github.com/hxx258456/ccgo@v0.0.5-0.20230213014102-48b35f46f66f/internal/subtle/aliasing.go

package subtle

import "unsafe"

// AnyOverlap reports whether x and y share memory at any (not necessarily
// corresponding) index. The memory beyond the slice length is ignored.
func AnyOverlap(x, y []byte) bool {
	return len(x) > 0 && len(y) > 0 &&
		uintptr(unsafe.Pointer(&x[0])) <= uintptr(unsafe.Pointer(&y[len(y)-1])) &&
		uintptr(unsafe.Pointer(&y[0])) <= uintptr(unsafe.Pointer(&x[len(x)-1]))
}

// InexactOverlap reports whether x and y share memory at any non-corresponding
// index. The memory beyond the slice length is ignored. Note that x and y can
// have different lengths and still not have any inexact overlap.
//
// InexactOverlap can be used to implement the requirements of the crypto/cipher
// AEAD, Block, BlockMode and Stream interfaces.
func InexactOverlap(x, y []byte) bool {
	if len(x) == 0 || len(y) == 0 || &x[0] == &y[0] {
		return false
	}
	return AnyOverlap(x, y)
}

// SliceForAppend takes a slice and a requested number of bytes. It returns a
// slice with the contents of the given slice followed by that many bytes and a
// second slice that aliases into it and contains only the extra bytes. If the
// original slice has sufficient capacity then no allocation is performed.
func SliceForAppend(in []byte, n int) (head, tail []byte) {
	if total := len(in) + n; cap(in) >= total {
		// 如果in的当前长度+n 依然在其in的容量范围内，则直接获取in的(len(in) + n)部分作为head
		head = in[:total]
	} else {
		// in的当前长度+n已经超过其容量，则重新申请内存生成head，并将in的(len(in))部分拷贝到新的head切片
		head = make([]byte, total)
		copy(head, in)
	}
	// 获取head切片的尾部
	tail = head[len(in):]
	return
}