wm/wm.sql

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\set ON_ERROR_STOP on
SET plpgsql.extra_errors TO 'all';
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-- detect_bends detects bends using the inflection angles. No corrections.
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drop function if exists detect_bends;
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create function detect_bends(line geometry, OUT bends geometry[]) as $$
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declare
pi real;
p geometry;
p1 geometry;
p2 geometry;
p3 geometry;
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bend geometry;
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prev_sign int4;
cur_sign int4;
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begin
pi = radians(180);
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-- the last vertex is iterated over twice, because the algorithm uses 3
-- vertices to calculate the angle between them.
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--
-- Given 3 vertices p1, p2, p3:
--
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-- p1___ ...
-- /
-- ... _____/
-- p3 p2
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--
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-- When looping over the line, p1 will be head (lead) vertex, p2 will be the
-- measured angle, and p3 will be trailing. The line that will be added to
-- the bend will always be [p3,p2].
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-- So once the p1 becomes the last vertex, the loop terminates, and the
-- [p2,p1] line will not have a chance to be added. So the loop adds the last
-- vertex twice, so it has a chance to become p2, and be added to the bend.
--
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for p in (
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(select geom from st_dumppoints(line) order by path[1] asc)
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union all
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(select geom from st_dumppoints(line) order by path[1] desc limit 1)
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) loop
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p3 = p2;
p2 = p1;
p1 = p;
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continue when p3 is null;
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cur_sign = sign(pi - st_angle(p1, p2, p2, p3));
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if bend is null then
bend = st_makeline(p3, p2);
else
bend = st_linemerge(st_union(bend, st_makeline(p3, p2)));
end if;
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if prev_sign + cur_sign = 0 then
if bend is not null then
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bends = bends || bend;
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end if;
bend = st_makeline(p3, p2);
end if;
prev_sign = cur_sign;
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end loop;
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-- the last line may be lost if there is no "final" inflection angle. Add it.
if (select count(1) >= 2 from st_dumppoints(bend)) then
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bends = bends || bend;
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end if;
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end
$$ language plpgsql;
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-- fix_gentle_inflections moves bend endpoints following "Gentle Inflection at
-- End of a Bend" section.
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--
-- The text does not specify how many vertices can be "adjusted"; it can
-- equally be one or many. This function is adjusting many, as long as the
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-- commulative inflection angle small (see variable below).
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--
-- The implementation could be significantly optimized to avoid `st_reverse`
-- and array reversals, trading for complexity in fix_gentle_inflections1.
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create or replace function fix_gentle_inflections(INOUT bends geometry[]) as $$
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declare
len int4;
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bends1 geometry[];
begin
len = array_length(bends, 1);
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bends = fix_gentle_inflections1(bends);
for i in 1..len loop
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bends1[i] = st_reverse(bends[len-i+1]);
end loop;
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bends1 = fix_gentle_inflections1(bends1);
for i in 1..len loop
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bends[i] = st_reverse(bends1[len-i+1]);
end loop;
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end
$$ language plpgsql;
-- fix_gentle_inflections1 fixes gentle inflections of an array of lines in
-- one direction. This is an implementation detail of fix_gentle_inflections.
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drop function if exists fix_gentle_inflections1;
create function fix_gentle_inflections1(INOUT bends geometry[]) as $$
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declare
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pi real;
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small_angle real;
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ptail geometry; -- tail point of tail bend
phead geometry[]; -- 3 tail points of head bend
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i int4; -- bends[i] is the current head
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begin
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pi = radians(180);
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-- the threshold when the angle is still "small", so gentle inflections can
-- be joined
small_angle := radians(30);
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for i in 2..array_length(bends, 1) loop
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-- Predicate: two bends will always share an edge. Assuming (A,B,C,D,E,F)
-- is a bend:
-- C________D
-- / \
-- \________/ \_______/
-- A B E F
--
-- Then edges (A,B) and (E,F) are shared with the neighboring bends.
--
--
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-- Assume this curve (figure `inflection-1`), going clockwise from A:
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--
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-- \______B
-- A `-------. C
-- |
-- G___ F |
-- / `-----.____+ D
-- E
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--
-- After processing the curve following the definition of a bend, the bend
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-- [A-E] would be detected. Assuming inflection point E and F are "small",
-- the bend needs to be extended by two edges to [A,G].
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select geom from st_dumppoints(bends[i-1])
order by path[1] asc limit 1 into ptail;
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while true loop
-- copy last 3 points of bends[i-1] (tail) to ptail
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select array(
select geom from st_dumppoints(bends[i]) order by path[1] asc limit 3
) into phead;
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-- if the bend got too short, stop processing it
exit when array_length(phead, 1) < 3;
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-- inflection angle between ptail[1:3] is "large", stop processing
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exit when abs(st_angle(phead[1], phead[2], phead[3]) - pi) > small_angle;
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-- distance from head's 1st vertex should be larger than from 2nd vertex
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exit when st_distance(ptail, phead[2]) < st_distance(ptail, phead[3]);
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-- Detected a gentle inflection.
-- Move head of the tail to the tail of head
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bends[i] = st_removepoint(bends[i], 0);
bends[i-1] = st_addpoint(bends[i-1], phead[3]);
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end loop;
end loop;
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end
$$ language plpgsql;
-- self_crossing eliminates self-crossing from the bends, following the
-- article's section "Self-line Crossing When Cutting a Bend".
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drop function if exists self_crossing;
create function self_crossing(
INOUT bends geometry[],
OUT mutated boolean
) as $$
declare
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i int4;
j int4;
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prev_length int4;
pi real;
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angle real;
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p0 geometry;
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p1 geometry;
p2 geometry;
p3 geometry;
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a geometry;
b geometry;
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bend geometry;
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multi geometry;
begin
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pi = radians(180);
mutated = false;
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-- go through the bends and find one where sum of inflection angle is >180
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for i in 1..array_length(bends, 1) loop
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angle = 0;
p1 = null;
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p2 = null;
p3 = null;
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for p0 in (
select geom from st_dumppoints(bends[i]) order by path[1] asc
) loop
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p3 = p2;
p2 = p1;
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p1 = p0;
continue when p3 is null;
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angle = angle + abs(pi - st_angle(p1, p2, p3));
end loop;
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continue when abs(angle) <= pi;
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-- sum of inflection angles for this bend is >180, so it may be
-- self-crossing. now try to find another bend in this line that
-- crosses an imaginary line of end-vertices
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p0 = st_pointn(bends[i], 1);
p1 = st_pointn(bends[i], -1);
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-- go through each bend in the given line, and see if has a potential to
-- cross bends[i]. optimization: we care only about bends which beginning
-- and end start at different sides of the plane, separated by endpoints
-- p0 and p1.
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j = 0;
while j < array_length(bends, 1) loop
j = j + 1;
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continue when i = j;
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p2 = st_pointn(bends[j], 1);
p3 = st_pointn(bends[j], -1);
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-- do end vertices of bend[i] cross bend[j]?
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a = st_pointn(bends[i], 1);
b = st_pointn(bends[i], -1);
multi = st_split(bends[j], st_makeline(a, b));
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continue when st_numgeometries(multi) = 1;
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continue when st_numgeometries(multi) = 2 and
(st_contains(bends[j], a) or st_contains(bends[j], b));
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-- stars are aligned, we are changing the bend
mutated = true;
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-- Sincere apologies to someone who will need to debug the block below.
-- To understand it, I suggest you take a pencil and paper, draw a
-- self-crossing bend (fig6 from the article works well), and figure out
-- what happens here, by hand.
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prev_length = array_length(bends, 1);
if j < i then
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-- remove first vertex of the following bend, because the last
-- segment is always duplicated with the i-th bend.
bends[i+1] = st_removepoint(bends[i+1], 0);
bends[j] = st_geometryn(multi, 1);
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bends[j] = st_setpoint(
bends[j], st_npoints(bends[j])-1,
st_pointn(bends[i], st_npoints(bends[i]))
);
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bends = bends[1:j] || bends[i+1:prev_length];
j = i;
else
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-- remove last vertex of the previous bend, because the last
-- segment is duplicated with the i'th bend.
bends[i-1] = st_removepoint(bends[i-1], st_npoints(bends[i-1])-1);
bends[i] = st_makeline(
st_pointn(bends[i], 1),
st_removepoint(st_geometryn(multi, st_numgeometries(multi)), 0)
);
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bends = bends[1:i] || bends[j+1:prev_length];
end if;
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j = j - prev_length + array_length(bends, 1);
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end loop;
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end loop;
end
$$ language plpgsql;
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-- ST_SimplifyWM simplifies a given geometry using Wang & Müller's
-- "Line Generalization Based on Analysis of Shape Characteristics" algorithm,
-- 1998.
drop function if exists ST_SimplifyWM;
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create function ST_SimplifyWM(geom geometry) returns geometry as $$
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declare
line geometry;
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lines geometry[];
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bends geometry[];
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mutated boolean;
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l_type text;
begin
l_type = st_geometrytype(geom);
if l_type = 'ST_LineString' then
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lines = array[geom];
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elseif l_type = 'ST_MultiLineString' then
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lines = array((select a.geom from st_dump(geom) a order by path[1] asc));
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else
raise 'Unknown geometry type %', l_type;
end if;
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foreach line in array lines loop
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mutated = true;
while mutated loop
bends = detect_bends(line);
bends = fix_gentle_inflections(bends);
select * from self_crossing(bends) into bends, mutated;
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line = st_linemerge(st_union(bends));
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end loop;
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end loop;
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if l_type = 'ST_LineString' then
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return st_linemerge(st_union(bends));
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elseif l_type = 'ST_MultiLineString' then
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return st_union(bends);
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end if;
end
$$ language plpgsql;
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drop function if exists ST_SimplifyWM_DEBUG;
create function ST_SimplifyWM_DEBUG(geom geometry) returns geometry as $$
declare
i integer;
line geometry;
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lines geometry[];
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bends geometry[];
mutated boolean;
l_type text;
begin
l_type = st_geometrytype(geom);
if l_type = 'ST_LineString' then
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lines = array[geom];
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elseif l_type = 'ST_MultiLineString' then
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lines = array((select a.geom from st_dump(geom) a order by path[1] asc));
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else
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raise 'Unsupported geometry type %', l_type;
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end if;
i = 1;
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mutated = true;
foreach line in array lines loop
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while mutated loop
execute format('create table if not exists integ_%safigures (way geometry)', i);
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-- if anyone has suggestions how to insert a variable to a table without
-- such hackery, I'll be glad to know
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execute format('insert into integ_%safigures select $1;', i) using (select unnest(array[line]));
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bends = detect_bends(line);
execute format('create table if not exists integ_%sbbends (i bigint, way geometry)', i);
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execute format('insert into integ_%sbbends (i, way) select generate_subscripts($1, 1), unnest($1)', i) using bends;
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bends = fix_gentle_inflections(bends);
execute format('create table if not exists integ_%scinflections (i bigint, way geometry)', i);
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execute format('insert into integ_%scinflections (i, way) select generate_subscripts($1, 1), unnest($1)', i) using bends;
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select * from self_crossing(bends) into bends, mutated;
execute format('create table if not exists integ_%sdselfcrossing (i bigint, way geometry)', i);
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execute format('insert into integ_%sdselfcrossing (i, way) select generate_subscripts($1, 1), unnest($1)', i) using bends;
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line = st_linemerge(st_union(bends));
i = i + 1;
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end loop;
end loop;
if l_type = 'ST_LineString' then
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return st_linemerge(st_union(bends));
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elseif l_type = 'ST_MultiLineString' then
return st_union(bends);
end if;
end
$$ language plpgsql;