package Slic3r::Print::Object;
use Moo;

use List::Util qw(min max sum first);
use Slic3r::Geometry qw(X Y Z PI scale unscale deg2rad rad2deg scaled_epsilon chained_path);
use Slic3r::Geometry::Clipper qw(diff diff_ex intersection intersection_ex union union_ex 
    offset offset_ex offset2 offset2_ex CLIPPER_OFFSET_SCALE JT_MITER);
use Slic3r::Print::State ':steps';
use Slic3r::Surface ':types';

has 'print'             => (is => 'ro', weak_ref => 1, required => 1);
has 'model_object'      => (is => 'ro', required => 1);
has 'region_volumes'    => (is => 'rw', default => sub { [] });  # by region_id
has 'copies'            => (is => 'ro');  # Slic3r::Point objects in scaled G-code coordinates
has 'config_overrides'  => (is => 'rw', default => sub { Slic3r::Config->new });
has 'config'            => (is => 'rw');
has 'layer_height_ranges' => (is => 'rw', default => sub { [] }); # [ z_min, z_max, layer_height ]

has 'size'              => (is => 'rw'); # XYZ in scaled coordinates
has '_copies_shift'     => (is => 'rw');  # scaled coordinates to add to copies (to compensate for the alignment operated when creating the object but still preserving a coherent API for external callers)
has '_shifted_copies'   => (is => 'rw');  # Slic3r::Point objects in scaled G-code coordinates in our coordinates
has 'layers'            => (is => 'rw', default => sub { [] });
has 'support_layers'    => (is => 'rw', default => sub { [] });
has 'fill_maker'        => (is => 'lazy');
has '_state'            => (is => 'ro', default => sub { Slic3r::Print::State->new });

sub BUILD {
    my $self = shift;
 	
 	$self->init_config;
 	
 	# translate meshes so that we work with smaller coordinates
 	{
 	    # compute the bounding box of the supplied meshes
 	    my @meshes = map $self->model_object->volumes->[$_]->mesh,
 	                    map @$_,
 	                    grep defined $_,
 	                    @{$self->region_volumes};
 	    my $bb = Slic3r::Geometry::BoundingBox->merge(map $_->bounding_box, @meshes);
 	    
 	    # Translate meshes so that our toolpath generation algorithms work with smaller
 	    # XY coordinates; this translation is an optimization and not strictly required.
 	    # However, this also aligns object to Z = 0, which on the contrary is required
 	    # since we don't assume input is already aligned.
 	    # We store the XY translation so that we can place copies correctly in the output G-code
 	    # (copies are expressed in G-code coordinates and this translation is not publicly exposed).
 	    $self->_copies_shift(Slic3r::Point->new_scale($bb->x_min, $bb->y_min));
        $self->_trigger_copies;
 	    
 	    # Scale the object size and store it
 	    my $scaled_bb = $bb->clone;
 	    $scaled_bb->scale(1 / &Slic3r::SCALING_FACTOR);
 	    $self->size($scaled_bb->size);
 	}
}

sub _build_fill_maker {
    my $self = shift;
    return Slic3r::Fill->new(bounding_box => $self->bounding_box);
}

sub _trigger_copies {
    my $self = shift;
    
    return if !defined $self->_copies_shift;
    
    # order copies with a nearest neighbor search and translate them by _copies_shift
    $self->_shifted_copies([
        map {
            my $c = $_->clone;
            $c->translate(@{ $self->_copies_shift });
            $c;
        } @{$self->copies}[@{chained_path($self->copies)}]
    ]);
    
    $self->print->_state->invalidate(STEP_SKIRT);
    $self->print->_state->invalidate(STEP_BRIM);
}

# in unscaled coordinates
sub add_copy {
    my ($self, $x, $y) = @_;
    push @{$self->copies}, Slic3r::Point->new_scale($x, $y);
    $self->_trigger_copies;
}

sub delete_last_copy {
    my ($self) = @_;
    pop @{$self->copies};
    $self->_trigger_copies;
}

sub delete_all_copies {
    my ($self) = @_;
    @{$self->copies} = ();
    $self->_trigger_copies;
}

sub init_config {
    my $self = shift;
    $self->config(Slic3r::Config->merge($self->print->config, $self->config_overrides));
}

sub layer_count {
    my $self = shift;
    return scalar @{ $self->layers };
}

sub bounding_box {
    my $self = shift;
    
    # since the object is aligned to origin, bounding box coincides with size
    return Slic3r::Geometry::BoundingBox->new_from_points([ map Slic3r::Point->new(@$_[X,Y]), [0,0], $self->size ]);
}

# this should be idempotent
sub slice {
    my $self = shift;
    my %params = @_;
    
    # init layers
    {
        @{$self->layers} = ();
    
        # make layers taking custom heights into account
        my $print_z = my $slice_z = my $height = my $id = 0;
    
        # add raft layers
        if ($self->config->raft_layers > 0) {
            $print_z += $self->config->get_value('first_layer_height');
            $print_z += $self->config->layer_height * ($self->config->raft_layers - 1);
            $id += $self->config->raft_layers;
        }
    
        # loop until we have at least one layer and the max slice_z reaches the object height
        my $max_z = unscale $self->size->[Z];
        while (!@{$self->layers} || ($slice_z - $height) <= $max_z) {
            # assign the default height to the layer according to the general settings
            $height = ($id == 0)
                ? $self->config->get_value('first_layer_height')
                : $self->config->layer_height;
        
            # look for an applicable custom range
            if (my $range = first { $_->[0] <= $slice_z && $_->[1] > $slice_z } @{$self->layer_height_ranges}) {
                $height = $range->[2];
        
                # if user set custom height to zero we should just skip the range and resume slicing over it
                if ($height == 0) {
                    $slice_z += $range->[1] - $range->[0];
                    next;
                }
            }
        
            $print_z += $height;
            $slice_z += $height/2;
        
            ### Slic3r::debugf "Layer %d: height = %s; slice_z = %s; print_z = %s\n", $id, $height, $slice_z, $print_z;
        
            push @{$self->layers}, Slic3r::Layer->new(
                object  => $self,
                id      => $id,
                height  => $height,
                print_z => $print_z,
                slice_z => $slice_z,
            );
            if (@{$self->layers} >= 2) {
                $self->layers->[-2]->upper_layer($self->layers->[-1]);
            }
            $id++;
        
            $slice_z += $height/2;   # add the other half layer
        }
    }
    
    # make sure all layers contain layer region objects for all regions
    my $regions_count = $self->print->regions_count;
    foreach my $layer (@{ $self->layers }) {
        $layer->region($_) for 0 .. ($regions_count-1);
    }
    
    # process facets
    for my $region_id (0..$#{$self->region_volumes}) {
        next if !defined $self->region_volumes->[$region_id];
        
        # compose mesh
        my $mesh;
        foreach my $volume_id (@{$self->region_volumes->[$region_id]}) {
            if (defined $mesh) {
                $mesh->merge($self->model_object->volumes->[$volume_id]->mesh);
            } else {
                $mesh = $self->model_object->volumes->[$volume_id]->mesh->clone;
            }
        }
        
        # transform mesh
        # we ignore the per-instance transformations currently and only 
        # consider the first one
        $self->model_object->instances->[0]->transform_mesh($mesh, 1);
        
        # align mesh to Z = 0 and apply XY shift
        $mesh->translate((map unscale(-$_), @{$self->_copies_shift}), -$self->model_object->bounding_box->z_min);
        
        {
            my $loops = $mesh->slice([ map $_->slice_z, @{$self->layers} ]);
            for my $layer_id (0..$#$loops) {
                my $layerm = $self->layers->[$layer_id]->regions->[$region_id];
                $layerm->make_surfaces($loops->[$layer_id]);
            }
            # TODO: read slicing_errors
        }
    }
    
    # remove last layer(s) if empty
    pop @{$self->layers} while @{$self->layers} && (!map @{$_->slices}, @{$self->layers->[-1]->regions});
    
    foreach my $layer (@{ $self->layers }) {
        # merge all regions' slices to get islands
        $layer->make_slices;
    }
    
    # detect slicing errors
    my $warning_thrown = 0;
    for my $i (0 .. $#{$self->layers}) {
        my $layer = $self->layers->[$i];
        next unless $layer->slicing_errors;
        if (!$warning_thrown) {
            warn "The model has overlapping or self-intersecting facets. I tried to repair it, "
                . "however you might want to check the results or repair the input file and retry.\n";
            $warning_thrown = 1;
        }
        
        # try to repair the layer surfaces by merging all contours and all holes from
        # neighbor layers
        Slic3r::debugf "Attempting to repair layer %d\n", $i;
        
        foreach my $region_id (0 .. $#{$layer->regions}) {
            my $layerm = $layer->region($region_id);
            
            my (@upper_surfaces, @lower_surfaces);
            for (my $j = $i+1; $j <= $#{$self->layers}; $j++) {
                if (!$self->layers->[$j]->slicing_errors) {
                    @upper_surfaces = @{$self->layers->[$j]->region($region_id)->slices};
                    last;
                }
            }
            for (my $j = $i-1; $j >= 0; $j--) {
                if (!$self->layers->[$j]->slicing_errors) {
                    @lower_surfaces = @{$self->layers->[$j]->region($region_id)->slices};
                    last;
                }
            }
            
            my $union = union_ex([
                map $_->expolygon->contour, @upper_surfaces, @lower_surfaces,
            ]);
            my $diff = diff_ex(
                [ map @$_, @$union ],
                [ map @{$_->expolygon->holes}, @upper_surfaces, @lower_surfaces, ],
            );
            
            $layerm->slices->clear;
            $layerm->slices->append(
                map Slic3r::Surface->new
                    (expolygon => $_, surface_type => S_TYPE_INTERNAL),
                    @$diff
            );
        }
            
        # update layer slices after repairing the single regions
        $layer->make_slices;
    }
    
    # remove empty layers from bottom
    my $first_object_layer_id = $self->config->raft_layers;
    while (@{$self->layers} && !@{$self->layers->[$first_object_layer_id]->slices}) {
        splice @{$self->layers}, $first_object_layer_id, 1;
        for (my $i = $first_object_layer_id; $i <= $#{$self->layers}; $i++) {
            $self->layers->[$i]->id($i);
        }
    }
    
    # simplify slices if required
    if ($self->config->resolution) {
        $self->_simplify_slices(scale($self->config->resolution));
    }
}

sub make_perimeters {
    my $self = shift;
    
    # compare each layer to the one below, and mark those slices needing
    # one additional inner perimeter, like the top of domed objects-
    
    # this algorithm makes sure that at least one perimeter is overlapping
    # but we don't generate any extra perimeter if fill density is zero, as they would be floating
    # inside the object - infill_only_where_needed should be the method of choice for printing
    # hollow objects
    if ($self->config->extra_perimeters && $self->config->perimeters > 0 && $self->config->fill_density > 0) {
        for my $region_id (0 .. ($self->print->regions_count-1)) {
            for my $layer_id (0 .. $self->layer_count-2) {
                my $layerm          = $self->layers->[$layer_id]->regions->[$region_id];
                my $upper_layerm    = $self->layers->[$layer_id+1]->regions->[$region_id];
                my $perimeter_spacing       = $layerm->perimeter_flow->scaled_spacing;
                
                my $overlap = $perimeter_spacing;  # one perimeter
                
                my $diff = diff(
                    offset([ map @{$_->expolygon}, @{$layerm->slices} ], -($self->config->perimeters * $perimeter_spacing)),
                    offset([ map @{$_->expolygon}, @{$upper_layerm->slices} ], -$overlap),
                );
                next if !@$diff;
                # if we need more perimeters, $diff should contain a narrow region that we can collapse
                
                # we use a higher miterLimit here to handle areas with acute angles
                # in those cases, the default miterLimit would cut the corner and we'd
                # get a triangle that would trigger a non-needed extra perimeter
                $diff = diff(
                    $diff,
                    offset2($diff, -$perimeter_spacing, +$perimeter_spacing, CLIPPER_OFFSET_SCALE, JT_MITER, 5),
                    1,
                );
                next if !@$diff;
                # diff contains the collapsed area
                
                foreach my $slice (@{$layerm->slices}) {
                    my $extra_perimeters = 0;
                    CYCLE: while (1) {
                        # compute polygons representing the thickness of the hypotetical new internal perimeter
                        # of our slice
                        $extra_perimeters++;
                        my $hypothetical_perimeter = diff(
                            offset($slice->expolygon->arrayref, -($perimeter_spacing * ($self->config->perimeters + $extra_perimeters-1))),
                            offset($slice->expolygon->arrayref, -($perimeter_spacing * ($self->config->perimeters + $extra_perimeters))),
                        );
                        last CYCLE if !@$hypothetical_perimeter;  # no extra perimeter is possible
                        
                        # only add the perimeter if there's an intersection with the collapsed area
                        last CYCLE if !@{ intersection($diff, $hypothetical_perimeter) };
                        
                        Slic3r::debugf "  adding one more perimeter at layer %d\n", $layer_id;
                        $slice->extra_perimeters($extra_perimeters);
                    }
                }
            }
        }
    }
    
    Slic3r::parallelize(
        threads => $self->config->threads,
        items => sub { 0 .. ($self->layer_count-1) },
        thread_cb => sub {
            my $q = shift;
            while (defined (my $layer_id = $q->dequeue)) {
                $self->layers->[$layer_id]->make_perimeters;
            }
        },
        collect_cb => sub {},
        no_threads_cb => sub {
            $_->make_perimeters for @{$self->layers};
        },
    );
    
    # simplify slices (both layer and region slices),
    # we only need the max resolution for perimeters
    ### This makes this method not-idempotent, so we keep it disabled for now.
    ###$self->_simplify_slices(&Slic3r::SCALED_RESOLUTION);
}

sub detect_surfaces_type {
    my $self = shift;
    Slic3r::debugf "Detecting solid surfaces...\n";
    
    for my $region_id (0 .. ($self->print->regions_count-1)) {
        for my $i (0 .. ($self->layer_count-1)) {
            my $layerm = $self->layers->[$i]->regions->[$region_id];
        
            # prepare a reusable subroutine to make surface differences
            my $difference = sub {
                my ($subject, $clip, $result_type) = @_;
                my $diff = diff(
                    [ map @$_, @$subject ],
                    [ map @$_, @$clip ],
                );
                
                # collapse very narrow parts (using the safety offset in the diff is not enough)
                my $offset = $layerm->perimeter_flow->scaled_width / 10;
                return map Slic3r::Surface->new(expolygon => $_, surface_type => $result_type),
                    @{ offset2_ex($diff, -$offset, +$offset) };
            };
            
            # comparison happens against the *full* slices (considering all regions)
            my $upper_layer = $self->layers->[$i+1];
            my $lower_layer = $i > 0 ? $self->layers->[$i-1] : undef;
            
            my (@bottom, @top, @internal) = ();
            
            # find top surfaces (difference between current surfaces
            # of current layer and upper one)
            if ($upper_layer) {
                @top = $difference->(
                    [ map $_->expolygon, @{$layerm->slices} ],
                    $upper_layer->slices,
                    S_TYPE_TOP,
                );
            } else {
                # if no upper layer, all surfaces of this one are solid
                # we clone surfaces because we're going to clear the slices collection
                @top = map $_->clone, @{$layerm->slices};
                $_->surface_type(S_TYPE_TOP) for @top;
            }
            
            # find bottom surfaces (difference between current surfaces
            # of current layer and lower one)
            if ($lower_layer) {
                # lower layer's slices are already Surface objects
                @bottom = $difference->(
                    [ map $_->expolygon, @{$layerm->slices} ],
                    $lower_layer->slices,
                    S_TYPE_BOTTOM,
                );
            } else {
                # if no lower layer, all surfaces of this one are solid
                # we clone surfaces because we're going to clear the slices collection
                @bottom = map $_->clone, @{$layerm->slices};
                $_->surface_type(S_TYPE_BOTTOM) for @bottom;
            }
            
            # now, if the object contained a thin membrane, we could have overlapping bottom
            # and top surfaces; let's do an intersection to discover them and consider them
            # as bottom surfaces (to allow for bridge detection)
            if (@top && @bottom) {
                my $overlapping = intersection_ex([ map $_->p, @top ], [ map $_->p, @bottom ]);
                Slic3r::debugf "  layer %d contains %d membrane(s)\n", $layerm->id, scalar(@$overlapping)
                    if $Slic3r::debug;
                @top = $difference->([map $_->expolygon, @top], $overlapping, S_TYPE_TOP);
            }
            
            # find internal surfaces (difference between top/bottom surfaces and others)
            @internal = $difference->(
                [ map $_->expolygon, @{$layerm->slices} ],
                [ map $_->expolygon, @top, @bottom ],
                S_TYPE_INTERNAL,
            );
            
            # save surfaces to layer
            $layerm->slices->clear;
            $layerm->slices->append(@bottom, @top, @internal);
            
            Slic3r::debugf "  layer %d has %d bottom, %d top and %d internal surfaces\n",
                $layerm->id, scalar(@bottom), scalar(@top), scalar(@internal) if $Slic3r::debug;
        }
        
        # clip surfaces to the fill boundaries
        foreach my $layer (@{$self->layers}) {
            my $layerm = $layer->regions->[$region_id];
            my $fill_boundaries = [ map $_->clone->p, @{$layerm->fill_surfaces} ];
            $layerm->fill_surfaces->clear;
            foreach my $surface (@{$layerm->slices}) {
                my $intersection = intersection_ex(
                    [ $surface->p ],
                    $fill_boundaries,
                );
                $layerm->fill_surfaces->append(map Slic3r::Surface->new
                    (expolygon => $_, surface_type => $surface->surface_type),
                    @$intersection);
            }
        }
    }
}

sub clip_fill_surfaces {
    my $self = shift;
    return unless $self->config->infill_only_where_needed;
    
    # We only want infill under ceilings; this is almost like an
    # internal support material.
    
    my $additional_margin = scale 3;
    
    my $overhangs = [];  # arrayref of polygons
    for my $layer_id (reverse 0..$#{$self->layers}) {
        my $layer = $self->layers->[$layer_id];
        my @layer_internal = ();
        my @new_internal = ();
        
        # clip this layer's internal surfaces to @overhangs
        foreach my $layerm (@{$layer->regions}) {
            # we assume that this step is run before bridge_over_infill() and combine_infill()
            # so these are the only internal types we might have
            my (@internal, @other) = ();
            foreach my $surface (map $_->clone, @{$layerm->fill_surfaces}) {
                $surface->surface_type == S_TYPE_INTERNAL
                    ? push @internal, $surface
                    : push @other, $surface;
            }
            
            # keep all the original internal surfaces to detect overhangs in this layer
            push @layer_internal, @internal;
            
            push @new_internal, my @new = map Slic3r::Surface->new(
                expolygon       => $_,
                surface_type    => S_TYPE_INTERNAL,
            ),
            @{intersection_ex(
                $overhangs,
                [ map $_->p, @internal ],
            )};
            
            $layerm->fill_surfaces->clear;
            $layerm->fill_surfaces->append(@new, @other);
        }
        
        # get this layer's overhangs defined as the full slice minus the internal infill
        # (thus we also consider perimeters)
        if ($layer_id > 0) {
            my $solid = diff(
                [ map @$_, @{$layer->slices} ],
                \@layer_internal,
            );
            $overhangs = offset($solid, +$additional_margin);
            push @$overhangs, @new_internal;  # propagate upper overhangs
        }
    }
}

sub bridge_over_infill {
    my $self = shift;
    return if $self->config->fill_density == 1;
    
    for my $layer_id (1..$#{$self->layers}) {
        my $layer       = $self->layers->[$layer_id];
        my $lower_layer = $self->layers->[$layer_id-1];
        
        foreach my $layerm (@{$layer->regions}) {
            # compute the areas needing bridge math 
            my @internal_solid = @{$layerm->fill_surfaces->filter_by_type(S_TYPE_INTERNALSOLID)};
            my @lower_internal = map @{$_->fill_surfaces->filter_by_type(S_TYPE_INTERNAL)}, @{$lower_layer->regions};
            my $to_bridge = intersection_ex(
                [ map $_->p, @internal_solid ],
                [ map $_->p, @lower_internal ],
            );
            next unless @$to_bridge;
            Slic3r::debugf "Bridging %d internal areas at layer %d\n", scalar(@$to_bridge), $layer_id;
            
            # build the new collection of fill_surfaces
            {
                my @new_surfaces = map $_->clone, grep $_->surface_type != S_TYPE_INTERNALSOLID, @{$layerm->fill_surfaces};
                push @new_surfaces, map Slic3r::Surface->new(
                        expolygon       => $_,
                        surface_type    => S_TYPE_INTERNALBRIDGE,
                    ), @$to_bridge;
                push @new_surfaces, map Slic3r::Surface->new(
                        expolygon       => $_,
                        surface_type    => S_TYPE_INTERNALSOLID,
                    ), @{diff_ex(
                        [ map $_->p, @internal_solid ],
                        [ map @$_, @$to_bridge ],
                        1,
                    )};
                $layerm->fill_surfaces->clear;
                $layerm->fill_surfaces->append(@new_surfaces);
            }
            
            # exclude infill from the layers below if needed
            # see discussion at https://github.com/alexrj/Slic3r/issues/240
            # Update: do not exclude any infill. Sparse infill is able to absorb the excess material.
            if (0) {
                my $excess = $layerm->extruders->{infill}->bridge_flow->width - $layerm->height;
                for (my $i = $layer_id-1; $excess >= $self->layers->[$i]->height; $i--) {
                    Slic3r::debugf "  skipping infill below those areas at layer %d\n", $i;
                    foreach my $lower_layerm (@{$self->layers->[$i]->regions}) {
                        my @new_surfaces = ();
                        # subtract the area from all types of surfaces
                        foreach my $group (@{$lower_layerm->fill_surfaces->group}) {
                            push @new_surfaces, map $group->[0]->clone(expolygon => $_),
                                @{diff_ex(
                                    [ map $_->p, @$group ],
                                    [ map @$_, @$to_bridge ],
                                )};
                            push @new_surfaces, map Slic3r::Surface->new(
                                expolygon       => $_,
                                surface_type    => S_TYPE_INTERNALVOID,
                            ), @{intersection_ex(
                                [ map $_->p, @$group ],
                                [ map @$_, @$to_bridge ],
                            )};
                        }
                        $lower_layerm->fill_surfaces->clear;
                        $lower_layerm->fill_surfaces->append(@new_surfaces);
                    }
                    
                    $excess -= $self->layers->[$i]->height;
                }
            }
        }
    }
}

sub process_external_surfaces {
    my ($self) = @_;
    
    for my $region_id (0 .. ($self->print->regions_count-1)) {
        $self->layers->[0]->regions->[$region_id]->process_external_surfaces(undef);
        for my $layer_id (1 .. ($self->layer_count-1)) {
            $self->layers->[$layer_id]->regions->[$region_id]->process_external_surfaces($self->layers->[$layer_id-1]);
        }
    }
}

sub discover_horizontal_shells {
    my $self = shift;
    
    Slic3r::debugf "==> DISCOVERING HORIZONTAL SHELLS\n";
    
    for my $region_id (0 .. ($self->print->regions_count-1)) {
        for (my $i = 0; $i < $self->layer_count; $i++) {
            my $layerm = $self->layers->[$i]->regions->[$region_id];
            
            if ($self->config->solid_infill_every_layers && $self->config->fill_density > 0
                && ($i % $self->config->solid_infill_every_layers) == 0) {
                $_->surface_type(S_TYPE_INTERNALSOLID) for @{$layerm->fill_surfaces->filter_by_type(S_TYPE_INTERNAL)};
            }
            
            EXTERNAL: foreach my $type (S_TYPE_TOP, S_TYPE_BOTTOM) {
                # find slices of current type for current layer
                # use slices instead of fill_surfaces because they also include the perimeter area
                # which needs to be propagated in shells; we need to grow slices like we did for
                # fill_surfaces though.  Using both ungrown slices and grown fill_surfaces will
                # not work in some situations, as there won't be any grown region in the perimeter 
                # area (this was seen in a model where the top layer had one extra perimeter, thus
                # its fill_surfaces were thinner than the lower layer's infill), however it's the best
                # solution so far. Growing the external slices by EXTERNAL_INFILL_MARGIN will put
                # too much solid infill inside nearly-vertical slopes.
                my $solid = [
                    (map $_->p, @{$layerm->slices->filter_by_type($type)}),
                    (map $_->p, @{$layerm->fill_surfaces->filter_by_type($type)}),
                ];
                next if !@$solid;
                Slic3r::debugf "Layer %d has %s surfaces\n", $i, ($type == S_TYPE_TOP) ? 'top' : 'bottom';
                
                my $solid_layers = ($type == S_TYPE_TOP)
                    ? $self->config->top_solid_layers
                    : $self->config->bottom_solid_layers;
                NEIGHBOR: for (my $n = ($type == S_TYPE_TOP) ? $i-1 : $i+1; 
                        abs($n - $i) <= $solid_layers-1; 
                        ($type == S_TYPE_TOP) ? $n-- : $n++) {
                    
                    next if $n < 0 || $n >= $self->layer_count;
                    Slic3r::debugf "  looking for neighbors on layer %d...\n", $n;
                    
                    my $neighbor_fill_surfaces = $self->layers->[$n]->regions->[$region_id]->fill_surfaces;
                    my @neighbor_fill_surfaces = map $_->clone, @$neighbor_fill_surfaces;  # clone because we will use these surfaces even after clearing the collection
                    
                    # find intersection between neighbor and current layer's surfaces
                    # intersections have contours and holes
                    # we update $solid so that we limit the next neighbor layer to the areas that were
                    # found on this one - in other words, solid shells on one layer (for a given external surface)
                    # are always a subset of the shells found on the previous shell layer
                    # this approach allows for DWIM in hollow sloping vases, where we want bottom
                    # shells to be generated in the base but not in the walls (where there are many
                    # narrow bottom surfaces): reassigning $solid will consider the 'shadow' of the 
                    # upper perimeter as an obstacle and shell will not be propagated to more upper layers
                    my $new_internal_solid = $solid = intersection(
                        $solid,
                        [ map $_->p, grep { ($_->surface_type == S_TYPE_INTERNAL) || ($_->surface_type == S_TYPE_INTERNALSOLID) } @neighbor_fill_surfaces ],
                        1,
                    );
                    next EXTERNAL if !@$new_internal_solid;
                    
                    # make sure the new internal solid is wide enough, as it might get collapsed when
                    # spacing is added in Fill.pm
                    {
                        # we use a higher miterLimit here to handle areas with acute angles
                        # in those cases, the default miterLimit would cut the corner and we'd
                        # get a triangle in $too_narrow; if we grow it below then the shell
                        # would have a different shape from the external surface and we'd still
                        # have the same angle, so the next shell would be grown even more and so on.
                        my $margin = 3 * $layerm->solid_infill_flow->scaled_width; # require at least this size
                        my $too_narrow = diff(
                            $new_internal_solid,
                            offset2($new_internal_solid, -$margin, +$margin, CLIPPER_OFFSET_SCALE, JT_MITER, 5),
                            1,
                        );
                        
                        # if some parts are going to collapse, use a different strategy according to fill density
                        if (@$too_narrow) {
                            if ($self->config->fill_density > 0) {
                                # if we have internal infill, grow the collapsing parts and add the extra area to 
                                # the neighbor layer as well as to our original surfaces so that we support this 
                                # additional area in the next shell too

                                # make sure our grown surfaces don't exceed the fill area
                                my @grown = @{intersection(
                                    offset($too_narrow, +$margin),
                                    [ map $_->p, @neighbor_fill_surfaces ],
                                )};
                                $new_internal_solid = $solid = [ @grown, @$new_internal_solid ];
                            } else {
                                # if we're printing a hollow object, we discard such small parts
                                $new_internal_solid = $solid = diff(
                                    $new_internal_solid,
                                    $too_narrow,
                                );
                            }
                        }
                    }
                    
                    # internal-solid are the union of the existing internal-solid surfaces
                    # and new ones
                    my $internal_solid = union_ex([
                        ( map $_->p, grep $_->surface_type == S_TYPE_INTERNALSOLID, @neighbor_fill_surfaces ),
                        @$new_internal_solid,
                    ]);
                    
                    # subtract intersections from layer surfaces to get resulting internal surfaces
                    my $internal = diff_ex(
                        [ map $_->p, grep $_->surface_type == S_TYPE_INTERNAL, @neighbor_fill_surfaces ],
                        [ map @$_, @$internal_solid ],
                        1,
                    );
                    Slic3r::debugf "    %d internal-solid and %d internal surfaces found\n",
                        scalar(@$internal_solid), scalar(@$internal);
                    
                    # assign resulting internal surfaces to layer
                    $neighbor_fill_surfaces->clear;
                    $neighbor_fill_surfaces->append(map Slic3r::Surface->new
                        (expolygon => $_, surface_type => S_TYPE_INTERNAL), @$internal);
                    
                    # assign new internal-solid surfaces to layer
                    $neighbor_fill_surfaces->append(map Slic3r::Surface->new
                        (expolygon => $_, surface_type => S_TYPE_INTERNALSOLID), @$internal_solid);
                    
                    # assign top and bottom surfaces to layer
                    foreach my $s (@{Slic3r::Surface::Collection->new(grep { ($_->surface_type == S_TYPE_TOP) || ($_->surface_type == S_TYPE_BOTTOM) } @neighbor_fill_surfaces)->group}) {
                        my $solid_surfaces = diff_ex(
                            [ map $_->p, @$s ],
                            [ map @$_, @$internal_solid, @$internal ],
                            1,
                        );
                        $neighbor_fill_surfaces->append(map $s->[0]->clone(expolygon => $_), @$solid_surfaces);
                    }
                }
            }
        }
    }
}

# combine fill surfaces across layers
sub combine_infill {
    my $self = shift;
    return unless $self->config->infill_every_layers > 1 && $self->config->fill_density > 0;
    my $every = $self->config->infill_every_layers;
    
    my $layer_count = $self->layer_count;
    my @layer_heights = map $self->layers->[$_]->height, 0 .. $layer_count-1;
    
    for my $region_id (0 .. ($self->print->regions_count-1)) {
        # limit the number of combined layers to the maximum height allowed by this regions' nozzle
        my $nozzle_diameter = $self->print->regions->[$region_id]->extruders->{infill}->nozzle_diameter;
        
        # define the combinations
        my @combine = ();   # layer_id => thickness in layers
        {
            my $current_height = my $layers = 0;
            for my $layer_id (1 .. $#layer_heights) {
                my $height = $self->layers->[$layer_id]->height;
                
                if ($current_height + $height >= $nozzle_diameter || $layers >= $every) {
                    $combine[$layer_id-1] = $layers;
                    $current_height = $layers = 0;
                }
                
                $current_height += $height;
                $layers++;
            }
        }
        
        # skip bottom layer
        for my $layer_id (1 .. $#combine) {
            next unless ($combine[$layer_id] // 1) > 1;
            my @layerms = map $self->layers->[$_]->regions->[$region_id],
                ($layer_id - ($combine[$layer_id]-1) .. $layer_id);
            
            # only combine internal infill
            for my $type (S_TYPE_INTERNAL) {
                # we need to perform a multi-layer intersection, so let's split it in pairs
                
                # initialize the intersection with the candidates of the lowest layer
                my $intersection = [ map $_->expolygon, @{$layerms[0]->fill_surfaces->filter_by_type($type)} ];
                
                # start looping from the second layer and intersect the current intersection with it
                for my $layerm (@layerms[1 .. $#layerms]) {
                    $intersection = intersection_ex(
                        [ map @$_, @$intersection ],
                        [ map @{$_->expolygon}, @{$layerm->fill_surfaces->filter_by_type($type)} ],
                    );
                }
                
                my $area_threshold = $layerms[0]->infill_area_threshold;
                @$intersection = grep $_->area > $area_threshold, @$intersection;
                next if !@$intersection;
                Slic3r::debugf "  combining %d %s regions from layers %d-%d\n",
                    scalar(@$intersection),
                    ($type == S_TYPE_INTERNAL ? 'internal' : 'internal-solid'),
                    $layer_id-($every-1), $layer_id;
                
                # $intersection now contains the regions that can be combined across the full amount of layers
                # so let's remove those areas from all layers
                
                 my @intersection_with_clearance = map @{$_->offset(
                       $layerms[-1]->solid_infill_flow->scaled_width    / 2
                     + $layerms[-1]->perimeter_flow->scaled_width / 2
                     # Because fill areas for rectilinear and honeycomb are grown 
                     # later to overlap perimeters, we need to counteract that too.
                     + (($type == S_TYPE_INTERNALSOLID || $self->config->fill_pattern =~ /(rectilinear|honeycomb)/)
                       ? $layerms[-1]->solid_infill_flow->scaled_width * &Slic3r::INFILL_OVERLAP_OVER_SPACING
                       : 0)
                     )}, @$intersection;

                
                foreach my $layerm (@layerms) {
                    my @this_type   = @{$layerm->fill_surfaces->filter_by_type($type)};
                    my @other_types = map $_->clone, grep $_->surface_type != $type, @{$layerm->fill_surfaces};
                    
                    my @new_this_type = map Slic3r::Surface->new(expolygon => $_, surface_type => $type),
                        @{diff_ex(
                            [ map $_->p, @this_type ],
                            [ @intersection_with_clearance ],
                        )};
                    
                    # apply surfaces back with adjusted depth to the uppermost layer
                    if ($layerm->id == $layer_id) {
                        push @new_this_type,
                            map Slic3r::Surface->new(
                                expolygon        => $_,
                                surface_type     => $type,
                                thickness        => sum(map $_->height, @layerms),
                                thickness_layers => scalar(@layerms),
                            ),
                            @$intersection;
                    } else {
                        # save void surfaces
                        push @this_type,
                            map Slic3r::Surface->new(expolygon => $_, surface_type => S_TYPE_INTERNALVOID),
                            @{intersection_ex(
                                [ map @{$_->expolygon}, @this_type ],
                                [ @intersection_with_clearance ],
                            )};
                    }
                    
                    $layerm->fill_surfaces->clear;
                    $layerm->fill_surfaces->append(@new_this_type, @other_types);
                }
            }
        }
    }
}

sub generate_support_material {
    my $self = shift;
    return unless ($self->config->support_material || $self->config->raft_layers > 0)
        && $self->layer_count >= 2;
    
    Slic3r::Print::SupportMaterial
        ->new(config => $self->config, flow => $self->print->support_material_flow)
        ->generate($self);
}

sub _simplify_slices {
    my ($self, $distance) = @_;
    
    foreach my $layer (@{$self->layers}) {
        $layer->slices->simplify($distance);
        $_->slices->simplify($distance) for @{$layer->regions};
    }
}

1;