my first D program (and benchmark against perl)

[Rikki Cattermole via Digitalmars-d-learn]

On 12/11/15 2:31 AM, perlancar wrote:
> Here's my first non-hello-world D program, which is a direct translation
> from the Perl version. I was trying to get a feel about D's performance:
>
> ---BEGIN asciitable.d---
> import std.string;
> import std.stdio;
>
> string fmttable(ref string[][] table) {
>      string res = "";
>
>      // column widths
>      int[] widths;
>
>      if (table.length == 0) return "";
>
>      widths.length = table[0].length;
>
>      for (int colnum=0; colnum < table[0].length; colnum++) {
>          int width = 0;
>          for (int rownum=0; rownum < table.length; rownum++) {
>              if (table[rownum][colnum].length > width)
>                  width = cast(int) table[rownum][colnum].length;
>          }
>          widths[colnum] = width;
>      }
>
>      for (int rownum=0; rownum < table.length; rownum++) {
>          res ~= "|";
>          for (int colnum=0; colnum < table[rownum].length; colnum++) {
>              res ~= leftJustify(table[rownum][colnum], widths[colnum]);
>              res ~= "|";
>          }
>          res ~= "\n";
>      }
>
>      return res;
> }
>
> void main() {
>      // tiny table (1x1)
>      /*
>      string[][] table = [
>          ["row1.1"],
>      ];
>      */
>
>      // small table (3x5)
>      string[][] table = [
>          ["row1.1", "row1.2  ", "row1.3"],
>          ["row2.1", "row2.2", "row2.3"],
>          ["row3.1", "row3.2", "row3.3  "],
>          ["row4.1", "row4.2", "row4.3"],
>          ["row5.1", "row5.2", "row5.3"],
>      ];
>
>      write(fmttable(table));
>      for (int i=0; i < 1000000; i++) {
>          fmttable(table);
>      }
> }
> ---END asciitable.d---
>
> Perl version:
>
> ---BEGIN asciitable.pl---
> #!/usr/bin/env perl
>
> sub fmttable {
>      my $table = shift;
>
>      my $res = "";
>
>      # column widths
>      my @widths;
>
>      if (@$table == 0) { return "" }
>
>      for my $colnum (0 .. $#{$table->[0]}) {
>          my $width = 0;
>          for my $rownum (0 .. $#{$table}) {
>              if (length($table->[$rownum][$colnum]) > $width) {
>                  $width = length($table->[$rownum][$colnum]);
>              }
>          }
>          $widths[$colnum] = $width;
>      }
>
>      for my $rownum (0..$#{$table}) {
>          $res .= "|";
>          for my $colnum (0..$#{$table->[$rownum]}) {
>              $res .= sprintf("%-".$widths[$colnum]."s|",
> $table->[$rownum][$colnum]);
>          }
>          $res .= "\n";
>      }
>      $res;
> }
>
> # tiny table (1x1)
> #my $table = [["row1.1"]];
>
> # small table (3x5)
> my $table = [
>      ["row1.1", "row1.2", "row1.3"],
>      ["row2.1", "row2.2  ", "row2.3"],
>      ["row3.1", "row3.2", "row3.3  "],
>      ["row4.1", "row4.2", "row4.3"],
>      ["row5.1", "row5.2", "row5.3"],
> ];
>
> print fmttable($table);
>
> for (1..1_000_000) {
>      fmttable($table);
> }
> ---END asciitable.pl---
>
> While I am quite impressed with how easy I was able to write D, I am not
> so impressed with the performance. Using rdmd (build 20151103), the D
> program runs in 17.127s while the Perl version runs in 11.391s (so the D
> version is quite a bit *slower* than Perl's). While using gdc (Debian
> 4.9.2-10), I am able to run it in 3.988s (only about 3x faster than
> Perl's version).
>
> I understand that string processing (concatenation, allocation) is quite
> optimized in Perl, I was wondering if the D version could still be sped
> up significantly?

I turned it into mostly using large allocations, instead of small ones.
Although I'd recommend using Appender instead of my custom functions for 
this.

Oh and for me, I got it at 2 secs, 513 ms, 397 μs, and 5 hnsecs. 
Unoptimized, using dmd.
When release mode is enabled on dmd: 1 sec, 550 ms, 838 μs, and 9 
hnsecs. So significant improvement even with dmds awful optimizer.

import std.string;
import std.stdio;

static string SPACES = " 
               ";

string fmttable(string[][] table) {
     char[] res;

     // column widths
     int[] widths;
     size_t totalSize;

     if (table.length == 0) return "";

     widths.length = table[0].length;

     foreach(colnum; 0 .. table[0].length) {
         int width = 0;
         size_t count;

         foreach(rownum; 0 .. table.length) {
             if (table[rownum][colnum].length > width)
                 width = cast(int) table[rownum][colnum].length;
             count += table[rownum].length;
         }

         totalSize += ((width + 1) * count) + 2;
         widths[colnum] = width;
     }

     char[] buffer = new char[](totalSize);

     void assignText(string toAdd) {
         if (res.length < buffer.length - toAdd.length) {
         } else {
             buffer.length += toAdd.length;
         }

         res = buffer[0 .. res.length + toAdd.length];
         res[$-toAdd.length .. $] = toAdd[];
     }


     foreach(rownum; 0 .. table.length) {
         assignText("|");
         foreach(colnum; 0 .. table[rownum].length) {
             assignText(SPACES[0 .. widths[colnum] - 
table[rownum][colnum].length]);
             assignText(table[rownum][colnum]);
             assignText("|");
         }
         assignText("\n");
     }

     return cast(string)res;
}

void main() {
     // tiny table (1x1)
     /*
     string[][] table = [
         ["row1.1"],
     ];
     */

     // small table (3x5)
     string[][] table = [
         ["row1.1", "row1.2  ", "row1.3"],
         ["row2.1", "row2.2", "row2.3"],
         ["row3.1", "row3.2", "row3.3  "],
         ["row4.1", "row4.2", "row4.3"],
         ["row5.1", "row5.2", "row5.3"],
     ];

import std.datetime : StopWatch, TickDuration, Duration;
StopWatch sw;
TickDuration start = sw.peek();
sw.start();

     write(fmttable(table));
     for (int i=0; i < 1000000; i++) {
         fmttable(table);
     }
sw.stop();

     writeln(cast(Duration)(sw.peek() - start));
}