wm/wm.sql

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\set ON_ERROR_STOP on
SET plpgsql.extra_errors TO 'all';
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-- wm_detect_bends detects bends using the inflection angles. No corrections.
drop function if exists wm_detect_bends;
create function wm_detect_bends(
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line geometry,
dbgname text default null,
dbggen integer default null,
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OUT bends geometry[]
) as $$
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declare
p geometry;
p1 geometry;
p2 geometry;
p3 geometry;
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bend geometry;
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prev_sign int4;
cur_sign int4;
l_type text;
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dbgpolygon geometry;
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begin
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l_type = st_geometrytype(line);
if l_type != 'ST_LineString' then
raise 'This function works with ST_LineString, got %', l_type;
end if;
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-- The last vertex is iterated over twice, because the algorithm uses 3
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-- 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
(select geom from st_dumppoints(line) order by path[1] asc)
union all
(select geom from st_dumppoints(line) order by path[1] desc limit 1)
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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if dbgname is not null then
for i in 1..array_length(bends, 1) loop
insert into wm_debug(stage, name, gen, nbend, way) values(
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'bbends', dbgname, dbggen, i, bends[i]);
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dbgpolygon = null;
if st_npoints(bends[i]) >= 3 then
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dbgpolygon = st_makepolygon(
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st_addpoint(bends[i],
st_startpoint(bends[i]))
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);
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end if;
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insert into wm_debug(stage, name, gen, nbend, way) values(
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'bbends-polygon', dbgname, dbggen, i, dbgpolygon);
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end loop;
end if;
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end
$$ language plpgsql;
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-- wm_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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-- cumulative inflection angle small (see variable below).
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--
-- The implementation could be significantly optimized to avoid `st_reverse`
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-- and array reversals, trading for complexity in wm_fix_gentle_inflections1.
drop function if exists wm_fix_gentle_inflections;
create function wm_fix_gentle_inflections(
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INOUT bends geometry[],
dbgname text default null,
dbggen integer default null
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) as $$
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declare
len int4;
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bends1 geometry[];
dbgpolygon geometry;
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begin
len = array_length(bends, 1);
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bends = wm_fix_gentle_inflections1(bends);
for i in 1..len loop
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bends1[i] = st_reverse(bends[len-i+1]);
end loop;
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bends1 = wm_fix_gentle_inflections1(bends1);
for i in 1..len loop
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bends[i] = st_reverse(bends1[len-i+1]);
end loop;
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if dbgname is not null then
for i in 1..array_length(bends, 1) loop
insert into wm_debug(stage, name, gen, nbend, way) values(
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'cinflections', dbgname, dbggen, i, bends[i]);
dbgpolygon = null;
if st_npoints(bends[i]) >= 3 then
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dbgpolygon = st_makepolygon(
st_addpoint(bends[i],
st_startpoint(bends[i]))
);
end if;
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insert into wm_debug(stage, name, gen, nbend, way) values(
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'cinflections-polygon', dbgname, dbggen, i, dbgpolygon);
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end loop;
end if;
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end
$$ language plpgsql;
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-- wm_fix_gentle_inflections1 fixes gentle inflections of an array of lines in
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-- one direction. An implementation detail of wm_fix_gentle_inflections.
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drop function if exists wm_fix_gentle_inflections1;
create function wm_fix_gentle_inflections1(INOUT bends geometry[]) as $$
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declare
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-- the threshold when the angle is still "small", so gentle inflections can
-- be joined
small_angle constant real default radians(45);
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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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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;
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-- wm_if_selfcross returns whether baseline of bendi crosses bendj.
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-- If it doesn't, returns a null geometry.
-- Otherwise, it will return the baseline split into a few parts where it
-- crosses bendj.
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drop function if exists wm_if_selfcross;
create function wm_if_selfcross(
bendi geometry,
bendj geometry
) returns geometry as $$
declare
a geometry;
b geometry;
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multi geometry;
begin
a = st_pointn(bendi, 1);
b = st_pointn(bendi, -1);
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multi = st_split(bendj, st_makeline(a, b));
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if st_numgeometries(multi) = 1 then
return null;
end if;
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if st_numgeometries(multi) = 2 and
(st_contains(bendj, a) or st_contains(bendj, b)) then
return null;
end if;
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return multi;
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end
$$ language plpgsql;
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-- wm_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 wm_self_crossing;
create function wm_self_crossing(
INOUT bends geometry[],
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dbgname text default null,
dbggen integer default null,
OUT mutated boolean
) as $$
declare
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i int4;
j int4;
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multi geometry;
begin
mutated = false;
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<<bendloop>>
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for i in 1..array_length(bends, 1) loop
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continue when abs(wm_inflection_angle(bends[i])) <= pi();
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-- sum of inflection angles for this bend is >180, so it may be
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-- self-crossing. Now try to find another bend in this line that
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-- crosses an imaginary line of end-vertices
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-- Go through each bend in the given line, and see if has a potential to
-- cross bends[i]. The line-cut process is different when i<j and i>j;
-- therefore there are two loops, one for each case.
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for j in 1..i-1 loop
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multi = wm_if_selfcross(bends[i], bends[j]);
continue when multi is null;
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mutated = true;
-- 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);
bends[j] = st_setpoint(
bends[j],
st_npoints(bends[j])-1,
st_pointn(bends[i], st_npoints(bends[i]))
);
bends = bends[1:j] || bends[i+1:];
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continue bendloop;
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end loop;
for j in reverse array_length(bends, 1)..i+1 loop
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multi = wm_if_selfcross(bends[i], bends[j]);
continue when multi is null;
mutated = true;
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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)
);
bends = bends[1:i] || bends[j+1:];
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continue bendloop;
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end loop;
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end loop;
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if dbgname is not null then
insert into wm_debug(stage, name, gen, nbend, way) values(
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'dcrossings', dbgname, dbggen, generate_subscripts(bends, 1),
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unnest(bends)
);
end if;
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end
$$ language plpgsql;
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drop function if exists wm_inflection_angle;
create function wm_inflection_angle (IN bend geometry, OUT angle real) as $$
declare
p0 geometry;
p1 geometry;
p2 geometry;
p3 geometry;
begin
angle = 0;
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for p0 in select geom from st_dumppoints(bend) order by path[1] asc loop
p3 = p2;
p2 = p1;
p1 = p0;
continue when p3 is null;
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angle = angle + abs(pi() - st_angle(p1, p2, p3));
end loop;
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end
$$ language plpgsql;
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drop function if exists wm_bend_attrs;
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drop function if exists wm_isolated_bends;
drop function if exists wm_elimination;
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drop function if exists wm_exaggeration;
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drop type if exists wm_t_bend_attrs;
create type wm_t_bend_attrs as (
bend geometry,
area real,
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adjsize real,
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baselinelength real,
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curvature real,
isolated boolean
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);
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create function wm_bend_attrs(
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bends geometry[],
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dbgname text default null,
dbggen integer default null
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) returns setof wm_t_bend_attrs as $$
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declare
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cmp float;
i int4;
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polygon geometry;
bend geometry;
res wm_t_bend_attrs;
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begin
for i in 1..array_length(bends, 1) loop
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bend = bends[i];
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res = null;
res.bend = bend;
res.adjsize = 0;
res.baselinelength = st_distance(st_startpoint(bend), st_endpoint(bend));
res.curvature = wm_inflection_angle(bend) / st_length(bend);
res.isolated = false;
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if st_numpoints(bend) >= 3 then
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res.adjsize = wm_adjsize(bend);
end if;
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if dbgname is not null then
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insert into wm_debug (stage, name, gen, nbend, way, props) values(
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'ebendattrs', dbgname, dbggen, i, bend,
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jsonb_build_object(
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'adjsize', res.adjsize,
'baselinelength', res.baselinelength,
'curvature', res.curvature
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)
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);
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end if;
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return next res;
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end loop;
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end;
$$ language plpgsql;
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-- sm_st_split a line by a point in a more robust way than st_split.
-- See https://trac.osgeo.org/postgis/ticket/2192
drop function if exists wm_st_split;
create function wm_st_split(
input geometry,
blade geometry
) returns geometry as $$
declare
type1 text;
type2 text;
begin
type1 = st_geometrytype(input);
type2 = st_geometrytype(blade);
if not (type1 = 'ST_LineString' and
type2 = 'ST_Point') then
raise 'Arguments must be LineString and Point, got: % and %', type1, type2;
end if;
return st_split(st_snap(input, blade, 0.00000001), blade);
end $$ language plpgsql;
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-- wm_exaggerate_bend exaggerates a given bend. Must be a simple linestring.
drop function if exists wm_exaggerate_bend;
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create function wm_exaggerate_bend(
INOUT bend geometry,
size float,
desired_size float
) as $$
declare
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scale constant float default 1.2; -- exaggeration enthusiasm
midpoint geometry; -- midpoint of the baseline
splitbend geometry; -- bend split across its half
bendm geometry; -- bend with coefficients to prolong the lines
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points geometry[];
begin
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if size = 0 then
raise 'invalid input: zero-area bend';
end if;
midpoint = st_lineinterpolatepoint(st_makeline(
st_pointn(bend, 1),
st_pointn(bend, -1)
), .5);
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while size < desired_size loop
splitbend = wm_st_split(bend, st_lineinterpolatepoint(bend, .5));
-- Convert bend to LINESTRINGM, where M is the fraction by how
-- much the point will be prolonged:
-- 1. draw a line between midpoint and the point on the bend.
-- 2. multiply the line length by M. Midpoint stays intact.
-- 3. the new set of lines form a new bend.
-- Uses linear interpolation; can be updated to gaussian or similar;
-- then interpolate manually instead of relying on st_addmeasure.
bendm = st_collect(
st_addmeasure(st_geometryn(splitbend, 1), 1, scale),
st_addmeasure(st_geometryn(splitbend, 2), scale, 1)
);
points = array((
select st_scale(
st_makepoint(st_x(geom), st_y(geom)),
st_makepoint(st_m(geom), st_m(geom)),
midpoint
)
from st_dumppoints(bendm)
order by path[1], path[2]
));
bend = st_setsrid(st_makeline(points), st_srid(bend));
size = wm_adjsize(bend);
end loop;
end
$$ language plpgsql;
-- wm_adjsize calculates adjusted size for a polygon. Can return 0.
drop function if exists wm_adjsize;
create function wm_adjsize(bend geometry, OUT adjsize float) as $$
declare
polygon geometry;
area float;
cmp float;
begin
adjsize = 0;
polygon = st_makepolygon(st_addpoint(bend, st_startpoint(bend)));
-- Compactness Index (cmp) is defined as "the ratio of the area of the
-- polygon over the circle whose circumference length is the same as the
-- length of the circumference of the polygon". I assume they meant the
-- area of the circle. So here goes:
-- 1. get polygon area P.
-- 2. get polygon perimeter = u. Pretend it's our circle's circumference.
-- 3. get A (area) of the circle from u: A = u^2/(4pi)
-- 4. divide P by A: cmp = P/A = P/(u^2/(4pi)) = 4pi*P/u^2
area = st_area(polygon);
cmp = 4*pi()*area/(st_perimeter(polygon)^2);
if cmp > 0 then
adjsize = (area*(0.75/cmp));
end if;
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end
$$ language plpgsql;
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-- wm_exaggeration is the Exaggeration Operator described in the WM paper.
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create function wm_exaggeration(
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INOUT bendattrs wm_t_bend_attrs[],
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dhalfcircle float,
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intersect_patience integer,
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dbgname text default null,
dbggen integer default null,
OUT mutated boolean
) as $$
declare
desired_size constant float default pi()*(dhalfcircle^2)/8;
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tmpbendattr wm_t_bend_attrs;
tmpint geometry;
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i integer;
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n integer;
last_id integer;
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begin
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mutated = false;
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<<bendloop>>
for i in 1..array_length(bendattrs, 1) loop
if bendattrs[i].isolated and bendattrs[i].adjsize < desired_size then
tmpbendattr.bend = wm_exaggerate_bend(
bendattrs[i].bend,
bendattrs[i].adjsize,
desired_size
);
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-- does tmpbendattrs.bend intersect with the previous or next
-- intersect_patience bends? If they do, abort exaggeration for this one.
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-- Do close-by bends intersect with this one? Special
-- handling first, because 2 vertices need to be removed before checking.
n = st_npoints(bendattrs[i-1].bend);
if n > 3 then
continue when st_intersects(tmpbendattr.bend,
st_removepoint(st_removepoint(bendattrs[i-1].bend, n-1), n-2));
end if;
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n = st_npoints(bendattrs[i+1].bend);
if n > 3 then
continue when st_intersects(tmpbendattr.bend,
st_removepoint(st_removepoint(bendattrs[i+1].bend, 0), 0));
end if;
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for n in -intersect_patience+1..intersect_patience-1 loop
continue when n in (-1, 0, 1);
continue when i+n < 1;
continue when i+n > array_length(bendattrs, 1);
-- More special handling: if the neigbhoring bend has 3 vertices, the
-- neighbor's neighbor may just touch the tmpbendattr.bend; in this
-- case, the nearest vertex should be removed before comparing.
tmpint = bendattrs[i+n].bend;
if st_npoints(tmpint) > 2 then
if n = -2 and st_npoints(bendattrs[i+n+1].bend) = 3 then
tmpint = st_removepoint(tmpint, st_npoints(tmpint)-1);
elsif n = 2 and st_npoints(bendattrs[i+n-1].bend) = 3 then
tmpint = st_removepoint(tmpint, 0);
end if;
end if;
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continue bendloop when st_intersects(tmpbendattr.bend, tmpint);
end loop;
-- No intersections within intersect_patience, mutate bend!
mutated = true;
bendattrs[i] = tmpbendattr;
-- remove last vertex of the previous bend and first vertex of the next
-- bend, because bends always share a line segment together this is
-- duplicated in a few places, because PostGIS does not allow (?)
-- mutating an array when passed to a function.
tmpbendattr.bend = st_removepoint(
bendattrs[i-1].bend,
st_npoints(bendattrs[i-1].bend)-1
);
bendattrs[i-1] = tmpbendattr;
tmpbendattr.bend = st_removepoint(bendattrs[i+1].bend, 0);
bendattrs[i+1] = tmpbendattr;
if dbgname is not null then
insert into wm_debug (stage, name, gen, nbend, way) values(
'gexaggeration', dbgname, dbggen, i, bendattrs[i].bend
);
end if;
end if;
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end loop;
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end $$ language plpgsql;
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create function wm_elimination(
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INOUT bendattrs wm_t_bend_attrs[],
dhalfcircle float,
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dbgname text default null,
dbggen integer default null,
OUT mutated boolean
) as $$
declare
desired_size constant float default pi()*(dhalfcircle^2)/8;
leftsize float;
rightsize float;
i int4;
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j int4;
tmpbendattr wm_t_bend_attrs;
dbgbends geometry[];
begin
mutated = false;
i = 1;
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while i < array_length(bendattrs, 1)-1 loop
i = i + 1;
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continue when bendattrs[i].adjsize = 0;
continue when bendattrs[i].adjsize > desired_size;
if i = 2 then
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leftsize = bendattrs[i].adjsize + 1;
else
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leftsize = bendattrs[i-1].adjsize;
end if;
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if i = array_length(bendattrs, 1)-1 then
rightsize = bendattrs[i].adjsize + 1;
else
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rightsize = bendattrs[i+1].adjsize;
end if;
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continue when bendattrs[i].adjsize >= leftsize;
continue when bendattrs[i].adjsize >= rightsize;
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-- Local minimum. Elminate bend!
mutated = true;
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tmpbendattr.bend = st_makeline(
st_pointn(bendattrs[i].bend, 1),
st_pointn(bendattrs[i].bend, -1)
);
bendattrs[i] = tmpbendattr;
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-- remove last vertex of the previous bend and
-- first vertex of the next bend, because bends always
-- share a line segment together
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tmpbendattr.bend = st_removepoint(
bendattrs[i-1].bend,
st_npoints(bendattrs[i-1].bend)-1
);
bendattrs[i-1] = tmpbendattr;
tmpbendattr.bend = st_removepoint(bendattrs[i+1].bend, 0);
bendattrs[i+1] = tmpbendattr;
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-- the next bend's adjsize is now messed up; it should not be taken
-- into consideration for other local minimas. Skip over 2.
i = i + 2;
end loop;
if dbgname is not null then
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for j in 1..array_length(bendattrs, 1) loop
dbgbends[j] = bendattrs[j].bend;
end loop;
insert into wm_debug(stage, name, gen, nbend, way) values(
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'helimination',
dbgname,
dbggen,
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generate_subscripts(dbgbends, 1),
unnest(dbgbends)
);
end if;
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end
$$ language plpgsql;
create function wm_isolated_bends(
INOUT bendattrs wm_t_bend_attrs[],
dbgname text default null,
dbggen integer default null
) as $$
declare
-- if neighbor's curvatures are within this fraction of the current bend
isolation_threshold constant real default 0.5;
this real;
skip_next bool;
res wm_t_bend_attrs;
i int4;
last_id integer;
begin
for i in 1..array_length(bendattrs, 1) loop
if dbgname is not null then
insert into wm_debug (stage, name, gen, nbend, way, props) values(
'fisolated_bends', dbgname, dbggen, i, bendattrs[i].bend,
jsonb_build_object('isolated', false)
) returning id into last_id;
end if;
-- first and last bends cannot be isolated
if i = 1 or i = array_length(bendattrs, 1) then
continue;
end if;
res = bendattrs[i];
if skip_next then
skip_next = false;
else
this = bendattrs[i].curvature * isolation_threshold;
if bendattrs[i-1].curvature < this and
bendattrs[i+1].curvature < this then
res.isolated = true;
bendattrs[i] = res;
skip_next = true;
if dbgname is not null then
update wm_debug
set props=props || jsonb_build_object('isolated', true)
where id=last_id;
end if;
end if;
end if;
end loop;
end
$$ language plpgsql;
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drop function if exists ST_SimplifyWM_Estimate;
create function ST_SimplifyWM_Estimate(
geom geometry,
OUT npoints bigint,
OUT secs bigint
) as $$
declare
lines geometry[];
l_type text;
begin
l_type = st_geometrytype(geom);
if l_type = 'ST_LineString' then
lines = array[geom];
elseif l_type = 'ST_MultiLineString' then
lines = array((select a.geom from st_dump(geom) a order by path[1] asc));
else
raise 'Unknown geometry type %', l_type;
end if;
npoints = 0;
for i in 1..array_length(lines, 1) loop
npoints = npoints + st_numpoints(lines[i]);
end loop;
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secs = npoints / 33;
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end
$$ 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.
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-- Input parameters:
-- - geom: ST_LineString or ST_MultiLineString: the geometry to be simplified
-- - dhalfcircle: the diameter of a half-circle, whose area is an approximate
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-- threshold for small bend elimination. If bend's area is larger than that,
-- the bend will be left alone.
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drop function if exists ST_SimplifyWM;
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create function ST_SimplifyWM(
geom geometry,
dhalfcircle float,
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intersect_patience integer default 10,
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dbgname text default null
) returns geometry as $$
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declare
gen integer;
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i integer;
j integer;
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line geometry;
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lines geometry[];
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bends geometry[];
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bendattrs wm_t_bend_attrs[];
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mutated boolean;
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l_type text;
begin
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if intersect_patience is null then
intersect_patience = 10;
end if;
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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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<<lineloop>>
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for i in 1..array_length(lines, 1) loop
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mutated = true;
gen = 1;
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while mutated loop
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if dbgname is not null then
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insert into wm_debug (stage, name, gen, nbend, way) values(
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'afigures', dbgname, gen, i, lines[i]);
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end if;
bends = wm_detect_bends(lines[i], dbgname, gen);
bends = wm_fix_gentle_inflections(bends, dbgname, gen);
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select * from wm_self_crossing(bends, dbgname, gen) into bends, mutated;
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if mutated then
lines[i] = st_linemerge(st_union(bends));
gen = gen + 1;
continue;
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end if;
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bendattrs = array((select wm_bend_attrs(bends, dbgname, gen)));
bendattrs = wm_isolated_bends(bendattrs, dbgname, gen);
select * from wm_exaggeration(
bendattrs, dhalfcircle, intersect_patience, dbgname, gen
) into bendattrs, mutated;
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-- TODO: wm_combination
if not mutated then
select * from wm_elimination(
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bendattrs, dhalfcircle, dbgname, gen) into bendattrs, mutated;
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end if;
if mutated then
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for j in 1..array_length(bendattrs, 1) loop
bends[j] = bendattrs[j].bend;
end loop;
lines[i] = st_linemerge(st_union(bends));
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if st_geometrytype(lines[i]) != 'ST_LineString' then
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-- For manual debugging:
--insert into wm_manual(name, way)
--select 'non-linestring-' || a.path[1], a.geom
--from st_dump(lines[i]) a
--order by a.path[1];
raise 'Got % (in %) instead of ST_LineString. '
'Does the exaggerated bend intersect with the line? '
'If so, try increasing intersect_patience.',
st_geometrytype(lines[i]), dbgname;
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--exit lineloop;
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end if;
gen = gen + 1;
continue;
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end if;
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end loop;
end loop;
if l_type = 'ST_LineString' then
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return st_linemerge(st_union(lines));
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elseif l_type = 'ST_MultiLineString' then
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return st_union(lines);
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end if;
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end
$$ language plpgsql;