fix(railways): respect layer tag and use ascending rails on terrain slopes

src/element_processing/railways.rs

@@ -3,6 +3,9 @@ use crate::bresenham::bresenham_line;
 use crate::osm_parser::ProcessedWay;
 use crate::world_editor::WorldEditor;
 
+/// Number of blocks per OSM layer level (matches highway elevation step).
+const LAYER_HEIGHT_STEP: i32 = 6;
+
 pub fn generate_railways(editor: &mut WorldEditor, element: &ProcessedWay) {
     if let Some(railway_type) = element.tags.get("railway") {
         if [
@@ -30,6 +33,17 @@ pub fn generate_railways(editor: &mut WorldEditor, element: &ProcessedWay) {
             }
         }
 
+        // Respect the OSM `layer` tag so elevated railways (bridges, overpasses)
+        // are placed above ground level instead of clipping through it.
+        let layer_value: i32 = element
+            .tags
+            .get("layer")
+            .and_then(|l| l.parse::<i32>().ok())
+            .unwrap_or(0)
+            .max(0); // underground (<0) is handled by the tunnel check above
+
+        let layer_offset = layer_value * LAYER_HEIGHT_STEP;
+
         for i in 1..element.nodes.len() {
             let prev_node = element.nodes[i - 1].xz();
             let cur_node = element.nodes[i].xz();
@@ -40,35 +54,121 @@ pub fn generate_railways(editor: &mut WorldEditor, element: &ProcessedWay) {
             for j in 0..smoothed_points.len() {
                 let (bx, _, bz) = smoothed_points[j];
 
-                editor.set_block(GRAVEL, bx, 0, bz, None, None);
+                // Base offsets from ground level.
+                // When layer_offset > 0 the rail floats above terrain (bridge/overpass).
+                let gravel_y = layer_offset;
+                let rail_y = layer_offset + 1;
+
+                // --- Terrain-slope detection (only for at-grade railways) ---
+                // When terrain is enabled and layer == 0 we try to place ascending
+                // rail variants so consecutive blocks stay visually connected even
+                // when the ground rises or falls by one block per step.
+                let prev_ground = if j > 0 {
+                    let (px, _, pz) = smoothed_points[j - 1];
+                    editor.get_ground_level(px, pz)
+                } else {
+                    editor.get_ground_level(bx, bz)
+                };
 
-                let prev = if j > 0 {
-                    Some(smoothed_points[j - 1])
+                let next_ground = if j + 1 < smoothed_points.len() {
+                    let (nx, _, nz) = smoothed_points[j + 1];
+                    editor.get_ground_level(nx, nz)
+                } else {
+                    editor.get_ground_level(bx, bz)
+                };
+
+                let current_ground = editor.get_ground_level(bx, bz);
+
+                // Fill the vertical gap under the rail when terrain rises steeply
+                // so there is always a solid gravel block supporting the track.
+                if layer_offset == 0 && prev_ground < current_ground {
+                    for fill_y in prev_ground..current_ground {
+                        editor.set_block_absolute(GRAVEL, bx, fill_y, bz, None, None);
+                    }
+                }
+
+                editor.set_block(GRAVEL, bx, gravel_y, bz, None, None);
+
+                let prev_xz = if j > 0 {
+                    let (px, _, pz) = smoothed_points[j - 1];
+                    Some((px, pz))
                 } else {
                     None
                 };
-                let next = if j < smoothed_points.len() - 1 {
-                    Some(smoothed_points[j + 1])
+                let next_xz = if j + 1 < smoothed_points.len() {
+                    let (nx, _, nz) = smoothed_points[j + 1];
+                    Some((nx, nz))
                 } else {
                     None
                 };
 
-                let rail_block = determine_rail_direction(
-                    (bx, bz),
-                    prev.map(|(x, _, z)| (x, z)),
-                    next.map(|(x, _, z)| (x, z)),
-                );
+                let rail_block = if layer_offset == 0 {
+                    determine_rail_with_slope(
+                        (bx, bz),
+                        prev_xz,
+                        next_xz,
+                        prev_ground,
+                        current_ground,
+                        next_ground,
+                    )
+                } else {
+                    determine_rail_direction((bx, bz), prev_xz, next_xz)
+                };
 
-                editor.set_block(rail_block, bx, 1, bz, None, None);
+                editor.set_block(rail_block, bx, rail_y, bz, None, None);
 
                 if bx % 4 == 0 {
-                    editor.set_block(OAK_LOG, bx, 0, bz, None, None);
+                    editor.set_block(OAK_LOG, bx, gravel_y, bz, None, None);
                 }
             }
         }
     }
 }
 
+/// Choose between a flat or ascending rail block based on the ground-level
+/// difference between the previous, current, and next track points.
+fn determine_rail_with_slope(
+    current: (i32, i32),
+    prev: Option<(i32, i32)>,
+    next: Option<(i32, i32)>,
+    prev_ground: i32,
+    current_ground: i32,
+    next_ground: i32,
+) -> Block {
+    // Ascending toward the *higher* neighbour.
+    if next_ground > current_ground {
+        if let Some((nx, nz)) = next {
+            return ascending_toward(current, (nx, nz));
+        }
+    }
+    if prev_ground > current_ground {
+        if let Some((px, pz)) = prev {
+            return ascending_toward(current, (px, pz));
+        }
+    }
+    // Flat section – fall back to standard direction logic.
+    determine_rail_direction(current, prev, next)
+}
+
+/// Return the ascending rail variant that climbs from `from` toward `to`.
+fn ascending_toward(from: (i32, i32), to: (i32, i32)) -> Block {
+    let (fx, fz) = from;
+    let (tx, tz) = to;
+    let dx = tx - fx;
+    let dz = tz - fz;
+    if dx.abs() >= dz.abs() {
+        if dx > 0 {
+            RAIL_ASCENDING_EAST
+        } else {
+            RAIL_ASCENDING_WEST
+        }
+    } else if dz < 0 {
+        RAIL_ASCENDING_NORTH
+    } else {
+        RAIL_ASCENDING_SOUTH
+    }
+}
+
 fn smooth_diagonal_rails(points: &[(i32, i32, i32)]) -> Vec<(i32, i32, i32)> {
     let mut smoothed = Vec::new();