Why not just assign value to target in method header.clone - go

Question

the official version
my question is What's the role of "sv"
func (h Header) Clone() Header {
if h == nil {
return nil
}
// Find total number of values.
nv := 0
for _, vv := range h {
nv += len(vv)
}
sv := make([]string, nv) // shared backing array for headers' values
h2 := make(Header, len(h))
for k, vv := range h {
n := copy(sv, vv)
h2[k] = sv[:n:n]
sv = sv[n:]
}
return h2
}
Why not just write it like this
just assgin value to h2 instead of creating a slice
for k, vv := range h {
// changed here
h2[k] = vv
}

Answer 1

It looks like Header is a map[string][]string, (maybe http.Header?). If it did what you suggested, then the new Header would be a shallow copy of the map, containing the original slices from the source Header. If the contents of the backing array of those slices are modified, the copied header would be modified as well. For instance:
s:=make([]string,0,10)
s=append(s,"a")
header[key]=s
h2:=header.Clone()
s=append(s,"b")
// Here, both header[key] and h2[key] has two elements, "a" and "b"
The prevent this, Clone is doing a deep copy, where each []string is also copied. That would need len(h) string slice allocations, one for each key. Instead, this algorithm uses one allocation, a single shared string slice containing all the strings of the original header.
The algorithm first counts the number of strings contained in the header, and allocates a string slice of that size. In the second for loop, it copies the strings from the value of a key to the shared string slice, creates a slice using that shared slice array and sets it as the value of the key, and then updates the shared slice to point to the next available empty slot. In the end, it is a deep-copy with a single memory allocation instead of len(h).