package List::MoreUtils;

use 5.008_001;
use strict;
use warnings;

my $have_xs;
our $VERSION = '0.430';

BEGIN
{
    unless (defined($have_xs))
    {
        ## no critic (ErrorHandling::RequireCheckingReturnValueOfEval)
        eval { require List::MoreUtils::XS; } unless $ENV{LIST_MOREUTILS_PP};
        ## no critic (ErrorHandling::RequireCarping)
        die $@ if $@ && defined $ENV{LIST_MOREUTILS_PP} && $ENV{LIST_MOREUTILS_PP} == 0;
        $have_xs = 0 + defined($INC{'List/MoreUtils/XS.pm'});
    }

    use List::MoreUtils::PP qw();
}

use Exporter::Tiny qw();

my @junctions = qw(any all none notall);
my @v0_22     = qw(
  true false
  firstidx lastidx
  insert_after insert_after_string
  apply indexes
  after after_incl before before_incl
  firstval lastval
  each_array each_arrayref
  pairwise natatime
  mesh uniq
  minmax part
  _XScompiled
);
my @v0_24  = qw(bsearch);
my @v0_33  = qw(sort_by nsort_by);
my @v0_400 = qw(one any_u all_u none_u notall_u one_u
  firstres onlyidx onlyval onlyres lastres
  singleton bsearchidx
);
my @v0_420 = qw(arrayify duplicates minmaxstr samples zip6 reduce_0 reduce_1 reduce_u
  listcmp frequency occurrences mode
  binsert bremove equal_range lower_bound upper_bound qsort
  slide slideatatime);

my @all_functions = (@junctions, @v0_22, @v0_24, @v0_33, @v0_400, @v0_420);

## no critic (TestingAndDebugging::ProhibitNoStrict)
no strict "refs";
if ($have_xs)
{
    my $x;
    for (@all_functions)
    {
        List::MoreUtils->can($_) or *$_ = $x if ($x = List::MoreUtils::XS->can($_));
    }
}
List::MoreUtils->can($_) or *$_ = List::MoreUtils::PP->can($_) for (@all_functions);
use strict;
## use critic (TestingAndDebugging::ProhibitNoStrict)
use parent qw(Exporter::Tiny);

my %alias_list = (
    v0_22 => {
        first_index => "firstidx",
        last_index  => "lastidx",
        first_value => "firstval",
        last_value  => "lastval",
        zip         => "mesh",
    },
    v0_33 => {
        distinct => "uniq",
    },
    v0_400 => {
        first_result  => "firstres",
        only_index    => "onlyidx",
        only_value    => "onlyval",
        only_result   => "onlyres",
        last_result   => "lastres",
        bsearch_index => "bsearchidx",
    },
    v0_420 => {
        bsearch_insert => "binsert",
        bsearch_remove => "bremove",
        zip_unflatten  => "zip6",
    },
);

our @EXPORT_OK   = (@all_functions, map { keys %$_ } values %alias_list);
our %EXPORT_TAGS = (
    all         => \@EXPORT_OK,
    'like_0.22' => [
        any_u    => {-as => 'any'},
        all_u    => {-as => 'all'},
        none_u   => {-as => 'none'},
        notall_u => {-as => 'notall'},
        @v0_22,
        keys %{$alias_list{v0_22}},
    ],
    'like_0.24' => [
        any_u    => {-as => 'any'},
        all_u    => {-as => 'all'},
        notall_u => {-as => 'notall'},
        'none',
        @v0_22,
        @v0_24,
        keys %{$alias_list{v0_22}},
    ],
    'like_0.33' => [
        @junctions,
        @v0_22,
        # v0_24 functions were omitted
        @v0_33,
        keys %{$alias_list{v0_22}},
        keys %{$alias_list{v0_33}},
    ],
);

for my $set (values %alias_list)
{
    for my $alias (keys %$set)
    {
        ## no critic (TestingAndDebugging::ProhibitNoStrict)
        no strict qw(refs);
        *$alias = __PACKAGE__->can($set->{$alias});
        ## use critic (TestingAndDebugging::ProhibitNoStrict)
    }
}
use strict;

=pod

=head1 NAME

List::MoreUtils - Provide the stuff missing in List::Util

=head1 SYNOPSIS

    # import specific functions

    use List::MoreUtils qw(any uniq);

    if ( any { /foo/ } uniq @has_duplicates ) {
        # do stuff
    }

    # import everything

    use List::MoreUtils ':all';

    # import by API

    # has "original" any/all/none/notall behavior
    use List::MoreUtils ':like_0.22';
    # 0.22 + bsearch
    use List::MoreUtils ':like_0.24';
    # has "simplified" any/all/none/notall behavior + (n)sort_by
    use List::MoreUtils ':like_0.33';

=head1 DESCRIPTION

B<List::MoreUtils> provides some trivial but commonly needed functionality on
lists which is not going to go into L<List::Util>.

All of the below functions are implementable in only a couple of lines of Perl
code. Using the functions from this module however should give slightly better
performance as everything is implemented in C. The pure-Perl implementation of
these functions only serves as a fallback in case the C portions of this module
couldn't be compiled on this machine.

=head1 EXPORTS

=head2 Default behavior

Nothing by default. To import all of this module's symbols use the C<:all> tag.
Otherwise functions can be imported by name as usual:

    use List::MoreUtils ':all';

    use List::MoreUtils qw{ any firstidx };

Because historical changes to the API might make upgrading List::MoreUtils
difficult for some projects, the legacy API is available via special import
tags.

=head2 Like version 0.22 (last release with original API)

This API was available from 2006 to 2009, returning undef for empty lists on
C<all>/C<any>/C<none>/C<notall>:

    use List::MoreUtils ':like_0.22';

This import tag will import all functions available as of version 0.22.
However, it will import C<any_u> as C<any>, C<all_u> as C<all>, C<none_u> as
C<none>, and C<notall_u> as C<notall>.

=head2 Like version 0.24 (first incompatible change)

This API was available from 2010 to 2011.  It changed the return value of C<none>
and added the C<bsearch> function.

    use List::MoreUtils ':like_0.24';

This import tag will import all functions available as of version 0.24.
However it will import C<any_u> as C<any>, C<all_u> as C<all>, and
C<notall_u> as C<notall>.  It will import C<none> as described in
the documentation below (true for empty list).

=head2 Like version 0.33 (second incompatible change)

This API was available from 2011 to 2014. It is widely used in several CPAN
modules and thus it's closest to the current API.  It changed the return values
of C<any>, C<all>, and C<notall>.  It added the C<sort_by> and C<nsort_by> functions
and the C<distinct> alias for C<uniq>.  It omitted C<bsearch>.

    use List::MoreUtils ':like_0.33';

This import tag will import all functions available as of version 0.33.  Note:
it will not import C<bsearch> for consistency with the 0.33 API.

=head1 FUNCTIONS

=head2 Junctions

=head3 I<Treatment of an empty list>

There are two schools of thought for how to evaluate a junction on an
empty list:

=over

=item *

Reduction to an identity (boolean)

=item *

Result is undefined (three-valued)

=back

In the first case, the result of the junction applied to the empty list is
determined by a mathematical reduction to an identity depending on whether
the underlying comparison is "or" or "and".  Conceptually:

                    "any are true"      "all are true"
                    --------------      --------------
    2 elements:     A || B || 0         A && B && 1
    1 element:      A || 0              A && 1
    0 elements:     0                   1

In the second case, three-value logic is desired, in which a junction
applied to an empty list returns C<undef> rather than true or false 

Junctions with a C<_u> suffix implement three-valued logic.  Those
without are boolean.

=head3 all BLOCK LIST

=head3 all_u BLOCK LIST

Returns a true value if all items in LIST meet the criterion given through
BLOCK. Sets C<$_> for each item in LIST in turn:

  print "All values are non-negative"
    if all { $_ >= 0 } ($x, $y, $z);

For an empty LIST, C<all> returns true (i.e. no values failed the condition)
and C<all_u> returns C<undef>.

Thus, C<< all_u(@list) >> is equivalent to C<< @list ? all(@list) : undef >>.

B<Note>: because Perl treats C<undef> as false, you must check the return value
of C<all_u> with C<defined> or you will get the opposite result of what you
expect.

=head3 any BLOCK LIST

=head3 any_u BLOCK LIST

Returns a true value if any item in LIST meets the criterion given through
BLOCK. Sets C<$_> for each item in LIST in turn:

  print "At least one non-negative value"
    if any { $_ >= 0 } ($x, $y, $z);

For an empty LIST, C<any> returns false and C<any_u> returns C<undef>.

Thus, C<< any_u(@list) >> is equivalent to C<< @list ? any(@list) : undef >>.

=head3 none BLOCK LIST

=head3 none_u BLOCK LIST

Logically the negation of C<any>. Returns a true value if no item in LIST meets
the criterion given through BLOCK. Sets C<$_> for each item in LIST in turn:

  print "No non-negative values"
    if none { $_ >= 0 } ($x, $y, $z);

For an empty LIST, C<none> returns true (i.e. no values failed the condition)
and C<none_u> returns C<undef>.

Thus, C<< none_u(@list) >> is equivalent to C<< @list ? none(@list) : undef >>.

B<Note>: because Perl treats C<undef> as false, you must check the return value
of C<none_u> with C<defined> or you will get the opposite result of what you
expect.

=head3 notall BLOCK LIST

=head3 notall_u BLOCK LIST

Logically the negation of C<all>. Returns a true value if not all items in LIST
meet the criterion given through BLOCK. Sets C<$_> for each item in LIST in
turn:

  print "Not all values are non-negative"
    if notall { $_ >= 0 } ($x, $y, $z);

For an empty LIST, C<notall> returns false and C<notall_u> returns C<undef>.

Thus, C<< notall_u(@list) >> is equivalent to C<< @list ? notall(@list) : undef >>.

=head3 one BLOCK LIST

=head3 one_u BLOCK LIST

Returns a true value if precisely one item in LIST meets the criterion
given through BLOCK. Sets C<$_> for each item in LIST in turn:

    print "Precisely one value defined"
        if one { defined($_) } @list;

Returns false otherwise.

For an empty LIST, C<one> returns false and C<one_u> returns C<undef>.

The expression C<one BLOCK LIST> is almost equivalent to
C<1 == true BLOCK LIST>, except for short-cutting.
Evaluation of BLOCK will immediately stop at the second true value.

=head2 Transformation

=head3 apply BLOCK LIST

Applies BLOCK to each item in LIST and returns a list of the values after BLOCK
has been applied. In scalar context, the last element is returned.  This
function is similar to C<map> but will not modify the elements of the input
list:

  my @list = (1 .. 4);
  my @mult = apply { $_ *= 2 } @list;
  print "\@list = @list\n";
  print "\@mult = @mult\n";
  __END__
  @list = 1 2 3 4
  @mult = 2 4 6 8

Think of it as syntactic sugar for

  for (my @mult = @list) { $_ *= 2 }

=head3 insert_after BLOCK VALUE LIST

Inserts VALUE after the first item in LIST for which the criterion in BLOCK is
true. Sets C<$_> for each item in LIST in turn.

  my @list = qw/This is a list/;
  insert_after { $_ eq "a" } "longer" => @list;
  print "@list";
  __END__
  This is a longer list

=head3 insert_after_string STRING VALUE LIST

Inserts VALUE after the first item in LIST which is equal to STRING. 

  my @list = qw/This is a list/;
  insert_after_string "a", "longer" => @list;
  print "@list";
  __END__
  This is a longer list

=head3 pairwise BLOCK ARRAY1 ARRAY2

Evaluates BLOCK for each pair of elements in ARRAY1 and ARRAY2 and returns a
new list consisting of BLOCK's return values. The two elements are set to C<$a>
and C<$b>.  Note that those two are aliases to the original value so changing
them will modify the input arrays.

  @a = (1 .. 5);
  @b = (11 .. 15);
  @x = pairwise { $a + $b } @a, @b;     # returns 12, 14, 16, 18, 20

  # mesh with pairwise
  @a = qw/a b c/;
  @b = qw/1 2 3/;
  @x = pairwise { ($a, $b) } @a, @b;    # returns a, 1, b, 2, c, 3

=head3 mesh ARRAY1 ARRAY2 [ ARRAY3 ... ]

=head3 zip ARRAY1 ARRAY2 [ ARRAY3 ... ]

Returns a list consisting of the first elements of each array, then
the second, then the third, etc, until all arrays are exhausted.

Examples:

  @x = qw/a b c d/;
  @y = qw/1 2 3 4/;
  @z = mesh @x, @y;         # returns a, 1, b, 2, c, 3, d, 4

  @a = ('x');
  @b = ('1', '2');
  @c = qw/zip zap zot/;
  @d = mesh @a, @b, @c;   # x, 1, zip, undef, 2, zap, undef, undef, zot

C<zip> is an alias for C<mesh>.

=head3 zip6

=head3 zip_unflatten

Returns a list of arrays consisting of the first elements of each array,
then the second, then the third, etc, until all arrays are exhausted.

  @x = qw/a b c d/;
  @y = qw/1 2 3 4/;
  @z = zip6 @x, @y;         # returns [a, 1], [b, 2], [c, 3], [d, 4]

  @a = ('x');
  @b = ('1', '2');
  @c = qw/zip zap zot/;
  @d = zip6 @a, @b, @c;     # [x, 1, zip], [undef, 2, zap], [undef, undef, zot]

C<zip_unflatten> is an alias for C<zip6>.

=head3 listcmp ARRAY0 ARRAY1 [ ARRAY2 ... ]

Returns an associative list of elements and every I<id> of the list it
was found in. Allows easy implementation of @a & @b, @a | @b, @a ^ @b and
so on.
Undefined entries in any given array are skipped.

  my @a = qw(one two three four five six seven eight nine ten eleven twelve thirteen);
  my @b = qw(two three five seven eleven thirteen seventeen);
  my @c = qw(one one two three five eight thirteen twentyone);
  my %cmp = listcmp @a, @b, @c; # returns (one => [0, 2], two => [0, 1, 2], three => [0, 1, 2], four => [0], ...)

  my @seq = (1, 2, 3);
  my @prim = (undef, 2, 3, 5);
  my @fib = (1, 1, 2);
  my %cmp = listcmp @seq, @prim, @fib;
  # returns ( 1 => [0, 2], 2 => [0, 1, 2], 3 => [0, 1], 5 => [1] )

=head3 arrayify LIST[,LIST[,LIST...]]

Returns a list consisting of each element of given arrays. Recursive arrays
are flattened, too.

  @a = (1, [[2], 3], 4, [5], 6, [7], 8, 9);
  @l = arrayify @a;         # returns 1, 2, 3, 4, 5, 6, 7, 8, 9

=head3 uniq LIST

=head3 distinct LIST

Returns a new list by stripping duplicate values in LIST by comparing
the values as hash keys, except that undef is considered separate from ''.
The order of elements in the returned list is the same as in LIST. In
scalar context, returns the number of unique elements in LIST.

  my @x = uniq 1, 1, 2, 2, 3, 5, 3, 4; # returns 1 2 3 5 4
  my $x = uniq 1, 1, 2, 2, 3, 5, 3, 4; # returns 5
  # returns "Mike", "Michael", "Richard", "Rick"
  my @n = distinct "Mike", "Michael", "Richard", "Rick", "Michael", "Rick"
  # returns "A8", "", undef, "A5", "S1"
  my @s = distinct "A8", "", undef, "A5", "S1", "A5", "A8"
  # returns "Giulia", "Giulietta", undef, "", 156, "GTA", "GTV", 159, "Brera", "4C"
  my @w = uniq "Giulia", "Giulietta", undef, "", 156, "GTA", "GTV", 159, "Brera", "4C", "Giulietta", "Giulia"

C<distinct> is an alias for C<uniq>.

B<RT#49800> can be used to give feedback about this behavior.

=head3 singleton LIST

Returns a new list by stripping values in LIST occurring more than once by
comparing the values as hash keys, except that undef is considered separate
from ''.  The order of elements in the returned list is the same as in LIST.
In scalar context, returns the number of elements occurring only once in LIST.

  my @x = singleton 1,1,2,2,3,4,5 # returns 3 4 5

=head3 duplicates LIST

Returns a new list by stripping values in LIST occurring less than twice by
comparing the values as hash keys, except that undef is considered separate
from ''.  The order of elements in the returned list is the same as in LIST.
In scalar context, returns the number of elements occurring more than once
in LIST.

  my @y = duplicates 1,1,2,4,7,2,3,4,6,9; #returns 1,2,4

=head3 frequency LIST

Returns an associative list of distinct values and the corresponding frequency.

  my @f = frequency values %radio_nrw; # returns (
  #  'Deutschlandfunk (DLF)' => 9, 'WDR 3' => 10,
  #  'WDR 4' => 11, 'WDR 5' => 14, 'WDR Eins Live' => 14,
  #  'Deutschlandradio Kultur' => 8,...)

=head3 occurrences LIST

Returns a new list of frequencies and the corresponding values from LIST.

  my @o = occurrences ((1) x 3, (2) x 4, (3) x 2, (4) x 7, (5) x 2, (6) x 4);
  #  @o = (undef, undef, [3, 5], [1], [2, 6], undef, undef, [4]);

=head3 mode LIST

Returns the modal value of LIST. In scalar context, just the modal value
is returned, in list context all probes occurring I<modal> times are returned,
too.

  my @m = mode ((1) x 3, (2) x 4, (3) x 2, (4) x 7, (5) x 2, (6) x 4, (7) x 3, (8) x 7);
  #  @m = (7, 4, 8) - bimodal LIST

=head3 slide BLOCK LIST

The function C<slide> operates on pairs of list elements like:

  my @s = slide { "$a and $b" } (0..3);
  # @s = ("0 and 1", "1 and 2", "2 and 3")

The idea behind this function is a kind of magnifying glass that is moved
along a list and calls C<BLOCK> every time the next list item is reached.

=head2 Partitioning

=head3 after BLOCK LIST

Returns a list of the values of LIST after (and not including) the point
where BLOCK returns a true value. Sets C<$_> for each element in LIST in turn.

  @x = after { $_ % 5 == 0 } (1..9);    # returns 6, 7, 8, 9

=head3 after_incl BLOCK LIST

Same as C<after> but also includes the element for which BLOCK is true.

=head3 before BLOCK LIST

Returns a list of values of LIST up to (and not including) the point where BLOCK
returns a true value. Sets C<$_> for each element in LIST in turn.

=head3 before_incl BLOCK LIST

Same as C<before> but also includes the element for which BLOCK is true.

=head3 part BLOCK LIST

Partitions LIST based on the return value of BLOCK which denotes into which
partition the current value is put.

Returns a list of the partitions thusly created. Each partition created is a
reference to an array.

  my $i = 0;
  my @part = part { $i++ % 2 } 1 .. 8;   # returns [1, 3, 5, 7], [2, 4, 6, 8]

You can have a sparse list of partitions as well where non-set partitions will
be undef:

  my @part = part { 2 } 1 .. 10;            # returns undef, undef, [ 1 .. 10 ]

Be careful with negative values, though:

  my @part = part { -1 } 1 .. 10;
  __END__
  Modification of non-creatable array value attempted, subscript -1 ...

Negative values are only ok when they refer to a partition previously created:

  my @idx  = ( 0, 1, -1 );
  my $i    = 0;
  my @part = part { $idx[$i++ % 3] } 1 .. 8; # [1, 4, 7], [2, 3, 5, 6, 8]

=head3 samples COUNT LIST

Returns a new list containing COUNT random samples from LIST. Is similar to
L<List::Util/shuffle>, but stops after COUNT.

  @r  = samples 10, 1..10; # same as shuffle
  @r2 = samples 5, 1..10; # gives 5 values from 1..10;

=head2 Iteration

=head3 each_array ARRAY1 ARRAY2 ...

Creates an array iterator to return the elements of the list of arrays ARRAY1,
ARRAY2 throughout ARRAYn in turn.  That is, the first time it is called, it
returns the first element of each array.  The next time, it returns the second
elements.  And so on, until all elements are exhausted.

This is useful for looping over more than one array at once:

  my $ea = each_array(@a, @b, @c);
  while ( my ($a, $b, $c) = $ea->() )   { .... }

The iterator returns the empty list when it reached the end of all arrays.

If the iterator is passed an argument of 'C<index>', then it returns
the index of the last fetched set of values, as a scalar.

=head3 each_arrayref LIST

Like each_array, but the arguments are references to arrays, not the
plain arrays.

=head3 natatime EXPR, LIST

Creates an array iterator, for looping over an array in chunks of
C<$n> items at a time.  (n at a time, get it?).  An example is
probably a better explanation than I could give in words.

Example:

  my @x = ('a' .. 'g');
  my $it = natatime 3, @x;
  while (my @vals = $it->())
  {
    print "@vals\n";
  }

This prints

  a b c
  d e f
  g

=head3 slideatatime STEP, WINDOW, LIST

Creates an array iterator, for looping over an array in chunks of
C<$windows-size> items at a time.

The idea behind this function is a kind of magnifying glass (finer
controllable compared to L</slide>) that is moved along a list.

Example:

  my @x = ('a' .. 'g');
  my $it = slideatatime 2, 3, @x;
  while (my @vals = $it->())
  {
    print "@vals\n";
  }

This prints

  a b c
  c d e
  e f g
  g

=head2 Searching

=head3 firstval BLOCK LIST

=head3 first_value BLOCK LIST

Returns the first element in LIST for which BLOCK evaluates to true. Each
element of LIST is set to C<$_> in turn. Returns C<undef> if no such element
has been found.

C<first_value> is an alias for C<firstval>.

=head3 onlyval BLOCK LIST

=head3 only_value BLOCK LIST

Returns the only element in LIST for which BLOCK evaluates to true. Sets
C<$_> for each item in LIST in turn. Returns C<undef> if no such element
has been found.

C<only_value> is an alias for C<onlyval>.

=head3 lastval BLOCK LIST

=head3 last_value BLOCK LIST

Returns the last value in LIST for which BLOCK evaluates to true. Each element
of LIST is set to C<$_> in turn. Returns C<undef> if no such element has been
found.

C<last_value> is an alias for C<lastval>.

=head3 firstres BLOCK LIST

=head3 first_result BLOCK LIST

Returns the result of BLOCK for the first element in LIST for which BLOCK
evaluates to true. Each element of LIST is set to C<$_> in turn. Returns
C<undef> if no such element has been found.

C<first_result> is an alias for C<firstres>.

=head3 onlyres BLOCK LIST

=head3 only_result BLOCK LIST

Returns the result of BLOCK for the first element in LIST for which BLOCK
evaluates to true. Sets C<$_> for each item in LIST in turn. Returns
C<undef> if no such element has been found.

C<only_result> is an alias for C<onlyres>.

=head3 lastres BLOCK LIST

=head3 last_result BLOCK LIST

Returns the result of BLOCK for the last element in LIST for which BLOCK
evaluates to true. Each element of LIST is set to C<$_> in turn. Returns
C<undef> if no such element has been found.

C<last_result> is an alias for C<lastres>.

=head3 indexes BLOCK LIST

Evaluates BLOCK for each element in LIST (assigned to C<$_>) and returns a list
of the indices of those elements for which BLOCK returned a true value. This is
just like C<grep> only that it returns indices instead of values:

  @x = indexes { $_ % 2 == 0 } (1..10);   # returns 1, 3, 5, 7, 9

=head3 firstidx BLOCK LIST

=head3 first_index BLOCK LIST

Returns the index of the first element in LIST for which the criterion in BLOCK
is true. Sets C<$_> for each item in LIST in turn:

  my @list = (1, 4, 3, 2, 4, 6);
  printf "item with index %i in list is 4", firstidx { $_ == 4 } @list;
  __END__
  item with index 1 in list is 4

Returns C<-1> if no such item could be found.

C<first_index> is an alias for C<firstidx>.

=head3 onlyidx BLOCK LIST

=head3 only_index BLOCK LIST

Returns the index of the only element in LIST for which the criterion
in BLOCK is true. Sets C<$_> for each item in LIST in turn:

    my @list = (1, 3, 4, 3, 2, 4);
    printf "uniqe index of item 2 in list is %i", onlyidx { $_ == 2 } @list;
    __END__
    unique index of item 2 in list is 4

Returns C<-1> if either no such item or more than one of these
has been found.

C<only_index> is an alias for C<onlyidx>.

=head3 lastidx BLOCK LIST

=head3 last_index BLOCK LIST

Returns the index of the last element in LIST for which the criterion in BLOCK
is true. Sets C<$_> for each item in LIST in turn:

  my @list = (1, 4, 3, 2, 4, 6);
  printf "item with index %i in list is 4", lastidx { $_ == 4 } @list;
  __END__
  item with index 4 in list is 4

Returns C<-1> if no such item could be found.

C<last_index> is an alias for C<lastidx>.

=head2 Sorting

=head3 sort_by BLOCK LIST

Returns the list of values sorted according to the string values returned by the
KEYFUNC block or function. A typical use of this may be to sort objects according
to the string value of some accessor, such as

  sort_by { $_->name } @people

The key function is called in scalar context, being passed each value in turn as
both $_ and the only argument in the parameters, @_. The values are then sorted
according to string comparisons on the values returned.
This is equivalent to

  sort { $a->name cmp $b->name } @people

except that it guarantees the name accessor will be executed only once per value.
One interesting use-case is to sort strings which may have numbers embedded in them
"naturally", rather than lexically.

  sort_by { s/(\d+)/sprintf "%09d", $1/eg; $_ } @strings

This sorts strings by generating sort keys which zero-pad the embedded numbers to
some level (9 digits in this case), helping to ensure the lexical sort puts them
in the correct order.

=head3 nsort_by BLOCK LIST

Similar to sort_by but compares its key values numerically.

=head3 qsort BLOCK ARRAY

This sorts the given array B<in place> using the given compare code. Except for
tiny compare code like C<< $a <=> $b >>, qsort is much faster than Perl's C<sort>
depending on the version.

Compared 5.8 and 5.26:

  my @rl;
  for(my $i = 0; $i < 1E6; ++$i) { push @rl, rand(1E5) }
  my $idx;

  sub ext_cmp { $_[0] <=> $_[1] }

  cmpthese( -60, {
      'qsort' => sub {
	  my @qrl = @rl;
	  qsort { ext_cmp($a, $b) } @qrl;
	  $idx = bsearchidx { ext_cmp($_, $rl[0]) } @qrl
      },
      'reverse qsort' => sub {
	  my @qrl = @rl;
	  qsort { ext_cmp($b, $a) } @qrl;
	  $idx = bsearchidx { ext_cmp($rl[0], $_) } @qrl
      },
      'sort' => sub {
	  my @srl = @rl;
	  @srl = sort { ext_cmp($a, $b) } @srl;
	  $idx = bsearchidx { ext_cmp($_, $rl[0]) } @srl
      },
      'reverse sort' => sub {
	  my @srl = @rl;
	  @srl = sort { ext_cmp($b, $a) } @srl;
	  $idx = bsearchidx { ext_cmp($rl[0], $_) } @srl
      },
  });

5.8 results

		  s/iter  reverse sort          sort reverse qsort         qsort
  reverse sort    6.21            --           -0%           -8%          -10%
  sort            6.19            0%            --           -7%          -10%
  reverse qsort   5.73            8%            8%            --           -2%
  qsort           5.60           11%           11%            2%            --

5.26 results

		s/iter  reverse sort          sort reverse qsort         qsort
  reverse sort    4.54            --           -0%          -96%          -96%
  sort            4.52            0%            --          -96%          -96%
  reverse qsort  0.203         2139%         2131%            --          -19%
  qsort          0.164         2666%         2656%           24%            --

Use it where external data sources might have to be compared (think of L<Unix::Statgrab>
"tables").

C<qsort> is available from List::MoreUtils::XS only. It's insane to maintain
a wrapper around Perl's sort nor having a pure Perl implementation. One could
create a flip-book in same speed as PP runs a qsort.

=head2 Searching in sorted Lists

=head3 bsearch BLOCK LIST

Performs a binary search on LIST which must be a sorted list of values. BLOCK
must return a negative value if the current element (stored in C<$_>) is smaller,
a positive value if it is bigger and zero if it matches.

Returns a boolean value in scalar context. In list context, it returns the element
if it was found, otherwise the empty list.

=head3 bsearchidx BLOCK LIST

=head3 bsearch_index BLOCK LIST

Performs a binary search on LIST which must be a sorted list of values. BLOCK
must return a negative value if the current element (stored in C<$_>) is smaller,
a positive value if it is bigger and zero if it matches.

Returns the index of found element, otherwise C<-1>.

C<bsearch_index> is an alias for C<bsearchidx>.

=head3 lower_bound BLOCK LIST

Returns the index of the first element in LIST which does not compare
I<less than val>. Technically it's the first element in LIST which does
not return a value below zero when passed to BLOCK.

  @ids = (1, 1, 2, 2, 3, 3, 3, 3, 3, 3, 4, 4, 4, 4, 5, 5, 6, 7, 7, 7, 8, 8, 9, 9, 9, 9, 9, 11, 13, 13, 13, 17);
  $lb = lower_bound { $_ <=> 2 } @ids; # returns 2
  $lb = lower_bound { $_ <=> 4 } @ids; # returns 10

lower_bound has a complexity of O(log n).

=head3 upper_bound BLOCK LIST

Returns the index of the first element in LIST which does not compare
I<greater than val>. Technically it's the first element in LIST which does
not return a value below or equal to zero when passed to BLOCK.

  @ids = (1, 1, 2, 2, 3, 3, 3, 3, 3, 3, 4, 4, 4, 4, 5, 5, 6, 7, 7, 7, 8, 8, 9, 9, 9, 9, 9, 11, 13, 13, 13, 17);
  $lb = upper_bound { $_ <=> 2 } @ids; # returns 4
  $lb = upper_bound { $_ <=> 4 } @ids; # returns 14

upper_bound has a complexity of O(log n).

=head3 equal_range BLOCK LIST

Returns a pair of indices containing the lower_bound and the upper_bound.

=head2 Operations on sorted Lists

=head3 binsert BLOCK ITEM LIST

=head3 bsearch_insert BLOCK ITEM LIST

Performs a binary search on LIST which must be a sorted list of values. BLOCK
must return a negative value if the current element (stored in C<$_>) is smaller,
a positive value if it is bigger and zero if it matches.

ITEM is inserted at the index where the ITEM should be placed (based on above
search). That means, it's inserted before the next bigger element.

  @l = (2,3,5,7);
  binsert { $_ <=> 4 }  4, @l; # @l = (2,3,4,5,7)
  binsert { $_ <=> 6 } 42, @l; # @l = (2,3,4,42,7)

You take care that the inserted element matches the compare result.

=head3 bremove BLOCK LIST

=head3 bsearch_remove BLOCK LIST

Performs a binary search on LIST which must be a sorted list of values. BLOCK
must return a negative value if the current element (stored in C<$_>) is smaller,
a positive value if it is bigger and zero if it matches.

The item at the found position is removed and returned.

  @l = (2,3,4,5,7);
  bremove { $_ <=> 4 }, @l; # @l = (2,3,5,7);

=head2 Counting and calculation

=head3 true BLOCK LIST

Counts the number of elements in LIST for which the criterion in BLOCK is true.
Sets C<$_> for  each item in LIST in turn:

  printf "%i item(s) are defined", true { defined($_) } @list;

=head3 false BLOCK LIST

Counts the number of elements in LIST for which the criterion in BLOCK is false.
Sets C<$_> for each item in LIST in turn:

  printf "%i item(s) are not defined", false { defined($_) } @list;

=head3 reduce_0 BLOCK LIST

Reduce LIST by calling BLOCK in scalar context for each element of LIST.
C<$a> contains the progressional result and is initialized with 0.
C<$b> contains the current processed element of LIST and C<$_> contains the
index of the element in C<$b>.

The idea behind reduce_0 is B<summation> (addition of a sequence of numbers).

=head3 reduce_1 BLOCK LIST

Reduce LIST by calling BLOCK in scalar context for each element of LIST.
C<$a> contains the progressional result and is initialized with 1.
C<$b> contains the current processed element of LIST and C<$_> contains the
index of the element in C<$b>.

The idea behind reduce_1 is product of a sequence of numbers.

=head3 reduce_u BLOCK LIST

Reduce LIST by calling BLOCK in scalar context for each element of LIST.
C<$a> contains the progressional result and is uninitialized.
C<$b> contains the current processed element of LIST and C<$_> contains the
index of the element in C<$b>.

This function has been added if one might need the extra of the index
value but need an individual initialization.

B<Use with caution>: In most cases L<List::Util/reduce> will do the
job better.

=head3 minmax LIST

Calculates the minimum and maximum of LIST and returns a two element list with
the first element being the minimum and the second the maximum. Returns the
empty list if LIST was empty.

The C<minmax> algorithm differs from a naive iteration over the list where each
element is compared to two values being the so far calculated min and max value
in that it only requires 3n/2 - 2 comparisons. Thus it is the most efficient
possible algorithm.

However, the Perl implementation of it has some overhead simply due to the fact
that there are more lines of Perl code involved. Therefore, LIST needs to be
fairly big in order for C<minmax> to win over a naive implementation. This
limitation does not apply to the XS version.

=head3 minmaxstr LIST

Computes the minimum and maximum of LIST using string compare and returns a
two element list with the first element being the minimum and the second the
maximum. Returns the empty list if LIST was empty.

The implementation is similar to C<minmax>.

=head1 ENVIRONMENT

When C<LIST_MOREUTILS_PP> is set, the module will always use the pure-Perl
implementation and not the XS one. This environment variable is really just
there for the test-suite to force testing the Perl implementation, and possibly
for reporting of bugs. I don't see any reason to use it in a production
environment.

=head1 MAINTENANCE

The maintenance goal is to preserve the documented semantics of the API;
bug fixes that bring actual behavior in line with semantics are allowed.
New API functions may be added over time.  If a backwards incompatible
change is unavoidable, we will attempt to provide support for the legacy
API using the same export tag mechanism currently in place.

This module attempts to use few non-core dependencies. Non-core
configuration and testing modules will be bundled when reasonable;
run-time dependencies will be added only if they deliver substantial
benefit.

=head1 CONTRIBUTING

While contributions are appreciated, a contribution should not cause more
effort for the maintainer than the contribution itself saves (see
L<Open Source Contribution Etiquette|http://tirania.org/blog/archive/2010/Dec-31.html>).

To get more familiar where help could be needed - see L<List::MoreUtils::Contributing>.

=head1 BUGS

There is a problem with a bug in 5.6.x perls. It is a syntax error to write
things like:

    my @x = apply { s/foo/bar/ } qw{ foo bar baz };

It has to be written as either

    my @x = apply { s/foo/bar/ } 'foo', 'bar', 'baz';

or

    my @x = apply { s/foo/bar/ } my @dummy = qw/foo bar baz/;

Perl 5.5.x and Perl 5.8.x don't suffer from this limitation.

If you have a functionality that you could imagine being in this module, please
drop me a line. This module's policy will be less strict than L<List::Util>'s
when it comes to additions as it isn't a core module.

When you report bugs, it would be nice if you could additionally give me the
output of your program with the environment variable C<LIST_MOREUTILS_PP> set
to a true value. That way I know where to look for the problem (in XS,
pure-Perl or possibly both).

=head1 SUPPORT

Bugs should always be submitted via the CPAN bug tracker.

You can find documentation for this module with the perldoc command.

    perldoc List::MoreUtils

You can also look for information at:

=over 4

=item * RT: CPAN's request tracker

L<https://rt.cpan.org/Dist/Display.html?Name=List-MoreUtils>

=item * AnnoCPAN: Annotated CPAN documentation

L<http://annocpan.org/dist/List-MoreUtils>

=item * CPAN Ratings

L<http://cpanratings.perl.org/dist/List-MoreUtils>

=item * MetaCPAN

L<https://metacpan.org/release/List-MoreUtils>

=item * CPAN Search

L<http://search.cpan.org/dist/List-MoreUtils/>

=item * Git Repository

L<https://github.com/perl5-utils/List-MoreUtils>

=back

=head2 Where can I go for help?

If you have a bug report, a patch or a suggestion, please open a new
report ticket at CPAN (but please check previous reports first in case
your issue has already been addressed) or open an issue on GitHub.

Report tickets should contain a detailed description of the bug or
enhancement request and at least an easily verifiable way of
reproducing the issue or fix. Patches are always welcome, too - and
it's cheap to send pull-requests on GitHub. Please keep in mind that
code changes are more likely accepted when they're bundled with an
approving test.

If you think you've found a bug then please read
"How to Report Bugs Effectively" by Simon Tatham:
L<http://www.chiark.greenend.org.uk/~sgtatham/bugs.html>.

=head2 Where can I go for help with a concrete version?

Bugs and feature requests are accepted against the latest version
only. To get patches for earlier versions, you need to get an
agreement with a developer of your choice - who may or not report the
issue and a suggested fix upstream (depends on the license you have
chosen).

=head2 Business support and maintenance

Generally, in volunteered projects, there is no right for support.
While every maintainer is happy to improve the provided software,
spare time is limited.

For those who have a use case which requires guaranteed support, one of
the maintainers should be hired or contracted.  For business support you
can contact Jens via his CPAN email address rehsackATcpan.org. Please
keep in mind that business support is neither available for free nor
are you eligible to receive any support based on the license distributed
with this package.

=head1 THANKS

=head2 Tassilo von Parseval

Credits go to a number of people: Steve Purkis for giving me namespace advice
and James Keenan and Terrence Branno for their effort of keeping the CPAN
tidier by making L<List::Utils> obsolete.

Brian McCauley suggested the inclusion of apply() and provided the pure-Perl
implementation for it.

Eric J. Roode asked me to add all functions from his module C<List::MoreUtil>
into this one. With minor modifications, the pure-Perl implementations of those
are by him.

The bunch of people who almost immediately pointed out the many problems with
the glitchy 0.07 release (Slaven Rezic, Ron Savage, CPAN testers).

A particularly nasty memory leak was spotted by Thomas A. Lowery.

Lars Thegler made me aware of problems with older Perl versions.

Anno Siegel de-orphaned each_arrayref().

David Filmer made me aware of a problem in each_arrayref that could ultimately
lead to a segfault.

Ricardo Signes suggested the inclusion of part() and provided the
Perl-implementation.

Robin Huston kindly fixed a bug in perl's MULTICALL API to make the
XS-implementation of part() work.

=head2 Jens Rehsack

Credits goes to all people contributing feedback during the v0.400
development releases.

Special thanks goes to David Golden who spent a lot of effort to develop
a design to support current state of CPAN as well as ancient software
somewhere in the dark. He also contributed a lot of patches to refactor
the API frontend to welcome any user of List::MoreUtils - from ancient
past to recently last used.

Toby Inkster provided a lot of useful feedback for sane importer code
and was a nice sounding board for API discussions.

Peter Rabbitson provided a sane git repository setup containing entire
package history.

=head1 TODO

A pile of requests from other people is still pending further processing in
my mailbox. This includes:

=over 4

=item * delete_index

=item * random_item

=item * random_item_delete_index

=item * list_diff_hash

=item * list_diff_inboth

=item * list_diff_infirst

=item * list_diff_insecond

These were all suggested by Dan Muey.

=item * listify

Always return a flat list when either a simple scalar value was passed or an
array-reference. Suggested by Mark Summersault.

=back

=head1 SEE ALSO

L<List::Util>, L<List::AllUtils>, L<List::UtilsBy>

=head1 AUTHOR

Jens Rehsack E<lt>rehsack AT cpan.orgE<gt>

Adam Kennedy E<lt>adamk@cpan.orgE<gt>

Tassilo von Parseval E<lt>tassilo.von.parseval@rwth-aachen.deE<gt>

=head1 COPYRIGHT AND LICENSE

Some parts copyright 2011 Aaron Crane.

Copyright 2004 - 2010 by Tassilo von Parseval

Copyright 2013 - 2017 by Jens Rehsack

All code added with 0.417 or later is licensed under the Apache License,
Version 2.0 (the "License"); you may not use this file except in compliance
with the License. You may obtain a copy of the License at

 http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.

All code until 0.416 is licensed under the same terms as Perl itself,
either Perl version 5.8.4 or, at your option, any later version of
Perl 5 you may have available.

=cut

1;