A bit more d-ified and uses foreach + foreach_reverse without allocating
an array.
```d
import std.stdio;
void main()
{
doRepetition(4, 3);
writeln("==========");
doRepetitionReversed(4, 3);
}
void doRepetition(const int n, const int m)
{
// combination total number is m, element is repeatable.
assert(n >= 1 && n < 10);
assert(m >= 1 && m < 10);
enum N = 10;
ubyte[10] a;
void inc(int index, int r, int start, int end)
{
if (index == r)
{
// get one unique combination result, not repeatable
foreach (j; 0 .. r)
{
writef!"%d "(a[j]);
}
writeln;
return;
}
foreach (i; start .. end)
{
a[index] = cast(ubyte) i;
inc(index + 1, r, i, end);
}
}
inc(0, m, 0, n);
}
void doRepetitionReversed(const int n, const int m)
{
// combination total number is m, element is repeatable.
assert(n >= 1 && n < 10);
assert(m >= 1 && m < 10);
enum N = 10;
ubyte[10] a;
void inc(int index, int r, int start, int end)
{
if (index == r)
{
// get one unique combination result, not repeatable
foreach_reverse (j; 0 .. r)
{
writef!"%d "(a[j]);
}
writeln;
return;
}
foreach_reverse (i; start .. end)
{
a[index] = cast(ubyte) i;
inc(index + 1, r, i, end);
}
}
inc(0, m, 0, n);
}
```
Output:
```
0 0 0
0 0 1
0 0 2
0 0 3
0 1 1
0 1 2
0 1 3
0 2 2
0 2 3
0 3 3
1 1 1
1 1 2
1 1 3
1 2 2
1 2 3
1 3 3
2 2 2
2 2 3
2 3 3
3 3 3
==========
3 3 3
3 3 2
3 2 2
2 2 2
3 3 1
3 2 1
2 2 1
3 1 1
2 1 1
1 1 1
3 3 0
3 2 0
2 2 0
3 1 0
2 1 0
1 1 0
3 0 0
2 0 0
1 0 0
0 0 0
```
