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fix fib zeros

master
Joey Hess 2012-04-02 11:18:32 -04:00
parent 33b34781c1
commit 5c55e80477
1 changed files with 10 additions and 10 deletions
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20
doc/plugins/write/tutorial.mdwn
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@ -81,8 +81,8 @@ textbook.
sub fib { sub fib {
my $num=shift; my $num=shift;
return 0 if $num == 1; return 0 if $num == 0;
return 1 if $num == 2; return 1 if $num == 1;
return fib($num - 1) + fib($num - 2); return fib($num - 1) + fib($num - 2);
} }
@ -92,7 +92,7 @@ And let's change the `preprocess` sub to use it:
sub preprocess { sub preprocess {
my %params=@_; my %params=@_;
my $num=$last++; my $num=++$last;
return fib($num); return fib($num);
} }
@ -102,7 +102,7 @@ Feel free to try it out with a simple page like this:
Looks like it works ok, doesn't it? That creates a page that lists: Looks like it works ok, doesn't it? That creates a page that lists:
1, 1, 3, 5, 8 1, 1, 2, 3, 5
But what happens if there are two pages that both use fib? Try it out. But what happens if there are two pages that both use fib? Try it out.
If ikiwiki builds both pages in one pass, the sequence will continue If ikiwiki builds both pages in one pass, the sequence will continue
@ -127,7 +127,7 @@ to start from 1 in every page that uses it.
sub preprocess { sub preprocess {
my %params=@_; my %params=@_;
my $page=$params{destpage}; my $page=$params{destpage};
my $num=$last{$page}++; my $num=++$last{$page};
return fib($num); return fib($num);
} }
@ -145,7 +145,7 @@ Just insert these lines of code inside `preprocess`, in the appropriate
spot: spot:
if (exists $params{seed}) { if (exists $params{seed}) {
$last{$page}=$params{seed}-1; $last{$page}=$params{seed};
} }
But this highlights another issue with the plugin. The `fib()` function is But this highlights another issue with the plugin. The `fib()` function is
@ -158,7 +158,7 @@ Now, we could try to fix `fib()` to run in constant time for any number,
but that's not the focus of this tutorial. Instead, let's concentrate on but that's not the focus of this tutorial. Instead, let's concentrate on
making the plugin use the existing function safely. A good first step would making the plugin use the existing function safely. A good first step would
be a guard on how high it will go. be a guard on how high it will go.
my %last; my %last;
sub preprocess { sub preprocess {
@ -167,7 +167,7 @@ be a guard on how high it will go.
if (exists $params{seed}) { if (exists $params{seed}) {
$last{$page}=$params{seed}-1; $last{$page}=$params{seed}-1;
} }
my $num=$last{$page}++; my $num=++$last{$page};
if ($num > 25) { if ($num > 25) {
error "can only calculate the first 25 numbers in the sequence"; error "can only calculate the first 25 numbers in the sequence";
} }
@ -178,10 +178,10 @@ Returning an error message like this is standard for preprocessor plugins,
so that the user can look at the built page and see what went wrong. so that the user can look at the built page and see what went wrong.
Are we done? Nope, there's still a security hole. Consider what `fib()` Are we done? Nope, there's still a security hole. Consider what `fib()`
does for numbers less than 1. Or for any number that's not an integer. In does for numbers less than 0. Or for any number that's not an integer. In
either case, it will run forever. Here's one way to fix that: either case, it will run forever. Here's one way to fix that:
if (int($num) != $num || $num < 1) { if (int($num) != $num || $num < 0) {
error "positive integers only, please"; error "positive integers only, please";
} }