From 79d72839fe75446bbeb7292a339d0129c157c51e Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Motiejus=20Jak=C5=A1tys?= <motiejus@uber.com>
Date: Thu, 4 Mar 2021 17:59:26 +0200
Subject: [PATCH] wip self-crossing

---
 IV/notes.txt |  4 ++--
 IV/tests.sql |  1 +
 IV/wm.sql    | 24 +++++++++++++++++++-----
 3 files changed, 22 insertions(+), 7 deletions(-)

diff --git a/IV/notes.txt b/IV/notes.txt
index 61c8b2c..90f5a39 100644
--- a/IV/notes.txt
+++ b/IV/notes.txt
@@ -12,13 +12,13 @@ after a few bends have been skipped. E.g. ends of A<->B cross the line, but
 
    ,______
   /       \
-  |___A   |    \  |
+  |___A   |    \  \
       \   |    B\ |             __
        \  |     | |            /  \
        /  |     | |___,---,___/A  |
       /   |     \_________________|
       \   |
-       \  |    \  |
+       \  |    \  \
        /  /    B\ |     _       __
   ----/  /      | |    / \     /  \
  / ,____/       | |___/   \___/A  |
diff --git a/IV/tests.sql b/IV/tests.sql
index 5ab4143..228e2fc 100644
--- a/IV/tests.sql
+++ b/IV/tests.sql
@@ -18,6 +18,7 @@ insert into figures (name, way) values ('fig3',ST_GeomFromText('LINESTRING(0 0,1
 insert into figures (name, way) values ('fig3-1',ST_GeomFromText('LINESTRING(0 0,12 0,13 4,20 2,20 0,32 0,33 10,38 16,43 15,44 10,44 0)'));
 insert into figures (name, way) values ('fig5',ST_GeomFromText('LINESTRING(0 39,19 52,27 77,26 104,41 115,49 115,65 103,65 75,53 45,63 15,91 0)'));
 insert into figures (name, way) values ('fig6',ST_GeomFromText('LINESTRING(84 47,91 59,114 64,122 80,116 92,110 93,106 106,117 118,136 107,135 76,120 45,125 39,141 39,147 32)'));
+insert into figures (name, way) values ('fig6-rev',st_reverse(st_translate((select way from figures where name='fig6'), 80, 0)));
 insert into figures (name, way) values ('inflection-1',ST_GeomFromText('LINESTRING(110 24,114 20,133 20,145 15,145 0,136 5,123 7,114 7,111 2)'));
 
 -- DETECT BENDS
diff --git a/IV/wm.sql b/IV/wm.sql
index 4379b5f..b115070 100644
--- a/IV/wm.sql
+++ b/IV/wm.sql
@@ -175,6 +175,8 @@ declare
   s2 real;
   s3 real;
   bend geometry;
+  this geometry;
+  multi geometry;
 begin
   pi = radians(180);
 
@@ -199,7 +201,6 @@ begin
     -- now try to find another bend in this line that crosses this one.
     p0 = st_pointn(bends[i], 1);
     p1 = st_pointn(bends[i], -1);
-    --this = st_makeline(st_pointn(bends[i], 1), st_pointn(bends[i], -1));
 
     -- go through each bend in this line, and see if has a potential to cross bends[i].
     -- optimization: we care only about bends which beginning and end start at different
@@ -210,14 +211,27 @@ begin
       p2 = st_pointn(bends[j], 1);
       p3 = st_pointn(bends[j], -1);
 
-      -- https://stackoverflow.com/questions/1560492
+      -- are p2 and p3 on the same side of (p0,p1)? vector multiplication
+      -- https://stackoverflow.com/questions/1560492/
       s2 = (st_x(p0)-st_x(p1)*(st_y(p2)-st_y(p1))-(st_y(p0)-st_y(p1))*(st_x(p2)-st_x(p1)));
       s3 = (st_x(p0)-st_x(p1)*(st_y(p3)-st_y(p1))-(st_y(p0)-st_y(p1))*(st_x(p3)-st_x(p1)));
       continue when sign(s2) = sign(s3);
 
-      -- bend j may be crossing bend i, and it has a chance to be "important" --
-      -- p2 and p3 are in different sides of the plane as delimited by p0 and p1.
-      -- now does it really cross the line (p0, p1)?
+      -- do end vertices of bend[i] cross bend[j]?
+      this = st_makeline(st_pointn(bends[i], 1), st_pointn(bends[i], -1));
+      multi = st_split(bends[j], this);
+      continue when st_numgeometries(multi) = 1;
+
+      -- self-crossing detected!
+      -- if j < i:
+      --   bends[j] = multi[1][1...n-1]; that will have all the vertices of bends[j],
+      --     except the crossing itself.
+      --   bends[j] = append(bends[j], bends[i][-1])
+      --   remove bends from bends[j+1] to bends[i] inclusive.
+      -- elif j > i:
+      --   remove bends from bends[i+1] to bends[j-1] inclusive.
+      --   bends[j] = multi[2][2..n]
+      --   bends[i]
 
     end loop;