Sort::Key::Top(3) select and sort top n elements

SYNOPSIS

use Sort::Key::Top (nkeytop top);
# select 5 first numbers by absolute value:
@top = nkeytop { abs \$_ } 5 => 1, 2, 7, 5, 5, 1, 78, 0, -2, -8, 2;
# ==> @top = (1, 2, 1, 0, -2)
# select 5 first numbers by absolute value and sort accordingly:
@top = nkeytopsort { abs \$_ } 5 => 1, 2, 7, 5, 5, 1, 78, 0, -2, -8, 2;
# ==> @top = (0, 1, 1, 2, -2)
# select 5 first words by lexicographic order:
@a = qw(cat fish bird leon penguin horse rat elephant squirrel dog);
@top = top 5 => @a;
# ==> @top = qw(cat fish bird elephant dog);

DESCRIPTION

The functions available from this module select the top n elements from a list using several common orderings and custom key extraction procedures.

They are all variations around

```  keytopsort { CALC_KEY(\$_) } \$n => @data;
```

In array context, this function calculates the ordering key for every element in @data using the expression inside the block. Then it selects and orders the \$n elements with the lower keys when compared lexicographically.

It is equivalent to the pure Perl expression:

```  (sort { CALC_KEY(\$a) cmp CALC_KEY(\$b) } @data)[0 .. \$n-1];
```

If \$n is negative, the last \$n elements from the bottom are selected:

```  topsort 3 => qw(foo doom me bar doz hello);
# ==> ('bar', 'doz', 'doom')
topsort -3 => qw(foo doom me bar doz hello);
# ==> ('foo', 'hello', 'me')
top 3 => qw(foo doom me bar doz hello);
# ==> ('doom', 'bar', 'doz')
top -3 => qw(foo doom me bar doz hello);
# ==> ('foo', 'me', 'hello')
```

In scalar context, the value returned by the functions on this module is the cutoff value allowing to select nth element from the array. For instance:

```  # n = 5;
scalar(topsort 5 => @data) eq (sort @data)[4]    # true
# n = -5;
scalar(topsort -5 => @data) eq (sort @data)[-5]  # true
```

Note that on scalar context, the "sort" variations (see below) are usually the right choice:

```  scalar topsort 3 => qw(me foo doz doom me bar hello); # ==> 'doz'
scalar top 3 => qw(me foo doz doom me bar hello); # ==> 'bar'
```

Note also, that the index is 1-based (it starts at one instead of at zero). The "atpos" set of functions explained below do the same and are 0-based.

Variations allow to:

- use the own values as the ordering keys
```  topsort 5 => qw(a b ab t uu g h aa aac);
# ==> a aa aac ab b
```
- use an array or hash index instead of a subroutine to extract the key
```  slottop 0, 2, [4], [1], [3], [2], [4];
# ==> [1], [2]
```
- return the selected values in the original order
```  top 5 => qw(a b ab t uu g h aa aac);
# ==> a b ab aa aac
```
- use a different ordering
For instance comparing the keys as numbers, using the locale configuration or in reverse order:

```  rnkeytop { length \$_ } 3 => qw(a ab aa aac b t uu g h);
# ==> ab aa aac
rnkeytopsort { length \$_ } 3 => qw(a ab aa aac b t uu g h);
# ==> aac ab aa
```

A prefix is used to indicate the required ordering:

(no prefix)
lexicographical ascending order
r
lexicographical descending order
l
lexicographical ascending order obeying locale configuration
r
lexicographical descending order obeying locale configuration
n
numerical ascending order
rn
numerical descending order
i
numerical ascending order but converting the keys to integers first
ri
numerical descending order but converting the keys to integers first
u
numerical ascending order but converting the keys to unsigned integers first
ru
numerical descending order but converting the keys to unsigned integers first
- select the head element from the list sorted
```  nhead 6, 7, 3, 8, 9, 9;
# ==> 3
nkeyhead { length \$_ } qw(a ab aa aac b t uu uiyii)
# ==> 'a'
```
- select the tail element from the list sorted
```  tail qw(a ab aa aac b t uu uiyii);
# ==> 'uu'
nkeytail { length \$_ } qw(a ab aa aac b t uu uiyii)
# ==> 'uiyii'
```
- select the element at position n from the list sorted
```  atpos 3, qw(a ab aa aac b t uu uiyii);
# ==> 'ab';
rnkeyatpos { abs \$_ } 2 => -0.3, 1.1, 4, 0.1, 0.9, -2;
# ==> 1.1
rnkeyatpos { abs \$_ } -2 => -0.3, 1.1, 4, 0.1, 0.9, -2;
# ==> -0.3
```

Note that for the "atpos" set of functions indexes start at zero.

- return a list composed by the elements with the first n ordered keys and then the remaining ones.
```  ikeypart { length \$_ } 3 => qw(a bbbb cc ddddd g fd);
# ==> a cc g bbbb ddddd fd
```
- return two arrays references, the first array containing the elements with the first n ordered keys and the second with the rest.
```  keypartref { length \$_ } 3 => qw(a bbbb cc ddddd g fd);
# ==> [a cc g] [bbbb ddddd fd]
```

The full list of available functions is:

```  top ltop ntop itop utop rtop rltop rntop ritop rutop
keytop lkeytop nkeytop ikeytop ukeytop rkeytop rlkeytop rnkeytop
rikeytop rukeytop
slottop lslottop nslottop islottop uslottop rslottop rlslottop rnslottop
rislottop ruslottop
topsort ltopsort ntopsort itopsort utopsort rtopsort rltopsort
rntopsort ritopsort rutopsort
keytopsort lkeytopsort nkeytopsort ikeytopsort ukeytopsort
rkeytopsort rlkeytopsort rnkeytopsort rikeytopsort rukeytopsort
slottopsort lslottopsort nslottopsort islottopsort uslottopsort
rslottopsort rlslottopsort rnslottopsort rislottopsort ruslottopsort
tail ltail ntail itail utail rtail rltail rntail ritail rutail
keytail lkeytail nkeytail ikeytail ukeytail rkeytail rlkeytail
rnkeytail rikeytail rukeytail
slottail lslottail nslottail islottail uslottail rslottail rlslottail
rnslottail rislottail ruslottail
atpos latpos natpos iatpos uatpos ratpos rlatpos rnatpos riatpos
ruatpos
keyatpos lkeyatpos nkeyatpos ikeyatpos ukeyatpos rkeyatpos
rlkeyatpos rnkeyatpos rikeyatpos rukeyatpos
slotatpos lslotatpos nslotatpos islotatpos uslotatpos rslotatpos
rlslotatpos rnslotatpos rislotatpos ruslotatpos
part lpart npart ipart upart rpart rlpart rnpart ripart
rupart
keypart lkeypart nkeypart ikeypart ukeypart rkeypart
rlkeypart rnkeypart rikeypart rukeypart
slotpart lslotpart nslotpart islotpart uslotpart rslotpart
rlslotpart rnslotpart rislotpart ruslotpart
```