void main()
{
    import std.stdio : writefln;
    int[char[2]] aa;
    auto arr = "ABBBA";

    // Iterate over all pairs in the string and observe each pair
    // ('A', 'B'), ('B', 'B'), ('B', 'A'), ...
    // String slicing doesn't allocate a copy
    foreach (i; 0 .. arr.length - 1)
        aa[arr[i .. $][0 .. 2]]++;

    foreach (key, value; aa)
        writefln("key: %s, value: %d", key, value);