假设我们有一结构体长下面这样：
const (
maxHeight = 20
heightIncrease = math.MaxUint32 / 3
)
const (
offsetSize = int(unsafe.Sizeof(uint32(0)))
nodeAlign = int(unsafe.Sizeof(uint64(0))) - 1
MaxNodeSize = int(unsafe.Sizeof(node{}))
)
type node struct {
value uint64
keyOffset uint32 // Immutable. No need to lock to access key.
keySize uint16 // Immutable. No need to lock to access key.
height uint16
tower [maxHeight]uint32
}
func putNode(height int) uint32 {
// Compute the amount of the tower that will never be used, since the height
// is less than maxHeight.
// 假设新node 的高度10
// unusedSize = (20 - 10) * 4 = 40
// 假设新node 的高度3
// unusedSize = (20 - 3) * 4 = 68
// 76
unusedSize := (maxHeight - height) * offsetSize
// Pad the allocation with enough bytes to ensure pointer alignment.
// 假设新node 的高度10
// 96 - 40 + 7 = 63
// 假设新node 的高度3
// 96 - 68 + 7 = 35
// MaxNodeSize - unusedSize 是tower[height] 要使用的 size 。
l := uint32(MaxNodeSize - unusedSize + nodeAlign)
n := s.allocate(l)
// Return the aligned offset.
m := (n + uint32(nodeAlign)) & ^uint32(nodeAlign)
return m
}