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iof-bird-daemon / filter / trie.c @ 5e173e9f

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/*
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 *        Filters: Trie for prefix sets
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 *
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 *        Copyright 2009 Ondrej Zajicek <santiago@crfreenet.org>
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 *
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 *        Can be freely distributed and used under the terms of the GNU GPL.
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 */
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/**
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 * DOC: Trie for prefix sets
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 *
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 * We use a (compressed) trie to represent prefix sets. Every node
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 * in the trie represents one prefix (&addr/&plen) and &plen also
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 * indicates the index of the bit in the address that is used to
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 * branch at the node. If we need to represent just a set of
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 * prefixes, it would be simple, but we have to represent a
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 * set of prefix patterns. Each prefix pattern consists of
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 * &ppaddr/&pplen and two integers: &low and &high, and a prefix
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 * &paddr/&plen matches that pattern if the first MIN(&plen, &pplen)
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 * bits of &paddr and &ppaddr are the same and &low <= &plen <= &high.
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 *
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 * We use a bitmask (&accept) to represent accepted prefix lengths
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 * at a node. As there are 33 prefix lengths (0..32 for IPv4), but
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 * there is just one prefix of zero length in the whole trie so we
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 * have &zero flag in &f_trie (indicating whether the trie accepts
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 * prefix 0.0.0.0/0) as a special case, and &accept bitmask
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 * represents accepted prefix lengths from 1 to 32.
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 *
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 * There are two cases in prefix matching - a match when the length
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 * of the prefix is smaller that the length of the prefix pattern,
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 * (&plen < &pplen) and otherwise. The second case is simple - we
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 * just walk through the trie and look at every visited node
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 * whether that prefix accepts our prefix length (&plen). The
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 * first case is tricky - we don't want to examine every descendant
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 * of a final node, so (when we create the trie) we have to propagate
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 * that information from nodes to their ascendants.
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 *
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 * Suppose that we have two masks (M1 and M2) for a node. Mask M1
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 * represents accepted prefix lengths by just the node and mask M2
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 * represents accepted prefix lengths by the node or any of its
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 * descendants. Therefore M2 is a bitwise or of M1 and children's
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 * M2 and this is a maintained invariant during trie building.
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 * Basically, when we want to match a prefix, we walk through the trie,
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 * check mask M1 for our prefix length and when we came to
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 * final node, we check mask M2.
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 *
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 * There are two differences in the real implementation. First,
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 * we use a compressed trie so there is a case that we skip our
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 * final node (if it is not in the trie) and we came to node that
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 * is either extension of our prefix, or completely out of path
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 * In the first case, we also have to check M2.
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 *
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 * Second, we really need not to maintain two separate bitmasks.
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 * Checks for mask M1 are always larger than &applen and we need
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 * just the first &pplen bits of mask M2 (if trie compression
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 * hadn't been used it would suffice to know just $applen-th bit),
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 * so we have to store them together in &accept mask - the first
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 * &pplen bits of mask M2 and then mask M1.
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 *
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 * There are four cases when we walk through a trie:
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 *
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 * - we are in NULL
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 * - we are out of path (prefixes are inconsistent)
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 * - we are in the wanted (final) node (node length == &plen)
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 * - we are beyond the end of path (node length > &plen)
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 * - we are still on path and keep walking (node length < &plen)
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 *
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 * The walking code in trie_match_prefix() is structured according to
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 * these cases.
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 */
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#include "nest/bird.h"
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#include "lib/string.h"
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#include "conf/conf.h"
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#include "filter/filter.h"
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/**
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 * f_new_trie - allocates and returns a new empty trie
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 * @lp: linear pool to allocate items from
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 * @node_size: node size to be used (&f_trie_node and user data)
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 */
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struct f_trie *
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f_new_trie(linpool *lp, uint node_size)
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{
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  struct f_trie * ret;
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  ret = lp_allocz(lp, sizeof(struct f_trie) + node_size);
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  ret->lp = lp;
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  ret->node_size = node_size;
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  return ret;
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}
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static inline struct f_trie_node *
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new_node(struct f_trie *t, int plen, ip_addr paddr, ip_addr pmask, ip_addr amask)
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{
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  struct f_trie_node *n = lp_allocz(t->lp, t->node_size);
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  n->plen = plen;
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  n->addr = paddr;
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  n->mask = pmask;
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  n->accept = amask;
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  return n;
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}
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static inline void
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attach_node(struct f_trie_node *parent, struct f_trie_node *child)
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{
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  parent->c[ipa_getbit(child->addr, parent->plen) ? 1 : 0] = child;
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}
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/**
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 * trie_add_prefix
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 * @t: trie to add to
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 * @net: IP network prefix
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 * @l: prefix lower bound
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 * @h: prefix upper bound
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 *
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 * Adds prefix (prefix pattern) @n to trie @t.  @l and @h are lower
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 * and upper bounds on accepted prefix lengths, both inclusive.
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 * 0 <= l, h <= 32 (128 for IPv6).
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 *
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 * Returns a pointer to the allocated node. The function can return a pointer to
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 * an existing node if @px and @plen are the same. If px/plen == 0/0 (or ::/0),
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 * a pointer to the root node is returned.
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 */
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void *
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trie_add_prefix(struct f_trie *t, net_addr *net, uint l, uint h)
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{
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  ip_addr px = net_prefix(net);
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  uint plen = net_pxlen(net);
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  if (net->type == NET_IP4)
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  {
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    const uint delta = IP6_MAX_PREFIX_LENGTH - IP4_MAX_PREFIX_LENGTH;
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    plen += delta;
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    l += delta;
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    h += delta;
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  }
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  if (l == 0)
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    t->zero = 1;
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  else
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    l--;
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  if (h < plen)
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    plen = h;
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  ip_addr amask = ipa_xor(ipa_mkmask(l), ipa_mkmask(h));
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  ip_addr pmask = ipa_mkmask(plen);
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  ip_addr paddr = ipa_and(px, pmask);
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  struct f_trie_node *o = NULL;
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  struct f_trie_node *n = t->root;
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  while (n)
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    {
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      ip_addr cmask = ipa_and(n->mask, pmask);
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      if (ipa_compare(ipa_and(paddr, cmask), ipa_and(n->addr, cmask)))
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        {
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          /* We are out of path - we have to add branching node 'b'
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             between node 'o' and node 'n', and attach new node 'a'
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             as the other child of 'b'. */
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          int blen = ipa_pxlen(paddr, n->addr);
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          ip_addr bmask = ipa_mkmask(blen);
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          ip_addr baddr = ipa_and(px, bmask);
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          /* Merge accept masks from children to get accept mask for node 'b' */
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          ip_addr baccm = ipa_and(ipa_or(amask, n->accept), bmask);
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          struct f_trie_node *a = new_node(t, plen, paddr, pmask, amask);
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          struct f_trie_node *b = new_node(t, blen, baddr, bmask, baccm);
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          attach_node(o, b);
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          attach_node(b, n);
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          attach_node(b, a);
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          return a;
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        }
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      if (plen < n->plen)
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        {
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          /* We add new node 'a' between node 'o' and node 'n' */
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          amask = ipa_or(amask, ipa_and(n->accept, pmask));
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          struct f_trie_node *a = new_node(t, plen, paddr, pmask, amask);
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          attach_node(o, a);
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          attach_node(a, n);
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          return a;
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        }
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      if (plen == n->plen)
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        {
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          /* We already found added node in trie. Just update accept mask */
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          n->accept = ipa_or(n->accept, amask);
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          return n;
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        }
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      /* Update accept mask part M2 and go deeper */
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      n->accept = ipa_or(n->accept, ipa_and(amask, n->mask));
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      /* n->plen < plen and plen <= 32 (128) */
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      o = n;
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      n = n->c[ipa_getbit(paddr, n->plen) ? 1 : 0];
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    }
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  /* We add new tail node 'a' after node 'o' */
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  struct f_trie_node *a = new_node(t, plen, paddr, pmask, amask);
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  attach_node(o, a);
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  return a;
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}
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static int
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trie_match_prefix(struct f_trie *t, ip_addr px, int plen)
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{
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  ip_addr pmask = ipa_mkmask(plen);
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  ip_addr paddr = ipa_and(px, pmask);
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  if (plen == 0)
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    return t->zero;
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  int plentest = plen - 1;
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  struct f_trie_node *n = t->root;
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  while(n)
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    {
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      ip_addr cmask = ipa_and(n->mask, pmask);
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      /* We are out of path */
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      if (ipa_compare(ipa_and(paddr, cmask), ipa_and(n->addr, cmask)))
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        return 0;
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      /* Check accept mask */
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      if (ipa_getbit(n->accept, plentest))
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        return 1;
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      /* We finished trie walk and still no match */
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      if (plen <= n->plen)
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        return 0;
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      /* Choose children */
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      n =  n->c[(ipa_getbit(paddr, n->plen)) ? 1 : 0];
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    }
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  return 0;
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}
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/**
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 * trie_match_net
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 * @t: trie
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 * @n: net address
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 *
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 * Tries to find a matching net in the trie such that
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 * prefix @n matches that prefix pattern. Returns 1 if there
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 * is such prefix pattern in the trie.
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 */
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int
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trie_match_net(struct f_trie *t, const net_addr *n)
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{
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  int add = 0;
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  switch (n->type) {
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    case NET_IP4:
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    case NET_VPN4: add = IP6_MAX_PREFIX_LENGTH - IP4_MAX_PREFIX_LENGTH;
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  }
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  return trie_match_prefix(t, net_prefix(n), net_pxlen(n) + add);
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}
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static int
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trie_node_same(struct f_trie_node *t1, struct f_trie_node *t2)
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{
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  if ((t1 == NULL) && (t2 == NULL))
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    return 1;
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  if ((t1 == NULL) || (t2 == NULL))
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    return 0;
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  if ((t1->plen != t2->plen) ||
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      (! ipa_equal(t1->addr, t2->addr)) ||
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      (! ipa_equal(t1->accept, t2->accept)))
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    return 0;
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  return trie_node_same(t1->c[0], t2->c[0]) && trie_node_same(t1->c[1], t2->c[1]);
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}
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/**
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 * trie_same
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 * @t1: first trie to be compared
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 * @t2: second one
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 *
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 * Compares two tries and returns 1 if they are same
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 */
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int
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trie_same(struct f_trie *t1, struct f_trie *t2)
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{
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  return (t1->zero == t2->zero) && trie_node_same(t1->root, t2->root);
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}
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static void
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trie_node_format(struct f_trie_node *t, buffer *buf)
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{
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  if (t == NULL)
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    return;
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  if (ipa_nonzero(t->accept))
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    buffer_print(buf, "%I/%d{%I}, ", t->addr, t->plen, t->accept);
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  trie_node_format(t->c[0], buf);
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  trie_node_format(t->c[1], buf);
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}
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/**
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 * trie_format
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 * @t: trie to be formatted
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 * @buf: destination buffer
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 *
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 * Prints the trie to the supplied buffer.
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 */
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void
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trie_format(struct f_trie *t, buffer *buf)
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{
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  buffer_puts(buf, "[");
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  if (t->zero)
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    buffer_print(buf, "%I/%d", IPA_NONE, 0);
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  trie_node_format(t->root, buf);
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  /* Undo last separator */
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  if (buf->pos[-1] != '[')
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    buf->pos -= 2;
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  buffer_puts(buf, "]");
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}