Implement --exhaustive y mode as another database access optimization

This one works best when you have a wide area with low height (e.g. 10000x200x10000)

README.rst

@@ -108,5 +108,6 @@ scales:
     Draw scales on specified image edges (letters *t b l r* meaning top, bottom, left and right), e.g. ``--scales tbr``
 
 exhaustive:
-    Select if database should be traversed exhaustively or using range queries, available: *never*, *y*, *full*, *auto*
+    | Select if database should be traversed exhaustively or using range queries, available: *never*, *y*, *full*, *auto*
-    Defaults to *auto*. You shouldn't need to change this, but doing so can improve rendering times on large maps.
+    | Defaults to *auto*. You shouldn't need to change this, but doing so can improve rendering times on large maps.
+    | For these optimizations to work it is important that you set ``min-y`` and ``max-y`` when you don't care about the world below e.g. -60 and above 1000 nodes.

TileGenerator.cpp

@@ -353,7 +353,7 @@ void TileGenerator::openDb(const std::string &input)
 			m_exhaustiveSearch = EXH_NEVER;
 		else if (blocks < 200000)
 			m_exhaustiveSearch = EXH_FULL;
-		else if (y_range < 600)
+		else if (y_range < 42)
 			m_exhaustiveSearch = EXH_Y;
 		else
 			m_exhaustiveSearch = EXH_NEVER;
@@ -364,8 +364,6 @@ void TileGenerator::openDb(const std::string &input)
 				" in worse performance." << std::endl;
 		}
 	}
-	if (m_exhaustiveSearch == EXH_Y)
-		m_exhaustiveSearch = EXH_NEVER; // (TODO remove when implemented)
 	assert(m_exhaustiveSearch != EXH_AUTO);
 }
 
@@ -511,6 +509,30 @@ void TileGenerator::renderMap()
 			}
 			postRenderRow(zPos);
 		}
+	} else if (m_exhaustiveSearch == EXH_Y) {
+#ifndef NDEBUG
+		std::cerr << "Exhaustively searching height of "
+			<< (yMax - (m_yMin / 16)) << " blocks" << std::endl;
+#endif
+		std::vector<BlockPos> positions;
+		positions.reserve(yMax - (m_yMin / 16));
+		for (auto it = m_positions.rbegin(); it != m_positions.rend(); ++it) {
+			int16_t zPos = it->first;
+			for (auto it2 = it->second.rbegin(); it2 != it->second.rend(); ++it2) {
+				int16_t xPos = *it2;
+
+				positions.clear();
+				for (int16_t yPos = m_yMin / 16; yPos < yMax; yPos++)
+					positions.emplace_back(xPos, yPos, zPos);
+
+				BlockList blockStack;
+				m_db->getBlocksByPos(blockStack, positions);
+				blockStack.sort();
+
+				renderSingle(xPos, zPos, blockStack);
+			}
+			postRenderRow(zPos);
+		}
 	} else if (m_exhaustiveSearch == EXH_FULL) {
 #ifndef NDEBUG
 		std::cerr << "Exhaustively searching "

include/TileGenerator.h

@@ -25,7 +25,7 @@ enum {
 
 enum {
 	EXH_NEVER, // Always use range queries
-	EXH_Y,     // Exhaustively search Y space, range queries for X/Z (future TODO)
+	EXH_Y,     // Exhaustively search Y space, range queries for X/Z
 	EXH_FULL,  // Exhaustively search entire requested geometry
 	EXH_AUTO,  // Automatically pick one of the previous modes
 };

minetestmapper.6

@@ -97,6 +97,11 @@ Draw scales on specified image edges (letters \fIt b l r\fP meaning top, bottom,
 Select if database should be traversed exhaustively or using range queries, available: \fInever\fP, \fIy\fP, \fIfull\fP, \fIauto\fP
 
 Defaults to \fIauto\fP. You shouldn't need to change this, but doing so can improve rendering times on large maps.
+For these optimizations to work it is important that you set
+.B min-y
+and
+.B max-y
+when you don't care about the world below e.g. -60 and above 1000 nodes.
 
 .SH MORE INFORMATION
 Website: https://github.com/minetest/minetestmapper