如何分配内存以映射指向golang中的切片

Question

Is there a way to allocate the memory of a map, which has at most Nmax keys, that points to a slice of maximum length Nmax?

I'm currently just specifying the max number of keys via make(map[int][]int,Nmax), but I'm not sure how to tell Go that each slice will be of maximum length Nmax because I don't know what the keys will be apriori.

I essentially have a bunch of sites with an integer population. I use the map to keep track of how many sites have a given population N. The bottleneck in my program appears to be runtime.memmove, which I'm guessing comes from constant resizing of the slice the map points to.

Answer 1

So given that your description of the problem is really rather vague, I'll just start by saying how I'd go about "managing" the map. To keep things simple, I'm going to wrap all the logic up in receiver functions, so wrapping the map up in a custom type:

type dataMap struct {
    data map[int][]int
    nmax int
}

func New(Nmax int) *dataMap {
    return &dataMap{
        data: make(map[int][]int, Nmax),
        nmax: Nmax,
    }
}

// Get - return slice for given key
func (d dataMap) Get(k int) []int {
    s, ok := d.data[k]
    if !ok {
        return nil // optionally return error
    }
    return s
}

// Set - set/append values to a given key - this is not safe for concurrent use
// if that's needed, add a RWMutex to the type
func (d *dataMap) Set(k int, vals ...int) error {
    s, ok := d.data[k]
    if !ok {
        s = make([]int, 0, d.nmax) // allocate slice of given length
    }
    // optionally check for nil-values + ensure we're not exceeding the nmax
    checked := make([]int, 0, len(vals))
    for i := range vals {
        if vals[i] != 0 {
            checked = append(checked, vals[i])
        }
    }
    if len(s) + len(checked) > d.nmax {
        return errors.New("max capacity exceeded")
    }
    s = append(s, checked...) // append values
    d.data[k] = s // update map
    return nil
}

This cuts down on needless memory (re-)allocation calls. It also ensures that I can get the length of any slice in the map in an O(1) operation, without having to worry about nil values:

myData := New(10)
fmt.Println(myData.Set(4, 1, 2, 3, 4))
fmt.Println(len(myData.Get(4))) // 4
fmt.Println(cap(myData.Get(4))) // 10
// nil-values are filtered out
myData.Set(4, 5, 6, 7, 0, 0, 0, 0)
fmt.Println(len(myData.Get(4))) // 7
fmt.Println(cap(myData.Get(4))) // 10
// exceeding capacity of 10
fmt.Println(myData.Set(4, 8, 9, 10, 11)) // max capacity exceeded

working demo

---

You could manage the capacity by using an array instead of a slice, but that does require you to manually keep track of the index/offset at which you want to start appending values. Generally speaking, you don't use arrays in golang lest in very, very specific cases. In this case, I'd just opt for a slice with a set cap. The advantage of this is that you could, for example have slices of different lengths. The result is very easy to test, too, because a type like this lends itself quite well to replacing it with an interface type

type DataContainer interface {
    Get(k int) []int
    Set(k int, vals ...int) error
    Declare(k, capacity int) error // error if k is already in use?
}