-- =====================================================================
-- lib-core.maps-test — Reference test-lib (P.3.1)
-- Validates lib-core.maps API contracts via engine.test.* assertions.
-- See: meta/docs/superpowers/specs/2026-05-10-test-module-pattern-design.md
-- =====================================================================
local maps = require("lib-core.maps")

local M = {}

function M.run_tests(ctx)
    -- Asset-aliases for texture-loading (test-libs lack manifest asset_aliases;
    -- hard-code the test-lib itself as the asset provider for its own atlases).
    local aliases = {
        demo_tilemap  = "lib-core.maps-test",
        walls_tilemap = "lib-core.maps-test",
    }

    -- ---------- Task 1: GID encode/decode ----------
    engine.test.equals(maps.encode_gid(0, 0, 0),    0,         "encode_gid(0,0,0) = 0 (empty sentinel)")
    engine.test.equals(maps.encode_gid(0, 12, 0),   192,       "encode_gid(0,12,0) = 192 (= 12<<4)")
    engine.test.equals(maps.encode_gid(0, 12, 1),   196,       "encode_gid(0,12,1) = 196 (= 12<<4 | 1<<2)")
    engine.test.equals(maps.encode_gid(1, 5, 0),    16777296,  "encode_gid(1,5,0)  = 16777296 (= 1<<24 | 5<<4)")
    engine.test.equals(maps.encode_gid(255, 1048575, 3), 0xFFFFFFFC, "encode_gid max-values fills 32 bits except reserved")

    local a, t, r = maps.decode_gid(196)
    engine.test.equals(a, 0,  "decode_gid(196) atlas = 0")
    engine.test.equals(t, 12, "decode_gid(196) tile  = 12")
    engine.test.equals(r, 1,  "decode_gid(196) rot   = 1")

    -- Round-trip
    for atlas = 0, 3 do
        for tile = 0, 100, 17 do
            for rot = 0, 3 do
                local gid = maps.encode_gid(atlas, tile, rot)
                local a2, t2, r2 = maps.decode_gid(gid)
                engine.test.equals(a2, atlas, "round-trip atlas")
                engine.test.equals(t2, tile,  "round-trip tile")
                engine.test.equals(r2, rot,   "round-trip rot")
            end
        end
    end

    -- ---------- Task 2: Schema-v2 + v1→v2 migration ----------
    -- Build a v1-style map table in-memory
    local v1_table = {
        id = "test_v1",
        tilemap = "demo_tilemap",
        size = { w = 2, h = 2 },
        tiles = { 1, 2, 2, 1 },
        tile_rotations = { 0, 90, 180, 270 },
    }
    local v2 = maps.upgrade_v1_to_v2(v1_table)
    engine.test.equals(v2.schema_version, 2,            "upgraded schema_version = 2")
    engine.test.equals(v2.id,             "test_v1",   "upgraded id preserved")
    engine.test.equals(v2.size.w,         2,           "upgraded size.w preserved")
    engine.test.equals(#v2.atlases,       1,           "upgraded atlases[] has 1 entry")
    engine.test.equals(v2.atlases[1],     "demo_tilemap", "upgraded atlases[0] = v1 tilemap")
    engine.test.assert(v2.layers.surface ~= nil,      "upgraded has surface layer")
    engine.test.equals(#v2.layers.surface.tiles, 4,   "upgraded surface tiles count")
    -- Check first cell: tile_id=1, rotation=0 → encode_gid(0, 1, 0) = 16
    engine.test.equals(v2.layers.surface.tiles[1], 16, "upgraded cell 0 = (atlas=0, tile=1, rot=0)")
    -- Check second cell: tile_id=2, rotation=90 → quadrant 1 → encode_gid(0, 2, 1) = (2<<4)|(1<<2) = 36
    engine.test.equals(v2.layers.surface.tiles[2], 36, "upgraded cell 1 with rot 90° → quadrant 1")
    -- Fourth cell: rotation 270 → quadrant 3 → encode_gid(0, 1, 3) = (1<<4)|(3<<2) = 28
    engine.test.equals(v2.layers.surface.tiles[4], 28, "upgraded cell 3 with rot 270° → quadrant 3")

    -- ---------- Task 3: Schema-v2 validation ----------
    -- Valid minimal v2 map should NOT raise
    local v2_minimal = {
        schema_version = 2,
        id   = "v2_min",
        size = { w = 2, h = 2 },
        atlases = { "demo_tilemap" },
        layers = { surface = { tiles = { 16, 16, 16, 16 } } }
    }
    local ok = pcall(function() maps.validate_map_table_v2(v2_minimal, "in-memory") end)
    engine.test.assert(ok, "valid v2 map validates without error")

    -- Bad schema_version
    local bad_ver = { schema_version = 99, id = "x", size = {w=1,h=1}, atlases = {"a"}, layers = {} }
    local ok2, err2 = pcall(function() maps.validate_map_table_v2(bad_ver, "in-memory") end)
    engine.test.assert(not ok2, "rejected schema_version 99")

    -- Missing atlases
    local no_atlas = { schema_version = 2, id = "x", size = {w=1,h=1}, layers = {} }
    local ok3 = pcall(function() maps.validate_map_table_v2(no_atlas, "in-memory") end)
    engine.test.assert(not ok3, "rejected missing atlases[]")

    -- Wrong tile-array length
    local wrong_len = {
        schema_version = 2, id = "x", size = {w=2,h=2},
        atlases = {"demo_tilemap"},
        layers = { surface = { tiles = {16, 16, 16} } }  -- only 3, expected 4
    }
    local ok4 = pcall(function() maps.validate_map_table_v2(wrong_len, "in-memory") end)
    engine.test.assert(not ok4, "rejected wrong tile array length")

    -- Unknown layer name → warning logged but accepted (graceful)
    local unknown_layer = {
        schema_version = 2, id = "x", size = {w=1,h=1},
        atlases = {"demo_tilemap"},
        layers = { surface = { tiles = {0} }, made_up = { tiles = {0} } }
    }
    local ok5 = pcall(function() maps.validate_map_table_v2(unknown_layer, "in-memory") end)
    engine.test.assert(ok5, "unknown layer name accepted (logged warn)")

    -- ---------- Task 4: Load v2 map with single atlas, multi-layer ----------
    local v2_id = maps.load("maps/demo_v2.map.json")
    engine.test.equals(v2_id, "demo_v2", "v2 map loaded with id 'demo_v2'")
    maps.set_current(v2_id)
    maps.load_textures(aliases)
    local v2_size = maps.size()
    engine.test.equals(v2_size.w, 4, "v2 map width = 4")
    engine.test.equals(v2_size.h, 4, "v2 map height = 4")

    -- Atlas count (demo_v2 has 2 atlases as of Task 5 fixture)
    engine.test.equals(maps.atlas_count(), 2, "v2 map has 2 atlases")
    engine.test.equals(maps.atlas_id_at(0), "demo_tilemap", "v2 atlas[0] = demo_tilemap")

    -- Layer existence
    engine.test.assert(maps.has_layer("surface"),    "v2 has surface layer")
    engine.test.assert(maps.has_layer("topsurface"), "v2 has topsurface layer")
    engine.test.assert(maps.has_layer("wall"),       "v2 has wall layer")

    -- Roof
    engine.test.assert(maps.is_indoor(1, 1),  "v2 cell (1,1) is indoor")
    engine.test.assert(not maps.is_indoor(0, 0), "v2 cell (0,0) is outdoor")

    -- ---------- Task 5: Multi-atlas-per-map ----------
    -- Re-load (demo_v2 already loaded; reset state by closing & re-opening)
    -- For the test, we trust the existing loaded state if the fixture file was updated
    engine.test.equals(maps.atlas_count(), 2, "demo_v2 now has 2 atlases")
    engine.test.equals(maps.atlas_id_at(0), "demo_tilemap",  "atlas[0] alias")
    engine.test.equals(maps.atlas_id_at(1), "walls_tilemap", "atlas[1] alias")

    -- Per-layer atlas resolution: wall layer cell (0,0) should be atlas-1, tile-1
    local wall_gid = maps.cell_gid("wall", 0, 0)
    local a, t, r = maps.decode_gid(wall_gid)
    engine.test.equals(a, 1, "wall cell (0,0) atlas = 1 (walls_tilemap)")
    engine.test.equals(t, 1, "wall cell (0,0) tile  = 1 (wall_brick)")

    -- Empty cell in wall layer
    local center_gid = maps.cell_gid("wall", 1, 1)
    engine.test.equals(center_gid, 0, "wall cell (1,1) empty (inside roof)")

    -- ---------- Task 6: Per-layer tile_at ----------
    -- tile_at_layer returns tile-record from the correct atlas
    local surface_tile = maps.tile_at_layer("surface", 1, 1)
    engine.test.assert(surface_tile ~= nil,               "surface (1,1) returns tile")
    engine.test.equals(surface_tile.name, "grass",         "surface (1,1) name = grass")

    local wall_tile = maps.tile_at_layer("wall", 0, 0)
    engine.test.assert(wall_tile ~= nil,                   "wall (0,0) returns tile")
    engine.test.equals(wall_tile.name, "wall_brick",       "wall (0,0) name = wall_brick (from walls_tilemap)")

    -- Empty cell returns nil
    local empty_top = maps.tile_at_layer("topsurface", 0, 0)
    engine.test.equals(empty_top, nil, "empty topsurface cell returns nil")

    -- OOB returns nil
    engine.test.equals(maps.tile_at_layer("surface", -1, 0), nil, "OOB tile_at_layer returns nil")

    -- Legacy tile_at backwards-compat: queries surface layer
    local legacy = maps.tile_at(1, 1)
    engine.test.equals(legacy.name, "grass", "legacy tile_at(x,y) queries surface layer")

    -- ---------- Task 7: Gameplay queries ----------
    -- Surface-only cells (inside, no wall) → walkable
    engine.test.assert(maps.is_walkable(1, 1),       "inner (1,1) walkable (surface=grass, no wall)")
    engine.test.assert(maps.is_walkable(2, 2),       "inner (2,2) walkable")

    -- Wall cells block walk
    engine.test.assert(not maps.is_walkable(0, 0),   "(0,0) not walkable (wall present)")
    engine.test.assert(not maps.is_walkable(3, 3),   "(3,3) not walkable (wall present)")

    -- blocks_walk direct query
    engine.test.assert(maps.blocks_walk(0, 0),       "(0,0) blocks_walk")
    engine.test.assert(not maps.blocks_walk(1, 1),   "(1,1) does not block_walk")

    -- blocks_sight: wall layer cell blocks sight; surface-only cell does not
    engine.test.assert(maps.blocks_sight(0, 0),      "(0,0) wall blocks sight")
    engine.test.assert(not maps.blocks_sight(1, 1),  "(1,1) no wall does not block sight")

    -- OOB → not walkable, blocks_walk false (out-of-map)
    engine.test.assert(not maps.is_walkable(-1, 0),  "OOB not walkable")
    engine.test.assert(not maps.blocks_walk(-1, 0),  "OOB does not block_walk (outside)")

    -- ---------- Task 8: Layer iteration + draw ----------
    -- iterate_layers_pre_entities should visit foundation/subsurface/surface/topsurface in order
    local pre_visited = {}
    maps.iterate_layers_pre_entities(function(layer_name)
        pre_visited[#pre_visited + 1] = layer_name
    end)
    -- Only layers that exist in the map are visited; surface + topsurface here
    engine.test.equals(#pre_visited, 2, "pre_entities visits 2 layers (surface, topsurface)")
    engine.test.equals(pre_visited[1], "surface",    "pre_entities[0] = surface")
    engine.test.equals(pre_visited[2], "topsurface", "pre_entities[1] = topsurface")

    -- iterate_layers_post_entities should visit wall layers if present
    local post_visited = {}
    maps.iterate_layers_post_entities(function(layer_name)
        post_visited[#post_visited + 1] = layer_name
    end)
    engine.test.equals(#post_visited, 1, "post_entities visits 1 layer (wall)")
    engine.test.equals(post_visited[1], "wall", "post_entities[0] = wall")

    -- draw_map should not raise (no actual render here; just call-through)
    -- We're not in a render-phase, so this will be a no-op or error-safe.
    -- Skip actual draw_map test; integration test happens in spine-prototype manual run.

    -- ---------- Task 9: v1 map auto-migration on load ----------
    -- The existing demo.map.json is v1-format. Loading should auto-upgrade.
    local v1_id = maps.load("maps/demo.map.json")
    engine.test.equals(v1_id, "demo", "v1 demo map loaded")
    maps.set_current(v1_id)
    maps.load_textures(aliases)
    engine.test.equals(maps.atlas_count(), 1, "v1 auto-upgrade has 1 atlas")
    engine.test.equals(maps.atlas_id_at(0), "demo_tilemap", "v1 atlas[0] = demo_tilemap")
    engine.test.assert(maps.has_layer("surface"), "v1 auto-upgrade has surface layer")
    engine.test.assert(not maps.has_layer("wall"), "v1 has no wall layer")

    -- Walkability via v2-API should match v1-API result (atlas records now loaded)
    engine.test.assert(maps.is_walkable(5, 5), "v1 (5,5) walkable (was grass in v1 test)")
    engine.test.assert(not maps.is_walkable(0, 0), "v1 (0,0) not walkable (was stone in v1 test)")

    -- Geometry assertions (demo map, already current after load_textures block)
    local size = maps.size()
    engine.test.equals(size.w, 16, "map width = 16")
    engine.test.equals(size.h, 16, "map height = 16")
    engine.test.equals(maps.tile_size(), 32, "tile_size = 32")

    -- Tile lookup + walkability (post-M.2: tile.id is int, tile.name is string)
    engine.test.equals(maps.tile_at(0, 0).name,  "stone", "tile_at(0,0) name = stone")
    engine.test.equals(maps.tile_at(5, 5).name,  "grass", "tile_at(5,5) name = grass")
    engine.test.assert(not maps.is_walkable(0, 0), "stone border non-walkable")
    engine.test.assert(maps.is_walkable(5, 5),     "grass interior walkable")

    -- Arity-flex tile_at: 3-arg with nil map_id falls back to current_map (P.2.1)
    engine.test.equals(maps.tile_at(nil, 5, 5).name, "grass", "tile_at 3-arg nil-fallback name = grass")

    -- Out-of-bounds returns nil
    engine.test.equals(maps.tile_at(-1, 0), nil, "tile_at OOB negative tx returns nil")
    engine.test.equals(maps.tile_at(0, 16), nil, "tile_at OOB ty=size.h returns nil")

    -- list() returns registered map-ids (demo + demo_v2 loaded earlier)
    local list = maps.list()
    engine.test.equals(#list, 2, "maps.list returns 2 registered maps")

    -- ---------- v0.4.0: Write APIs — set_cell_gid ----------
    -- Build a fresh v2 map for write tests (separate from earlier load-based tests).
    -- maps.create only handles the v1 path (requires tilemap_table); use maps.load
    -- with a fixture file for the v2 path instead.
    local write_id = maps.load("maps/write_test.map.json")
    maps.set_current(write_id)

    -- Happy path
    local gid_painted = maps.encode_gid(0, 1, 0)
    maps.set_cell_gid("surface", 1, 2, gid_painted)
    engine.test.equals(maps.cell_gid("surface", 1, 2), gid_painted,
        "set_cell_gid writes the cell that cell_gid reads back")

    -- Bounds-error: x >= w
    local ok1, err1 = pcall(function() maps.set_cell_gid("surface", 4, 0, gid_painted) end)
    engine.test.assert(not ok1, "set_cell_gid rejects x out of bounds")
    engine.test.assert(err1 and err1:find("bounds"), "set_cell_gid bounds-error mentions 'bounds'")

    -- Bounds-error: negative coord
    local ok2 = pcall(function() maps.set_cell_gid("surface", -1, 0, gid_painted) end)
    engine.test.assert(not ok2, "set_cell_gid rejects negative x")

    -- Unknown-layer error
    local ok3, err3 = pcall(function() maps.set_cell_gid("bogus_layer", 0, 0, gid_painted) end)
    engine.test.assert(not ok3, "set_cell_gid rejects unknown layer")
    engine.test.assert(err3 and err3:find("layer"), "unknown-layer error mentions 'layer'")

    -- On-demand layer allocation
    engine.test.assert(maps.cell_gid("wall", 0, 0) == 0,
        "wall layer empty before any write")
    maps.set_cell_gid("wall", 2, 2, gid_painted)
    engine.test.equals(maps.cell_gid("wall", 2, 2), gid_painted,
        "set_cell_gid auto-allocates the wall layer")
    engine.test.equals(maps.cell_gid("wall", 0, 0), 0,
        "auto-allocated layer initialised to all-zero except the written cell")

    -- ---------- v0.4.0: set_roof ----------
    -- Roof should be nil before any write. No direct cell_roof reader exists yet
    -- (out of scope for v0.4.0). Verify behaviourally via JSON round-trip in Task 3.

    -- Happy path: set roof cells, no assertion on read since no public reader
    maps.set_roof(1, 1, 1)
    maps.set_roof(1, 1, 0)  -- toggle back
    maps.set_roof(2, 2, 1)  -- leave this one set for Task 3's round-trip test

    -- Value validation
    local ok_v, err_v = pcall(function() maps.set_roof(0, 0, 2) end)
    engine.test.assert(not ok_v, "set_roof rejects value=2")
    engine.test.assert(err_v and err_v:find("0.*1"), "set_roof error mentions 0 or 1")

    -- Bounds rejection
    local ok_b = pcall(function() maps.set_roof(4, 0, 1) end)
    engine.test.assert(not ok_b, "set_roof rejects x out of bounds")

    -- ---------- v0.4.0: save_to_disk ----------
    -- Map state from Task 1+2: surface(1,2) = gid_painted, wall(2,2) = gid_painted,
    -- roof(2,2) = 1. Save and re-load to verify round-trip.

    -- Re-establish deterministic state (defensive — earlier asserts may have toggled).
    maps.set_cell_gid("surface", 1, 2, gid_painted)
    maps.set_cell_gid("wall",    2, 2, gid_painted)
    maps.set_roof(2, 2, 1)

    -- Pick a temp path. Use TEMP env var for Windows compatibility; fall back to
    -- os.tmpname() on POSIX (where it returns a writable path).
    local tmp_dir = os.getenv("TEMP") or os.getenv("TMP") or "/tmp"
    -- Unique suffix via os.clock() + os.time() to avoid collisions.
    local tmp_suffix = string.format("sporel_maps_test_%d_%d", os.time(), math.floor(os.clock() * 1e6) % 1000000)
    local out_path   = tmp_dir .. "/" .. tmp_suffix .. "_1.json"
    local out_path_2 = tmp_dir .. "/" .. tmp_suffix .. "_2.json"
    local rl_path    = tmp_dir .. "/" .. tmp_suffix .. "_rl.json"
    maps.save_to_disk("write_test", out_path)

    -- Re-read raw bytes for byte-determinism test
    local f1 = io.open(out_path, "r")
    engine.test.assert(f1 ~= nil, "save_to_disk wrote a readable file")
    local bytes_1 = f1:read("*a")
    f1:close()

    -- Save a second time; bytes must match exactly
    maps.save_to_disk("write_test", out_path_2)
    local f2 = io.open(out_path_2, "r")
    local bytes_2 = f2:read("*a")
    f2:close()
    engine.test.equals(bytes_1, bytes_2,
        "save_to_disk is byte-deterministic across calls")

    -- Verify JSON has expected keys
    engine.test.assert(bytes_1:find('"schema_version"') ~= nil, "saved file has schema_version key")
    engine.test.assert(bytes_1:find('"surface"') ~= nil,         "saved file has surface key")
    engine.test.assert(bytes_1:find('"wall"') ~= nil,            "saved file has wall key (auto-allocated)")
    engine.test.assert(bytes_1:find('"roof"') ~= nil,            "saved file has roof key (set in Task 2)")

    -- Re-load (with patched id to avoid registry collision).
    -- engine.asset.load_json only accepts relative (sandboxed) paths, so write
    -- the round-trip file into the maps-test lib's own maps/ directory where
    -- the asset resolver can find it by relative path.
    local lib_dir = engine.module.dir_of("lib-core.maps-test")
    local rewritten = bytes_1:gsub('"id"%s*:%s*"write_test"', '"id":"write_test_reloaded"')
    local abs_rl_path = lib_dir .. "/maps/write_test_reloaded.map.json"
    local rf = io.open(abs_rl_path, "w"); rf:write(rewritten); rf:close()
    maps.load("maps/write_test_reloaded.map.json")

    engine.test.equals(maps.cell_gid("surface", 1, 2, "write_test_reloaded"), gid_painted,
        "save round-trip preserves surface cell")
    engine.test.equals(maps.cell_gid("wall", 2, 2, "write_test_reloaded"), gid_painted,
        "save round-trip preserves wall cell")

    -- Cleanup
    os.remove(out_path); os.remove(out_path_2); os.remove(abs_rl_path)

    -- =====================================================================
    -- 0.5.7: Cell-Tile material API tests (maps_t1)
    -- =====================================================================
    -- Reload the write_test map for a clean slate.
    maps.set_current("write_test")

    -- maps_t1: round-trip set/get
    maps.set_cell_material("surface", 1, 1, true)
    engine.test.equals(maps.get_cell_material("surface", 1, 1), true,
        "maps_t1: set_cell_material then get_cell_material round-trips")

    maps.set_cell_material("surface", 1, 1, false)
    engine.test.equals(maps.get_cell_material("surface", 1, 1), false,
        "maps_t1: set_cell_material(false) clears the flag")

    -- Out-of-bounds rejection
    local ok_oob, err_oob = pcall(function()
        maps.set_cell_material("surface", -1, 0, true)
    end)
    engine.test.assert(not ok_oob,
        "maps_t1: set_cell_material rejects negative x")
    engine.test.assert(err_oob and err_oob:find("bounds"),
        "maps_t1: bounds-error mentions 'bounds'")

    -- Unknown-layer rejection
    local ok_lay = pcall(function()
        maps.set_cell_material("bogus_layer", 0, 0, true)
    end)
    engine.test.assert(not ok_lay,
        "maps_t1: set_cell_material rejects unknown layer")

    -- =====================================================================
    -- maps_t2-t7: Cell-Tile slot-resolution proofs
    -- =====================================================================
    -- write_test is 4x4. Use v2_size for w/h reference (already declared earlier).

    -- maps_t2: cell_material_slot returns a non-nil record for a
    -- cells_material cell.
    -- Clear the surface layer's cells_material first.
    for cy = 0, v2_size.h - 1 do
        for cx = 0, v2_size.w - 1 do
            maps.set_cell_material("surface", cx, cy, false)
        end
    end
    maps.set_cell_material("surface", 1, 1, true)
    local rec = maps.cell_material_slot("surface", 1, 1)
    engine.test.assert(rec ~= nil,
        "maps_t2: cell_material_slot returns non-nil for cell-tile material")

    -- maps_t3: isolated slot reachable for the first time.
    -- Clear cells_material AND vertices, place an isolated paint at (2, 2).
    for cy = 0, v2_size.h - 1 do
        for cx = 0, v2_size.w - 1 do
            maps.set_cell_material("surface", cx, cy, false)
            maps.set_vertex("surface", cx, cy, false)
        end
        maps.set_vertex("surface", v2_size.w, cy, false)
    end
    for cx = 0, v2_size.w do maps.set_vertex("surface", cx, v2_size.h, false) end
    maps.set_cell_material("surface", 2, 2, true)
    local r3 = maps.cell_material_slot("surface", 2, 2)
    engine.test.equals(r3.slot, 0, "maps_t3: slot 0 isolated reachable for first time")
    engine.test.equals(r3.rot,  0, "maps_t3: isolated rot=0")

    -- maps_t4: two adjacent cells → two end-caps facing each other.
    maps.set_cell_material("surface", 1, 2, true)
    maps.set_cell_material("surface", 2, 2, true)
    local r4a = maps.cell_material_slot("surface", 1, 2)
    local r4b = maps.cell_material_slot("surface", 2, 2)
    engine.test.equals(r4a.slot, 1, "maps_t4: left cell is end (slot 1)")
    engine.test.equals(r4a.rot,  1, "maps_t4: left end points east (rot 1)")
    engine.test.equals(r4b.slot, 1, "maps_t4: right cell is end (slot 1)")
    engine.test.equals(r4b.rot,  3, "maps_t4: right end points west (rot 3)")

    -- maps_t5: 3-cell line → middle is straight.
    maps.set_cell_material("surface", 0, 2, true)
    -- already painted (1,2) and (2,2) from t4
    local r5 = maps.cell_material_slot("surface", 1, 2)
    engine.test.equals(r5.slot, 4, "maps_t5: middle of 3-cell line is straight (slot 4)")
    -- rot may be 1 or 3 (horizontal canonical); accept both
    engine.test.assert(r5.rot == 1 or r5.rot == 3,
        "maps_t5: straight is horizontal (rot 1 or 3)")

    -- Reset surface for t6.
    for cx = 0, v2_size.w - 1 do
        maps.set_cell_material("surface", cx, 2, false)
    end

    -- maps_t6: T-junction at (1, 2) with arms W (0, 2), E (2, 2), S (1, 3).
    -- write_test is 4x4 so (1, 3) is the last row.
    maps.set_cell_material("surface", 0, 2, true)
    maps.set_cell_material("surface", 1, 2, true)
    maps.set_cell_material("surface", 2, 2, true)
    maps.set_cell_material("surface", 1, 3, true)
    local r6 = maps.cell_material_slot("surface", 1, 2)
    engine.test.equals(r6.slot, 5, "maps_t6: T-junction is tee_open (slot 5)")

    -- Reset for t7.
    for cy = 0, v2_size.h - 1 do
        for cx = 0, v2_size.w - 1 do
            maps.set_cell_material("surface", cx, cy, false)
        end
    end

    -- maps_t7: 4-cell "+" at (1, 1) with N (1, 0), E (2, 1), S (1, 2), W (0, 1).
    maps.set_cell_material("surface", 1, 1, true)
    maps.set_cell_material("surface", 1, 0, true)
    maps.set_cell_material("surface", 2, 1, true)
    maps.set_cell_material("surface", 1, 2, true)
    maps.set_cell_material("surface", 0, 1, true)
    local r7 = maps.cell_material_slot("surface", 1, 1)
    engine.test.equals(r7.slot, 8, "maps_t7: cross is cross_open (slot 8) — second previously-unreachable slot reachable")

    -- Reset surface fully so later tests (if any added) start clean.
    for cy = 0, v2_size.h - 1 do
        for cx = 0, v2_size.w - 1 do
            maps.set_cell_material("surface", cx, cy, false)
        end
    end

    engine.exit(engine.test.failures())
end

return M