Mapgen v6: Various mudflow code improvements and bugfixes (#8805)

Calculate 'i' inside 'y' loop to avoid 'i' and 'y' becoming
out of step due to the uses of 'continue'.
Simplify calculation of 'p2d' when coordinates are inverted.
Remove some unnecessary and unreliable 'index out of
voxelmanip' checks.
Move calculation of '&em' out of loops.
For 'Loop further down until not air' code, use y coordinate
to detect being out of voxelmanip, instead of checking index
which did not detect that.
Add and improve comments.
Indent the 'for (;; y--) {' loop.
Improve format and fix codestyle issues.

1 files changed, 76 insertions, 91 deletions

diff --git a/src/mapgen/mapgen_v6.cpp b/src/mapgen/mapgen_v6.cpp
index 28ca8faa2..4e876fc53 100644
--- a/src/mapgen/mapgen_v6.cpp
+++ b/src/mapgen/mapgen_v6.cpp
@@ -538,7 +538,7 @@ void MapgenV6::makeChunk(BlockMakeData *data)
 	updateHeightmap(node_min, node_max);
 
 	const s16 max_spread_amount = MAP_BLOCKSIZE;
-	// Limit dirt flow area by 1 because mud is flown into neighbors.
+	// Limit dirt flow area by 1 because mud is flowed into neighbors
 	s16 mudflow_minpos = -max_spread_amount + 1;
 	s16 mudflow_maxpos = central_area_size.X + max_spread_amount - 2;
 
@@ -773,6 +773,14 @@ void MapgenV6::addMud()
 
 void MapgenV6::flowMud(s16 &mudflow_minpos, s16 &mudflow_maxpos)
 {
+	const v3s16 &em = vm->m_area.getExtent();
+	static const v3s16 dirs4[4] = {
+		v3s16(0, 0, 1), // Back
+		v3s16(1, 0, 0), // Right
+		v3s16(0, 0, -1), // Front
+		v3s16(-1, 0, 0), // Left
+	};
+	
 	// Iterate twice
 	for (s16 k = 0; k < 2; k++) {
 		for (s16 z = mudflow_minpos; z <= mudflow_maxpos; z++)
@@ -781,120 +789,97 @@ void MapgenV6::flowMud(s16 &mudflow_minpos, s16 &mudflow_maxpos)
 			v2s16 p2d;
 			// Invert coordinates on second iteration to process columns in
 			// opposite order, to avoid a directional bias.
-			if (k == 1) {
-				p2d = v2s16(node_min.X, node_min.Z) + v2s16(
-					mudflow_maxpos - (x - mudflow_minpos),
-					mudflow_maxpos - (z - mudflow_minpos));
-			} else {
+			if (k == 1)
+				p2d = v2s16(node_max.X, node_max.Z) - v2s16(x, z);
+			else
 				p2d = v2s16(node_min.X, node_min.Z) + v2s16(x, z);
-			}
 
-			const v3s16 &em = vm->m_area.getExtent();
-			u32 i = vm->m_area.index(p2d.X, node_max.Y, p2d.Y);
 			s16 y = node_max.Y;
 
 			while (y >= node_min.Y) {
+				for (;; y--) {
+					u32 i = vm->m_area.index(p2d.X, y, p2d.Y);
+					MapNode *n = nullptr;
+
+					// Find next mud node in mapchunk column
+					for (; y >= node_min.Y; y--) {
+						n = &vm->m_data[i];
+						if (n->getContent() == c_dirt ||
+								n->getContent() == c_dirt_with_grass ||
+								n->getContent() == c_gravel)
+							break;
 
-			for (;; y--) {
-				MapNode *n = NULL;
-				// Find mud
-				for (; y >= node_min.Y; y--) {
-					n = &vm->m_data[i];
-					if (n->getContent() == c_dirt ||
-							n->getContent() == c_dirt_with_grass ||
-							n->getContent() == c_gravel)
+						VoxelArea::add_y(em, i, -1);
+					}
+					if (y < node_min.Y)
+						// No mud found in mapchunk column, process the next column
 						break;
 
-					VoxelArea::add_y(em, i, -1);
-				}
-
-				// Stop if out of area
-				//if(vmanip.m_area.contains(i) == false)
-				if (y < node_min.Y)
-					break;
-
-				if (n->getContent() == c_dirt ||
-						n->getContent() == c_dirt_with_grass) {
-					// Make it exactly mud
-					n->setContent(c_dirt);
-
-					// Don't flow it if the stuff under it is not mud
-					{
+					if (n->getContent() == c_dirt || n->getContent() == c_dirt_with_grass) {
+						// Convert dirt_with_grass to dirt
+						n->setContent(c_dirt);
+						// Don't flow mud if the stuff under it is not mud,
+						// to leave at least 1 node of mud.
 						u32 i2 = i;
 						VoxelArea::add_y(em, i2, -1);
-						// Cancel if out of area
-						if (!vm->m_area.contains(i2))
-							continue;
 						MapNode *n2 = &vm->m_data[i2];
 						if (n2->getContent() != c_dirt &&
 								n2->getContent() != c_dirt_with_grass)
+							// Find next mud node in column
 							continue;
 					}
-				}
 
-				static const v3s16 dirs4[4] = {
-					v3s16(0, 0, 1), // back
-					v3s16(1, 0, 0), // right
-					v3s16(0, 0, -1), // front
-					v3s16(-1, 0, 0), // left
-				};
-
-				// Check that upper is walkable. Cancel
-				// dropping if upper keeps it in place.
-				u32 i3 = i;
-				VoxelArea::add_y(em, i3, 1);
-				MapNode *n3 = NULL;
-
-				if (vm->m_area.contains(i3)) {
-					n3 = &vm->m_data[i3];
+					// Check if node above is walkable. If so, cancel
+					// flowing as if node above keeps it in place.
+					u32 i3 = i;
+					VoxelArea::add_y(em, i3, 1);
+					MapNode *n3 = &vm->m_data[i3];
 					if (ndef->get(*n3).walkable)
+						// Find next mud node in column
 						continue;
-				}
 
-				// Drop mud on side
-				for (const v3s16 &dirp : dirs4) {
-					u32 i2 = i;
-					// Move to side
-					VoxelArea::add_p(em, i2, dirp);
-					// Fail if out of area
-					if (!vm->m_area.contains(i2))
-						continue;
-					// Check that side is air
-					MapNode *n2 = &vm->m_data[i2];
-					if (ndef->get(*n2).walkable)
-						continue;
-					// Check that under side is air
-					VoxelArea::add_y(em, i2, -1);
-					if (!vm->m_area.contains(i2))
-						continue;
-					n2 = &vm->m_data[i2];
-					if (ndef->get(*n2).walkable)
-						continue;
-					// Loop further down until not air
-					bool dropped_to_unknown = false;
-					do {
+					// Drop mud on one side
+					for (const v3s16 &dirp : dirs4) {
+						u32 i2 = i;
+						// Move to side
+						VoxelArea::add_p(em, i2, dirp);
+						// Check that side is air
+						MapNode *n2 = &vm->m_data[i2];
+						if (ndef->get(*n2).walkable)
+							continue;
+
+						// Check that under side is air
 						VoxelArea::add_y(em, i2, -1);
 						n2 = &vm->m_data[i2];
-						// if out of known area
-						if (!vm->m_area.contains(i2) ||
-								n2->getContent() == CONTENT_IGNORE) {
-							dropped_to_unknown = true;
-							break;
-						}
-					} while (!ndef->get(*n2).walkable);
-					// Loop one up so that we're in air
-					VoxelArea::add_y(em, i2, 1);
-
-					// Move mud to new place. Outside mapchunk remove
-					// any decorations above removed or placed mud.
-					if (!dropped_to_unknown)
-						moveMud(i, i2, i3, p2d, em);
+						if (ndef->get(*n2).walkable)
+							continue;
 
-					// Done
-					break;
+						// Loop further down until not air
+						s16 y2 = y - 1; // y of i2
+						bool dropped_to_unknown = false;
+						do {
+							y2--;
+							VoxelArea::add_y(em, i2, -1);
+							n2 = &vm->m_data[i2];
+							// If out of area or in ungenerated world
+							if (y2 < full_node_min.Y || n2->getContent() == CONTENT_IGNORE) {
+								dropped_to_unknown = true;
+								break;
+							}
+						} while (!ndef->get(*n2).walkable);
+
+						if (!dropped_to_unknown) {
+							// Move up one so that we're in air
+							VoxelArea::add_y(em, i2, 1);
+							// Move mud to new place, and if outside mapchunk remove
+							// any decorations above removed or placed mud.
+							moveMud(i, i2, i3, p2d, em);
+						}
+						// Done, find next mud node in column
+						break;
+					}
 				}
 			}
-			}
 		}
 	}
 }