) recurse into subtrees.
     */
    for (i = 0; i < nkids; i++) {
        void *lower = (i == 0 ? lowbound : node->elems[i-1]);
        void *higher = (i >= nelems ? highbound : node->elems[i]);
        chknode(ctx, level+1, node->kids[i], lower, higher);
    }
}

void verify(void) {
    chkctx ctx;
    enum234 e;
    int i;
    void *p;

    ctx.treedepth = -1;                /* depth unknown yet */
    ctx.elemcount = 0;                 /* no elements seen yet */
    /*
     * Verify validity of tree properties.
     */
    if (tree->root)
        chknode(&ctx, 0, tree->root, NULL, NULL);
    printf("tree depth: %d\n", ctx.treedepth);
    /*
     * Enumerate the tree and ensure it matches up to the array.
     */
    for (i = 0, p = first234(tree, &e);
         p;
         i++, p = next234(&e)) {
        if (i >= arraylen)
            error("tree contains more than %d elements", arraylen);
        if (array[i] != p)
            error("enum at position %d: array says %s, tree says %s",
                   i, array[i], p);
    }
    if (i != ctx.elemcount) {
        error("tree really contains %d elements, enum gave %d",
               i, ctx.elemcount);
    }
    if (i < arraylen) {
        error("enum gave only %d elements, array has %d", i, arraylen);
    }
}

void addtest(void *elem) {
    int i, j;
    void *retval, *realret;

    if (arraysize < arraylen+1) {
        arraysize = arraylen+1+256;
        array = (array == NULL ? malloc(arraysize*sizeof(*array)) :
                 realloc(array, arraysize*sizeof(*array)));
    }

    i = 0;
    while (i < arraylen && cmp(elem, array[i]) > 0)
        i++;
    /* now i points to the first element >= elem */
    if (i < arraylen && !cmp(elem, array[i]))
        retval = array[i];             /* expect that returned not elem */
    else {
        retval = elem;                  /* expect elem returned (success) */
        for (j = arraylen; j > i; j--)
            array[j] = array[j-1];
        array[i] = elem;                /* add elem to array */
        arraylen++;
    }

    realret = add234(tree, elem);
    if (realret != retval) {
        error("add: retval was %p expected %p", realret, retval);
    }

    verify();
}

void deltest(void *elem) {
    int i;

    i = 0;
    while (i < arraylen && cmp(elem, array[i]) > 0)
        i++;
    /* now i points to the first element >= elem */
    if (i >= arraylen || cmp(elem, array[i]) != 0)
        return;                        /* don't do it! */
    else {
        while (i < arraylen-1) {
            array[i] = array[i+1];
            i++;
        }
        arraylen--;                    /* delete elem from array */
    }

    del234(tree, elem);

    verify();
}

/* A sample data set and test utility. Designed for pseudo-randomness,
 * and yet repeatability. */

/*
 * This random number generator uses the `portable implementation'
 * given in ANSI C99 draft N869. It assumes `unsigned' is 32 bits;
 * change it if not.
 */
int randomnumber(unsigned *seed) {
    *seed *= 1103515245;
    *seed += 12345;
    return ((*seed) / 65536) % 32768;
}

int mycmp(void *av, void *bv) {
    char const *a = (char const *)av;
    char const *b = (char const *)bv;
    return strcmp(a, b);
}

#define lenof(x) ( sizeof((x)) / sizeof(*(x)) )

char *strings[] = {
    "a", "ab", "absque", "coram", "de",
    "palam", "clam", "cum", "ex", "e",
    "sine", "tenus", "pro", "prae",
    "banana", "carrot", "cabbage", "broccoli", "onion", "zebra",
    "penguin", "blancmange", "pangolin", "whale", "hedgehog",
    "giraffe", "peanut", "bungee", "foo", "bar", "baz", "quux",
    "murfl", "spoo", "breen", "flarn", "octothorpe",
    "snail", "tiger", "elephant", "octopus", "warthog", "armadillo",
    "aardvark", "wyvern", "dragon", "elf", "dwarf", "orc", "goblin",
    "pixie", "basilisk", "warg", "ape", "lizard", "newt", "shopkeeper",
    "wand", "ring", "amulet"
};

#define NSTR lenof(strings)

int main(void) {
    int in[NSTR];
    int i, j;
    unsigned seed = 0;

    for (i = 0; i < NSTR; i++) in[i] = 0;
    array = NULL;
    arraylen = arraysize = 0;
    tree = newtree234(mycmp);
    cmp = mycmp;

    verify();
    for (i = 0; i < 10000; i++) {
        j = randomnumber(&seed);
        j %= NSTR;
        printf("trial: %d\n", i);
        if (in[j]) {
            printf("deleting %s (%d)\n", strings[j], j);
            deltest(strings[j]);
            in[j] = 0;
        } else {
            printf("adding %s (%d)\n", strings[j], j);
            addtest(strings[j]);
            in[j] = 1;
        }
    }

    while (arraylen > 0) {
        j = randomnumber(&seed);
        j %= arraylen;
        deltest(array[j]);
    }

    return 0;
}

#endif