Koha/C4/HoldsQueue.pm

package C4::HoldsQueue;

# Copyright 2011 Catalyst IT
#
# This file is part of Koha.
#
# Koha is free software; you can redistribute it and/or modify it
# under the terms of the GNU General Public License as published by
# the Free Software Foundation; either version 3 of the License, or
# (at your option) any later version.
#
# Koha is distributed in the hope that it will be useful, but
# WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
# GNU General Public License for more details.
#
# You should have received a copy of the GNU General Public License
# along with Koha; if not, see <http://www.gnu.org/licenses>.

# FIXME: expand perldoc, explain intended logic

use strict;
use warnings;

use C4::Context;
use C4::Search;
use C4::Items;
use C4::Circulation;
use C4::Members;
use C4::Biblio;
use Koha::DateUtils;

use List::Util qw(shuffle);
use List::MoreUtils qw(any);
use Data::Dumper;

use vars qw(@ISA @EXPORT @EXPORT_OK %EXPORT_TAGS);
BEGIN {
    require Exporter;
    @ISA = qw(Exporter);
    @EXPORT_OK = qw(
        &CreateQueue
        &GetHoldsQueueItems

        &TransportCostMatrix
        &UpdateTransportCostMatrix
     );
}


=head1 FUNCTIONS

=head2 TransportCostMatrix

  TransportCostMatrix();

Returns Transport Cost Matrix as a hashref <to branch code> => <from branch code> => cost

=cut

sub TransportCostMatrix {
    my $dbh   = C4::Context->dbh;
    my $transport_costs = $dbh->selectall_arrayref("SELECT * FROM transport_cost",{ Slice => {} });

    my $today = dt_from_string();
    my $calendars;
    my %transport_cost_matrix;
    foreach (@$transport_costs) {
        my $from     = $_->{frombranch};
        my $to       = $_->{tobranch};
        my $cost     = $_->{cost};
        my $disabled = $_->{disable_transfer};
        $transport_cost_matrix{$to}{$from} = {
            cost             => $cost,
            disable_transfer => $disabled
        };

        if ( C4::Context->preference("HoldsQueueSkipClosed") ) {
            $calendars->{$from} ||= Koha::Calendar->new( branchcode => $from );
            $transport_cost_matrix{$to}{$from}{disable_transfer} ||=
              $calendars->{$from}->is_holiday( $today );
        }

    }
    return \%transport_cost_matrix;
}

=head2 UpdateTransportCostMatrix

  UpdateTransportCostMatrix($records);

Updates full Transport Cost Matrix table. $records is an arrayref of records.
Records: { frombranch => <code>, tobranch => <code>, cost => <figure>, disable_transfer => <0,1> }

=cut

sub UpdateTransportCostMatrix {
    my ($records) = @_;
    my $dbh   = C4::Context->dbh;

    my $sth = $dbh->prepare("INSERT INTO transport_cost (frombranch, tobranch, cost, disable_transfer) VALUES (?, ?, ?, ?)");

    $dbh->do("TRUNCATE TABLE transport_cost");
    foreach (@$records) {
        my $cost = $_->{cost};
        my $from = $_->{frombranch};
        my $to = $_->{tobranch};
        if ($_->{disable_transfer}) {
            $cost ||= 0;
        }
        elsif ( !defined ($cost) || ($cost !~ m/(0|[1-9][0-9]*)(\.[0-9]*)?/o) ) {
            warn  "Invalid $from -> $to cost $cost - must be a number >= 0, disablig";
            $cost = 0;
            $_->{disable_transfer} = 1;
        }
        $sth->execute( $from, $to, $cost, $_->{disable_transfer} ? 1 : 0 );
    }
}

=head2 GetHoldsQueueItems

  GetHoldsQueueItems($branch);

Returns hold queue for a holding branch. If branch is omitted, then whole queue is returned

=cut

sub GetHoldsQueueItems {
    my ($branchlimit) = @_;
    my $dbh   = C4::Context->dbh;

    my @bind_params = ();
    my $query = q/SELECT tmp_holdsqueue.*, biblio.author, items.ccode, items.itype, biblioitems.itemtype, items.location, items.enumchron, items.cn_sort, biblioitems.publishercode,biblio.copyrightdate,biblioitems.publicationyear,biblioitems.pages,biblioitems.size,biblioitems.publicationyear,biblioitems.isbn,items.copynumber
                  FROM tmp_holdsqueue
                       JOIN biblio      USING (biblionumber)
                  LEFT JOIN biblioitems USING (biblionumber)
                  LEFT JOIN items       USING (  itemnumber)
                /;
    if ($branchlimit) {
        $query .=" WHERE tmp_holdsqueue.holdingbranch = ?";
        push @bind_params, $branchlimit;
    }
    $query .= " ORDER BY ccode, location, cn_sort, author, title, pickbranch, reservedate";
    my $sth = $dbh->prepare($query);
    $sth->execute(@bind_params);
    my $items = [];
    while ( my $row = $sth->fetchrow_hashref ){
        my $record = GetMarcBiblio($row->{biblionumber});
        if ($record){
            $row->{subtitle} = [ map { $_->{subfield} } @{ GetRecordValue( 'subtitle', $record, '' ) } ];
            $row->{parts} = GetRecordValue('parts',$record,'')->[0]->{subfield};
            $row->{numbers} = GetRecordValue('numbers',$record,'')->[0]->{subfield};
        }

        # return the bib-level or item-level itype per syspref
        if (!C4::Context->preference('item-level_itypes')) {
            $row->{itype} = $row->{itemtype};
        }
        delete $row->{itemtype};

        push @$items, $row;
    }
    return $items;
}

=head2 CreateQueue

  CreateQueue();

Top level function that turns reserves into tmp_holdsqueue and hold_fill_targets.

=cut

sub CreateQueue {
    my $dbh   = C4::Context->dbh;

    $dbh->do("DELETE FROM tmp_holdsqueue");  # clear the old table for new info
    $dbh->do("DELETE FROM hold_fill_targets");

    my $total_bibs            = 0;
    my $total_requests        = 0;
    my $total_available_items = 0;
    my $num_items_mapped      = 0;

    my $branches_to_use;
    my $transport_cost_matrix;
    my $use_transport_cost_matrix = C4::Context->preference("UseTransportCostMatrix");
    if ($use_transport_cost_matrix) {
        $transport_cost_matrix = TransportCostMatrix();
        unless (keys %$transport_cost_matrix) {
            warn "UseTransportCostMatrix set to yes, but matrix not populated";
            undef $transport_cost_matrix;
        }
    }
    unless ($transport_cost_matrix) {
        $branches_to_use = load_branches_to_pull_from();
    }

    my $bibs_with_pending_requests = GetBibsWithPendingHoldRequests();

    foreach my $biblionumber (@$bibs_with_pending_requests) {
        $total_bibs++;
        my $hold_requests   = GetPendingHoldRequestsForBib($biblionumber);
        my $available_items = GetItemsAvailableToFillHoldRequestsForBib($biblionumber, $branches_to_use);
        $total_requests        += scalar(@$hold_requests);
        $total_available_items += scalar(@$available_items);

        my $item_map = MapItemsToHoldRequests($hold_requests, $available_items, $branches_to_use, $transport_cost_matrix);
        $item_map  or next;
        my $item_map_size = scalar(keys %$item_map)
          or next;

        $num_items_mapped += $item_map_size;
        CreatePicklistFromItemMap($item_map);
        AddToHoldTargetMap($item_map);
        if (($item_map_size < scalar(@$hold_requests  )) and
            ($item_map_size < scalar(@$available_items))) {
            # DOUBLE CHECK, but this is probably OK - unfilled item-level requests
            # FIXME
            #warn "unfilled requests for $biblionumber";
            #warn Dumper($hold_requests), Dumper($available_items), Dumper($item_map);
        }
    }
}

=head2 GetBibsWithPendingHoldRequests

  my $biblionumber_aref = GetBibsWithPendingHoldRequests();

Return an arrayref of the biblionumbers of all bibs
that have one or more unfilled hold requests.

=cut

sub GetBibsWithPendingHoldRequests {
    my $dbh = C4::Context->dbh;

    my $bib_query = "SELECT DISTINCT biblionumber
                     FROM reserves
                     WHERE found IS NULL
                     AND priority > 0
                     AND reservedate <= CURRENT_DATE()
                     AND suspend = 0
                     ";
    my $sth = $dbh->prepare($bib_query);

    $sth->execute();
    my $biblionumbers = $sth->fetchall_arrayref();

    return [ map { $_->[0] } @$biblionumbers ];
}

=head2 GetPendingHoldRequestsForBib

  my $requests = GetPendingHoldRequestsForBib($biblionumber);

Returns an arrayref of hashrefs to pending, unfilled hold requests
on the bib identified by $biblionumber.  The following keys
are present in each hashref:

    biblionumber
    borrowernumber
    itemnumber
    priority
    branchcode
    reservedate
    reservenotes
    borrowerbranch

The arrayref is sorted in order of increasing priority.

=cut

sub GetPendingHoldRequestsForBib {
    my $biblionumber = shift;

    my $dbh = C4::Context->dbh;

    my $request_query = "SELECT biblionumber, borrowernumber, itemnumber, priority, reserves.branchcode,
                                reservedate, reservenotes, borrowers.branchcode AS borrowerbranch, itemtype
                         FROM reserves
                         JOIN borrowers USING (borrowernumber)
                         WHERE biblionumber = ?
                         AND found IS NULL
                         AND priority > 0
                         AND reservedate <= CURRENT_DATE()
                         AND suspend = 0
                         ORDER BY priority";
    my $sth = $dbh->prepare($request_query);
    $sth->execute($biblionumber);

    my $requests = $sth->fetchall_arrayref({});
    return $requests;

}

=head2 GetItemsAvailableToFillHoldRequestsForBib

  my $available_items = GetItemsAvailableToFillHoldRequestsForBib($biblionumber, $branches_ar);

Returns an arrayref of items available to fill hold requests
for the bib identified by C<$biblionumber>.  An item is available
to fill a hold request if and only if:

    * it is not on loan
    * it is not withdrawn
    * it is not marked notforloan
    * it is not currently in transit
    * it is not lost
    * it is not sitting on the hold shelf
    * it is not damaged (unless AllowHoldsOnDamagedItems is on)

=cut

sub GetItemsAvailableToFillHoldRequestsForBib {
    my ($biblionumber, $branches_to_use) = @_;

    my $dbh = C4::Context->dbh;
    my $items_query = "SELECT itemnumber, homebranch, holdingbranch, itemtypes.itemtype AS itype
                       FROM items ";

    if (C4::Context->preference('item-level_itypes')) {
        $items_query .=   "LEFT JOIN itemtypes ON (itemtypes.itemtype = items.itype) ";
    } else {
        $items_query .=   "JOIN biblioitems USING (biblioitemnumber)
                           LEFT JOIN itemtypes USING (itemtype) ";
    }
    $items_query .=   "WHERE items.notforloan = 0
                       AND holdingbranch IS NOT NULL
                       AND itemlost = 0
                       AND withdrawn = 0";
    $items_query .= "  AND damaged = 0" unless C4::Context->preference('AllowHoldsOnDamagedItems');
    $items_query .= "  AND items.onloan IS NULL
                       AND (itemtypes.notforloan IS NULL OR itemtypes.notforloan = 0)
                       AND itemnumber NOT IN (
                           SELECT itemnumber
                           FROM reserves
                           WHERE biblionumber = ?
                           AND itemnumber IS NOT NULL
                           AND (found IS NOT NULL OR priority = 0)
                        )
                       AND items.biblionumber = ?";

    my @params = ($biblionumber, $biblionumber);
    if ($branches_to_use && @$branches_to_use) {
        $items_query .= " AND holdingbranch IN (" . join (",", map { "?" } @$branches_to_use) . ")";
        push @params, @$branches_to_use;
    }
    my $sth = $dbh->prepare($items_query);
    $sth->execute(@params);

    my $itm = $sth->fetchall_arrayref({});
    my @items = grep { ! scalar GetTransfers($_->{itemnumber}) } @$itm;
    return [ grep {
        my $rule = GetBranchItemRule($_->{homebranch}, $_->{itype});
        $_->{holdallowed} = $rule->{holdallowed};
        $_->{hold_fulfillment_policy} = $rule->{hold_fulfillment_policy};
    } @items ];
}

=head2 MapItemsToHoldRequests

  MapItemsToHoldRequests($hold_requests, $available_items, $branches, $transport_cost_matrix)

=cut

sub MapItemsToHoldRequests {
    my ($hold_requests, $available_items, $branches_to_use, $transport_cost_matrix) = @_;


    # handle trival cases
    return unless scalar(@$hold_requests) > 0;
    return unless scalar(@$available_items) > 0;

    # identify item-level requests
    my %specific_items_requested = map { $_->{itemnumber} => 1 }
                                   grep { defined($_->{itemnumber}) }
                                   @$hold_requests;

    # group available items by itemnumber
    my %items_by_itemnumber = map { $_->{itemnumber} => $_ } @$available_items;

    # items already allocated
    my %allocated_items = ();

    # map of items to hold requests
    my %item_map = ();

    # figure out which item-level requests can be filled
    my $num_items_remaining = scalar(@$available_items);
    foreach my $request (@$hold_requests) {
        last if $num_items_remaining == 0;

        # is this an item-level request?
        if (defined($request->{itemnumber})) {
            # fill it if possible; if not skip it
            if (
                    exists $items_by_itemnumber{ $request->{itemnumber} }
                and not exists $allocated_items{ $request->{itemnumber} }
                and ( # Don't fill item level holds that contravene the hold pickup policy at this time
                    ( $items_by_itemnumber{ $request->{itemnumber} }->{hold_fulfillment_policy} eq 'any' )
                    || ( $request->{branchcode} eq $items_by_itemnumber{ $request->{itemnumber} }->{ $items_by_itemnumber{ $request->{itemnumber} }->{hold_fulfillment_policy} }  )
                and ( !$request->{itemtype} # If hold itemtype is set, item's itemtype must match
                    || $items_by_itemnumber{ $request->{itemnumber} }->{itype} eq $request->{itemtype} )
                )

              )
            {

                $item_map{ $request->{itemnumber} } = {
                    borrowernumber => $request->{borrowernumber},
                    biblionumber   => $request->{biblionumber},
                    holdingbranch  => $items_by_itemnumber{ $request->{itemnumber} }->{holdingbranch},
                    pickup_branch  => $request->{branchcode} || $request->{borrowerbranch},
                    item_level     => 1,
                    reservedate    => $request->{reservedate},
                    reservenotes   => $request->{reservenotes},
                };
                $allocated_items{ $request->{itemnumber} }++;
                $num_items_remaining--;
            }
        } else {
            # it's title-level request that will take up one item
            $num_items_remaining--;
        }
    }

    # group available items by branch
    my %items_by_branch = ();
    foreach my $item (@$available_items) {
        next unless $item->{holdallowed};

        push @{ $items_by_branch{ $item->{holdingbranch} } }, $item
          unless exists $allocated_items{ $item->{itemnumber} };
    }
    return \%item_map unless keys %items_by_branch;

    # now handle the title-level requests
    $num_items_remaining = scalar(@$available_items) - scalar(keys %allocated_items);
    my $pull_branches;
    foreach my $request (@$hold_requests) {
        last if $num_items_remaining == 0;
        next if defined($request->{itemnumber}); # already handled these

        # look for local match first
        my $pickup_branch = $request->{branchcode} || $request->{borrowerbranch};
        my ($itemnumber, $holdingbranch);

        my $holding_branch_items = $items_by_branch{$pickup_branch};
        if ( $holding_branch_items ) {
            foreach my $item (@$holding_branch_items) {
                if (
                    $request->{borrowerbranch} eq $item->{homebranch}
                    && ( ( $item->{hold_fulfillment_policy} eq 'any' ) # Don't fill item level holds that contravene the hold pickup policy at this time
                        || $request->{branchcode} eq $item->{ $item->{hold_fulfillment_policy} } )
                    && ( !$request->{itemtype} # If hold itemtype is set, item's itemtype must match
                        || $items_by_itemnumber{ $request->{itemnumber} }->{itype} eq $request->{itemtype} )
                  )
                {
                    $itemnumber = $item->{itemnumber};
                    last;
                }
            }
            $holdingbranch = $pickup_branch;
        }
        elsif ($transport_cost_matrix) {
            $pull_branches = [keys %items_by_branch];
            $holdingbranch = least_cost_branch( $pickup_branch, $pull_branches, $transport_cost_matrix );
            if ( $holdingbranch ) {

                my $holding_branch_items = $items_by_branch{$holdingbranch};
                foreach my $item (@$holding_branch_items) {
                    next if $request->{borrowerbranch} ne $item->{homebranch};

                    # Don't fill item level holds that contravene the hold pickup policy at this time
                    next unless $item->{hold_fulfillment_policy} eq 'any'
                        || $request->{branchcode} eq $item->{ $item->{hold_fulfillment_policy} };

                    # If hold itemtype is set, item's itemtype must match
                    next unless ( !$request->{itemtype}
                        || $item->{itype} eq $request->{itemtype} );

                    $itemnumber = $item->{itemnumber};
                    last;
                }
            }
            else {
                next;
            }
        }

        unless ($itemnumber) {
            # not found yet, fall back to basics
            if ($branches_to_use) {
                $pull_branches = $branches_to_use;
            } else {
                $pull_branches = [keys %items_by_branch];
            }

            # Try picking items where the home and pickup branch match first
            PULL_BRANCHES:
            foreach my $branch (@$pull_branches) {
                my $holding_branch_items = $items_by_branch{$branch}
                  or next;

                $holdingbranch ||= $branch;
                foreach my $item (@$holding_branch_items) {
                    next if $pickup_branch ne $item->{homebranch};
                    next if ( $item->{holdallowed} == 1 && $item->{homebranch} ne $request->{borrowerbranch} );

                    # Don't fill item level holds that contravene the hold pickup policy at this time
                    next unless $item->{hold_fulfillment_policy} eq 'any'
                        || $request->{branchcode} eq $item->{ $item->{hold_fulfillment_policy} };

                    # If hold itemtype is set, item's itemtype must match
                    next unless ( !$request->{itemtype}
                        || $item->{itype} eq $request->{itemtype} );

                    $itemnumber = $item->{itemnumber};
                    $holdingbranch = $branch;
                    last PULL_BRANCHES;
                }
            }

            # Now try items from the least cost branch based on the transport cost matrix or StaticHoldsQueueWeight
            unless ( $itemnumber ) {
                foreach my $current_item ( @{ $items_by_branch{$holdingbranch} } ) {
                    if ( $holdingbranch && ( $current_item->{holdallowed} == 2 || $request->{borrowerbranch} eq $current_item->{homebranch} ) ) {

                        # Don't fill item level holds that contravene the hold pickup policy at this time
                        next unless $current_item->{hold_fulfillment_policy} eq 'any'
                            || $request->{branchcode} eq $current_item->{ $current_item->{hold_fulfillment_policy} };

                        # If hold itemtype is set, item's itemtype must match
                        next unless ( !$request->{itemtype}
                            || $current_item->{itype} eq $request->{itemtype} );

                        $itemnumber = $current_item->{itemnumber};
                        last; # quit this loop as soon as we have a suitable item
                    }
                }
            }

            # Now try for items for any item that can fill this hold
            unless ( $itemnumber ) {
                PULL_BRANCHES2:
                foreach my $branch (@$pull_branches) {
                    my $holding_branch_items = $items_by_branch{$branch}
                      or next;

                    foreach my $item (@$holding_branch_items) {
                        next if ( $item->{holdallowed} == 1 && $item->{homebranch} ne $request->{borrowerbranch} );

                        # Don't fill item level holds that contravene the hold pickup policy at this time
                        next unless $item->{hold_fulfillment_policy} eq 'any'
                            || $request->{branchcode} eq $item->{ $item->{hold_fulfillment_policy} };

                        # If hold itemtype is set, item's itemtype must match
                        next unless ( !$request->{itemtype}
                            || $item->{itype} eq $request->{itemtype} );

                        $itemnumber = $item->{itemnumber};
                        $holdingbranch = $branch;
                        last PULL_BRANCHES2;
                    }
                }
            }
        }

        if ($itemnumber) {
            my $holding_branch_items = $items_by_branch{$holdingbranch}
              or die "Have $itemnumber, $holdingbranch, but no items!";
            @$holding_branch_items = grep { $_->{itemnumber} != $itemnumber } @$holding_branch_items;
            delete $items_by_branch{$holdingbranch} unless @$holding_branch_items;

            $item_map{$itemnumber} = {
                borrowernumber => $request->{borrowernumber},
                biblionumber => $request->{biblionumber},
                holdingbranch => $holdingbranch,
                pickup_branch => $pickup_branch,
                item_level => 0,
                reservedate => $request->{reservedate},
                reservenotes => $request->{reservenotes},
            };
            $num_items_remaining--;
        }
    }
    return \%item_map;
}

=head2 CreatePickListFromItemMap

=cut

sub CreatePicklistFromItemMap {
    my $item_map = shift;

    my $dbh = C4::Context->dbh;

    my $sth_load=$dbh->prepare("
        INSERT INTO tmp_holdsqueue (biblionumber,itemnumber,barcode,surname,firstname,phone,borrowernumber,
                                    cardnumber,reservedate,title, itemcallnumber,
                                    holdingbranch,pickbranch,notes, item_level_request)
        VALUES (?,?,?,?,?,?,?,?,?,?,?,?,?,?,?)
    ");

    foreach my $itemnumber  (sort keys %$item_map) {
        my $mapped_item = $item_map->{$itemnumber};
        my $biblionumber = $mapped_item->{biblionumber};
        my $borrowernumber = $mapped_item->{borrowernumber};
        my $pickbranch = $mapped_item->{pickup_branch};
        my $holdingbranch = $mapped_item->{holdingbranch};
        my $reservedate = $mapped_item->{reservedate};
        my $reservenotes = $mapped_item->{reservenotes};
        my $item_level = $mapped_item->{item_level};

        my $item = GetItem($itemnumber);
        my $barcode = $item->{barcode};
        my $itemcallnumber = $item->{itemcallnumber};

        my $borrower = GetMember('borrowernumber'=>$borrowernumber);
        my $cardnumber = $borrower->{'cardnumber'};
        my $surname = $borrower->{'surname'};
        my $firstname = $borrower->{'firstname'};
        my $phone = $borrower->{'phone'};

        my $bib = GetBiblioData($biblionumber);
        my $title = $bib->{title};

        $sth_load->execute($biblionumber, $itemnumber, $barcode, $surname, $firstname, $phone, $borrowernumber,
                           $cardnumber, $reservedate, $title, $itemcallnumber,
                           $holdingbranch, $pickbranch, $reservenotes, $item_level);
    }
}

=head2 AddToHoldTargetMap

=cut

sub AddToHoldTargetMap {
    my $item_map = shift;

    my $dbh = C4::Context->dbh;

    my $insert_sql = q(
        INSERT INTO hold_fill_targets (borrowernumber, biblionumber, itemnumber, source_branchcode, item_level_request)
                               VALUES (?, ?, ?, ?, ?)
    );
    my $sth_insert = $dbh->prepare($insert_sql);

    foreach my $itemnumber (keys %$item_map) {
        my $mapped_item = $item_map->{$itemnumber};
        $sth_insert->execute($mapped_item->{borrowernumber}, $mapped_item->{biblionumber}, $itemnumber,
                             $mapped_item->{holdingbranch}, $mapped_item->{item_level});
    }
}

# Helper functions, not part of any interface

sub _trim {
    return $_[0] unless $_[0];
    $_[0] =~ s/^\s+//;
    $_[0] =~ s/\s+$//;
    $_[0];
}

sub load_branches_to_pull_from {
    my @branches_to_use;

    my $static_branch_list = C4::Context->preference("StaticHoldsQueueWeight");
    @branches_to_use = map { _trim($_) } split( /,/, $static_branch_list )
      if $static_branch_list;

    @branches_to_use =
      Koha::Database->new()->schema()->resultset('Branch')
      ->get_column('branchcode')->all()
      unless (@branches_to_use);

    @branches_to_use = shuffle(@branches_to_use)
      if C4::Context->preference("RandomizeHoldsQueueWeight");

    my $today = dt_from_string();
    if ( C4::Context->preference('HoldsQueueSkipClosed') ) {
        @branches_to_use = grep {
            !Koha::Calendar->new( branchcode => $_ )
              ->is_holiday( $today )
        } @branches_to_use;
    }

    return \@branches_to_use;
}

sub least_cost_branch {

    #$from - arrayref
    my ($to, $from, $transport_cost_matrix) = @_;

    # Nothing really spectacular: supply to branch, a list of potential from branches
    # and find the minimum from - to value from the transport_cost_matrix
    return $from->[0] if ( @$from == 1 && $transport_cost_matrix->{$to}{$from->[0]}->{disable_transfer} != 1 );

    # If the pickup library is in the list of libraries to pull from,
    # return that library right away, it is obviously the least costly
    return ($to) if any { $_ eq $to } @$from;

    my ($least_cost, @branch);
    foreach (@$from) {
        my $cell = $transport_cost_matrix->{$to}{$_};
        next if $cell->{disable_transfer};

        my $cost = $cell->{cost};
        next unless defined $cost; # XXX should this be reported?

        unless (defined $least_cost) {
            $least_cost = $cost;
            push @branch, $_;
            next;
        }

        next if $cost > $least_cost;

        if ($cost == $least_cost) {
            push @branch, $_;
            next;
        }

        @branch = ($_);
        $least_cost = $cost;
    }

    return $branch[0];

    # XXX return a random @branch with minimum cost instead of the first one;
    # return $branch[0] if @branch == 1;
}


1;