Fun with binary

    #!/usr/bin/rdmd

    import std.math, std.stdio, std.algorithm;

    long[40][40] choose;
    long[40][40] choose_at_most;

    long fun(long n,long m){
      long dig=1<<30;
      long pos=31;
      long res=0;

      while (pos){
        if (choose_at_most[pos-1][m] <= n) {
          n -= choose_at_most[pos-1][m];
          res += dig;
          --m;
        }
        dig>>=1;
        pos--;
      }

      return res;
    }

    string binary(long n, bool trail=true){
      return n? binary(n/2,false) ~ (n&1?'1':'0'): trail? "0": "";
    }

    void main(){
      choose[0][0]=1;

      for (int i=0; i<40; i++)
        for (int j=0; j<40; j++)
          choose[i][j]=(i?choose[i-1][j]+(j?choose[i-1][j-1]:0):j==0),
          choose_at_most[i][j]=choose[i][j]+(j?choose_at_most[i][j-1]:0);

      writeln(choose_at_most[0][0]);

      for (int i=0; i<50; i++){
        writefln("%3d = %s",cast(int)fun(i,3),fun(i,3).binary);
      }
    }