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Challenges 5 by 5 knight's tour string validator

Perl 5, 93 bytes I am disappointed to discover that just doing the obvious coordinate calculations requires fewer bytes than my lookup table approach. 😢 Reads a string from stdin. Exits 0 if vali...

posted 4mo ago by jhnc‭ · edited 3mo ago by jhnc‭

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#4: Post edited by

jhnc‭ · 2026-06-12T16:29:46Z (3 months ago)
why did I think grouping assignments was shorter than doing individually?! borrow m90 idea

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# [Perl 5], 93 bytes
I am disappointed to discover that just doing the obvious coordinate calculations requires fewer bytes than my lookup table approach. 😢
Reads a string from stdin. Exits 0 if valid tour, 1 if invalid.
~~-6 bytes from m90.~~
```
perl -ne'map{$_=ord;$x=$_%5;$y=int$_/5;exit 1if$i++&&2-abs(($x-$X)*($y-$Y));$X=$x;$Y=$y}/./g'
```
```
echo 'MJCFQXODGPWTIBKVSHENYRULA' | perl -ne'
# walk the positions
map {
# convert character to ascii ordinal value.
# conveniently A is 65 which is a multiple of 5
# so no offset is needed for modulo to work
$_ = ord;
# derive x/y coordinates
$x = $_%5;
$y = int $_/5;
# after the first move, abort if not a legal move
# a legal move differs by 1 or 2 in x and 2 or 1 in y
# from the previous position
# borrowing @m90's observation, if we multiply together
~~# we just need to for check +/- 2~~
exit 1 if $i++ && 2 - abs(($x-$X)*($y-$Y));
# current position becomes previous position
$X = $x;
$Y = $y
} /./g
'
```
[Try it online!][TIO-mqb50ny9]
[Perl 5]: https://www.perl.org/
[TIO-mqb50ny9]: https://tio.run/##dZBZS8NAFIXf51cM9aqtGoNLXzpEyNLs@9YEhFI11YAmIZNCi/SvW6cVgwq@3eW7Z86Zpmhfx7sj3BW0wy@LtiooRStaYLp6oPi0WJfdKUGswfsSv2NohcFy8UqL@2pA8HPd1fiprgq8RftN164OCwSlMBwR9PhSvzUENW1ZdUs8OOZubinm7vDgAuZMlseXZzxPdm@L5h3mQt0@EVgLMD8eE9gI7Abm/JgcXr4ql1Cen5@cXHOLBzocwpqDbHQ2hA0H@WhEIBNgTSAXYLPlL/nnv5kI2tucfFlhKZjFhlncOaasBpmnaP4sNiQrjfSpm4eJLeIJ3qdBop2EuTvVo9SSjHjma4qXBapsOt/Ar6ufcgw4/BT6R0C0e8I2JNOfubIaMynRS0JNcbIgt6Y94aqKFrKBHmS@KdvMLMOi1JF6IvR0K1Viw07k3NV8KXPMaDoTA7UndNbakZlJVugpqRq4cm7Ejq8lPfFRN11ZV3THVZ8 "Perl 5 – Try It Online"

# [Perl 5], 93 bytes
I am disappointed to discover that just doing the obvious coordinate calculations requires fewer bytes than my lookup table approach. 😢
Reads a string from stdin. Exits 0 if valid tour, 1 if invalid.
-6 bytes from @m90.
```
perl -ne'map{$_=ord;$x=$_%5;$y=int$_/5;exit 1if$i++&&2-abs(($x-$X)*($y-$Y));$X=$x;$Y=$y}/./g'
```
```
echo 'MJCFQXODGPWTIBKVSHENYRULA' | perl -ne'
# walk the positions
map {
# convert character to ascii ordinal value.
# conveniently A is 65 which is a multiple of 5
# so no offset is needed for modulo to work
$_ = ord;
# derive x/y coordinates
$x = $_%5;
$y = int $_/5;
# after the first move, abort if not a legal move
# a legal move differs by 1 or 2 in x and 2 or 1 in y
# from the previous position
# borrowing @m90's observation, if we multiply together
# we just need to check for +/- 2
exit 1 if $i++ && 2 - abs(($x-$X)*($y-$Y));
# current position becomes previous position
$X = $x;
$Y = $y
} /./g
'
```
[Try it online!][TIO-mqb50ny9]
[Perl 5]: https://www.perl.org/
[TIO-mqb50ny9]: https://tio.run/##dZBZS8NAFIXf51cM9aqtGoNLXzpEyNLs@9YEhFI11YAmIZNCi/SvW6cVgwq@3eW7Z86Zpmhfx7sj3BW0wy@LtiooRStaYLp6oPi0WJfdKUGswfsSv2NohcFy8UqL@2pA8HPd1fiprgq8RftN164OCwSlMBwR9PhSvzUENW1ZdUs8OOZubinm7vDgAuZMlseXZzxPdm@L5h3mQt0@EVgLMD8eE9gI7Abm/JgcXr4ql1Cen5@cXHOLBzocwpqDbHQ2hA0H@WhEIBNgTSAXYLPlL/nnv5kI2tucfFlhKZjFhlncOaasBpmnaP4sNiQrjfSpm4eJLeIJ3qdBop2EuTvVo9SSjHjma4qXBapsOt/Ar6ufcgw4/BT6R0C0e8I2JNOfubIaMynRS0JNcbIgt6Y94aqKFrKBHmS@KdvMLMOi1JF6IvR0K1Viw07k3NV8KXPMaDoTA7UndNbakZlJVugpqRq4cm7Ejq8lPfFRN11ZV3THVZ8 "Perl 5 – Try It Online"

#3: Post edited by

jhnc‭ · 2026-06-12T16:28:32Z (3 months ago)
why did I think grouping assignments was shorter than doing individually?! borrow m90 idea

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~~# [Perl 5], 107 bytes~~
I am disappointed to discover that just doing the obvious coordinate calculations requires fewer bytes than my lookup table approach. 😢
Reads a string from stdin. Exits 0 if valid tour, 1 if invalid.
```
~~perl -ne'map{$_=ord;($x,$y)=($_%5,int$_/5);$_=$x-$X.abs($y-$Y);exit~~
~~1if$i++&&!/12|21/;($X,$Y)=($x,$y)}/./g'~~
```
```
echo 'MJCFQXODGPWTIBKVSHENYRULA' | perl -ne'
# walk the positions
map {
# convert character to ascii ordinal value.
# conveniently A is 65 which is a multiple of 5
# so no offset is needed for modulo to work
$_ = ord;
~~# calculate its x/y coordinates~~
($x, $y) = ( $_%5, int $_/5 );
# a legal move differs by 1 or 2 in x and 2 or 1 in y
# from the previous position
~~# board has 5 rows/cols so any difference is single-digit~~
# due to operator precedence, doing abs here is shorter
# than testing for a minus below: /1-?2|2-?1/
~~$_ = $x-$X . abs($y-$Y);~~
~~# after the first move, abort if not a legal move~~
~~exit 1 if $i++ && !/12|21/;~~
# current position becomes previous position
~~($X,$Y) = ($x,$y)~~
} /./g
'
```
~~[Try it online!][TIO-mps06n9n]~~
[Perl 5]: https://www.perl.org/
[TIO-mps06n9n]: https://tio.run/##dZBZS8NAFIXf51eMdVpbbRqq9sUhQpZm37cmIJSqaQ3YJGRSqKh/3TpxCSr4NnfOdw/n3CqrH2eHY9hkpIEPq7rICAE7kkGyuyXwJNvnzQkGdIDtEz5DVHO99eqRZDdFD8NN2ZTwviwy@Apapal3HwJAOTccYXD3UG4rDKo6L5o17PWZi0sCmWvYG6MltWXh5JRl8WG7qp7RkivrezxE@zF6GnFDtOzPxnQNLdnZCFMV7RmUTFa3ZIieGJSOcJsITPM1ys/OBoMjdnr@cj5lqUMypjL35fTKTtjN34YYtKGvPoPRTjRwRQMfLF2UvcSRFHcRaoIRB@rcTv3I5OEVbLsB3oz81J6rQWwIWrhwFclJPFnUrW/g19ZPOwp83A38Y8CbHWFqgu4ubFEOqRXvRL4iWYmXGvOOsGVJ8emH6iWuLpo0LMWC2BI6wndUI5ZCzYzE1FZcIbH0YL7gPbkjVDqagZ4Ihu9IsezZYqqFlqtEHfFWVk1eFuTAFO8 "Perl 5 – Try It Online"

# [Perl 5], 93 bytes
I am disappointed to discover that just doing the obvious coordinate calculations requires fewer bytes than my lookup table approach. 😢
Reads a string from stdin. Exits 0 if valid tour, 1 if invalid.
-6 bytes from m90.
```
perl -ne'map{$_=ord;$x=$_%5;$y=int$_/5;exit 1if$i++&&2-abs(($x-$X)*($y-$Y));$X=$x;$Y=$y}/./g'
```
```
echo 'MJCFQXODGPWTIBKVSHENYRULA' | perl -ne'
# walk the positions
map {
# convert character to ascii ordinal value.
# conveniently A is 65 which is a multiple of 5
# so no offset is needed for modulo to work
$_ = ord;
# derive x/y coordinates
$x = $_%5;
$y = int $_/5;
# after the first move, abort if not a legal move
# a legal move differs by 1 or 2 in x and 2 or 1 in y
# from the previous position
# borrowing @m90's observation, if we multiply together
# we just need to for check +/- 2
exit 1 if $i++ && 2 - abs(($x-$X)*($y-$Y));
# current position becomes previous position
$X = $x;
$Y = $y
} /./g
'
```
[Try it online!][TIO-mqb50ny9]
[Perl 5]: https://www.perl.org/
[TIO-mqb50ny9]: https://tio.run/##dZBZS8NAFIXf51cM9aqtGoNLXzpEyNLs@9YEhFI11YAmIZNCi/SvW6cVgwq@3eW7Z86Zpmhfx7sj3BW0wy@LtiooRStaYLp6oPi0WJfdKUGswfsSv2NohcFy8UqL@2pA8HPd1fiprgq8RftN164OCwSlMBwR9PhSvzUENW1ZdUs8OOZubinm7vDgAuZMlseXZzxPdm@L5h3mQt0@EVgLMD8eE9gI7Abm/JgcXr4ql1Cen5@cXHOLBzocwpqDbHQ2hA0H@WhEIBNgTSAXYLPlL/nnv5kI2tucfFlhKZjFhlncOaasBpmnaP4sNiQrjfSpm4eJLeIJ3qdBop2EuTvVo9SSjHjma4qXBapsOt/Ar6ufcgw4/BT6R0C0e8I2JNOfubIaMynRS0JNcbIgt6Y94aqKFrKBHmS@KdvMLMOi1JF6IvR0K1Viw07k3NV8KXPMaDoTA7UndNbakZlJVugpqRq4cm7Ejq8lPfFRN11ZV3THVZ8 "Perl 5 – Try It Online"

#2: Post edited by

jhnc‭ · 2026-05-30T07:05:35Z (4 months ago)
first abs() is not required since leading minus will be ignored in regex

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~~# [Perl 5], 112 bytes~~
I am disappointed to discover that just doing the obvious coordinate calculations requires fewer bytes than my lookup table approach. 😢
Reads a string from stdin. Exits 0 if valid tour, 1 if invalid.
```
~~perl -ne'map{$_=ord;($x,$y)=($_%5,int$_/5);$_=abs($x-$X).abs($y-$Y);exit~~
1if$i++&&!/12|21/;($X,$Y)=($x,$y)}/./g'
```
```
echo 'MJCFQXODGPWTIBKVSHENYRULA' | perl -ne'
# walk the positions
map {
# convert character to ascii ordinal value.
# conveniently A is 65 which is a multiple of 5
# so no offset is needed for modulo to work
$_ = ord;
# calculate its x/y coordinates
~~($x,$y) = ($_%5,int$_/5);~~
# a legal move differs by 1 or 2 in x and 2 or 1 in y
# from the previous position
$_ = abs($x-$X).abs($y-$Y);
# after the first move, abort if not a legal move
exit 1 if $i++ && !/12|21/;
# current position becomes previous position
($X,$Y) = ($x,$y)
} /./g
'
```
~~[Try it online!][TIO-mppj4qby]~~
[Perl 5]: https://www.perl.org/
[TIO-mppj4qby]: https://tio.run/##dZBZS8NAFIXf51eMddRGm4aqfXGIkKXZ960JCKVqqgFNQiYFpfavWydVgwq@3Tvnu4dzps6bp@nuELY5aeHjsilzQsCa5JCsbwk8yV@K9gQDusBuhBuIGn6wWj6R/KYcYPhQtRW8r8ocbkGntM16LwBU8EMGg7vH6rnGoG6Ksl3BwRF7cUkgew0HI7Sgthwcn3Ic3j0v6w1a8FVzj4foZYReGX6IFkfTET1DC27KYKoubwkVWZQy4/34yqKMwV0sMClWqDg7Oz4@4Cbnb@cTjtqkIyrzX3Zbbsw9/K2JQZf86jMdLUZT1zT1zjYkxU9dWfXmkS6aSajNnCyILQFewa4gEKw4yJyZFiamqEdzT5Xd1Fckw/4Gfl39tKPA/vPAPwaC1ROWLhre3JGUiFoJbhyosp36mTnrCUeR1YA@aH7qGZJFw1IsTGyxJwJXMxM50q1YyhzVE1PbCGdzwVd6QqOrFRqpaAaunCi@I2V6ZHtq3BPvVd0WVUl2bPkB "Perl 5 – Try It Online"

# [Perl 5], 107 bytes
I am disappointed to discover that just doing the obvious coordinate calculations requires fewer bytes than my lookup table approach. 😢
Reads a string from stdin. Exits 0 if valid tour, 1 if invalid.
```
perl -ne'map{$_=ord;($x,$y)=($_%5,int$_/5);$_=$x-$X.abs($y-$Y);exit
1if$i++&&!/12|21/;($X,$Y)=($x,$y)}/./g'
```
```
echo 'MJCFQXODGPWTIBKVSHENYRULA' | perl -ne'
# walk the positions
map {
# convert character to ascii ordinal value.
# conveniently A is 65 which is a multiple of 5
# so no offset is needed for modulo to work
$_ = ord;
# calculate its x/y coordinates
($x, $y) = ( $_%5, int $_/5 );
# a legal move differs by 1 or 2 in x and 2 or 1 in y
# from the previous position
# board has 5 rows/cols so any difference is single-digit
# due to operator precedence, doing abs here is shorter
# than testing for a minus below: /1-?2|2-?1/
$_ = $x-$X . abs($y-$Y);
# after the first move, abort if not a legal move
exit 1 if $i++ && !/12|21/;
# current position becomes previous position
($X,$Y) = ($x,$y)
} /./g
'
```
[Try it online!][TIO-mps06n9n]
[Perl 5]: https://www.perl.org/
[TIO-mps06n9n]: https://tio.run/##dZBZS8NAFIXf51eMdVpbbRqq9sUhQpZm37cmIJSqaQ3YJGRSqKh/3TpxCSr4NnfOdw/n3CqrH2eHY9hkpIEPq7rICAE7kkGyuyXwJNvnzQkGdIDtEz5DVHO99eqRZDdFD8NN2ZTwviwy@Apapal3HwJAOTccYXD3UG4rDKo6L5o17PWZi0sCmWvYG6MltWXh5JRl8WG7qp7RkivrezxE@zF6GnFDtOzPxnQNLdnZCFMV7RmUTFa3ZIieGJSOcJsITPM1ys/OBoMjdnr@cj5lqUMypjL35fTKTtjN34YYtKGvPoPRTjRwRQMfLF2UvcSRFHcRaoIRB@rcTv3I5OEVbLsB3oz81J6rQWwIWrhwFclJPFnUrW/g19ZPOwp83A38Y8CbHWFqgu4ubFEOqRXvRL4iWYmXGvOOsGVJ8emH6iWuLpo0LMWC2BI6wndUI5ZCzYzE1FZcIbH0YL7gPbkjVDqagZ4Ihu9IsezZYqqFlqtEHfFWVk1eFuTAFO8 "Perl 5 – Try It Online"

#1: Initial revision by

jhnc‭ · 2026-05-28T13:37:54Z (4 months ago)

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# [Perl 5], 112 bytes

I am disappointed to discover that just doing the obvious coordinate calculations requires fewer bytes than my lookup table approach. 😢

Reads a string from stdin. Exits 0 if valid tour, 1 if invalid.

```
perl -ne'map{$_=ord;($x,$y)=($_%5,int$_/5);$_=abs($x-$X).abs($y-$Y);exit
1if$i++&&!/12|21/;($X,$Y)=($x,$y)}/./g'
```


```
echo 'MJCFQXODGPWTIBKVSHENYRULA' | perl -ne'
    # walk the positions
    map {
        # convert character to ascii ordinal value.
        # conveniently A is 65 which is a multiple of 5
        #   so no offset is needed for modulo to work
        $_ = ord;

        # calculate its x/y coordinates
        ($x,$y) = ($_%5,int$_/5);

        # a legal move differs by 1 or 2 in x and 2 or 1 in y
        #   from the previous position
        $_ = abs($x-$X).abs($y-$Y);

        # after the first move, abort if not a legal move
        exit 1 if $i++ && !/12|21/;

        # current position becomes previous position
        ($X,$Y) = ($x,$y)
    } /./g
'
```

[Try it online!][TIO-mppj4qby]

[Perl 5]: https://www.perl.org/
[TIO-mppj4qby]: https://tio.run/##dZBZS8NAFIXf51eMddRGm4aqfXGIkKXZ960JCKVqqgFNQiYFpfavWydVgwq@3Tvnu4dzps6bp@nuELY5aeHjsilzQsCa5JCsbwk8yV@K9gQDusBuhBuIGn6wWj6R/KYcYPhQtRW8r8ocbkGntM16LwBU8EMGg7vH6rnGoG6Ksl3BwRF7cUkgew0HI7Sgthwcn3Ic3j0v6w1a8FVzj4foZYReGX6IFkfTET1DC27KYKoubwkVWZQy4/34yqKMwV0sMClWqDg7Oz4@4Cbnb@cTjtqkIyrzX3Zbbsw9/K2JQZf86jMdLUZT1zT1zjYkxU9dWfXmkS6aSajNnCyILQFewa4gEKw4yJyZFiamqEdzT5Xd1Fckw/4Gfl39tKPA/vPAPwaC1ROWLhre3JGUiFoJbhyosp36mTnrCUeR1YA@aH7qGZJFw1IsTGyxJwJXMxM50q1YyhzVE1PbCGdzwVd6QqOrFRqpaAaunCi@I2V6ZHtq3BPvVd0WVUl2bPkB "Perl 5 – Try It Online"

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