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net/art: make each strideTable track the IP prefix it represents

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This is a prerequisite for path compression, so that insert/delete
can determine when compression occurred.

Updates #7781

Signed-off-by: David Anderson <danderson@tailscale.com>
This commit is contained in:
David Anderson
2023-04-06 11:20:33 -07:00
committed by Dave Anderson
parent 45b5d0983c
commit 486195edf0
3 changed files with 59 additions and 8 deletions
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34
net/art/stride_table.go
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@@ -8,6 +8,7 @@ import (
"fmt"
"io"
"math/bits"
"net/netip"
"strconv"
"strings"
)
@@ -32,6 +33,9 @@ type strideEntry[T any] struct {
// The leaves of the binary tree are host routes (/8s). Each parent is a
// successively larger prefix that encompasses its children (/7 through /0).
type strideTable[T any] struct {
// prefix is the prefix represented by the 0/0 route of this strideTable. It
// is used in multi-level tables to support path compression.
prefix netip.Prefix
// entries is the nodes of the binary tree, laid out in a flattened array.
//
// The array indices are arranged by the prefixIndex function, such that the
@@ -76,7 +80,9 @@ func (t *strideTable[T]) deleteChild(idx int) {
func (t *strideTable[T]) getOrCreateChild(addr uint8) *strideTable[T] {
idx := hostIndex(addr)
if t.entries[idx].child == nil {
t.entries[idx].child = new(strideTable[T])
t.entries[idx].child = &strideTable[T]{
prefix: childPrefixOf(t.prefix, addr),
}
t.refs++
}
return t.entries[idx].child
@@ -229,3 +235,29 @@ func formatPrefixTable(addr uint8, len int) string {
}
return fmt.Sprintf("%3d/%d", addr, len)
}
// childPrefixOf returns the child prefix of parent whose final byte
// is stride. The parent prefix must be byte-aligned
// (i.e. parent.Bits() must be a multiple of 8), and be no more
// specific than /24 for IPv4 or /120 for IPv6.
//
// For example, childPrefixOf("192.168.0.0/16", 8) == "192.168.8.0/24".
func childPrefixOf(parent netip.Prefix, stride uint8) netip.Prefix {
l := parent.Bits()
if l%8 != 0 {
panic("parent prefix is not 8-bit aligned")
}
if l >= parent.Addr().BitLen() {
panic("parent prefix cannot be extended further")
}
off := l / 8
if parent.Addr().Is4() {
bs := parent.Addr().As4()
bs[off] = stride
return netip.PrefixFrom(netip.AddrFrom4(bs), l+8)
} else {
bs := parent.Addr().As16()
bs[off] = stride
return netip.PrefixFrom(netip.AddrFrom16(bs), l+8)
}
}