mirror of
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e92adfe5e4
Values are still turned into pointers internally to maintain the invariants of strideTable, but from the user's perspective it's now possible to tbl.Insert(pfx, true) rather than tbl.Insert(pfx, ptr.To(true)). Updates #7781 Signed-off-by: David Anderson <danderson@tailscale.com>
642 lines
23 KiB
Go
642 lines
23 KiB
Go
// Copyright (c) Tailscale Inc & AUTHORS
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// SPDX-License-Identifier: BSD-3-Clause
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// Package art provides a routing table that implements the Allotment Routing
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// Table (ART) algorithm by Donald Knuth, as described in the paper by Yoichi
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// Hariguchi.
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//
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// ART outperforms the traditional radix tree implementations for route lookups,
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// insertions, and deletions.
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//
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// For more information, see Yoichi Hariguchi's paper:
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// https://cseweb.ucsd.edu//~varghese/TEACH/cs228/artlookup.pdf
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package art
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import (
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"bytes"
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"encoding/binary"
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"fmt"
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"io"
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"math/bits"
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"net/netip"
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"strings"
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"sync"
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)
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const (
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debugInsert = false
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debugDelete = false
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)
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// Table is an IPv4 and IPv6 routing table.
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type Table[T any] struct {
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v4 strideTable[T]
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v6 strideTable[T]
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initOnce sync.Once
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}
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func (t *Table[T]) init() {
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t.initOnce.Do(func() {
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t.v4.prefix = netip.PrefixFrom(netip.IPv4Unspecified(), 0)
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t.v6.prefix = netip.PrefixFrom(netip.IPv6Unspecified(), 0)
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})
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}
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func (t *Table[T]) tableForAddr(addr netip.Addr) *strideTable[T] {
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if addr.Is6() {
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return &t.v6
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}
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return &t.v4
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}
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// Get does a route lookup for addr and returns the associated value, or nil if
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// no route matched.
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func (t *Table[T]) Get(addr netip.Addr) (ret T, ok bool) {
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t.init()
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// Ideally we would use addr.AsSlice here, but AsSlice is just
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// barely complex enough that it can't be inlined, and that in
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// turn causes the slice to escape to the heap. Using As16 and
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// manual slicing here helps the compiler keep Get alloc-free.
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st := t.tableForAddr(addr)
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rawAddr := addr.As16()
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bs := rawAddr[:]
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if addr.Is4() {
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bs = bs[12:]
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}
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i := 0
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// With path compression, we might skip over some address bits while walking
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// to a strideTable leaf. This means the leaf answer we find might not be
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// correct, because path compression took us down the wrong subtree. When
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// that happens, we have to backtrack and figure out which most specific
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// route further up the tree is relevant to addr, and return that.
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//
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// So, as we walk down the stride tables, each time we find a non-nil route
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// result, we have to remember it and the associated strideTable prefix.
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//
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// We could also deal with this edge case of path compression by checking
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// the strideTable prefix on each table as we descend, but that means we
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// have to pay N prefix.Contains checks on every route lookup (where N is
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// the number of strideTables in the path), rather than only paying M prefix
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// comparisons in the edge case (where M is the number of strideTables in
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// the path with a non-nil route of their own).
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const maxDepth = 16
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type prefixAndRoute struct {
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prefix netip.Prefix
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route T
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}
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strideMatch := make([]prefixAndRoute, 0, maxDepth)
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findLeaf:
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for {
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rt, rtOK, child := st.getValAndChild(bs[i])
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if rtOK {
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// This strideTable contains a route that may be relevant to our
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// search, remember it.
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strideMatch = append(strideMatch, prefixAndRoute{st.prefix, rt})
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}
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if child == nil {
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// No sub-routes further down, the last thing we recorded
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// in strideRoutes is tentatively the result, barring
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// misdirection from path compression.
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break findLeaf
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}
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st = child
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// Path compression means we may be skipping over some intermediate
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// tables. We have to skip forward to whatever depth st now references.
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i = st.prefix.Bits() / 8
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}
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// Walk backwards through the hits we recorded in strideRoutes and
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// stridePrefixes, returning the first one whose subtree matches addr.
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//
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// In the common case where path compression did not mislead us, we'll
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// return on the first loop iteration because the last route we recorded was
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// the correct most-specific route.
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for i := len(strideMatch) - 1; i >= 0; i-- {
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if m := strideMatch[i]; m.prefix.Contains(addr) {
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return m.route, true
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}
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}
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// We either found no route hits at all (both previous loops terminated
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// immediately), or we went on a wild goose chase down a compressed path for
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// the wrong prefix, and also found no usable routes on the way back up to
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// the root. This is a miss.
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return ret, false
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}
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// Insert adds pfx to the table, with value val.
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// If pfx is already present in the table, its value is set to val.
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func (t *Table[T]) Insert(pfx netip.Prefix, val T) {
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t.init()
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// The standard library doesn't enforce normalized prefixes (where
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// the non-prefix bits are all zero). These algorithms require
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// normalized prefixes, so do it upfront.
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pfx = pfx.Masked()
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if debugInsert {
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defer func() {
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fmt.Printf("%s", t.debugSummary())
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}()
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fmt.Printf("\ninsert: start pfx=%s\n", pfx)
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}
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st := t.tableForAddr(pfx.Addr())
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// This algorithm is full of off-by-one headaches that boil down
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// to the fact that pfx.Bits() has (2^n)+1 values, rather than
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// just 2^n. For example, an IPv4 prefix length can be 0 through
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// 32, which is 33 values.
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//
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// This extra possible value creates a lot of problems as we do
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// bits and bytes math to traverse strideTables below. So, we
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// treat the default route 0/0 specially here, that way the rest
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// of the logic goes back to having 2^n values to reason about,
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// which can be done in a nice and regular fashion with no edge
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// cases.
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if pfx.Bits() == 0 {
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if debugInsert {
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fmt.Printf("insert: default route\n")
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}
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st.insert(0, 0, val)
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return
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}
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// No matter what we do as we traverse strideTables, our final
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// action will be to insert the last 1-8 bits of pfx into a
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// strideTable somewhere.
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//
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// We calculate upfront the byte position of the end of the
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// prefix; the number of bits within that byte that contain prefix
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// data; and the prefix of the strideTable into which we'll
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// eventually insert.
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//
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// We need this in a couple different branches of the code below,
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// and because the possible values are 1-indexed (1 through 32 for
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// ipv4, 1 through 128 for ipv6), the math is very slightly
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// unusual to account for the off-by-one indexing. Do it once up
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// here, with this large comment, rather than reproduce the subtle
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// math in multiple places further down.
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finalByteIdx := (pfx.Bits() - 1) / 8
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finalBits := pfx.Bits() - (finalByteIdx * 8)
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finalStridePrefix, err := pfx.Addr().Prefix(finalByteIdx * 8)
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if err != nil {
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panic(fmt.Sprintf("invalid prefix requested: %s/%d", pfx.Addr(), finalByteIdx*8))
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}
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if debugInsert {
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fmt.Printf("insert: finalByteIdx=%d finalBits=%d finalStridePrefix=%s\n", finalByteIdx, finalBits, finalStridePrefix)
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}
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// The strideTable we want to insert into is potentially at the
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// end of a chain of strideTables, each one encoding 8 bits of the
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// prefix.
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//
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// We're expecting to walk down a path of tables, although with
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// prefix compression we may end up skipping some links in the
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// chain, or taking wrong turns and having to course correct.
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//
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// As we walk down the tree, byteIdx is the byte of bs we're
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// currently examining to choose our next step, and numBits is the
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// number of bits that remain in pfx, starting with the byte at
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// byteIdx inclusive.
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bs := pfx.Addr().AsSlice()
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byteIdx := 0
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numBits := pfx.Bits()
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for {
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if debugInsert {
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fmt.Printf("insert: loop byteIdx=%d numBits=%d st.prefix=%s\n", byteIdx, numBits, st.prefix)
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}
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if numBits <= 8 {
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if debugInsert {
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fmt.Printf("insert: existing leaf st.prefix=%s addr=%d/%d\n", st.prefix, bs[finalByteIdx], finalBits)
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}
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// We've reached the end of the prefix, whichever
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// strideTable we're looking at now is the place where we
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// need to insert.
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st.insert(bs[finalByteIdx], finalBits, val)
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return
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}
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// Otherwise, we need to go down at least one more level of
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// strideTables. With prefix compression, each level of
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// descent can have one of three outcomes: we find a place
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// where prefix compression is possible; a place where prefix
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// compression made us take a "wrong turn"; or a point along
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// our intended path that we have to keep following.
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child, created := st.getOrCreateChild(bs[byteIdx])
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switch {
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case created:
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// The subtree we need for pfx doesn't exist yet. The rest
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// of the path, if we were to create it, will consist of a
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// bunch of strideTables with a single child each. We can
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// use path compression to elide those intermediates, and
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// jump straight to the final strideTable that hosts this
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// prefix.
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child.prefix = finalStridePrefix
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child.insert(bs[finalByteIdx], finalBits, val)
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if debugInsert {
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fmt.Printf("insert: new leaf st.prefix=%s child.prefix=%s addr=%d/%d\n", st.prefix, child.prefix, bs[finalByteIdx], finalBits)
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}
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return
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case !prefixStrictlyContains(child.prefix, pfx):
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// child already exists, but its prefix does not contain
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// our destination. This means that the path between st
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// and child was compressed by a previous insertion, and
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// somewhere in the (implicit) compressed path we took a
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// wrong turn, into the wrong part of st's subtree.
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//
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// This is okay, because pfx and child.prefix must have a
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// common ancestor node somewhere between st and child. We
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// can figure out what node that is, and materialize it.
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//
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// Once we've done that, we can immediately complete the
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// remainder of the insertion in one of two ways, without
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// further traversal. See a little further down for what
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// those are.
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if debugInsert {
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fmt.Printf("insert: wrong turn, pfx=%s child.prefix=%s\n", pfx, child.prefix)
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}
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intermediatePrefix, addrOfExisting, addrOfNew := computePrefixSplit(child.prefix, pfx)
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intermediate := &strideTable[T]{prefix: intermediatePrefix} // TODO: make this whole thing be st.AddIntermediate or something?
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st.setChild(bs[byteIdx], intermediate)
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intermediate.setChild(addrOfExisting, child)
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if debugInsert {
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fmt.Printf("insert: new intermediate st.prefix=%s intermediate.prefix=%s child.prefix=%s\n", st.prefix, intermediate.prefix, child.prefix)
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}
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// Now, we have a chain of st -> intermediate -> child.
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//
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// pfx either lives in a different child of intermediate,
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// or in intermediate itself. For example, if we created
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// the intermediate 1.2.0.0/16, pfx=1.2.3.4/32 would have
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// to go into a new child of intermediate, but
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// pfx=1.2.0.0/18 would go into intermediate directly.
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if remain := pfx.Bits() - intermediate.prefix.Bits(); remain <= 8 {
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// pfx lives in intermediate.
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if debugInsert {
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fmt.Printf("insert: into intermediate intermediate.prefix=%s addr=%d/%d\n", intermediate.prefix, bs[finalByteIdx], finalBits)
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}
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intermediate.insert(bs[finalByteIdx], finalBits, val)
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} else {
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// pfx lives in a different child subtree of
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// intermediate. By definition this subtree doesn't
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// exist at all, otherwise we'd never have entered
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// this entire "wrong turn" codepath in the first
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// place.
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//
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// This means we can apply prefix compression as we
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// create this new child, and we're done.
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st, created = intermediate.getOrCreateChild(addrOfNew)
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if !created {
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panic("new child path unexpectedly exists during path decompression")
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}
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st.prefix = finalStridePrefix
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st.insert(bs[finalByteIdx], finalBits, val)
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if debugInsert {
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fmt.Printf("insert: new child st.prefix=%s addr=%d/%d\n", st.prefix, bs[finalByteIdx], finalBits)
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}
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}
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return
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default:
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// An expected child table exists along pfx's
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// path. Continue traversing downwards.
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st = child
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byteIdx = child.prefix.Bits() / 8
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numBits = pfx.Bits() - child.prefix.Bits()
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if debugInsert {
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fmt.Printf("insert: descend st.prefix=%s\n", st.prefix)
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}
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}
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}
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}
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// Delete removes pfx from the table, if it is present.
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func (t *Table[T]) Delete(pfx netip.Prefix) {
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t.init()
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// The standard library doesn't enforce normalized prefixes (where
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// the non-prefix bits are all zero). These algorithms require
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// normalized prefixes, so do it upfront.
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pfx = pfx.Masked()
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if debugDelete {
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defer func() {
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fmt.Printf("%s", t.debugSummary())
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}()
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fmt.Printf("\ndelete: start pfx=%s table:\n%s", pfx, t.debugSummary())
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}
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st := t.tableForAddr(pfx.Addr())
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// This algorithm is full of off-by-one headaches, just like
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// Insert. See the comment in Insert for more details. Bottom
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// line: we handle the default route as a special case, and that
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// simplifies the rest of the code slightly.
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if pfx.Bits() == 0 {
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if debugDelete {
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fmt.Printf("delete: default route\n")
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}
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st.delete(0, 0)
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return
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}
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// Deletion may drive the refcount of some strideTables down to
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// zero. We need to clean up these dangling tables, so we have to
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// keep track of which tables we touch on the way down, and which
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// strideEntry index each child is registered in.
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//
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// Note that the strideIndex and strideTables entries are off-by-one.
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// The child table pointer is recorded at i+1, but it is referenced by a
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// particular index in the parent table, at index i.
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//
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// In other words: entry number strideIndexes[0] in
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// strideTables[0] is the same pointer as strideTables[1].
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//
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// This results in some slightly odd array accesses further down
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// in this code, because in a single loop iteration we have to
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// write to strideTables[N] and strideIndexes[N-1].
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strideIdx := 0
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strideTables := [16]*strideTable[T]{st}
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strideIndexes := [15]uint8{}
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// Similar to Insert, navigate down the tree of strideTables,
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// looking for the one that houses this prefix. This part is
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// easier than with insertion, since we can bail if the path ends
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// early or takes an unexpected detour. However, unlike
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// insertion, there's a whole post-deletion cleanup phase later
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// on.
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//
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// As we walk down the tree, byteIdx is the byte of bs we're
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// currently examining to choose our next step, and numBits is the
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// number of bits that remain in pfx, starting with the byte at
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// byteIdx inclusive.
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bs := pfx.Addr().AsSlice()
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byteIdx := 0
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numBits := pfx.Bits()
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for numBits > 8 {
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if debugDelete {
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fmt.Printf("delete: loop byteIdx=%d numBits=%d st.prefix=%s\n", byteIdx, numBits, st.prefix)
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}
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child := st.getChild(bs[byteIdx])
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if child == nil {
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// Prefix can't exist in the table, because one of the
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// necessary strideTables doesn't exist.
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if debugDelete {
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fmt.Printf("delete: missing necessary child pfx=%s\n", pfx)
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}
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return
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}
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strideIndexes[strideIdx] = bs[byteIdx]
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strideTables[strideIdx+1] = child
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strideIdx++
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// Path compression means byteIdx can jump forwards
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// unpredictably. Recompute the next byte to look at from the
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// child we just found.
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byteIdx = child.prefix.Bits() / 8
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numBits = pfx.Bits() - child.prefix.Bits()
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st = child
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if debugDelete {
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fmt.Printf("delete: descend st.prefix=%s\n", st.prefix)
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}
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}
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// We reached a leaf stride table that seems to be in the right
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// spot. But path compression might have led us to the wrong
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// table.
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if !prefixStrictlyContains(st.prefix, pfx) {
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// Wrong table, the requested prefix can't exist since its
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// path led us to the wrong place.
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if debugDelete {
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fmt.Printf("delete: wrong leaf table pfx=%s\n", pfx)
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}
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return
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}
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if debugDelete {
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fmt.Printf("delete: delete from st.prefix=%s addr=%d/%d\n", st.prefix, bs[byteIdx], numBits)
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}
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if routeExisted := st.delete(bs[byteIdx], numBits); !routeExisted {
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// We're in the right strideTable, but pfx wasn't in
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// it. Refcounts haven't changed, so we can skip cleanup.
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if debugDelete {
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fmt.Printf("delete: prefix not present pfx=%s\n", pfx)
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}
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return
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}
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// st.delete reduced st's refcount by one. This table may now be
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// reclaimable, and depending on how we can reclaim it, the parent
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// tables may also need to be reclaimed. This loop ends as soon as
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// an iteration takes no action, or takes an action that doesn't
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// alter the parent table's refcounts.
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//
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// We start our walk back at strideTables[strideIdx], which
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// contains st.
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for strideIdx > 0 {
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cur := strideTables[strideIdx]
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if debugDelete {
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fmt.Printf("delete: GC? strideIdx=%d st.prefix=%s\n", strideIdx, cur.prefix)
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}
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if cur.routeRefs > 0 {
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// the strideTable has other route entries, it cannot be
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// deleted or compacted.
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if debugDelete {
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fmt.Printf("delete: has other routes st.prefix=%s\n", cur.prefix)
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}
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return
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}
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switch cur.childRefs {
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case 0:
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// no routeRefs and no childRefs, this table can be
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// deleted. This will alter the parent table's refcount,
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// so we'll have to look at it as well (in the next loop
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// iteration).
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if debugDelete {
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fmt.Printf("delete: remove st.prefix=%s\n", cur.prefix)
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}
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strideTables[strideIdx-1].deleteChild(strideIndexes[strideIdx-1])
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strideIdx--
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case 1:
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// This table has no routes, and a single child. Compact
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// this table out of existence by making the parent point
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// directly at the one child. This does not affect the
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// parent's refcounts, so the parent can't be eligible for
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// deletion or compaction, and we can stop.
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child := strideTables[strideIdx].findFirstChild() // only 1 child exists, by definition
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parent := strideTables[strideIdx-1]
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|
if debugDelete {
|
|
fmt.Printf("delete: compact parent.prefix=%s st.prefix=%s child.prefix=%s\n", parent.prefix, cur.prefix, child.prefix)
|
|
}
|
|
strideTables[strideIdx-1].setChild(strideIndexes[strideIdx-1], child)
|
|
return
|
|
default:
|
|
// This table has two or more children, so it's acting as a "fork in
|
|
// the road" between two prefix subtrees. It cannot be deleted, and
|
|
// thus no further cleanups are possible.
|
|
if debugDelete {
|
|
fmt.Printf("delete: fork table st.prefix=%s\n", cur.prefix)
|
|
}
|
|
return
|
|
}
|
|
}
|
|
}
|
|
|
|
// debugSummary prints the tree of allocated strideTables in t, with each
|
|
// strideTable's refcount.
|
|
func (t *Table[T]) debugSummary() string {
|
|
t.init()
|
|
var ret bytes.Buffer
|
|
fmt.Fprintf(&ret, "v4: ")
|
|
strideSummary(&ret, &t.v4, 4)
|
|
fmt.Fprintf(&ret, "v6: ")
|
|
strideSummary(&ret, &t.v6, 4)
|
|
return ret.String()
|
|
}
|
|
|
|
func strideSummary[T any](w io.Writer, st *strideTable[T], indent int) {
|
|
fmt.Fprintf(w, "%s: %d routes, %d children\n", st.prefix, st.routeRefs, st.childRefs)
|
|
indent += 4
|
|
st.treeDebugStringRec(w, 1, indent)
|
|
for addr, child := range st.children {
|
|
if child == nil {
|
|
continue
|
|
}
|
|
fmt.Fprintf(w, "%s%d/8 (%02x/8): ", strings.Repeat(" ", indent), addr, addr)
|
|
strideSummary(w, child, indent)
|
|
}
|
|
}
|
|
|
|
// prefixStrictlyContains reports whether child is a prefix within
|
|
// parent, but not parent itself.
|
|
func prefixStrictlyContains(parent, child netip.Prefix) bool {
|
|
return parent.Overlaps(child) && parent.Bits() < child.Bits()
|
|
}
|
|
|
|
// computePrefixSplit returns the smallest common prefix that contains
|
|
// both a and b. lastCommon is 8-bit aligned, with aStride and bStride
|
|
// indicating the value of the 8-bit stride immediately following
|
|
// lastCommon.
|
|
//
|
|
// computePrefixSplit is used in constructing an intermediate
|
|
// strideTable when a new prefix needs to be inserted in a compressed
|
|
// table. It can be read as: given that a is already in the table, and
|
|
// b is being inserted, what is the prefix of the new intermediate
|
|
// strideTable that needs to be created, and at what addresses in that
|
|
// new strideTable should a and b's subsequent strideTables be
|
|
// attached?
|
|
//
|
|
// Note as a special case, this can be called with a==b. An example of
|
|
// when this happens:
|
|
// - We want to insert the prefix 1.2.0.0/16
|
|
// - A strideTable exists for 1.2.0.0/16, because another child
|
|
// prefix already exists (e.g. 1.2.3.4/32)
|
|
// - The 1.0.0.0/8 strideTable does not exist, because path
|
|
// compression removed it.
|
|
//
|
|
// In this scenario, the caller of computePrefixSplit ends up making a
|
|
// "wrong turn" while traversing strideTables: it was looking for the
|
|
// 1.0.0.0/8 table, but ended up at the 1.2.0.0/16 table. When this
|
|
// happens, it will invoke computePrefixSplit(1.2.0.0/16, 1.2.0.0/16),
|
|
// and we return 1.0.0.0/8 as the missing intermediate.
|
|
func computePrefixSplit(a, b netip.Prefix) (lastCommon netip.Prefix, aStride, bStride uint8) {
|
|
a = a.Masked()
|
|
b = b.Masked()
|
|
if a.Bits() == 0 || b.Bits() == 0 {
|
|
panic("computePrefixSplit called with a default route")
|
|
}
|
|
if a.Addr().Is4() != b.Addr().Is4() {
|
|
panic("computePrefixSplit called with mismatched address families")
|
|
}
|
|
|
|
minPrefixLen := a.Bits()
|
|
if b.Bits() < minPrefixLen {
|
|
minPrefixLen = b.Bits()
|
|
}
|
|
|
|
commonBits := commonBits(a.Addr(), b.Addr(), minPrefixLen)
|
|
// We want to know how many 8-bit strides are shared between a and
|
|
// b. Naively, this would be commonBits/8, but this introduces an
|
|
// off-by-one error. This is due to the way our ART stores
|
|
// prefixes whose length falls exactly on a stride boundary.
|
|
//
|
|
// Consider 192.168.1.0/24 and 192.168.0.0/16. commonBits
|
|
// correctly reports that these prefixes have their first 16 bits
|
|
// in common. However, in the ART they only share 1 common stride:
|
|
// they both use the 192.0.0.0/8 strideTable, but 192.168.0.0/16
|
|
// is stored as 168/8 within that table, and not as 0/0 in the
|
|
// 192.168.0.0/16 table.
|
|
//
|
|
// So, when commonBits matches the length of one of the inputs and
|
|
// falls on a boundary between strides, the strideTable one
|
|
// further up from commonBits/8 is the one we need to create,
|
|
// which means we have to adjust the stride count down by one.
|
|
if commonBits == minPrefixLen {
|
|
commonBits--
|
|
}
|
|
commonStrides := commonBits / 8
|
|
lastCommon, err := a.Addr().Prefix(commonStrides * 8)
|
|
if err != nil {
|
|
panic(fmt.Sprintf("computePrefixSplit constructing common prefix: %v", err))
|
|
}
|
|
if a.Addr().Is4() {
|
|
aStride = a.Addr().As4()[commonStrides]
|
|
bStride = b.Addr().As4()[commonStrides]
|
|
} else {
|
|
aStride = a.Addr().As16()[commonStrides]
|
|
bStride = b.Addr().As16()[commonStrides]
|
|
}
|
|
return lastCommon, aStride, bStride
|
|
}
|
|
|
|
// commonBits returns the number of common leading bits of a and b.
|
|
// If the number of common bits exceeds maxBits, it returns maxBits
|
|
// instead.
|
|
func commonBits(a, b netip.Addr, maxBits int) int {
|
|
if a.Is4() != b.Is4() {
|
|
panic("commonStrides called with mismatched address families")
|
|
}
|
|
var common int
|
|
// The following implements an old bit-twiddling trick to compute
|
|
// the number of common leading bits: if you XOR two numbers
|
|
// together, equal bits become 0 and unequal bits become 1. You
|
|
// can then count the number of leading zeros (which is a single
|
|
// instruction on modern CPUs) to get the answer.
|
|
//
|
|
// This code is a little more complex than just XOR + count
|
|
// leading zeros, because IPv4 and IPv6 are different sizes, and
|
|
// for IPv6 we have to do the math in two 64-bit chunks because Go
|
|
// lacks a uint128 type.
|
|
if a.Is4() {
|
|
aNum, bNum := ipv4AsUint(a), ipv4AsUint(b)
|
|
common = bits.LeadingZeros32(aNum ^ bNum)
|
|
} else {
|
|
aNumHi, aNumLo := ipv6AsUint(a)
|
|
bNumHi, bNumLo := ipv6AsUint(b)
|
|
common = bits.LeadingZeros64(aNumHi ^ bNumHi)
|
|
if common == 64 {
|
|
common += bits.LeadingZeros64(aNumLo ^ bNumLo)
|
|
}
|
|
}
|
|
if common > maxBits {
|
|
common = maxBits
|
|
}
|
|
return common
|
|
}
|
|
|
|
// ipv4AsUint returns ip as a uint32.
|
|
func ipv4AsUint(ip netip.Addr) uint32 {
|
|
bs := ip.As4()
|
|
return binary.BigEndian.Uint32(bs[:])
|
|
}
|
|
|
|
// ipv6AsUint returns ip as a pair of uint64s.
|
|
func ipv6AsUint(ip netip.Addr) (uint64, uint64) {
|
|
bs := ip.As16()
|
|
return binary.BigEndian.Uint64(bs[:8]), binary.BigEndian.Uint64(bs[8:])
|
|
}
|