fix fib zeros

diff --git a/doc/plugins/write/tutorial.mdwn b/doc/plugins/write/tutorial.mdwn
index 5345f71f257c2979065189942ac77ce1074fa1e6..1912c8a2f37041ee54d4187957d7a664c6909573 100644 (file)
--- a/doc/plugins/write/tutorial.mdwn
+++ b/doc/plugins/write/tutorial.mdwn
@@ -81,8 +81,8 @@ textbook.
 
        sub fib {
                my $num=shift;
-               return 0 if $num == 1;
-               return 1 if $num == 2;
+               return 0 if $num == 0;
+               return 1 if $num == 1;
                return fib($num - 1) + fib($num - 2);
        }
 
@@ -92,7 +92,7 @@ And let's change the `preprocess` sub to use it:
 
        sub preprocess {
                my %params=@_;
-               my $num=$last++;
+               my $num=++$last;
                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:
 
-       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.
 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 {
                my %params=@_;
                my $page=$params{destpage};
-               my $num=$last{$page}++;
+               my $num=++$last{$page};
                return fib($num);
        }
 
@@ -145,7 +145,7 @@ Just insert these lines of code inside `preprocess`, in the appropriate
 spot:
 
                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
@@ -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
 making the plugin use the existing function safely. A good first step would
 be a guard on how high it will go.
-       
+
        my %last;
 
        sub preprocess {
@@ -167,7 +167,7 @@ be a guard on how high it will go.
                if (exists $params{seed}) {
                        $last{$page}=$params{seed}-1;
                }
-               my $num=$last{$page}++;
+               my $num=++$last{$page};
                if ($num > 25) {
                        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.
 
 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:
 
-               if (int($num) != $num || $num < 1) {
+               if (int($num) != $num || $num < 0) {
                        error "positive integers only, please";
                }