#!/usr/bin/perl -w
# This script takes a number n on the command line and edits html
# containing rows of a table so that comma-separated lists of numbers
# in the last cell of each row has n of its numbers highlighted in red.
# I am using this script to select which homework problems to grade.

$usage = "usage: $0 number\n";
if(@ARGV != 1)  {
    die $usage;
}

# Split the line into chunks containing either a problem or everything else on the line

@chunks = ();
$avoidIndices = ();
$numProbs = shift @ARGV;
while($line = <STDIN>)  {
    @words = split(/<td>/, $line);
    $lastword = pop(@words);
    push(@chunks, join("<td>", @words));
    push(@avoidIndices, $#chunks);
    @curProbs = split(/ *, */, $lastword);
    for($i = 0; $i <= $#curProbs - 1; $i++)  {
        if($i == 0)  {
            $curProbs[$i] =~ s/^/<td>/;
        }
        $curProbs[$i] =~ s/[0-9]+[a-z]*/$&,/;
    }
    @chunks = (@chunks, @curProbs);
}

# Make a list of indices, avoiding non-problem chunks

@allIndices = 0 .. $#chunks;

for $avoidIndex (@avoidIndices)  {
    $allIndices[$avoidIndex] = -1;
}

$i = 0;
while($i <= $#allIndices)  {
    if($allIndices[$i] == -1)  {
        @allIndices = (@allIndices[0..$i-1], @allIndices[$i+1..$#allIndices]);
    }
    else  {
        $i++;
    }
}

# Change the color for the indicated number of problems

if($numProbs > @allIndices)  {
   die "can't select $numProbs of " . @allIndices . " problems\n";
}

@selectedIndices = ();
for($i = 0; $i < $numProbs; $i++)  {
    $j = int(rand(@allIndices));
    push(@selectedIndices, $allIndices[$j]);
    @allIndices = (@allIndices[0..$j-1], @allIndices[$j+1..$#allIndices]);
}
for $i (sort(@selectedIndices))  {
    $chunks[$i] =~ s/([0-9]+) */<font color=red>$1<\/font>/;
}

# Print the results

print @chunks;