From ea7216641b25c8a2168ab970ae16efc1320cb04c Mon Sep 17 00:00:00 2001
From: Eric Fischer <enf@pobox.com>
Date: Mon, 27 Oct 2014 16:00:16 -0700
Subject: [PATCH] If the requested tile doesn't exist, try climbing up to a
 lower zoom

---
 decode.cc | 50 ++++++++++++++++++++++++++++++++------------------
 1 file changed, 32 insertions(+), 18 deletions(-)

diff --git a/decode.cc b/decode.cc
index 5fc539d..5a57d90 100644
--- a/decode.cc
+++ b/decode.cc
@@ -105,32 +105,46 @@ void handle(std::string message, int z, unsigned x, unsigned y) {
 
 void decode(char *fname, int z, unsigned x, unsigned y) {
 	sqlite3 *db;
+	int oz = z;
+	unsigned ox = x, oy = y;
 
 	if (sqlite3_open(fname, &db) != SQLITE_OK) {
 		fprintf(stderr, "%s: %s\n", fname,  sqlite3_errmsg(db));
 		exit(EXIT_FAILURE);
 	}
 
-	const char *sql = "SELECT tile_data from tiles where zoom_level = ? and tile_column = ? and tile_row = ?;";
-	sqlite3_stmt *stmt;
-	if (sqlite3_prepare_v2(db, sql, -1, &stmt, NULL) != SQLITE_OK) {
-		fprintf(stderr, "%s: select failed: %s\n", fname, sqlite3_errmsg(db));
-		exit(EXIT_FAILURE);
+	int handled = 0;
+	while (z >= 0 && !handled) {
+		const char *sql = "SELECT tile_data from tiles where zoom_level = ? and tile_column = ? and tile_row = ?;";
+		sqlite3_stmt *stmt;
+		if (sqlite3_prepare_v2(db, sql, -1, &stmt, NULL) != SQLITE_OK) {
+			fprintf(stderr, "%s: select failed: %s\n", fname, sqlite3_errmsg(db));
+			exit(EXIT_FAILURE);
+		}
+
+		sqlite3_bind_int(stmt, 1, z);
+		sqlite3_bind_int(stmt, 2, x);
+		sqlite3_bind_int(stmt, 3, (1LL << z) - 1 - y);
+
+		while (sqlite3_step(stmt) == SQLITE_ROW) {
+			int len = sqlite3_column_bytes(stmt, 0);
+			const char *s = (const char *) sqlite3_column_blob(stmt, 0);
+
+			if (z != oz) {
+				fprintf(stderr, "%s: Warning: using tile %d/%u/%u instead of %d/%u/%u\n", fname, z, x, y, oz, ox, oy);
+			}
+
+			handle(std::string(s, len), z, x, y);
+			handled = 1;
+		}
+
+		sqlite3_finalize(stmt);
+
+		z--;
+		x /= 2;
+		y /= 2;
 	}
 
-	sqlite3_bind_int(stmt, 1, z);
-	sqlite3_bind_int(stmt, 2, x);
-	sqlite3_bind_int(stmt, 3, (1LL << z) - 1 - y);
-
-	while (sqlite3_step(stmt) == SQLITE_ROW) {
-		int len = sqlite3_column_bytes(stmt, 0);
-		const char *s = (const char *) sqlite3_column_blob(stmt, 0);
-
-		handle(std::string(s, len), z, x, y);
-	}
-
-	sqlite3_finalize(stmt);
-
         if (sqlite3_close(db) != SQLITE_OK) {
                 fprintf(stderr, "%s: could not close database: %s\n", fname, sqlite3_errmsg(db));
                 exit(EXIT_FAILURE);