From 4cb56a6b11ee0d0769b1672943e60ea98a613827 Mon Sep 17 00:00:00 2001
From: Eric Fischer <enf@pobox.com>
Date: Mon, 27 Oct 2014 12:56:51 -0700
Subject: [PATCH] The indexing side of buffering features at the edges of tiles

---
 geojson.c | 42 ++++++++++++++++++++++++++++++++----------
 tile.cc   |  4 ++++
 tile.h    |  1 +
 3 files changed, 37 insertions(+), 10 deletions(-)

diff --git a/geojson.c b/geojson.c
index a2f63dd9..3fd22e7e 100644
--- a/geojson.c
+++ b/geojson.c
@@ -342,6 +342,7 @@ void read_json(FILE *f, char *fname, char *layername, int maxzoom, int minzoom,
 			}
 
 			int z = maxzoom;
+			int buffer = 10;
 
 			unsigned cx = bbox[0] / 2 + bbox[2] / 2;
 			unsigned cy = bbox[1] / 2 + bbox[3] / 2;
@@ -353,18 +354,39 @@ void read_json(FILE *f, char *fname, char *layername, int maxzoom, int minzoom,
 				ix.fpos = start;
 				fwrite_check(&ix, sizeof(struct index), 1, indexfile, fname, jp);
 			} else {
-				unsigned x, y;
-				for (x = bbox[0] >> (32 - z); x <= bbox[2] >> (32 - z); x++) {
-					for (y = bbox[1] >> (32 - z); y <= bbox[3] >> (32 - z); y++) {
-						struct index ix;
+				for (z = maxzoom; z >= 1; z--) {
+					unsigned x, y;
+					for (x = (bbox[0] - (buffer << (32 - z - 8))) >> (32 - z); x <= (bbox[2] + (buffer << (32 - z - 8))) >> (32 - z); x++) {
+						for (y = (bbox[1] - (buffer << (32 - z - 8))) >> (32 - z); y <= (bbox[3] + (buffer << (32 - z - 8))) >> (32 - z); y++) {
+							if (z != maxzoom) {
+								// There must be a clearer way to write this, but the intent is
+								// not to add an additional index for a low-zoom tile
+								// if one of its children was already part of the
+								// buffered bounding box for the child's zoom.
 
-						if (x == cx >> (32 - z) && y == cy >> (32 - z)) {
-							ix.index = encode(cx, cy);
-						} else {
-							ix.index = encode(x << (32 - z), y << (32 - z));
+								// So we are comparing this tile's x and y to the edges of the
+								// bounding box at the next zoom down, but divided by two
+								// to get it back into this zoom's tile coordinate scheme
+
+								if ((x >= ((bbox[0] - (buffer << (32 - (z + 1) - 8))) >> (32 - (z + 1)) >> 1)) &&
+							            (x <= ((bbox[2] + (buffer << (32 - (z + 1) - 8))) >> (32 - (z + 1)) >> 1)) &&
+								    (y >= ((bbox[1] - (buffer << (32 - (z + 1) - 8))) >> (32 - (z + 1)) >> 1)) &&
+								    (y <= ((bbox[3] + (buffer << (32 - (z + 1) - 8))) >> (32 - (z + 1)) >> 1))) {
+									continue;
+								}
+							}
+
+							struct index ix;
+
+							if (x == cx >> (32 - z) && y == cy >> (32 - z)) {
+								ix.index = encode(cx, cy);
+							} else {
+								ix.index = encode(x << (32 - z), y << (32 - z));
+							}
+							ix.fpos = start;
+							ix.maxzoom = z;
+							fwrite_check(&ix, sizeof(struct index), 1, indexfile, fname, jp);
 						}
-						ix.fpos = start;
-						fwrite_check(&ix, sizeof(struct index), 1, indexfile, fname, jp);
 					}
 				}
 			}
diff --git a/tile.cc b/tile.cc
index 41dff855..ae4023ea 100644
--- a/tile.cc
+++ b/tile.cc
@@ -366,6 +366,10 @@ long long write_tile(struct index *start, struct index *end, char *metabase, uns
 
 		struct index *i;
 		for (i = start; i < end; i++) {
+			if (z > i->maxzoom) {
+				continue;
+			}
+
 			struct pool_val *pv = pool_long_long(&dup, &i->fpos, 0);
 			if (pv->n == 0) {
 				continue;
diff --git a/tile.h b/tile.h
index cb4a5be6..f1d7d736 100644
--- a/tile.h
+++ b/tile.h
@@ -20,6 +20,7 @@ struct pool_val *deserialize_string(char **f, struct pool *p, int type);
 struct index {
         unsigned long long index;
         long long fpos;
+	int maxzoom;
 };
 
 long long write_tile(struct index *start, struct index *end, char *metabase, unsigned *file_bbox, int z, unsigned x, unsigned y, int detail, int basezoom, struct pool *file_keys, char *layername, sqlite3 *outdb, double droprate);