How can I map my large yet sparse enum to array indices in C-CodePudding

I have an enum class, now I want to turn them into array, so that any given enum I could return the corresponding object like below:

PeopleInfo fetch(enum e) {
    return arr[e];
}

Sounds easy, 1-1 mapping would do the trick, but today my enum is relative large and sparse like

enum class People: {
    American_start = 0x0,
    John,
    Aaron, 
    ...
    Asian_start = 0x10000,
    Yen_Huan,
    Kang_Hsuan,
    European_start = 0x20000,
    ...
}

As you can see, in order to preserve the space for additional people to join in the future, these enum apparently cannot use 1-1 mapping here, that's just a total waste of memory, what I need is an array with size of all people listed, so I can get the info I want

Does anybody has good idea how to do so?

CodePudding user response：

You can create an index that isn't sparse, so that you can use an array that doesn't have any holes in it.

PeopleInfo fetch(enum e) {
    index = e;
    if (index >= European_start)
        index -= European_start - Asian_end;
    if (index >= Asian_start)
        index -= Asian_start - American_end;
    if (index >= American_start)
        index -= American_start;
    return arr[index];
}

CodePudding user response：

You can use a 2D-array where the columns are the continents. Then you can identify the continent by the upper bytes and objects by the lower bytes.

SomeObject arr[NUMBER_OF_CONTINENTS][MAX_NUMBER_OF_OBJECTS];
PeopleInfo fetch(enum e) {
    return arr[e >> 16][e & 0xffff];
}

Note: You might waste some memory, when one continent has a lot more objects than the others.