Loops versus ranges

[bearophile via Digitalmars-d-learn]

A case where the usage of ranges (UFCS chains) leads to very bad 
performance:


import std.stdio: writeln;
import std.algorithm: map, join;

uint count1, count2;

const(int)[] foo1(in int[] data, in int i, in int max) {
     count1++;

     if (i < max) {
         typeof(return) result;
         foreach (immutable n; data)
             result ~= foo1(data, i + 1, max);
         return result;
     } else {
         return data;
     }
}

const(int)[] foo2(in int[] data, in int i, in int max) {
     count2++;

     if (i < max) {
         return data.map!(n => foo2(data, i + 1, max)).join;
     } else {
         return data;
     }
}

void main() {
     const r1 = foo1([1, 2, 3, 4, 5], 1, 7);
     writeln(count1); // 19531
     const r2 = foo2([1, 2, 3, 4, 5], 1, 7);
     writeln(count2); // 1111111
     assert(r1 == r2); // Results are equally correct.
}


Can you tell why? :-)

Bye,
bearophile