gb: the collision tile is the lower left of a map tile s quadrant

Measured on the cartridge, not derived: a map tile is 2x2 screen tiles and
CheckTilePassable matches one id, and the screen agrees with the lower-left tile
of the quadrant. Viridian Forest s (4, 32) reads 3, the second row of its
block, where the first row holds bash4. On open ground most quadrants hold one id
four times over, so the upper-left guess reads correctly on a town and falls
apart in a forest -- which is why the decode is cross-checked against the screen
before it is trusted.

services/flysim/crates/flybrain-gb/src/pokemon_red/fake_wram.rs

@@ -404,7 +404,9 @@ impl Wram {
             for x in 0..width * 2 {
                 let Some(block) = blocks.get((y / 2) * width + (x / 2)) else { continue };
                 let Some(tiles) = blockset.get(usize::from(*block)) else { continue };
-                let tile = tiles[(y % 2) * 2 * 4 + (x % 2) * 2];
+                // The lower-left tile of the map tile's own quadrant, which is the one the
+                // cartridge's collision read uses (`mapgrid::ANCHOR_ROW`).
+                let tile = tiles[((y % 2) * 2 + 1) * 4 + (x % 2) * 2];
                 self.map_tile(x as u8, y as u8, tile);
             }
         }

services/flysim/crates/flybrain-gb/src/pokemon_red/mapgrid.rs

@@ -49,6 +49,22 @@ pub const MAP_BORDER: usize = 3;
 /// which is why `wCurMapWidth` in blocks is `map_size().width` in tiles divided by this.
 pub const TILES_PER_BLOCK: u8 = 2;
 
+/// Which of a quadrant's two rows the collision read takes its tile id from: the lower one.
+///
+/// A map tile is a 2x2 patch of screen tiles and only one of the four is ever asked about, because
+/// `CheckTilePassable` matches a single tile id. Which one is **measured, not derived**: the
+/// decoded ids were compared against the screen buffer on the cartridge, tile by tile, and the
+/// screen agrees with the lower-left tile of each quadrant and not the upper-left -- Viridian
+/// Forest's (4, 32) reads `$23`, the second row of its block, where the first row holds `$04`
+/// (`tests/rom_map_grid.rs`, which is the test that pins it).
+///
+/// That is the same corner `_GetTileAndCoordsInFrontOfPlayer` reads at screen `(8, 9)` for the
+/// tile the player is standing on: the view is aligned so that the player's own 2x2 begins on
+/// screen row 8, so row 9 is its lower half. An upper-left decode still answers, and answers
+/// plausibly -- on the open ground of a town most quadrants hold one tile id four times over --
+/// which is why the cross-check in [`super::state::map_grid`] is not optional.
+const ANCHOR_ROW: usize = 1;
+
 /// Tileset ids the tile-pair lists name (`constants/tileset_constants.asm`, counted in the order
 /// that file declares them: OVERWORLD 0 … FOREST 3 … CAVERN 17).
 pub mod tileset {
@@ -143,10 +159,10 @@ pub fn decode(map: u8, width_blocks: u8, height_blocks: u8, blocks: &[u8], tiles
             let Some(block) = blocks.get(block_index).copied() else {
                 continue;
             };
-            // The top left screen tile of the map tile's own 2x2 quadrant of the block, which is
-            // the corner every collision read in the game uses.
+            // The screen tile the collision read uses, inside the map tile's own 2x2 quadrant of
+            // the block: the **lower** left one ([`ANCHOR_ROW`]).
             let column = usize::from(x % TILES_PER_BLOCK) * usize::from(TILES_PER_BLOCK);
-            let row = usize::from(y % TILES_PER_BLOCK) * usize::from(TILES_PER_BLOCK);
+            let row = usize::from(y % TILES_PER_BLOCK) * usize::from(TILES_PER_BLOCK) + ANCHOR_ROW;
             let Some(tile) = tiles.tile(block, column, row) else {
                 continue;
             };

services/flysim/crates/flybrain-gb/src/pokemon_red/mapgrid/tests.rs

@@ -16,11 +16,13 @@ use crate::pokemon_red::macros::state::{Facing, Walkable};
 fn blockset() -> Vec<u8> {
     let floor = [FLOOR; 16];
     let wall = [WALL; 16];
+    // The four ids sit on the rows the decode reads -- the *lower* row of each 2x2 quadrant
+    // (`ANCHOR_ROW`) -- and the rows it does not read hold ids that would be wrong answers.
     let quadrants = [
-        NORTH_WEST, 0x90, NORTH_EAST, 0x91, //
-        0x92, 0x93, 0x94, 0x95, //
-        SOUTH_WEST, 0x96, SOUTH_EAST, 0x97, //
-        0x98, 0x99, 0x9a, 0x9b,
+        0x90, 0x91, 0x92, 0x93, //
+        NORTH_WEST, 0x94, NORTH_EAST, 0x95, //
+        0x96, 0x97, 0x98, 0x99, //
+        SOUTH_WEST, 0x9a, SOUTH_EAST, 0x9b,
     ];
     let mut out = Vec::new();
     out.extend_from_slice(&floor);