From 8c5681a58249639ec050b5431d393d8ed9565abe Mon Sep 17 00:00:00 2001
From: Eric Fischer <enf@pobox.com>
Date: Fri, 9 Oct 2015 16:45:34 -0700
Subject: [PATCH] Still not quite a round trip from GeoJSON to GeoJSON, but
 getting closer

---
 decode.cc | 74 ++++++++++++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 73 insertions(+), 1 deletion(-)

diff --git a/decode.cc b/decode.cc
index f082f57..692a4be 100644
--- a/decode.cc
+++ b/decode.cc
@@ -173,7 +173,7 @@ void handle(std::string message, int z, unsigned x, unsigned y) {
 						ops.push_back(draw(op, lon, lat));
 					}
 				} else {
-					ops.push_back(draw(op, 0, 27));
+					ops.push_back(draw(op, 0, 0));
 				}
 			}
 
@@ -227,7 +227,79 @@ void handle(std::string message, int z, unsigned x, unsigned y) {
 					printf(" ] ]");
 				}
 			} else if (feat.type() == VT_POLYGON) {
+				std::vector<std::vector<draw> > rings;
+				std::vector<double> areas;
 
+				for (unsigned i = 0; i < ops.size(); i++) {
+					if (ops[i].op == VT_MOVETO) {
+						rings.push_back(std::vector<draw>());
+						areas.push_back(0);
+					}
+
+					int n = rings.size() - 1;
+					if (n >= 0) {
+						rings[n].push_back(ops[i]);
+					}
+				}
+
+				int outer = 0;
+
+				for (unsigned i = 0; i < rings.size(); i++) {
+					double area = 0;
+					for (unsigned k = 0; k < rings[i].size(); k++) {
+						if (rings[i][k].op != VT_CLOSEPATH) {
+							area += rings[i][k].lon * rings[i][(k + 1) % rings[i].size()].lat;
+							area -= rings[i][k].lat * rings[i][(k + 1) % rings[i].size()].lon;
+						}
+					}
+
+					areas[i] = area;
+					if (areas[i] <= 0) {
+						outer++;
+					}
+
+					// printf("area %f\n", area / .00000274 / .00000274);
+				}
+
+				if (outer > 1) {
+					printf("\"type\": \"MultiPolygon\", \"coordinates\": [ [ [ ");
+				} else {
+					printf("\"type\": \"Polygon\", \"coordinates\": [ [ ");
+				}
+
+				int state = 0;
+				for (unsigned i = 0; i < rings.size(); i++) {
+					if (areas[i] <= 0) {
+						if (state != 0) {
+							// new multipolygon
+							printf(" ] ], [ [ ");
+						}
+						state = 1;
+					}
+
+					if (state == 2) {
+						// new ring in the same polygon
+						printf(" ], [ ");
+					}
+
+					for (unsigned j = 0; j < rings[i].size(); j++) {
+						if (rings[i][j].op != VT_CLOSEPATH) {
+							if (j != 0) {
+								printf(", ");
+							}
+
+							printf("[ %f, %f ]", rings[i][j].lon, rings[i][j].lat);
+						}
+					}
+
+					state = 2;
+				}
+
+				if (outer > 1) {
+					printf(" ] ] ]");
+				} else {
+					printf(" ] ]");
+				}
 			}
 
 			printf(" } }\n");