Sheet Ore: Eliminate crash caused by PcgRandom range max < min

In the calculation of y_start,
when 'column height max' was large it caused
nmin.Y + max_height > nmax.Y - max_height
Now, in this situation y_start is set to the
midpoint between nmin.Y and nmax.Y

Limit y0 and y1 to between nmin.Y and nmax.Y,
otherwise index calculation, which has no checks for limits,
places them at unwanted locations in the voxelmanip

1 files changed, 9 insertions, 4 deletions

diff --git a/src/mg_ore.cpp b/src/mg_ore.cpp
index 6b6e0d7a7..257901614 100644
--- a/src/mg_ore.cpp
+++ b/src/mg_ore.cpp
@@ -179,7 +179,12 @@ void OreSheet::generate(MMVManip *vm, int mapseed, u32 blockseed,
 	MapNode n_ore(c_ore, 0, ore_param2);
 
 	u16 max_height = column_height_max;
-	int y_start = pr.range(nmin.Y + max_height, nmax.Y - max_height);
+	int y_start_min = nmin.Y + max_height;
+	int y_start_max = nmax.Y - max_height;
+
+	int y_start = y_start_min < y_start_max ?
+		pr.range(y_start_min, y_start_max) :
+		(y_start_min + y_start_max) / 2;
 
 	if (!noise) {
 		int sx = nmax.X - nmin.X + 1;
@@ -204,10 +209,10 @@ void OreSheet::generate(MMVManip *vm, int mapseed, u32 blockseed,
 
 		u16 height = pr.range(column_height_min, column_height_max);
 		int ymidpoint = y_start + noiseval;
-		int y0 = ymidpoint - height * (1 - column_midpoint_factor);
-		int y1 = y0 + height;
+		int y0 = MYMAX(nmin.Y, ymidpoint - height * (1 - column_midpoint_factor));
+		int y1 = MYMIN(nmax.Y, y0 + height - 1);
 
-		for (int y = y0; y < y1; y++) {
+		for (int y = y0; y <= y1; y++) {
 			u32 i = vm->m_area.index(x, y, z);
 			if (!vm->m_area.contains(i))
 				continue;