slices: handle aliasing cases in Insert/Replace

Handle cases where the inserted slice is actually part of the slice
that is being inserted into.

Requires a bit more work, but no more allocations. (Compare to #494536.)

Not entirely sure this is worth the complication.

Fixes golang#60138

Change-Id: Ia72c872b04309b99025e6ca5a4a326ebed2abb69
Reviewed-on: https://go-review.googlesource.com/c/go/+/494817
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Cuong Manh Le <[email protected]>
Reviewed-by: Keith Randall <[email protected]>
Run-TryBot: Keith Randall <[email protected]>
Reviewed-by: Bryan Mills <[email protected]>

Author: randall77

src/go/build/deps_test.go

@@ -46,9 +46,13 @@ var depsRules = `
 	  internal/goexperiment, internal/goos,
 	  internal/goversion, internal/nettrace, internal/platform,
 	  log/internal,
-	  maps, slices, unicode/utf8, unicode/utf16, unicode,
+	  maps, unicode/utf8, unicode/utf16, unicode,
 	  unsafe;
 
+	# slices depends on unsafe for overlapping check.
+	unsafe
+	< slices;
+
 	# These packages depend only on internal/goarch and unsafe.
 	internal/goarch, unsafe
 	< internal/abi;

src/slices/slices.go

@@ -5,6 +5,10 @@
 // Package slices defines various functions useful with slices of any type.
 package slices
 
+import (
+	"unsafe"
+)
+
 // Equal reports whether two slices are equal: the same length and all
 // elements equal. If the lengths are different, Equal returns false.
 // Otherwise, the elements are compared in increasing index order, and the
@@ -81,22 +85,83 @@ func ContainsFunc[E any](s []E, f func(E) bool) bool {
 // Insert panics if i is out of range.
 // This function is O(len(s) + len(v)).
 func Insert[S ~[]E, E any](s S, i int, v ...E) S {
-	tot := len(s) + len(v)
-	if tot <= cap(s) {
-		s2 := s[:tot]
-		copy(s2[i+len(v):], s[i:])
+	m := len(v)
+	if m == 0 {
+		return s
+	}
+	n := len(s)
+	if i == n {
+		return append(s, v...)
+	}
+	if n+m > cap(s) {
+		// Use append rather than make so that we bump the size of
+		// the slice up to the next storage class.
+		// This is what Grow does but we don't call Grow because
+		// that might copy the values twice.
+		s2 := append(S(nil), make(S, n+m)...)
+		copy(s2, s[:i])
 		copy(s2[i:], v)
+		copy(s2[i+m:], s[i:])
 		return s2
 	}
-	// Use append rather than make so that we bump the size of
-	// the slice up to the next storage class.
-	// This is what Grow does but we don't call Grow because
-	// that might copy the values twice.
-	s2 := append(S(nil), make(S, tot)...)
-	copy(s2, s[:i])
-	copy(s2[i:], v)
-	copy(s2[i+len(v):], s[i:])
-	return s2
+	s = s[:n+m]
+
+	// before:
+	// s: aaaaaaaabbbbccccccccdddd
+	//            ^   ^       ^   ^
+	//            i  i+m      n  n+m
+	// after:
+	// s: aaaaaaaavvvvbbbbcccccccc
+	//            ^   ^       ^   ^
+	//            i  i+m      n  n+m
+	//
+	// a are the values that don't move in s.
+	// v are the values copied in from v.
+	// b and c are the values from s that are shifted up in index.
+	// d are the values that get overwritten, never to be seen again.
+
+	if !overlaps(v, s[i+m:]) {
+		// Easy case - v does not overlap either the c or d regions.
+		// (It might be in some of a or b, or elsewhere entirely.)
+		// The data we copy up doesn't write to v at all, so just do it.
+
+		copy(s[i+m:], s[i:])
+
+		// Now we have
+		// s: aaaaaaaabbbbbbbbcccccccc
+		//            ^   ^       ^   ^
+		//            i  i+m      n  n+m
+		// Note the b values are duplicated.
+
+		copy(s[i:], v)
+
+		// Now we have
+		// s: aaaaaaaavvvvbbbbcccccccc
+		//            ^   ^       ^   ^
+		//            i  i+m      n  n+m
+		// That's the result we want.
+		return s
+	}
+
+	// The hard case - v overlaps c or d. We can't just shift up
+	// the data because we'd move or clobber the values we're trying
+	// to insert.
+	// So instead, write v on top of d, then rotate.
+	copy(s[n:], v)
+
+	// Now we have
+	// s: aaaaaaaabbbbccccccccvvvv
+	//            ^   ^       ^   ^
+	//            i  i+m      n  n+m
+
+	rotateRight(s[i:], m)
+
+	// Now we have
+	// s: aaaaaaaavvvvbbbbcccccccc
+	//            ^   ^       ^   ^
+	//            i  i+m      n  n+m
+	// That's the result we want.
+	return s
 }
 
 // Delete removes the elements s[i:j] from s, returning the modified slice.
@@ -143,18 +208,89 @@ func DeleteFunc[S ~[]E, E any](s S, del func(E) bool) S {
 // modified slice. Replace panics if s[i:j] is not a valid slice of s.
 func Replace[S ~[]E, E any](s S, i, j int, v ...E) S {
 	_ = s[i:j] // verify that i:j is a valid subslice
+
+	if i == j {
+		return Insert(s, i, v...)
+	}
+	if j == len(s) {
+		return append(s[:i], v...)
+	}
+
 	tot := len(s[:i]) + len(v) + len(s[j:])
-	if tot <= cap(s) {
-		s2 := s[:tot]
-		copy(s2[i+len(v):], s[j:])
+	if tot > cap(s) {
+		// Too big to fit, allocate and copy over.
+		s2 := append(S(nil), make(S, tot)...) // See Insert
+		copy(s2, s[:i])
 		copy(s2[i:], v)
+		copy(s2[i+len(v):], s[j:])
 		return s2
 	}
-	s2 := make(S, tot)
-	copy(s2, s[:i])
-	copy(s2[i:], v)
-	copy(s2[i+len(v):], s[j:])
-	return s2
+
+	r := s[:tot]
+
+	if i+len(v) <= j {
+		// Easy, as v fits in the deleted portion.
+		copy(r[i:], v)
+		if i+len(v) != j {
+			copy(r[i+len(v):], s[j:])
+		}
+		return r
+	}
+
+	// We are expanding (v is bigger than j-i).
+	// The situation is something like this:
+	// (example has i=4,j=8,len(s)=16,len(v)=6)
+	// s: aaaaxxxxbbbbbbbbyy
+	//        ^   ^       ^ ^
+	//        i   j  len(s) tot
+	// a: prefix of s
+	// x: deleted range
+	// b: more of s
+	// y: area to expand into
+
+	if !overlaps(r[i+len(v):], v) {
+		// Easy, as v is not clobbered by the first copy.
+		copy(r[i+len(v):], s[j:])
+		copy(r[i:], v)
+		return r
+	}
+
+	// This is a situation where we don't have a single place to which
+	// we can copy v. Parts of it need to go to two different places.
+	// We want to copy the prefix of v into y and the suffix into x, then
+	// rotate |y| spots to the right.
+	//
+	//        v[2:]      v[:2]
+	//         |           |
+	// s: aaaavvvvbbbbbbbbvv
+	//        ^   ^       ^ ^
+	//        i   j  len(s) tot
+	//
+	// If either of those two destinations don't alias v, then we're good.
+	y := len(v) - (j - i) // length of y portion
+
+	if !overlaps(r[i:j], v) {
+		copy(r[i:j], v[y:])
+		copy(r[len(s):], v[:y])
+		rotateRight(r[i:], y)
+		return r
+	}
+	if !overlaps(r[len(s):], v) {
+		copy(r[len(s):], v[:y])
+		copy(r[i:j], v[y:])
+		rotateRight(r[i:], y)
+		return r
+	}
+
+	// Now we know that v overlaps both x and y.
+	// That means that the entirety of b is *inside* v.
+	// So we don't need to preserve b at all; instead we
+	// can copy v first, then copy the b part of v out of
+	// v to the right destination.
+	k := startIdx(v, s[j:])
+	copy(r[i:], v)
+	copy(r[i+len(v):], r[i+k:])
+	return r
 }
 
 // Clone returns a copy of the slice.
@@ -224,3 +360,90 @@ func Grow[S ~[]E, E any](s S, n int) S {
 func Clip[S ~[]E, E any](s S) S {
 	return s[:len(s):len(s)]
 }
+
+// Rotation algorithm explanation:
+//
+// rotate left by 2
+// start with
+//   0123456789
+// split up like this
+//   01 234567 89
+// swap first 2 and last 2
+//   89 234567 01
+// join first parts
+//   89234567 01
+// recursively rotate first left part by 2
+//   23456789 01
+// join at the end
+//   2345678901
+//
+// rotate left by 8
+// start with
+//   0123456789
+// split up like this
+//   01 234567 89
+// swap first 2 and last 2
+//   89 234567 01
+// join last parts
+//   89 23456701
+// recursively rotate second part left by 6
+//   89 01234567
+// join at the end
+//   8901234567
+
+// TODO: There are other rotate algorithms.
+// This algorithm has the desirable property that it moves each element exactly twice.
+// The triple-reverse algorithm is simpler and more cache friendly, but takes more writes.
+// The follow-cycles algorithm can be 1-write but it is not very cache friendly.
+
+// rotateLeft rotates b left by n spaces.
+// s_final[i] = s_orig[i+r], wrapping around.
+func rotateLeft[S ~[]E, E any](s S, r int) {
+	for r != 0 && r != len(s) {
+		if r*2 <= len(s) {
+			swap(s[:r], s[len(s)-r:])
+			s = s[:len(s)-r]
+		} else {
+			swap(s[:len(s)-r], s[r:])
+			s, r = s[len(s)-r:], r*2-len(s)
+		}
+	}
+}
+func rotateRight[S ~[]E, E any](s S, r int) {
+	rotateLeft(s, len(s)-r)
+}
+
+// swap swaps the contents of x and y. x and y must be equal length and disjoint.
+func swap[S ~[]E, E any](x, y S) {
+	for i := 0; i < len(x); i++ {
+		x[i], y[i] = y[i], x[i]
+	}
+}
+
+// overlaps reports whether the memory ranges a[0:len(a)] and b[0:len(b)] overlap.
+func overlaps[S ~[]E, E any](a, b S) bool {
+	if len(a) == 0 || len(b) == 0 {
+		return false
+	}
+	elemSize := unsafe.Sizeof(a[0])
+	if elemSize == 0 {
+		return false
+	}
+	// TODO: use a runtime/unsafe facility once one becomes available. See issue 12445.
+	// Also see crypto/internal/alias/alias.go:AnyOverlap
+	return uintptr(unsafe.Pointer(&a[0])) <= uintptr(unsafe.Pointer(&b[len(b)-1]))+(elemSize-1) &&
+		uintptr(unsafe.Pointer(&b[0])) <= uintptr(unsafe.Pointer(&a[len(a)-1]))+(elemSize-1)
+}
+
+// startIdx returns the index in haystack where the needle starts.
+// prerequisite: the needle must be aliased entirely inside the haystack.
+func startIdx[S ~[]E, E any](haystack, needle S) int {
+	p := &needle[0]
+	for i := range haystack {
+		if p == &haystack[i] {
+			return i
+		}
+	}
+	// TODO: what if the overlap is by a non-integral number of Es?
+	panic("needle not found")
+}