From 40457fc0ddfcf586747a3069c79aa3ca28e91aad Mon Sep 17 00:00:00 2001
From: Kuhnovic <68320206+Kuhnovic@users.noreply.github.com>
Date: Fri, 8 Aug 2025 09:52:01 +0200
Subject: [PATCH] Change: Separate ships travelling in opposite direction
 (#14493)

---
 src/pathfinder/yapf/yapf_ship.cpp | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/src/pathfinder/yapf/yapf_ship.cpp b/src/pathfinder/yapf/yapf_ship.cpp
index a47df5abb3..9698cc603f 100644
--- a/src/pathfinder/yapf/yapf_ship.cpp
+++ b/src/pathfinder/yapf/yapf_ship.cpp
@@ -355,6 +355,23 @@ public:
 		return 0;
 	}
 
+	/**
+	 * Whether the provided direction is a preferred direction for a given tile. This is used to separate ships travelling in opposite directions.
+	 * @param tile Tile of current node.
+	 * @param td Trackdir of current node.
+	 * @returns true if a preferred direction, false otherwise.
+	 */
+	inline static bool IsPreferredShipDirection(TileIndex tile, Trackdir td)
+	{
+		const bool odd_x = TileX(tile) & 1;
+		const bool odd_y = TileY(tile) & 1;
+		if (td == TRACKDIR_X_NE) return odd_y;
+		if (td == TRACKDIR_X_SW) return !odd_y;
+		if (td == TRACKDIR_Y_NW) return odd_x;
+		if (td == TRACKDIR_Y_SE) return !odd_x;
+		return (odd_x ^ odd_y) ^ HasBit(TRACKDIR_BIT_RIGHT_N | TRACKDIR_BIT_LEFT_S | TRACKDIR_BIT_UPPER_W | TRACKDIR_BIT_LOWER_E, td);
+	}
+
 	/**
 	 * Called by YAPF to calculate the cost from the origin to the given node.
 	 * Calculates only the cost of given node, adds it to the parent node cost
@@ -376,6 +393,9 @@ public:
 			c += count * 3 * YAPF_TILE_LENGTH;
 		}
 
+		/* Encourage separation between ships traveling in different directions. */
+		if (!IsPreferredShipDirection(n.GetTile(), n.GetTrackdir())) c += YAPF_TILE_LENGTH;
+
 		/* Skipped tile cost for aqueducts. */
 		c += YAPF_TILE_LENGTH * tf->tiles_skipped;