build: Fix lcov-inject to consider different coverage percentages

The current code was not taking into account the different coverage
ranges, with their different colors, and different images for the
completion bar. Refactor the code into functions, while we are at it.

doc/lcov-inject

@@ -41,30 +41,59 @@ my $tmpl = sprintf '
       <td class="coverFile"><a href="%s">%s</a></td>
       <td class="coverBar" align="center">
         <table border=0 cellspacing=0 cellpadding=1>
-          <tr><td class="coverBarOutline">
-              <img src="ruby.png" width=%.0f height=10 alt="%.1f"><img src="snow.png" width=%.0f height=10 alt="%.1f">
-          </td></tr>
+          <tr><td class="coverBarOutline">%s</td></tr>
         </table>
       </td>
-      <td class="coverPerLo">%.1f&nbsp;%%</td>
-      <td class="coverNumLo">%d / %d</td>
-      <td class="coverPerLo">%.1f&nbsp;%%</td>
-      <td class="coverNumLo">%d / %d</td>
-      <td class="coverPerLo">%.1f&nbsp;%%</td>
-      <td class="coverNumLo">%d / %d</td>
+      %s
+      %s
+      %s
     </tr>
     <tr>
-', "$dir/coverage.html", $dir,
-   $s->{total}{percentage}, $s->{total}{percentage},
-   100 - $s->{total}{percentage}, $s->{total}{percentage},
-   $s->{total}{percentage}, $s->{total}{covered}, $s->{total}{total},
-   $s->{subroutine}{percentage},
-   $s->{subroutine}{covered}, $s->{subroutine}{total},
-   $s->{branch}{percentage}, $s->{branch}{covered}, $s->{branch}{total};
+', "$dir/coverage.html", $dir, bar_html($s->{total}{percentage}),
+   box_html($s->{total}), box_html($s->{subroutine}), box_html($s->{branch});
 
 while (<>) {
     s/^(.*<td .*href="src\/index\.html">.*)$/$tmpl$1/;
     print;
 }
 
+sub bar_image {
+    my ($num) = @_;
+
+    return 'emerald.png' if $num >= 90;
+    return 'ruby.png' if $num < 75;
+    return 'amber.png';
+}
+
+sub bar_html {
+    my ($num) = @_;
+
+    my $html = sprintf '<img src="%s" width=%.0f height=10 alt="%.1f">',
+                       bar_image($num), $num, $num;
+
+    if ($num < 100) {
+        $html .= sprintf '<img src="snow.png" width=%.0f height=10 alt="%.1f">',
+                         100 - $num, $num;
+    }
+
+    return $html;
+}
+
+sub box_rating {
+    my ($num) = @_;
+
+    return 'Hi' if $num >= 90;
+    return 'Lo' if $num < 75;
+    return 'Med';
+}
+
+sub box_html {
+    my ($stats) = @_;
+
+    return sprintf '<td class="coverPer%s">%.1f&nbsp;%%</td>\n' .
+                   '<td class="coverNum%s">%d / %d</td>',
+        box_rating($stats->{percentage}), $stats->{percentage},
+        box_rating($stats->{percentage}), $stats->{covered}, $stats->{total};
+}
+
 1;