wip almost

Signed-off-by: David Anderson <danderson@tailscale.com>

net/art/stride_table.go

@@ -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
@@ -44,10 +48,10 @@ type strideTable[T any] struct {
 	// memory trickery to store the refcount, but this is Go, where we don't
 	// store random bits in pointers lest we confuse the GC)
 	entries [lastHostIndex + 1]strideEntry[T]
-	// refs is the number of route entries and child strideTables referenced by
-	// this table. It is used in the multi-layered logic to determine when this
-	// table is empty and can be deleted.
-	refs int
+	// routeRefs is the number of route entries in this table.
+	routeRefs uint16
+	// childRefs is the number of child strideTables referenced by this table.
+	childRefs uint16
 }
 
 const (
@@ -68,18 +72,36 @@ func (t *strideTable[T]) getChild(addr uint8) (child *strideTable[T], idx int) {
 // obtained via a call to getChild.
 func (t *strideTable[T]) deleteChild(idx int) {
 	t.entries[idx].child = nil
-	t.refs--
+	t.childRefs--
+}
+
+func (t *strideTable[T]) setChild(addr uint8, child *strideTable[T]) {
+	idx := hostIndex(addr)
+	if t.entries[idx].child == nil {
+		t.childRefs++
+	}
+	t.entries[idx].child = child
+}
+
+func (t *strideTable[T]) setChildByIdx(idx int, child *strideTable[T]) {
+	if t.entries[idx].child == nil {
+		t.childRefs++
+	}
+	t.entries[idx].child = child
 }
 
 // getOrCreateChild returns the child strideTable for addr, creating it if
 // necessary.
-func (t *strideTable[T]) getOrCreateChild(addr uint8) *strideTable[T] {
+func (t *strideTable[T]) getOrCreateChild(addr uint8) (child *strideTable[T], created bool) {
 	idx := hostIndex(addr)
 	if t.entries[idx].child == nil {
-		t.entries[idx].child = new(strideTable[T])
-		t.refs++
+		t.entries[idx].child = &strideTable[T]{
+			prefix: childPrefixOf(t.prefix, addr),
+		}
+		t.childRefs++
+		return t.entries[idx].child, true
 	}
-	return t.entries[idx].child
+	return t.entries[idx].child, false
 }
 
 func (t *strideTable[T]) getValAndChild(addr uint8) (*T, *strideTable[T]) {
@@ -87,6 +109,15 @@ func (t *strideTable[T]) getValAndChild(addr uint8) (*T, *strideTable[T]) {
 	return t.entries[idx].value, t.entries[idx].child
 }
 
+func (t *strideTable[T]) findFirstChild() *strideTable[T] {
+	for i := firstHostIndex; i <= lastHostIndex; i++ {
+		if child := t.entries[i].child; child != nil {
+			return child
+		}
+	}
+	return nil
+}
+
 // allot updates entries whose stored prefixIndex matches oldPrefixIndex, in the
 // subtree rooted at idx. Matching entries have their stored prefixIndex set to
 // newPrefixIndex, and their value set to val.
@@ -127,7 +158,7 @@ func (t *strideTable[T]) insert(addr uint8, prefixLen int, val *T) {
 	if oldIdx != idx {
 		// This route entry was freshly created (not just updated), that's a new
 		// reference.
-		t.refs++
+		t.routeRefs++
 	}
 	return
 }
@@ -144,7 +175,7 @@ func (t *strideTable[T]) delete(addr uint8, prefixLen int) *T {
 
 	parentIdx := idx >> 1
 	t.allot(idx, idx, t.entries[parentIdx].prefixIndex, t.entries[parentIdx].value)
-	t.refs--
+	t.routeRefs--
 	return val
 }
 
@@ -183,7 +214,7 @@ func (t *strideTable[T]) treeDebugString() string {
 func (t *strideTable[T]) treeDebugStringRec(w io.Writer, idx, indent int) {
 	addr, len := inversePrefixIndex(idx)
 	if t.entries[idx].prefixIndex != 0 && t.entries[idx].prefixIndex == idx {
-		fmt.Fprintf(w, "%s%d/%d (%d/%d) = %v\n", strings.Repeat(" ", indent), addr, len, addr, len, *t.entries[idx].value)
+		fmt.Fprintf(w, "%s%d/%d (%02x/%d) = %v\n", strings.Repeat(" ", indent), addr, len, addr, len, *t.entries[idx].value)
 		indent += 2
 	}
 	if idx >= firstHostIndex {
@@ -229,3 +260,31 @@ func formatPrefixTable(addr uint8, len int) string {
 	}
 	return fmt.Sprintf("%3d/%d", addr, len)
 }
+
+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)
+	}
+}
+
+func mustPrefix(addr netip.Addr, bits int) netip.Prefix {
+	pfx, err := addr.Prefix(bits)
+	if err != nil {
+		panic(fmt.Sprintf("invalid prefix requested: %s/%d", addr, bits))
+	}
+	return pfx
+}

net/art/stride_table_test.go

@@ -7,6 +7,7 @@ import (
 	"bytes"
 	"fmt"
 	"math/rand"
+	"net/netip"
 	"sort"
 	"strings"
 	"testing"
@@ -100,7 +101,7 @@ func TestStrideTableInsertShuffled(t *testing.T) {
 		for _, route := range routes2 {
 			rt2.insert(route.addr, route.len, route.val)
 		}
-		if diff := cmp.Diff(rt, rt2, cmp.AllowUnexported(strideTable[int]{}, strideEntry[int]{})); diff != "" {
+		if diff := cmp.Diff(rt, rt2, cmpDiffOpts...); diff != "" {
 			t.Errorf("tables ended up different with different insertion order (-got+want):\n%s\n\nOrder 1: %v\nOrder 2: %v", diff, formatSlowEntriesShort(routes), formatSlowEntriesShort(routes2))
 		}
 
@@ -108,7 +109,7 @@ func TestStrideTableInsertShuffled(t *testing.T) {
 		for _, route := range routes2 {
 			rtZero2.insert(route.addr, route.len, &zero)
 		}
-		if diff := cmp.Diff(rtZero, rtZero2, cmp.AllowUnexported(strideTable[int]{}, strideEntry[int]{})); diff != "" {
+		if diff := cmp.Diff(rtZero, rtZero2, cmpDiffOpts...); diff != "" {
 			t.Errorf("tables with identical vals ended up different with different insertion order (-got+want):\n%s\n\nOrder 1: %v\nOrder 2: %v", diff, formatSlowEntriesShort(routes), formatSlowEntriesShort(routes2))
 		}
 	}
@@ -180,7 +181,7 @@ func TestStrideTableDeleteShuffle(t *testing.T) {
 		for _, route := range toDelete2 {
 			rt2.delete(route.addr, route.len)
 		}
-		if diff := cmp.Diff(rt, rt2, cmp.AllowUnexported(strideTable[int]{}, strideEntry[int]{})); diff != "" {
+		if diff := cmp.Diff(rt, rt2, cmpDiffOpts...); diff != "" {
 			t.Errorf("tables ended up different with different deletion order (-got+want):\n%s\n\nOrder 1: %v\nOrder 2: %v", diff, formatSlowEntriesShort(toDelete), formatSlowEntriesShort(toDelete2))
 		}
 
@@ -191,7 +192,7 @@ func TestStrideTableDeleteShuffle(t *testing.T) {
 		for _, route := range toDelete2 {
 			rtZero2.delete(route.addr, route.len)
 		}
-		if diff := cmp.Diff(rtZero, rtZero2, cmp.AllowUnexported(strideTable[int]{}, strideEntry[int]{})); diff != "" {
+		if diff := cmp.Diff(rtZero, rtZero2, cmpDiffOpts...); diff != "" {
 			t.Errorf("tables with identical vals ended up different with different deletion order (-got+want):\n%s\n\nOrder 1: %v\nOrder 2: %v", diff, formatSlowEntriesShort(toDelete), formatSlowEntriesShort(toDelete2))
 		}
 	}
@@ -382,3 +383,8 @@ func formatSlowEntriesShort[T any](ents []slowEntry[T]) string {
 	}
 	return "[" + strings.Join(ret, " ") + "]"
 }
+
+var cmpDiffOpts = []cmp.Option{
+	cmp.AllowUnexported(strideTable[int]{}, strideEntry[int]{}),
+	cmp.Comparer(func(a, b netip.Prefix) bool { return a == b }),
+}

net/art/table.go

@@ -16,147 +16,573 @@ import (
 	"bytes"
 	"fmt"
 	"io"
+	"math/bits"
 	"net/netip"
 	"strings"
+	"sync"
+)
+
+const (
+	debugInsert = false
+	debugDelete = false
 )
 
 // Table is an IPv4 and IPv6 routing table.
 type Table[T any] struct {
-	v4 strideTable[T]
-	v6 strideTable[T]
+	v4       strideTable[T]
+	v6       strideTable[T]
+	initOnce sync.Once
+}
+
+func (t *Table[T]) init() {
+	t.initOnce.Do(func() {
+		t.v4.prefix = netip.PrefixFrom(netip.IPv4Unspecified(), 0)
+		t.v6.prefix = netip.PrefixFrom(netip.IPv6Unspecified(), 0)
+	})
 }
 
 // Get does a route lookup for addr and returns the associated value, or nil if
 // no route matched.
 func (t *Table[T]) Get(addr netip.Addr) *T {
+	t.init()
 	st := &t.v4
 	if addr.Is6() {
 		st = &t.v6
 	}
 
-	var ret *T
-	for _, stride := range addr.AsSlice() {
-		rt, child := st.getValAndChild(stride)
+	i := 0
+	bs := addr.AsSlice()
+	// With path compression, we might skip over some address bits while walking
+	// to a strideTable leaf. This means the leaf answer we find might not be
+	// correct, because path compression took us down the wrong subtree. When
+	// that happens, we have to backtrack and figure out which most specific
+	// route further up the tree is relevant to addr, and return that.
+	//
+	// So, as we walk down the stride tables, each time we find a non-nil route
+	// result, we have to remember it and the associated strideTable prefix.
+	//
+	// We could also deal with this edge case of path compression by checking
+	// the strideTable prefix on each table as we descend, but that means we
+	// have to pay N prefix.Contains checks on every route lookup (where N is
+	// the number of strideTables in the path), rather than only paying M prefix
+	// comparisons in the edge case (where M is the number of strideTables in
+	// the path with a non-nil route of their own).
+	strideIdx := 0
+	stridePrefixes := [16]netip.Prefix{}
+	strideRoutes := [16]*T{}
+findLeaf:
+	for {
+		rt, child := st.getValAndChild(bs[i])
 		if rt != nil {
-			// Found a more specific route than whatever we found previously,
-			// keep a note.
-			ret = rt
+			// This strideTable contains a route that may be relevant to our
+			// search, remember it.
+			stridePrefixes[strideIdx] = st.prefix
+			strideRoutes[strideIdx] = rt
+			strideIdx++
 		}
 		if child == nil {
-			// No sub-routes further down, whatever we have recorded in ret is
-			// the result.
-			return ret
+			// No sub-routes further down, the last thing we recorded in
+			// strideRoutes is tentatively the result, barring path compression
+			// misdirection.
+			break findLeaf
 		}
 		st = child
+		// Path compression means we may be skipping over some intermediate
+		// tables. We have to skip forward to whatever depth st now references.
+		i = st.prefix.Bits() / 8
 	}
 
-	// Unreachable because Insert/Delete won't allow the leaf strideTables to
-	// have children, so we must return via the nil check in the loop.
-	panic("unreachable")
+	// Walk backwards through the hits we recorded in strideRoutes and
+	// stridePrefixes, returning the first one whose subtree matches addr.
+	//
+	// In the common case where path compression did not mislead us, we'll
+	// return on the first loop iteration because the last route we recorded was
+	// the correct most-specific route.
+	for strideIdx > 0 {
+		strideIdx--
+		if stridePrefixes[strideIdx].Contains(addr) {
+			return strideRoutes[strideIdx]
+		}
+	}
+
+	// We either found no route hits at all (both previous loops terminated
+	// immediately), or we went on a wild goose chase down a compressed path for
+	// the wrong prefix, and also found no usable routes on the way back up to
+	// the root. This is a miss.
+	return nil
 }
 
 // Insert adds pfx to the table, with value val.
 // If pfx is already present in the table, its value is set to val.
 func (t *Table[T]) Insert(pfx netip.Prefix, val *T) {
+	t.init()
 	if val == nil {
 		panic("Table.Insert called with nil value")
 	}
+
+	if debugInsert {
+		defer func() {
+			fmt.Printf("%s", t.debugSummary())
+		}()
+	}
+
+	// The standard library doesn't enforce normalized prefixes (where
+	// the non-prefix bits are all zero). Our algorithms all require
+	// normalized prefixes though, so do it upfront.
+	pfx = pfx.Masked()
+
+	if debugInsert {
+		fmt.Printf("\ninsert: start pfx=%s\n", pfx)
+	}
+
 	st := &t.v4
 	if pfx.Addr().Is6() {
 		st = &t.v6
 	}
-	bs := pfx.Addr().AsSlice()
-	i := 0
-	numBits := pfx.Bits()
 
-	// The strideTable we want to insert into is potentially at the end of a
-	// chain of parent tables, each one encoding successive 8 bits of the
-	// prefix. Navigate downwards, allocating child tables as needed, until we
-	// find the one this prefix belongs in.
-	for numBits > 8 {
-		st = st.getOrCreateChild(bs[i])
-		i++
-		numBits -= 8
+	// This algorithm is full of off-by-one headaches that boil down
+	// to the fact that pfx.Bits() has (2^n)+1 values, rather than
+	// just 2^n. For example, an IPv4 prefix length can be 0 through
+	// 32, which is 33 values.
+	//
+	// This extra possible value creates all kinds of headaches as we
+	// do bits and bytes math to traverse strideTables below. So, we
+	// treat the default route 0/0 specially here, that way the rest
+	// of the logic goes back to having 2^n values to reason about,
+	// which can be done in a nice and regular fashion with no edge
+	// cases.
+	if pfx.Bits() == 0 {
+		if debugInsert {
+			fmt.Printf("insert: default route\n")
+		}
+		st.insert(0, 0, val)
+		return
+	}
+
+	bs := pfx.Addr().AsSlice()
+
+	// No matter what we do as we traverse strideTables, our final
+	// action will be to insert the last 1-8 bits of pfx into a
+	// strideTable somewhere.
+	//
+	// We calculate upfront the byte position in bs of the end of the
+	// prefix; the number of bits within that byte that contain prefix
+	// data; and the prefix owned by the strideTable into which we'll
+	// eventually insert.
+	//
+	// We need this in a couple different branches of the code below,
+	// and because the possible values are 1-indexed (1 through 32 for
+	// ipv4, 1 through 128 for ipv6), the math is very slightly
+	// unusual to account for the off-by-one indexing. Do it once up
+	// here, with this large comment, rather than reproduce the subtle
+	// math in multiple places further down.
+	finalByteIdx := (pfx.Bits() - 1) / 8
+	finalBits := pfx.Bits() - (finalByteIdx * 8)
+	finalStridePrefix := mustPrefix(pfx.Addr(), finalByteIdx*8)
+	if debugInsert {
+		fmt.Printf("insert: finalByteIdx=%d finalBits=%d finalStridePrefix=%s\n", finalByteIdx, finalBits, finalStridePrefix)
+	}
+
+	// The strideTable we want to insert into is potentially at the
+	// end of a chain of strideTables, each one encoding 8 bits of the
+	// prefix.
+	//
+	// We're expecting to walk down a path of tables, although with
+	// prefix compression we may end up skipping some links in the
+	// chain, or taking wrong turns and having to course correct.
+	//
+	// As we walk down the tree, byteIdx is the byte of bs we're
+	// currently examining to choose our next step, and numBits is the
+	// number of bits that remain in pfx, starting with the byte at
+	// byteIdx inclusive.
+	byteIdx := 0
+	numBits := pfx.Bits()
+	for {
+		if debugInsert {
+			fmt.Printf("insert: loop byteIdx=%d numBits=%d st.prefix=%s\n", byteIdx, numBits, st.prefix)
+		}
+		if numBits <= 8 {
+			if debugInsert {
+				fmt.Printf("insert: existing leaf st.prefix=%s addr=%d/%d\n", st.prefix, bs[finalByteIdx], finalBits)
+			}
+			// We've reached the end of the prefix, whichever
+			// strideTable we're looking at now is the place where we
+			// need to insert.
+			st.insert(bs[finalByteIdx], finalBits, val)
+			return
+		}
+
+		// Otherwise, we need to go down at least one more level of
+		// strideTables. With prefix compression, each level of
+		// descent can have one of three outcomes: we find a place
+		// where prefix compression is possible; a place where prefix
+		// compression made us take a "wrong turn"; or a point along
+		// our intended path that we have to keep following.
+		child, created := st.getOrCreateChild(bs[byteIdx])
+		switch {
+		case created:
+			// The subtree we need for pfx doesn't exist yet. The rest
+			// of the path, if we were to create it, will consist of a
+			// bunch of strideTables with a single child. We can use
+			// path compression to elide those intermediates, and jump
+			// straight to the final strideTable that hosts this
+			// prefix.
+			child.prefix = finalStridePrefix
+			child.insert(bs[finalByteIdx], finalBits, val)
+			if debugInsert {
+				fmt.Printf("insert: new leaf st.prefix=%s child.prefix=%s addr=%d/%d\n", st.prefix, child.prefix, bs[finalByteIdx], finalBits)
+			}
+			return
+		case child.prefix == pfx:
+			// Edge case, /16 vs. /24
+			// Still fucked, rerun TestDeleteCompare to figure out why
+			intermediatePrefix, _ := pfx.Addr().Prefix(pfx.Bits() - 8)
+			intermediate := &strideTable[T]{prefix: intermediatePrefix}
+			st.setChild(bs[byteIdx], intermediate)
+			intermediate.setChild(bs[child.prefix.Bits()/8], child)
+			intermediate.insert(bs[finalByteIdx], finalBits, val)
+			return
+		case !prefixContains(child.prefix, pfx):
+			// child already exists, but its prefix does not contain
+			// pfx. This means that the path between st and child was
+			// compressed by a previous insertion, and somewhere in
+			// the (implicit) compressed path we took a wrong turn,
+			// into the wrong part of st's subtree.
+			//
+			// This is okay, because pfx and child.prefix must have a
+			// common ancestor node somewhere between st and child. We
+			// can figure out what node that is, materialize it
+			// between st and child, and resume from there.
+			intermediatePrefix, addrOfExisting, addrOfNew := computePrefixSplit(child.prefix, pfx)
+			intermediate := &strideTable[T]{prefix: intermediatePrefix} // TODO: make this whole thing be st.AddIntermediate or something?
+			st.setChild(bs[byteIdx], intermediate)
+			intermediate.setChild(addrOfExisting, child)
+
+			if debugInsert {
+				fmt.Printf("insert: new intermediate st.prefix=%s intermediate.prefix=%s child.prefix=%s\n", st.prefix, intermediate.prefix, child.prefix)
+			}
+
+			// Now, we have a chain of st -> intermediate -> child.
+			//
+			// pfx either lives in a different child of intermediate,
+			// or in intermediate itself. For example, if we created
+			// the intermediate 1.2.0.0/16, pfx=1.2.3.4/32 would have
+			// to go into a new child of intermediate, but
+			// pfx=1.2.0.0/18 would go into intermediate directly.
+			if remain := pfx.Bits() - intermediate.prefix.Bits(); remain <= 8 {
+				if debugInsert {
+					fmt.Printf("insert: into intermediate intermediate.prefix=%s addr=%d/%d\n", intermediate.prefix, bs[finalByteIdx], finalBits)
+				}
+				// pfx lives in intermediate.
+				intermediate.insert(bs[finalByteIdx], finalBits, val)
+			} else {
+				// pfx lives in a different child subtree of
+				// intermediate. By definition this subtree doesn't
+				// exist at all, otherwise we'd never have entereed
+				// this entire "wrong turn" codepath in the first
+				// place.
+				//
+				// This means we can apply prefix compression as we
+				// create this new child, and we're done.
+				st, created = intermediate.getOrCreateChild(addrOfNew)
+				if !created {
+					panic("new child path unexpectedly exists during path decompression")
+				}
+				st.prefix = finalStridePrefix
+				st.insert(bs[finalByteIdx], finalBits, val)
+				if debugInsert {
+					fmt.Printf("insert: new child st.prefix=%s addr=%d/%d\n", st.prefix, bs[finalByteIdx], finalBits)
+				}
+			}
+			return
+		default:
+			// An expected child table exists along pfx's
+			// path. Continue traversing downwards.
+			st = child
+			byteIdx = child.prefix.Bits() / 8
+			numBits = pfx.Bits() - child.prefix.Bits()
+			if debugInsert {
+				fmt.Printf("insert: descend st.prefix=%s\n", st.prefix)
+			}
+		}
 	}
-	// Finally, insert the remaining 0-8 bits of the prefix into the child
-	// table.
-	st.insert(bs[i], numBits, val)
 }
 
 // Delete removes pfx from the table, if it is present.
 func (t *Table[T]) Delete(pfx netip.Prefix) {
+	t.init()
+
+	// The standard library doesn't enforce normalized prefixes (where
+	// the non-prefix bits are all zero). Our algorithms all require
+	// normalized prefixes though, so do it upfront.
+	pfx = pfx.Masked()
+
+	if debugDelete {
+		defer func() {
+			fmt.Printf("%s", t.debugSummary())
+		}()
+		fmt.Printf("\ndelete: start pfx=%s table:\n%s", pfx, t.debugSummary())
+	}
+
 	st := &t.v4
 	if pfx.Addr().Is6() {
 		st = &t.v6
 	}
-	bs := pfx.Addr().AsSlice()
-	i := 0
-	numBits := pfx.Bits()
 
-	// Deletion may drive the refcount of some strideTables down to zero. We
-	// need to clean up these dangling tables, so we have to keep track of which
-	// tables we touch on the way down, and which strideEntry index each child
-	// is registered in.
+	// Deletion may drive the refcount of some strideTables down to
+	// zero. We need to clean up these dangling tables, so we have to
+	// keep track of which tables we touch on the way down, and which
+	// strideEntry index each child is registered in.
+	strideIdx := 0
 	strideTables := [16]*strideTable[T]{st}
-	var strideIndexes [16]int
+	strideIndexes := [16]int{}
 
-	// Similar to Insert, navigate down the tree of strideTables, looking for
-	// the one that houses the last 0-8 bits of the prefix to delete.
+	// Similar to Insert, navigate down the tree of strideTables,
+	// looking for the one that houses this prefix. This part is
+	// easier than with insertion, since we can bail if the path ends
+	// early or takes an unexpected detour.  However, unlike
+	// insertion, there's a whole post-deletion cleanup phase later
+	// on.
 	//
-	// The only difference is that here, we don't create missing child tables.
-	// If a child necessary to pfx is missing, then the pfx cannot exist in the
-	// Table, and we can exit early.
+	// As we walk down the tree, byteIdx is the byte of bs we're
+	// currently examining to choose our next step, and numBits is the
+	// number of bits that remain in pfx, starting with the byte at
+	// byteIdx inclusive.
+	bs := pfx.Addr().AsSlice()
+	byteIdx := 0
+	numBits := pfx.Bits()
 	for numBits > 8 {
-		child, idx := st.getChild(bs[i])
+		if debugDelete {
+			fmt.Printf("delete: loop byteIdx=%d numBits=%d st.prefix=%s\n", byteIdx, numBits, st.prefix)
+		}
+		child, idx := st.getChild(bs[byteIdx])
 		if child == nil {
 			// Prefix can't exist in the table, one of the necessary
-			// strideTables doesn't exit.
+			// strideTables doesn't exist.
+			if debugDelete {
+				fmt.Printf("delete: missing needed child pfx=%s\n", pfx)
+			}
 			return
 		}
 		// Note that the strideIndex and strideTables entries are off-by-one.
 		// The child table pointer is recorded at i+1, but it is referenced by a
 		// particular index in the parent table, at index i.
-		strideIndexes[i] = idx
-		i++
-		strideTables[i] = child
-		numBits -= 8
+		strideIndexes[strideIdx] = idx
+		strideTables[strideIdx+1] = child
+		strideIdx++
+
+		// Path compression means byteIdx can jump forwards
+		// unpredictably. Recompute the next byte to look at from the
+		// child we just found.
+		byteIdx = child.prefix.Bits() / 8
+		numBits = pfx.Bits() - child.prefix.Bits()
 		st = child
+
+		if debugDelete {
+			fmt.Printf("delete: descend st.prefix=%s\n", st.prefix)
+		}
 	}
-	if st.delete(bs[i], numBits) == nil {
-		// Prefix didn't exist in the expected strideTable, refcount hasn't
-		// changed, no need to run through cleanup.
+
+	// We reached a leaf stride table that seems to be in the right
+	// spot. But path compression might have led us to the wrong
+	// table. Or, we might be in the right place, but the strideTable
+	// just doesn't contain the prefix at all.
+	if !prefixContains(st.prefix, pfx) {
+		// Wrong table, the requested prefix can't exist since its
+		// path led us to the wrong place.
+		if debugDelete {
+			fmt.Printf("delete: wrong leaf table pfx=%s\n", pfx)
+		}
+		return
+	}
+	if debugDelete {
+		fmt.Printf("delete: delete from st.prefix=%s addr=%d/%d\n", st.prefix, bs[byteIdx], numBits)
+	}
+	if st.delete(bs[byteIdx], numBits) == nil {
+		// We're in the right strideTable, but pfx wasn't in
+		// it. Refcounts haven't changed, so no need to run through
+		// cleanup.
+		if debugDelete {
+			fmt.Printf("delete: prefix not present pfx=%s\n", pfx)
+		}
 		return
 	}
 
-	// st.delete reduced st's refcount by one, so we may be hanging onto a chain
-	// of redundant strideTables. Walk back up the path we recorded in the
-	// descent loop, deleting tables until we encounter one that still has other
-	// refs (or we hit the root strideTable, which is never deleted).
-	for i > 0 && strideTables[i].refs == 0 {
-		strideTables[i-1].deleteChild(strideIndexes[i-1])
-		i--
+	// st.delete reduced st's refcount by one. This table may now be
+	// reclaimable, and depending on how we can reclaim it, the parent
+	// tables may also need to be considered for reclamation. This
+	// loop ends as soon as an iteration takes no action, or takes an
+	// action that doesn't alter the parent table's refcounts.
+	//
+	// We start our walk back at strideTables[strideIdx], which
+	// contains st.
+	for strideIdx > 0 {
+		cur := strideTables[strideIdx]
+		if debugDelete {
+			fmt.Printf("delete: GC strideIdx=%d st.prefix=%s\n", strideIdx, cur.prefix)
+		}
+		if cur.routeRefs > 0 {
+			// the strideTable has route entries, it cannot be deleted
+			// or compacted.
+			if debugDelete {
+				fmt.Printf("delete: has other routes st.prefix=%s\n", cur.prefix)
+			}
+			return
+		}
+		switch cur.childRefs {
+		case 0:
+			// no routeRefs and no childRefs, this table can be
+			// deleted. This will alter the parent table's refcount,
+			// so we'll have to look at it as well (in the next loop
+			// iteration).
+			if debugDelete {
+				fmt.Printf("delete: remove st.prefix=%s\n", cur.prefix)
+			}
+			strideTables[strideIdx-1].deleteChild(strideIndexes[strideIdx-1])
+			strideIdx--
+		case 1:
+			// This table has no routes, and a single child. Compact
+			// this table out of existence by making the parent point
+			// directly at the one child. This does not affect the
+			// parent's refcounts, so the parent can't be eligible for
+			// deletion or compaction, and we can stop.
+			child := strideTables[strideIdx].findFirstChild()
+			parent := strideTables[strideIdx-1]
+			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].setChildByIdx(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
+		}
 	}
 }
 
+func (t *Table[T]) numStrides() int {
+	seen := map[*strideTable[T]]bool{}
+	return t.numStridesRec(seen, &t.v4) + t.numStridesRec(seen, &t.v6)
+}
+
+func (t *Table[T]) numStridesRec(seen map[*strideTable[T]]bool, st *strideTable[T]) int {
+	ret := 1
+	if st.childRefs == 0 {
+		return ret
+	}
+	for i := firstHostIndex; i <= lastHostIndex; i++ {
+		if c := st.entries[i].child; c != nil && !seen[c] {
+			seen[c] = true
+			ret += t.numStridesRec(seen, c)
+		}
+	}
+	return ret
+}
+
 // 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, 0)
+	strideSummary(&ret, &t.v4, 4)
 	fmt.Fprintf(&ret, "v6: ")
-	strideSummary(&ret, &t.v6, 0)
+	strideSummary(&ret, &t.v6, 4)
 	return ret.String()
 }
 
 func strideSummary[T any](w io.Writer, st *strideTable[T], indent int) {
-	fmt.Fprintf(w, "%d refs\n", st.refs)
-	indent += 2
+	fmt.Fprintf(w, "%s: %d routes, %d children\n", st.prefix, st.routeRefs, st.childRefs)
+	indent += 4
+	st.treeDebugStringRec(w, 1, indent)
 	for i := firstHostIndex; i <= lastHostIndex; i++ {
 		if child := st.entries[i].child; child != nil {
 			addr, len := inversePrefixIndex(i)
-			fmt.Fprintf(w, "%s%d/%d: ", strings.Repeat(" ", indent), addr, len)
+			fmt.Fprintf(w, "%s%d/%d (%02x/%d): ", strings.Repeat(" ", indent), addr, len, addr, len)
 			strideSummary(w, child, indent)
 		}
 	}
 }
+
+func prefixContains(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 host addresses in that new strideTable should a and b's subsequent
+// strideTables be attached?
+func computePrefixSplit(a, b netip.Prefix) (lastCommon netip.Prefix, aStride, bStride uint8) {
+	a = a.Masked()
+	b = b.Masked()
+	if a == b {
+		panic("computePrefixSplit called with identical prefixes")
+	}
+	if a.Addr().Is4() != b.Addr().Is4() {
+		panic("computePrefixSplit called with mismatched address families")
+	}
+
+	minPrefixLen := a.Bits()
+	if b.Bits() < minPrefixLen {
+		minPrefixLen = b.Bits()
+	}
+
+	commonStrides := commonStrides(a.Addr(), b.Addr(), minPrefixLen)
+	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
+}
+
+func commonStrides(a, b netip.Addr, maxBits int) int {
+	if a.Is4() != b.Is4() {
+		panic("commonStrides called with mismatched address families")
+	}
+	var common int
+	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 / 8
+}
+
+func ipv4AsUint(ip netip.Addr) uint32 {
+	bs := ip.As4()
+	return uint32(bs[0])<<24 | uint32(bs[1])<<16 | uint32(bs[2])<<8 | uint32(bs[3])
+}
+
+func ipv6AsUint(ip netip.Addr) (uint64, uint64) {
+	bs := ip.As16()
+	hi := uint64(bs[0])<<56 | uint64(bs[1])<<48 | uint64(bs[2])<<40 | uint64(bs[3])<<32 | uint64(bs[4])<<24 | uint64(bs[5])<<16 | uint64(bs[6])<<8 | uint64(bs[7])
+	lo := uint64(bs[8])<<56 | uint64(bs[9])<<48 | uint64(bs[10])<<40 | uint64(bs[11])<<32 | uint64(bs[12])<<24 | uint64(bs[13])<<16 | uint64(bs[14])<<8 | uint64(bs[15])
+	return hi, lo
+}

net/art/table_test.go

@@ -16,7 +16,524 @@ import (
 	"tailscale.com/types/ptr"
 )
 
+func TestRegression(t *testing.T) {
+	tbl := &Table[int]{}
+	slow := slowPrefixTable[int]{}
+	p := netip.MustParsePrefix
+
+	v := ptr.To(1)
+	tbl.Insert(p("226.205.197.0/24"), v)
+	slow.insert(p("226.205.197.0/24"), v)
+	v = ptr.To(2)
+	tbl.Insert(p("226.205.0.0/16"), v)
+	slow.insert(p("226.205.0.0/16"), v)
+
+	probe := netip.MustParseAddr("226.205.121.152")
+	got, want := tbl.Get(probe), slow.get(probe)
+	if got != want {
+		t.Fatalf("got %v, want %v", got, want)
+	}
+}
+
 func TestInsert(t *testing.T) {
+	tbl := &Table[int]{}
+	p := netip.MustParsePrefix
+
+	// Create a new leaf strideTable, with compressed path
+	tbl.Insert(p("192.168.0.1/32"), ptr.To(1))
+	checkRoutes(t, tbl, []tableTest{
+		{"192.168.0.1", 1},
+		{"192.168.0.2", -1},
+		{"192.168.0.3", -1},
+		{"192.168.0.255", -1},
+		{"192.168.1.1", -1},
+		{"192.170.1.1", -1},
+		{"192.180.0.1", -1},
+		{"192.180.3.5", -1},
+		{"10.0.0.5", -1},
+		{"10.0.0.15", -1},
+	})
+
+	// Insert into previous leaf, no tree changes
+	tbl.Insert(p("192.168.0.2/32"), ptr.To(2))
+	checkRoutes(t, tbl, []tableTest{
+		{"192.168.0.1", 1},
+		{"192.168.0.2", 2},
+		{"192.168.0.3", -1},
+		{"192.168.0.255", -1},
+		{"192.168.1.1", -1},
+		{"192.170.1.1", -1},
+		{"192.180.0.1", -1},
+		{"192.180.3.5", -1},
+		{"10.0.0.5", -1},
+		{"10.0.0.15", -1},
+	})
+
+	// Insert into previous leaf, unaligned prefix covering the /32s
+	tbl.Insert(p("192.168.0.0/26"), ptr.To(7))
+	checkRoutes(t, tbl, []tableTest{
+		{"192.168.0.1", 1},
+		{"192.168.0.2", 2},
+		{"192.168.0.3", 7},
+		{"192.168.0.255", -1},
+		{"192.168.1.1", -1},
+		{"192.170.1.1", -1},
+		{"192.180.0.1", -1},
+		{"192.180.3.5", -1},
+		{"10.0.0.5", -1},
+		{"10.0.0.15", -1},
+	})
+
+	// Create a different leaf elsewhere
+	tbl.Insert(p("10.0.0.0/27"), ptr.To(3))
+	checkRoutes(t, tbl, []tableTest{
+		{"192.168.0.1", 1},
+		{"192.168.0.2", 2},
+		{"192.168.0.3", 7},
+		{"192.168.0.255", -1},
+		{"192.168.1.1", -1},
+		{"192.170.1.1", -1},
+		{"192.180.0.1", -1},
+		{"192.180.3.5", -1},
+		{"10.0.0.5", 3},
+		{"10.0.0.15", 3},
+	})
+
+	// Insert that creates a new intermediate table and a new child
+	tbl.Insert(p("192.168.1.1/32"), ptr.To(4))
+	checkRoutes(t, tbl, []tableTest{
+		{"192.168.0.1", 1},
+		{"192.168.0.2", 2},
+		{"192.168.0.3", 7},
+		{"192.168.0.255", -1},
+		{"192.168.1.1", 4},
+		{"192.170.1.1", -1},
+		{"192.180.0.1", -1},
+		{"192.180.3.5", -1},
+		{"10.0.0.5", 3},
+		{"10.0.0.15", 3},
+	})
+
+	// Insert that creates a new intermediate table but no new child
+	tbl.Insert(p("192.170.0.0/16"), ptr.To(5))
+	checkRoutes(t, tbl, []tableTest{
+		{"192.168.0.1", 1},
+		{"192.168.0.2", 2},
+		{"192.168.0.3", 7},
+		{"192.168.0.255", -1},
+		{"192.168.1.1", 4},
+		{"192.170.1.1", 5},
+		{"192.180.0.1", -1},
+		{"192.180.3.5", -1},
+		{"10.0.0.5", 3},
+		{"10.0.0.15", 3},
+	})
+
+	// New leaf in a different subtree, so the next insert can test a
+	// variant of decompression.
+	tbl.Insert(p("192.180.0.1/32"), ptr.To(8))
+	checkRoutes(t, tbl, []tableTest{
+		{"192.168.0.1", 1},
+		{"192.168.0.2", 2},
+		{"192.168.0.3", 7},
+		{"192.168.0.255", -1},
+		{"192.168.1.1", 4},
+		{"192.170.1.1", 5},
+		{"192.180.0.1", 8},
+		{"192.180.3.5", -1},
+		{"10.0.0.5", 3},
+		{"10.0.0.15", 3},
+	})
+
+	// Insert that creates a new intermediate table but no new child,
+	// with an unaligned intermediate
+	tbl.Insert(p("192.180.0.0/21"), ptr.To(9))
+	checkRoutes(t, tbl, []tableTest{
+		{"192.168.0.1", 1},
+		{"192.168.0.2", 2},
+		{"192.168.0.3", 7},
+		{"192.168.0.255", -1},
+		{"192.168.1.1", 4},
+		{"192.170.1.1", 5},
+		{"192.180.0.1", 8},
+		{"192.180.3.5", 9},
+		{"10.0.0.5", 3},
+		{"10.0.0.15", 3},
+	})
+
+	// Insert a default route, those have their own codepath.
+	tbl.Insert(p("0.0.0.0/0"), ptr.To(6))
+	checkRoutes(t, tbl, []tableTest{
+		{"192.168.0.1", 1},
+		{"192.168.0.2", 2},
+		{"192.168.0.3", 7},
+		{"192.168.0.255", 6},
+		{"192.168.1.1", 4},
+		{"192.170.1.1", 5},
+		{"192.180.0.1", 8},
+		{"192.180.3.5", 9},
+		{"10.0.0.5", 3},
+		{"10.0.0.15", 3},
+	})
+
+	// Now all of the above again, but for IPv6.
+
+	// Create a new leaf strideTable, with compressed path
+	tbl.Insert(p("ff:aaaa::1/128"), ptr.To(1))
+	checkRoutes(t, tbl, []tableTest{
+		{"ff:aaaa::1", 1},
+		{"ff:aaaa::2", -1},
+		{"ff:aaaa::3", -1},
+		{"ff:aaaa::255", -1},
+		{"ff:aaaa:aaaa::1", -1},
+		{"ff:aaaa:aaaa:bbbb::1", -1},
+		{"ff:cccc::1", -1},
+		{"ff:cccc::ff", -1},
+		{"ffff:bbbb::5", -1},
+		{"ffff:bbbb::15", -1},
+	})
+
+	// Insert into previous leaf, no tree changes
+	tbl.Insert(p("ff:aaaa::2/128"), ptr.To(2))
+	checkRoutes(t, tbl, []tableTest{
+		{"ff:aaaa::1", 1},
+		{"ff:aaaa::2", 2},
+		{"ff:aaaa::3", -1},
+		{"ff:aaaa::255", -1},
+		{"ff:aaaa:aaaa::1", -1},
+		{"ff:aaaa:aaaa:bbbb::1", -1},
+		{"ff:cccc::1", -1},
+		{"ff:cccc::ff", -1},
+		{"ffff:bbbb::5", -1},
+		{"ffff:bbbb::15", -1},
+	})
+
+	// Insert into previous leaf, unaligned prefix covering the /128s
+	tbl.Insert(p("ff:aaaa::/125"), ptr.To(7))
+	checkRoutes(t, tbl, []tableTest{
+		{"ff:aaaa::1", 1},
+		{"ff:aaaa::2", 2},
+		{"ff:aaaa::3", 7},
+		{"ff:aaaa::255", -1},
+		{"ff:aaaa:aaaa::1", -1},
+		{"ff:aaaa:aaaa:bbbb::1", -1},
+		{"ff:cccc::1", -1},
+		{"ff:cccc::ff", -1},
+		{"ffff:bbbb::5", -1},
+		{"ffff:bbbb::15", -1},
+	})
+
+	// Create a different leaf elsewhere
+	tbl.Insert(p("ffff:bbbb::/120"), ptr.To(3))
+	checkRoutes(t, tbl, []tableTest{
+		{"ff:aaaa::1", 1},
+		{"ff:aaaa::2", 2},
+		{"ff:aaaa::3", 7},
+		{"ff:aaaa::255", -1},
+		{"ff:aaaa:aaaa::1", -1},
+		{"ff:aaaa:aaaa:bbbb::1", -1},
+		{"ff:cccc::1", -1},
+		{"ff:cccc::ff", -1},
+		{"ffff:bbbb::5", 3},
+		{"ffff:bbbb::15", 3},
+	})
+
+	// Insert that creates a new intermediate table and a new child
+	tbl.Insert(p("ff:aaaa:aaaa::1/128"), ptr.To(4))
+	checkRoutes(t, tbl, []tableTest{
+		{"ff:aaaa::1", 1},
+		{"ff:aaaa::2", 2},
+		{"ff:aaaa::3", 7},
+		{"ff:aaaa::255", -1},
+		{"ff:aaaa:aaaa::1", 4},
+		{"ff:aaaa:aaaa:bbbb::1", -1},
+		{"ff:cccc::1", -1},
+		{"ff:cccc::ff", -1},
+		{"ffff:bbbb::5", 3},
+		{"ffff:bbbb::15", 3},
+	})
+
+	// Insert that creates a new intermediate table but no new child
+	tbl.Insert(p("ff:aaaa:aaaa:bb00::/56"), ptr.To(5))
+	checkRoutes(t, tbl, []tableTest{
+		{"ff:aaaa::1", 1},
+		{"ff:aaaa::2", 2},
+		{"ff:aaaa::3", 7},
+		{"ff:aaaa::255", -1},
+		{"ff:aaaa:aaaa::1", 4},
+		{"ff:aaaa:aaaa:bbbb::1", 5},
+		{"ff:cccc::1", -1},
+		{"ff:cccc::ff", -1},
+		{"ffff:bbbb::5", 3},
+		{"ffff:bbbb::15", 3},
+	})
+
+	// New leaf in a different subtree, so the next insert can test a
+	// variant of decompression.
+	tbl.Insert(p("ff:cccc::1/128"), ptr.To(8))
+	checkRoutes(t, tbl, []tableTest{
+		{"ff:aaaa::1", 1},
+		{"ff:aaaa::2", 2},
+		{"ff:aaaa::3", 7},
+		{"ff:aaaa::255", -1},
+		{"ff:aaaa:aaaa::1", 4},
+		{"ff:aaaa:aaaa:bbbb::1", 5},
+		{"ff:cccc::1", 8},
+		{"ff:cccc::ff", -1},
+		{"ffff:bbbb::5", 3},
+		{"ffff:bbbb::15", 3},
+	})
+
+	// Insert that creates a new intermediate table but no new child,
+	// with an unaligned intermediate
+	tbl.Insert(p("ff:cccc::/37"), ptr.To(9))
+	checkRoutes(t, tbl, []tableTest{
+		{"ff:aaaa::1", 1},
+		{"ff:aaaa::2", 2},
+		{"ff:aaaa::3", 7},
+		{"ff:aaaa::255", -1},
+		{"ff:aaaa:aaaa::1", 4},
+		{"ff:aaaa:aaaa:bbbb::1", 5},
+		{"ff:cccc::1", 8},
+		{"ff:cccc::ff", 9},
+		{"ffff:bbbb::5", 3},
+		{"ffff:bbbb::15", 3},
+	})
+
+	// Insert a default route, those have their own codepath.
+	tbl.Insert(p("::/0"), ptr.To(6))
+	checkRoutes(t, tbl, []tableTest{
+		{"ff:aaaa::1", 1},
+		{"ff:aaaa::2", 2},
+		{"ff:aaaa::3", 7},
+		{"ff:aaaa::255", 6},
+		{"ff:aaaa:aaaa::1", 4},
+		{"ff:aaaa:aaaa:bbbb::1", 5},
+		{"ff:cccc::1", 8},
+		{"ff:cccc::ff", 9},
+		{"ffff:bbbb::5", 3},
+		{"ffff:bbbb::15", 3},
+	})
+}
+
+func checkRoutes(t *testing.T, tbl *Table[int], tt []tableTest) {
+	t.Helper()
+	for _, tc := range tt {
+		v := tbl.Get(netip.MustParseAddr(tc.addr))
+		if v == nil && tc.want != -1 {
+			t.Errorf("lookup %q got nil, want %d", tc.addr, tc.want)
+		}
+		if v != nil && *v != tc.want {
+			t.Errorf("lookup %q got %d, want %d", tc.addr, *v, tc.want)
+		}
+	}
+}
+
+func checkSize(t *testing.T, tbl *Table[int], want int) {
+	t.Helper()
+	if got := tbl.numStrides(); got != want {
+		t.Errorf("wrong table size, got %d strides want %d", got, want)
+	}
+}
+
+type tableTest struct {
+	addr string
+	want int
+}
+
+func TestDelete(t *testing.T) {
+	t.Parallel()
+	p := netip.MustParsePrefix
+
+	t.Run("prefix_in_root", func(t *testing.T) {
+		// Add/remove prefix from root table.
+		tbl := &Table[int]{}
+		checkSize(t, tbl, 2)
+
+		tbl.Insert(p("10.0.0.0/8"), ptr.To(1))
+		checkRoutes(t, tbl, []tableTest{
+			{"10.0.0.1", 1},
+			{"255.255.255.255", -1},
+		})
+		checkSize(t, tbl, 2)
+		tbl.Delete(p("10.0.0.0/8"))
+		checkRoutes(t, tbl, []tableTest{
+			{"10.0.0.1", -1},
+			{"255.255.255.255", -1},
+		})
+		checkSize(t, tbl, 2)
+	})
+
+	t.Run("prefix_in_leaf", func(t *testing.T) {
+		// Create, then delete a single leaf table.
+		tbl := &Table[int]{}
+		checkSize(t, tbl, 2)
+
+		tbl.Insert(p("192.168.0.1/32"), ptr.To(1))
+		checkRoutes(t, tbl, []tableTest{
+			{"192.168.0.1", 1},
+			{"255.255.255.255", -1},
+		})
+		checkSize(t, tbl, 3)
+		tbl.Delete(p("192.168.0.1/32"))
+		checkRoutes(t, tbl, []tableTest{
+			{"192.168.0.1", -1},
+			{"255.255.255.255", -1},
+		})
+		checkSize(t, tbl, 2)
+	})
+
+	t.Run("intermediate_no_routes", func(t *testing.T) {
+		// Create an intermediate with 2 children, then delete one leaf.
+		tbl := &Table[int]{}
+		checkSize(t, tbl, 2)
+		tbl.Insert(p("192.168.0.1/32"), ptr.To(1))
+		tbl.Insert(p("192.180.0.1/32"), ptr.To(2))
+		checkRoutes(t, tbl, []tableTest{
+			{"192.168.0.1", 1},
+			{"192.180.0.1", 2},
+			{"192.40.0.1", -1},
+		})
+		checkSize(t, tbl, 5) // 2 roots, 1 intermediate, 2 leaves
+		tbl.Delete(p("192.180.0.1/32"))
+		checkRoutes(t, tbl, []tableTest{
+			{"192.168.0.1", 1},
+			{"192.180.0.1", -1},
+			{"192.40.0.1", -1},
+		})
+		checkSize(t, tbl, 3) // 2 roots, 1 leaf
+	})
+
+	t.Run("intermediate_with_route", func(t *testing.T) {
+		// Same, but the intermediate carries a route as well.
+		tbl := &Table[int]{}
+		checkSize(t, tbl, 2)
+		tbl.Insert(p("192.168.0.1/32"), ptr.To(1))
+		tbl.Insert(p("192.180.0.1/32"), ptr.To(2))
+		tbl.Insert(p("192.0.0.0/10"), ptr.To(3))
+		checkRoutes(t, tbl, []tableTest{
+			{"192.168.0.1", 1},
+			{"192.180.0.1", 2},
+			{"192.40.0.1", 3},
+			{"192.255.0.1", -1},
+		})
+		checkSize(t, tbl, 5) // 2 roots, 1 intermediate, 2 leaves
+		tbl.Delete(p("192.180.0.1/32"))
+		checkRoutes(t, tbl, []tableTest{
+			{"192.168.0.1", 1},
+			{"192.180.0.1", -1},
+			{"192.40.0.1", 3},
+			{"192.255.0.1", -1},
+		})
+		checkSize(t, tbl, 4) // 2 roots, 1 intermediate w/route, 1 leaf
+	})
+
+	t.Run("intermediate_many_leaves", func(t *testing.T) {
+		// Intermediate with 3 leaves, then delete one leaf.
+		tbl := &Table[int]{}
+		checkSize(t, tbl, 2)
+		tbl.Insert(p("192.168.0.1/32"), ptr.To(1))
+		tbl.Insert(p("192.180.0.1/32"), ptr.To(2))
+		tbl.Insert(p("192.200.0.1/32"), ptr.To(3))
+		checkRoutes(t, tbl, []tableTest{
+			{"192.168.0.1", 1},
+			{"192.180.0.1", 2},
+			{"192.200.0.1", 3},
+			{"192.255.0.1", -1},
+		})
+		checkSize(t, tbl, 6) // 2 roots, 1 intermediate, 3 leaves
+		tbl.Delete(p("192.180.0.1/32"))
+		checkRoutes(t, tbl, []tableTest{
+			{"192.168.0.1", 1},
+			{"192.180.0.1", -1},
+			{"192.200.0.1", 3},
+			{"192.255.0.1", -1},
+		})
+		checkSize(t, tbl, 5) // 2 roots, 1 intermediate, 2 leaves
+	})
+
+	t.Run("nosuchprefix_missing_child", func(t *testing.T) {
+		// Delete non-existent prefix, missing strideTable path.
+		tbl := &Table[int]{}
+		checkSize(t, tbl, 2)
+		tbl.Insert(p("192.168.0.1/32"), ptr.To(1))
+		checkRoutes(t, tbl, []tableTest{
+			{"192.168.0.1", 1},
+			{"192.255.0.1", -1},
+		})
+		checkSize(t, tbl, 3) // 2 roots, 1 leaf
+		tbl.Delete(p("200.0.0.0/32")) // lookup miss in root
+		checkRoutes(t, tbl, []tableTest{
+			{"192.168.0.1", 1},
+			{"192.255.0.1", -1},
+		})
+		checkSize(t, tbl, 3) // 2 roots, 1 leaf
+	})
+
+	t.Run("nosuchprefix_wrong_turn", func(t *testing.T) {
+		// Delete non-existent prefix, strideTable path exists but
+		// with a wrong turn.
+		tbl := &Table[int]{}
+		checkSize(t, tbl, 2)
+		tbl.Insert(p("192.168.0.1/32"), ptr.To(1))
+		checkRoutes(t, tbl, []tableTest{
+			{"192.168.0.1", 1},
+			{"192.255.0.1", -1},
+		})
+		checkSize(t, tbl, 3) // 2 roots, 1 leaf
+		tbl.Delete(p("192.40.0.0/32")) // finds wrong child
+		checkRoutes(t, tbl, []tableTest{
+			{"192.168.0.1", 1},
+			{"192.255.0.1", -1},
+		})
+		checkSize(t, tbl, 3) // 2 roots, 1 leaf
+	})
+
+	t.Run("nosuchprefix_not_in_leaf", func(t *testing.T) {
+		// Delete non-existent prefix, strideTable path exists but
+		// leaf doesn't contain route.
+		tbl := &Table[int]{}
+		checkSize(t, tbl, 2)
+		tbl.Insert(p("192.168.0.1/32"), ptr.To(1))
+		checkRoutes(t, tbl, []tableTest{
+			{"192.168.0.1", 1},
+			{"192.255.0.1", -1},
+		})
+		checkSize(t, tbl, 3) // 2 roots, 1 leaf
+		tbl.Delete(p("192.168.0.5/32")) // right leaf, no route
+		checkRoutes(t, tbl, []tableTest{
+			{"192.168.0.1", 1},
+			{"192.255.0.1", -1},
+		})
+		checkSize(t, tbl, 3) // 2 roots, 1 leaf
+	})
+
+	t.Run("intermediate_with_deleted_route", func(t *testing.T) {
+		// Intermediate table loses its last route and becomes
+		// compactable.
+		tbl := &Table[int]{}
+		checkSize(t, tbl, 2)
+		tbl.Insert(p("192.168.0.1/32"), ptr.To(1))
+		tbl.Insert(p("192.168.0.0/22"), ptr.To(2))
+		checkRoutes(t, tbl, []tableTest{
+			{"192.168.0.1", 1},
+			{"192.168.0.2", 2},
+			{"192.255.0.1", -1},
+		})
+		checkSize(t, tbl, 4) // 2 roots, 1 intermediate w/route, 1 leaf
+		tbl.Delete(p("192.168.0.0/22"))
+		checkRoutes(t, tbl, []tableTest{
+			{"192.168.0.1", 1},
+			{"192.168.0.2", -1},
+			{"192.255.0.1", -1},
+		})
+		checkSize(t, tbl, 3) // 2 roots, 1 leaf
+	})
+}
+
+func TestInsertCompare(t *testing.T) {
+	// Create large route tables repeatedly, and compare Table's
+	// behavior to a naive and slow but correct implementation.
 	t.Parallel()
 	pfxs := randomPrefixes(10_000)
 
@@ -58,7 +575,7 @@ func TestInsert(t *testing.T) {
 
 func TestInsertShuffled(t *testing.T) {
 	t.Parallel()
-	pfxs := randomPrefixes(10_000)
+	pfxs := randomPrefixes(100)
 
 	rt := Table[int]{}
 	for _, pfx := range pfxs {
@@ -79,6 +596,9 @@ func TestInsertShuffled(t *testing.T) {
 			a := randomAddr()
 			val1 := rt.Get(a)
 			val2 := rt2.Get(a)
+			if val1 == nil && val2 == nil {
+				continue
+			}
 			if (val1 == nil && val2 != nil) || (val1 != nil && val2 == nil) || (*val1 != *val2) {
 				t.Errorf("get(%q) = %s, want %s", a, printIntPtr(val2), printIntPtr(val1))
 			}
@@ -86,7 +606,7 @@ func TestInsertShuffled(t *testing.T) {
 	}
 }
 
-func TestDelete(t *testing.T) {
+func TestDeleteCompare(t *testing.T) {
 	t.Parallel()
 
 	const (
@@ -104,6 +624,18 @@ func TestDelete(t *testing.T) {
 	toDelete := append([]slowPrefixEntry[int](nil), all4[deleteCut:]...)
 	toDelete = append(toDelete, all6[deleteCut:]...)
 
+	defer func() {
+		if t.Failed() {
+			for _, pfx := range pfxs {
+				fmt.Println(pfx.pfx, pfx.val)
+			}
+			fmt.Println("")
+			for _, pfx := range toDelete {
+				fmt.Println(pfx.pfx, pfx.val)
+			}
+		}
+	}()
+
 	slow := slowPrefixTable[int]{pfxs}
 	fast := Table[int]{}