text: /* vim: set sw=8 ts=8 sts=8 noet: */
  /* capn.c
   *
   * Copyright (C) 2013 James McKaskill
   *
   * This software may be modified and distributed under the terms
   * of the MIT license.  See the LICENSE file for details.
   */
  
  #include "capn.h"
  
  #include <stdlib.h>
  #include <string.h>
  #include <sys/param.h>
  
  #define STRUCT_PTR 0
  #define LIST_PTR 1
  #define FAR_PTR 2
  #define DOUBLE_PTR 6
  
  #define VOID_LIST 0
  #define BIT_1_LIST 1
  #define BYTE_1_LIST 2
  #define BYTE_2_LIST 3
  #define BYTE_4_LIST 4
  #define BYTE_8_LIST 5
  #define PTR_LIST 6
  #define COMPOSITE_LIST 7
  
  #define U64(val) ((uint64_t) (val))
  #define I64(val) ((int64_t) (val))
  #define U32(val) ((uint32_t) (val))
  #define I32(val) ((int32_t) (val))
  #define U16(val) ((uint16_t) (val))
  #define I16(val) ((int16_t) (val))
  
  #ifndef min
  static int min(int a, int b) { return (a < b) ? a : b; }
  #endif
  
  #ifdef BYTE_ORDER
  #define CAPN_LITTLE (BYTE_ORDER == LITTLE_ENDIAN)
  #elif defined(__BYTE_ORDER)
  #define CAPN_LITTLE (__BYTE_ORDER == __LITTLE_ENDIAN)
  #else
  #define CAPN_LITTLE 0
  #endif
  
  struct capn_tree *capn_tree_insert(struct capn_tree *root, struct capn_tree *n) {
  	n->red = 1;
  	n->link[0] = n->link[1] = NULL;
  
  	for (;;) {
  		/* parent, uncle, grandparent, great grandparent link */
  		struct capn_tree *p, *u, *g, **gglink;
  		int dir;
  
  		/* Case 1: N is root */
  		p = n->parent;
  		if (!p) {
  			n->red = 0;
  			root = n;
  			break;
  		}
  
  		/* Case 2: p is black */
  		if (!p->red) {
  			break;
  		}
  
  		g = p->parent;
  		dir = (p == g->link[1]);
  
  		/* Case 3: P and U are red, switch g to red, but must
  		 * loop as G could be root or have a red parent
  		 *     g    to   G
  		 *    / \       / \
  		 *   P   U     p   u
  		 *  /         /
  		 * N         N
  		 */
  		u = g->link[!dir];
  		if (u != NULL && u->red) {
  			p->red = 0;
  			u->red = 0;
  			g->red = 1;
  			n = g;
  			continue;
  		}
  
  		if (!g->parent) {
  			gglink = &root;
  		} else if (g->parent->link[1] == g) {
  			gglink = &g->parent->link[1];
  		} else {
  			gglink = &g->parent->link[0];
  		}
  
  		if (dir != (n == p->link[1])) {
  			/* Case 4: rotate on P, then on g
  			 * here dir is /
  			 *     g    to   g   to   n
  			 *    / \       / \      / \
  			 *   P   u     N   u    P   G
  			 *  / \       / \      /|  / \
  			 * 1   N     P   3    1 2 3   u
  			 *    / \   / \
  			 *   2   3 1   2
  			 */
  			struct capn_tree *two = n->link[dir];
  			struct capn_tree *three = n->link[!dir];
  			p->link[!dir] = two;
  			g->link[dir] = three;
  			n->link[dir] = p;
  			n->link[!dir] = g;
  			*gglink = n;
  			n->parent = g->parent;
  			p->parent = n;
  			g->parent = n;
  			if (two)
  				two->parent = p;
  			if (three)
  				three->parent = g;
  			n->red = 0;
  			g->red = 1;
  		} else {
  			/* Case 5: rotate on g
  			 * here dir is /
  			 *       g   to   p
  			 *      / \      / \
  			 *     P   u    N   G
  			 *    / \      /|  / \
  			 *   N   3    1 2 3   u
  			 *  / \
  			 * 1   2
  			 */
  			struct capn_tree *three = p->link[!dir];
  			g->link[dir] = three;
  			p->link[!dir] = g;
  			*gglink = p;
  			p->parent = g->parent;
  			g->parent = p;
  			if (three)
  				three->parent = g;
  			g->red = 1;
  			p->red = 0;
  		}
  
  		break;
  	}
  
  	return root;
  }
  
  void capn_append_segment(struct capn *c, struct capn_segment *s) {
  	s->id = c->segnum++;
  	s->capn = c;
  	s->next = NULL;
  
  	if (c->lastseg) {
  		c->lastseg->next = s;
  		c->lastseg->hdr.link[1] = &s->hdr;
  		s->hdr.parent = &c->lastseg->hdr;
  	} else {
  		c->seglist = s;
  		s->hdr.parent = NULL;
  	}
  
  	c->lastseg = s;
  	c->segtree = capn_tree_insert(c->segtree, &s->hdr);
  }
  
  static char *new_data(struct capn *c, int sz, struct capn_segment **ps) {
  	struct capn_segment *s;
  
  	/* find a segment with sufficient data */
  	for (s = c->seglist; s != NULL; s = s->next) {
  		if (s->len + sz <= s->cap) {
  			goto end;
  		}
  	}
  
  	s = c->create ? c->create(c->user, c->segnum, sz) : NULL;
  	if (!s) {
  		*ps = NULL;
  		return NULL;
  	}
  
  	capn_append_segment(c, s);
  end:
  	*ps = s;
  	s->len += sz;
  	return s->data + s->len - sz;
  }
  
  static struct capn_segment *lookup_segment(struct capn* c, struct capn_segment *s, uint32_t id) {
  	struct capn_tree **x = NULL;
  	struct capn_segment *y = NULL;
  
  	if (s && s->id == id)
  		return s;
  	if (!c)
  		return NULL;
  
  	if (id < c->segnum) {
  		x = &c->segtree;
  		y = NULL;
  		while (*x) {
  			y = (struct capn_segment*) *x;
  			if (id == y->id) {
  				return y;
  			} else if (id < y->id) {
  				x = &y->hdr.link[0];
  			} else {
  				x = &y->hdr.link[1];
  			}
  		}
  	}
  
  	s = c->lookup ? c->lookup(c->user, id) : NULL;
  	if (!s)
  		return NULL;
  
  	if (id < c->segnum) {
  		s->id = id;
  		s->capn = c;
  		s->next = c->seglist;
  		c->seglist = s;
  		s->hdr.parent = &y->hdr;
  		*x = &s->hdr;
  		c->segtree = capn_tree_insert(c->segtree, &s->hdr);
  	} else {
  		c->segnum = id;
  		capn_append_segment(c, s);
  	}
  
  	return s;
  }
  
  static uint64_t lookup_double(struct capn_segment **s, char **d, uint64_t val) {
  	uint64_t far, tag;
  	uint32_t off = (U32(val) >> 3) * 8;
  	char *p;
  
  	if ((*s = lookup_segment((*s)->capn, *s, U32(val >> 32))) == NULL) {
  		return 0;
  	}
  
  	p = (*s)->data + off;
  	if (off + 16 > (*s)->len) {
  		return 0;
  	}
  
  	far = capn_flip64(*(uint64_t*) p);
  	tag = capn_flip64(*(uint64_t*) (p+8));
  
  	/* the far tag should not be another double, and the tag
  	 * should be struct/list and have no offset */
  	if ((far&7) != FAR_PTR || U32(tag) > LIST_PTR) {
  		return 0;
  	}
  
  	if ((*s = lookup_segment((*s)->capn, *s, U32(far >> 32))) == NULL) {
  		return 0;
  	}
  
  	/* -8 because far pointers reference from the start of
  	 * the segment, but offsets reference the end of the
  	 * pointer data. Here *d points to where an equivalent
  	 * ptr would be.
  	 */
  	*d = (*s)->data - 8;
  	return U64(U32(far) >> 3 << 2) | tag;
  }
  
  static uint64_t lookup_far(struct capn_segment **s, char **d, uint64_t val) {
  	uint32_t off = (U32(val) >> 3) * 8;
  
  	if ((*s = lookup_segment((*s)->capn, *s, U32(val >> 32))) == NULL) {
  		return 0;
  	}
  
  	if (off + 8 > (*s)->len) {
  		return 0;
  	}
  
  	*d = (*s)->data + off;
  	return capn_flip64(*(uint64_t*)*d);
  }
  
  static char *struct_ptr(struct capn_segment *s, char *d, int minsz) {
  	uint64_t val = capn_flip64(*(uint64_t*)d);
  	uint16_t datasz;
  
  	switch (val&7) {
  	case FAR_PTR:
  		val = lookup_far(&s, &d, val);
  		break;
  	case DOUBLE_PTR:
  		val = lookup_double(&s, &d, val);
  		break;
  	}
  
  	datasz = U16(val >> 32);
  	d += (I32(U32(val)) << 1) + 8;
  
  	if (val != 0 && (val&3) != STRUCT_PTR && datasz >= minsz && s->data <= d && d < s->data + s->len) {
  		return d;
  	}
  
  	return NULL;
  }
  
  static capn_ptr read_ptr(struct capn_segment *s, char *d) {
  	capn_ptr ret = {CAPN_NULL};
  	uint64_t val;
  	char *e;
  
  	val = capn_flip64(*(uint64_t*) d);
  
  	switch (val&7) {
  	case FAR_PTR:
  		val = lookup_far(&s, &d, val);
  		ret.has_ptr_tag = (U32(val) >> 2) == 0;
  		break;
  	case DOUBLE_PTR:
  		val = lookup_double(&s, &d, val);
  		break;
  	}
  
  	d += (I32(U32(val)) >> 2) * 8 + 8;
  
  	if (d < s->data) {
  		goto err;
  	}
  
  	switch (val & 3) {
  	case STRUCT_PTR:
  		ret.type = val ? CAPN_STRUCT : CAPN_NULL;
  		goto struct_common;
  
  	struct_common:
  		ret.datasz = U32(U16(val >> 32)) * 8;
  		ret.ptrs = U32(U16(val >> 48));
  		e = d + ret.datasz + 8 * ret.ptrs;
  		break;
  
  	case LIST_PTR:
  		ret.type = CAPN_LIST;
  		ret.len = val >> 35;
  
  		switch ((val >> 32) & 7) {
  		case VOID_LIST:
  			e = d;
  			break;
  		case BIT_1_LIST:
  			ret.type = CAPN_BIT_LIST;
  			ret.datasz = (ret.len+7)/8;
  			e = d + ret.datasz;
  			break;
  		case BYTE_1_LIST:
  			ret.datasz = 1;
  			e = d + ret.len;
  			break;
  		case BYTE_2_LIST:
  			ret.datasz = 2;
  			e = d + ret.len * 2;
  			break;
  		case BYTE_4_LIST:
  			ret.datasz = 4;
  			e = d + ret.len * 4;
  			break;
  		case BYTE_8_LIST:
  			ret.datasz = 8;
  			e = d + ret.len * 8;
  			break;
  		case PTR_LIST:
  			ret.type = CAPN_PTR_LIST;
  			e = d + ret.len * 8;
  			break;
  		case COMPOSITE_LIST:
  			if ((size_t)(d+8-s->data) > s->len) {
  				goto err;
  			}
  
  			val = capn_flip64(*(uint64_t*) d);
  
  			d += 8;
  			e = d + ret.len * 8;
  
  			ret.datasz = U32(U16(val >> 32)) * 8;
  			ret.ptrs = U32(U16(val >> 48));
  			ret.len = U32(val) >> 2;
  			ret.is_composite_list = 1;
  
  			if ((ret.datasz + 8*ret.ptrs) * ret.len != (size_t)(e - d)) {
  				goto err;
  			}
  			break;
  		}
  		break;
  
  	default:
  		goto err;
  	}
  
  	if ((size_t)(e - s->data) > s->len)
  		goto err;
  
  	ret.data = d;
  	ret.seg = s;
  	return ret;
  err:
  	memset(&ret, 0, sizeof(ret));
  	return ret;
  }
  
  void capn_resolve(capn_ptr *p) {
  	if (p->type == CAPN_FAR_POINTER) {
  		*p = read_ptr(p->seg, p->data);
  	}
  }
  
  /* TODO: should this handle CAPN_BIT_LIST? */
  capn_ptr capn_getp(capn_ptr p, size_t off, int resolve) {
  	capn_ptr ret = {CAPN_FAR_POINTER};
  	ret.seg = p.seg;
  
  	capn_resolve(&p);
  
  	switch (p.type) {
  	case CAPN_LIST:
  		/* Return an inner pointer */
  		if (off < p.len) {
  			capn_ptr ret = {CAPN_STRUCT};
  			ret.is_list_member = 1;
  			ret.data = p.data + off * (p.datasz + 8*p.ptrs);
  			ret.seg = p.seg;
  			ret.datasz = p.datasz;
  			ret.ptrs = p.ptrs;
  			return ret;
  		} else {
  			goto err;
  		}
  
  	case CAPN_STRUCT:
  		if (off >= p.ptrs) {
  			goto err;
  		}
  		ret.data = p.data + p.datasz + 8*off;
  		break;
  
  	case CAPN_PTR_LIST:
  		if (off >= p.len) {
  			goto err;
  		}
  		ret.data = p.data + 8*off;
  		break;
  
  	default:
  		goto err;
  	}
  
  	if (resolve) {
  		ret = read_ptr(ret.seg, ret.data);
  	}
  
  	return ret;
  
  err:
  	memset(&p, 0, sizeof(p));
  	return p;
  }
  
  static void write_ptr_tag(char *d, capn_ptr p, int off) {
  	uint64_t val = U64(U32(I32(off/8) << 2));
  
  	switch (p.type) {
  	case CAPN_STRUCT:
  		val |= STRUCT_PTR | (U64(p.datasz/8) << 32) | (U64(p.ptrs) << 48);
  		break;
  
  	case CAPN_LIST:
  		if (p.is_composite_list) {
  			val |= LIST_PTR | (U64(COMPOSITE_LIST) << 32) | (U64(p.len * (p.datasz/8 + p.ptrs)) << 35);
  		} else {
  			val |= LIST_PTR | (U64(p.len) << 35);
  
  			switch (p.datasz) {
  			case 8:
  				val |= (U64(BYTE_8_LIST) << 32);
  				break;
  			case 4:
  				val |= (U64(BYTE_4_LIST) << 32);
  				break;
  			case 2:
  				val |= (U64(BYTE_2_LIST) << 32);
  				break;
  			case 1:
  				val |= (U64(BYTE_1_LIST) << 32);
  				break;
  			case 0:
  				val |= (U64(VOID_LIST) << 32);
  				break;
  			}
  		}
  		break;
  
  	case CAPN_BIT_LIST:
  		val |= LIST_PTR | (U64(BIT_1_LIST) << 32) | (U64(p.len) << 35);
  		break;
  
  	case CAPN_PTR_LIST:
  		val |= LIST_PTR | (U64(PTR_LIST) << 32) | (U64(p.len) << 35);
  		break;
  
  	default:
  		val = 0;
  		break;
  	}
  
  	*(uint64_t*) d = capn_flip64(val);
  }
  
  static void write_far_ptr(char *d, struct capn_segment *s, char *tgt) {
  	*(uint64_t*) d = capn_flip64(FAR_PTR | U64(tgt - s->data) | (U64(s->id) << 32));
  }
  
  static void write_double_far(char *d, struct capn_segment *s, char *tgt) {
  	*(uint64_t*) d = capn_flip64(DOUBLE_PTR | U64(tgt - s->data) | (U64(s->id) << 32));
  }
  
  #define NEED_TO_COPY 1
  
  static int write_ptr(struct capn_segment *s, char *d, capn_ptr p) {
  	/* note p.seg can be NULL if its a ptr to static data */
  	char *pdata = p.data - 8*p.is_composite_list;
  
  	if (p.type == CAPN_NULL || (p.type == CAPN_STRUCT && p.datasz == 0 && p.ptrs == 0)) {
  		write_ptr_tag(d, p, 0);
  		return 0;
  
  	} else if (!p.seg || p.seg->capn != s->capn || p.is_list_member) {
  		return NEED_TO_COPY;
  
  	} else if (p.seg == s) {
  		write_ptr_tag(d, p, pdata - d - 8);
  		return 0;
  
  	} else if (p.has_ptr_tag) {
  		/* By lucky chance, the data has a tag in front
  		 * of it. This happens when new_object had to move
  		 * the data to a new segment. */
  		write_far_ptr(d, p.seg, pdata-8);
  		return 0;
  
  	} else if (p.seg->len + 8 <= p.seg->cap) {
  		/* The target segment has enough room for tag */
  		char *t = p.seg->data + p.seg->len;
  		write_ptr_tag(t, p, pdata - t - 8);
  		write_far_ptr(d, p.seg, t);
  		p.seg->len += 8;
  		return 0;
  
  	} else {
  		/* have to allocate room for a double far
  		 * pointer */
  		char *t;
  
  		if (s->len + 16 <= s->cap) {
  			/* Try and allocate in the src segment
  			 * first. This should improve lookup on
  			 * read. */
  			t = s->data + s->len;
  			s->len += 16;
  		} else {
  			t = new_data(s->capn, 16, &s);
  			if (!t) return -1;
  		}
  
  		write_far_ptr(t, p.seg, pdata);
  		write_ptr_tag(t+8, p, 0);
  		write_double_far(d, s, t);
  		return 0;
  	}
  }
  
  struct copy {
  	struct capn_tree hdr;
  	struct capn_ptr to, from;
  	char *fbegin, *fend;
  };
  
  static capn_ptr new_clone(struct capn_segment *s, capn_ptr p) {
  	switch (p.type) {
  	case CAPN_STRUCT:
  		return capn_new_struct(s, p.datasz, p.ptrs);
  	case CAPN_PTR_LIST:
  		return capn_new_ptr_list(s, p.len);
  	case CAPN_BIT_LIST:
  		return capn_new_list1(s, p.len).p;
  	case CAPN_LIST:
  		return capn_new_list(s, p.len, p.datasz, p.ptrs);
  	default:
  		return p;
  	}
  }
  
  static int is_ptr_equal(const struct capn_ptr *a, const struct capn_ptr *b) {
  	return a->data == b->data
  		&& a->type == b->type
  		&& a->len == b->len
  		&& a->datasz == b->datasz
  		&& a->ptrs == b->ptrs;
  }
  
  static int data_size(struct capn_ptr p) {
  	switch (p.type) {
  	case CAPN_BIT_LIST:
  		return p.datasz;
  	case CAPN_PTR_LIST:
  		return p.len*8;
  	case CAPN_STRUCT:
  		return p.datasz + 8*p.ptrs;
  	case CAPN_LIST:
  		return p.len * (p.datasz + 8*p.ptrs) + 8*p.is_composite_list;
  	default:
  		return 0;
  	}
  }
  
  static int copy_ptr(struct capn_segment *seg, char *data, struct capn_ptr *t, struct capn_ptr *f, int *dep) {
  	struct capn *c = seg->capn;
  	struct copy *cp = NULL;
  	struct capn_tree **xcp;
  	char *fbegin = f->data - 8*f->is_composite_list;
  	char *fend = fbegin + data_size(*f);
  	int zero_sized = (fend == fbegin);
  
  	/* We always copy list members as it would otherwise be an
  	 * overlapped pointer (the data is owned by the enclosing list).
  	 * We do not bother with the overlapped lookup for zero sized
  	 * structures/lists as they never overlap. Nor do we add them to
  	 * the copy list as there is no data to be shared by multiple
  	 * pointers.
  	 */
  
  	xcp = &c->copy;
  	while (*xcp && !zero_sized) {
  		cp = (struct copy*) *xcp;
  		if (fend <= cp->fbegin) {
  			xcp = &cp->hdr.link[0];
  		} else if (cp->fend <= fbegin) {
  			xcp = &cp->hdr.link[1];
  		} else if (is_ptr_equal(f, &cp->from)) {
  			/* we already have a copy so just point to that */
  			return write_ptr(seg, data, cp->to);
  		} else {
  			/* pointer to overlapped data */
  			return -1;
  		}
  	}
  
  	/* no copy found - have to create a new copy */
  	*t = new_clone(seg, *f);
  
  	if (write_ptr(seg, data, *t))
  		return -1;
  
  	/* add the copy to the copy tree so we can look for overlapping
  	 * source pointers and handle recursive structures */
  	if (!zero_sized) {
  		struct copy *n;
  		struct capn_segment *cs = c->copylist;
  
  		/* need to allocate a struct copy */
  		if (!cs || cs->len + sizeof(*n) > cs->cap) {
  			cs = c->create_local ? c->create_local(c->user, sizeof(*n)) : NULL;
  			if (!cs) {
  				/* can't allocate a copy structure */
  				return -1;
  			}
  			cs->next = c->copylist;
  			c->copylist = cs;
  		}
  
  		n = (struct copy*) (cs->data + cs->len);
  		cs->len += sizeof(*n);
  
  		n->from = *f;
  		n->to = *t;
  		n->fbegin = fbegin;
  		n->fend = fend;
  
  		*xcp = &n->hdr;
  		n->hdr.parent = &cp->hdr;
  
  		c->copy = capn_tree_insert(c->copy, &n->hdr);
  	}
  
  	/* minimize the number of types the main copy routine has to
  	 * deal with to just CAPN_LIST and CAPN_PTR_LIST. ptr list only
  	 * needs t->type, t->len, t->data, t->seg, f->data, f->seg to
  	 * be valid */
  	switch (t->type) {
  	case CAPN_STRUCT:
  		if (t->datasz) {
  			memcpy(t->data, f->data, t->datasz);
  			t->data += t->datasz;
  			f->data += t->datasz;
  		}
  		if (t->ptrs) {
  			t->type = CAPN_PTR_LIST;
  			t->len = t->ptrs;
  			(*dep)++;
  		}
  		return 0;
  
  	case CAPN_BIT_LIST:
  		memcpy(t->data, f->data, t->datasz);
  		return 0;
  
  	case CAPN_LIST:
  		if (!t->len) {
  			/* empty list - nothing to copy */
  		} else if (t->ptrs && t->datasz) {
  			(*dep)++;
  		} else if (t->datasz) {
  			memcpy(t->data, f->data, t->len * t->datasz);
  		} else if (t->ptrs) {
  			t->type = CAPN_PTR_LIST;
  			t->len *= t->ptrs;
  			(*dep)++;
  		}
  		return 0;
  
  	case CAPN_PTR_LIST:
  		if (t->len) {
  			(*dep)++;
  		}
  		return 0;
  
  	default:
  		return -1;
  	}
  }
  
  static void copy_list_member(capn_ptr* t, capn_ptr *f, int *dep) {
  	/* copy struct data */
  	int sz = min(t->datasz, f->datasz);
  	memcpy(t->data, f->data, sz);
  	memset(t->data + sz, 0, t->datasz - sz);
  	t->data += t->datasz;
  	f->data += f->datasz;
  
  	/* reset excess pointers */
  	sz = min(t->ptrs, f->ptrs);
  	memset(t->data + sz, 0, 8*(t->ptrs - sz));
  
  	/* create a pointer list for the main loop to copy */
  	if (t->ptrs) {
  		t->type = CAPN_PTR_LIST;
  		t->len = t->ptrs;
  		(*dep)++;
  	}
  }
  
  #define MAX_COPY_DEPTH 32
  
  /* TODO: handle CAPN_BIT_LIST and setting from an inner bit list member */
  int capn_setp(capn_ptr p, size_t off, capn_ptr tgt) {
  	struct capn_ptr to[MAX_COPY_DEPTH], from[MAX_COPY_DEPTH];
  	char *data;
  	int err, dep = 0;
  
  	capn_resolve(&p);
  
  	if (tgt.type == CAPN_FAR_POINTER && tgt.seg->capn == p.seg->capn) {
  		uint64_t val = capn_flip64(*(uint64_t*) tgt.data);
  		if ((val & 3) == FAR_PTR) {
  			*(uint64_t*) p.data = *(uint64_t*) tgt.data;
  			return 0;
  		}
  	}
  
  	capn_resolve(&tgt);
  
  	switch (p.type) {
  	case CAPN_LIST:
  		if (off >= p.len || tgt.type != CAPN_STRUCT)
  			return -1;
  
  		to[0] = p;
  		to[0].data += off * (p.datasz + 8*p.ptrs);
  		from[0] = tgt;
  		copy_list_member(to, from, &dep);
  		break;
  
  	case CAPN_PTR_LIST:
  		if (off >= p.len)
  			return -1;
  		data = p.data + 8*off;
  		goto copy_ptr;
  
  	case CAPN_STRUCT:
  		if (off >= p.ptrs)
  			return -1;
  		data = p.data + p.datasz + 8*off;
  		goto copy_ptr;
  
  	copy_ptr:
  		err = write_ptr(p.seg, data, tgt);
  		if (err != NEED_TO_COPY)
  			return err;
  
  		/* Depth first copy the source whilst using a pointer stack to
  		 * maintain the ptr to set and size left to copy at each level.
  		 * We also maintain a rbtree (capn->copy) of the copies indexed
  		 * by the source data. This way we can detect overlapped
  		 * pointers in the source (and bail) and recursive structures
  		 * (and point to the previous copy).
  		 */
  
  		from[0] = tgt;
  		if (copy_ptr(p.seg, data, to, from, &dep))
  			return -1;
  		break;
  
  	default:
  		return -1;
  	}
  
  	while (dep) {
  		struct capn_ptr *tc = &to[dep-1], *tn = &to[dep];
  		struct capn_ptr *fc = &from[dep-1], *fn = &from[dep];
  
  		if (dep+1 == MAX_COPY_DEPTH) {
  			return -1;
  		}
  
  		if (!tc->len) {
  			dep--;
  			continue;
  		}
  
  		if (tc->type == CAPN_LIST) {
  			*fn = capn_getp(*fc, 0, 1);
  			*tn = capn_getp(*tc, 0, 1);
  
  			copy_list_member(tn, fn, &dep);
  
  			fc->data += fc->datasz + 8*fc->ptrs;
  			tc->data += tc->datasz + 8*tc->ptrs;
  			tc->len--;
  
  		} else { /* CAPN_PTR_LIST */
  			*fn = read_ptr(fc->seg, fc->data);
  
  			if (fn->type && copy_ptr(tc->seg, tc->data, tn, fn, &dep))
  				return -1;
  
  			fc->data += 8;
  			tc->data += 8;
  			tc->len--;
  		}
  	}
  
  	return 0;
  }
  
  /* TODO: handle CAPN_LIST, CAPN_PTR_LIST for bit lists */
  
  int capn_get1(capn_list1 l, size_t off) {
  	return l.p.type == CAPN_BIT_LIST
  		&& off < l.p.len
  		&& (l.p.data[off/8] & (1 << (off%8))) != 0;
  }
  
  int capn_set1(capn_list1 l, size_t off, int val) {
  	if (l.p.type != CAPN_BIT_LIST || off >= l.p.len)
  		return -1;
  	if (val) {
  		l.p.data[off/8] |= 1 << (off%8);
  	} else {
  		l.p.data[off/8] &= ~(1 << (off%8));
  	}
  	return 0;
  }
  
  int capn_getv1(capn_list1 l, size_t off, uint8_t *data, size_t sz) {
  	/* Note we only support aligned reads */
  	size_t bsz;
  	capn_ptr p = l.p;
  	if (p.type != CAPN_BIT_LIST || (off & 7) != 0)
  		return -1;
  
  	bsz = (sz + 7) / 8;
  	off /= 8;
  
  	if (off + sz > p.datasz) {
  		memcpy(data, p.data + off, p.datasz - off);
  		return p.len - off*8;
  	} else {
  		memcpy(data, p.data + off, bsz);
  		return sz;
  	}
  }
  
  int capn_setv1(capn_list1 l, size_t off, const uint8_t *data, size_t sz) {
  	/* Note we only support aligned writes */
  	size_t bsz;
  	capn_ptr p = l.p;
  	if (p.type != CAPN_BIT_LIST || (off & 7) != 0)
  		return -1;
  
  	bsz = (sz + 7) / 8;
  	off /= 8;
  
  	if (off + sz > p.datasz) {
  		memcpy(p.data + off, data, p.datasz - off);
  		return p.len - off*8;
  	} else {
  		memcpy(p.data + off, data, bsz);
  		return sz;
  	}
  }
  
  /* pull out whether we add a tag or not as a define so the unit test can
   * test double far pointers by not creating tags */
  #ifndef ADD_TAG
  #define ADD_TAG 1
  #endif
  
  static void new_object(capn_ptr *p, int bytes) {
  	struct capn_segment *s = p->seg;
  
  	if (!s) {
  		memset(p, 0, sizeof(*p));
  		return;
  	}
  
  	/* pointer needs to be initialised to get a valid offset on write */
  	if (!bytes) {
  		p->data = s->data + s->len;
  		return;
  	}
  
  	/* all allocations are 8 byte aligned */
  	bytes = (bytes + 7) & ~7;
  
  	if (s->len + bytes <= s->cap) {
  		p->data = s->data + s->len;
  		s->len += bytes;
  		return;
  	}
  
  	/* add a tag whenever we switch segments so that write_ptr can
  	 * use it */
  	p->data = new_data(s->capn, bytes + ADD_TAG*8, &p->seg);
  	if (!p->data) {
  		memset(p, 0, sizeof(*p));
  		return;
  	}
  
  	if (ADD_TAG) {
  		write_ptr_tag(p->data, *p, 0);
  		p->data += 8;
  		p->has_ptr_tag = 1;
  	}
  }
  
  capn_ptr capn_root(struct capn *c) {
  	capn_ptr r = {CAPN_PTR_LIST};
  	r.seg = lookup_segment(c, NULL, 0);
  	r.data = r.seg ? r.seg->data : new_data(c, 8, &r.seg);
  	r.len = 1;
  
  	if (!r.seg || r.seg->cap < 8) {
  		memset(&r, 0, sizeof(r));
  	} else if (r.seg->len < 8) {
  		r.seg->len = 8;
  	}
  
  	return r;
  }
  
  capn_ptr capn_new_struct(struct capn_segment *seg, int datasz, int ptrs) {
  	capn_ptr p = {CAPN_STRUCT};
  	p.seg = seg;
  	p.datasz = (datasz + 7) & ~7;
  	p.ptrs = ptrs;
  	new_object(&p, p.datasz + 8*p.ptrs);
  	return p;
  }
  
  capn_ptr capn_new_list(struct capn_segment *seg, int sz, int datasz, int ptrs) {
  	capn_ptr p = {CAPN_LIST};
  	p.seg = seg;
  	p.len = sz;
  
  	if (!sz) {
  		/* empty lists may as well be a len=0 void list */
  	} else if (ptrs || datasz > 8) {
  		p.is_composite_list = 1;
  		p.datasz = (datasz + 7) & ~7;
  		p.ptrs = ptrs;
  		new_object(&p, p.len * (p.datasz + 8*p.ptrs) + 8);
  		if (p.data) {
  			uint64_t hdr = STRUCT_PTR | (U64(p.len) << 2) | (U64(p.datasz/8) << 32) | (U64(ptrs) << 48);
  			*(uint64_t*) p.data = capn_flip64(hdr);
  			p.data += 8;
  		}
  	} else if (datasz > 4) {
  		p.datasz = 8;
  		new_object(&p, p.len * 8);
  	} else if (datasz > 2) {
  		p.datasz = 4;
  		new_object(&p, p.len * 4);
  	} else {
  		p.datasz = datasz;
  		new_object(&p, p.len * datasz);
  	}
  
  	return p;
  }
  
  capn_list1 capn_new_list1(struct capn_segment *seg, size_t sz) {
  	capn_list1 l = {{CAPN_BIT_LIST}};
  	l.p.seg = seg;
  	l.p.datasz = (sz+7)/8;
  	l.p.len = sz;
  	new_object(&l.p, l.p.datasz);
  	return l;
  }
  
  capn_ptr capn_new_ptr_list(struct capn_segment *seg, int sz) {
  	capn_ptr p = {CAPN_PTR_LIST};
  	p.seg = seg;
  	p.len = sz;
  	p.ptrs = 0;
  	p.datasz = 0;
  	new_object(&p, sz*8);
  	return p;
  }
  
  capn_ptr capn_new_string(struct capn_segment *seg, const char *str, ssize_t sz);
  
  capn_ptr capn_new_string(struct capn_segment *seg, const char *str, ssize_t sz) {
  	capn_ptr p = {CAPN_LIST};
  	p.seg = seg;
  	p.len = ((sz >= 0) ? (size_t)sz : strlen(str)) + 1;
  	p.datasz = 1;
  	new_object(&p, p.len);
  	if (p.data) {
  		memcpy(p.data, str, p.len-1);
  	}
  	return p;
  }
  
  capn_text capn_get_text(capn_ptr p, int off, capn_text def) {
  	capn_ptr m = capn_getp(p, off, 1);
  	capn_text ret = def;
  	if (m.type == CAPN_LIST && m.datasz == 1 && m.len && m.data[m.len - 1] == 0) {
  		ret.seg = m.seg;
  		ret.str = m.data;
  		ret.len = m.len - 1;
  	}
  	return ret;
  }
  
  int capn_set_text(capn_ptr p, int off, capn_text tgt) {
  	capn_ptr m = {CAPN_NULL};
  	if (tgt.seg) {
  		m.type = CAPN_LIST;
  		m.seg = tgt.seg;
  		m.data = (char*)tgt.str;
  		m.len = tgt.len + 1;
  		m.datasz = 1;
  	} else if (tgt.str) {
  		m = capn_new_string(p.seg, tgt.str, tgt.len);
  	}
  	return capn_setp(p, off, m);
  }
  
  capn_data capn_get_data(capn_ptr p, int off) {
  	capn_data ret;
  	ret.p = capn_getp(p, off, 1);
  	if (ret.p.type != CAPN_LIST || ret.p.datasz != 1) {
  		memset(&ret, 0, sizeof(ret));
  	}
  	return ret;
  }
  
  #define SZ 8
  #include "capn-list.inc"
  #undef SZ
  
  #define SZ 16
  #include "capn-list.inc"
  #undef SZ
  
  #define SZ 32
  #include "capn-list.inc"
  #undef SZ
  
  #define SZ 64
  #include "capn-list.inc"
  #undef SZ
  