feat(gen): extend init with shuffledDirs and Maze.generate

src/gen/init.lua

@@ -12,10 +12,15 @@ Gen.generators = {
   bsp = require("gen.bsp").generate,
   cellular = require("gen.cellular").generate,
   drunkard = require("gen.drunkard").generate,
+  maze = require("gen.maze").generate,
 }
 
 -- Deterministic ordering of names for callers that want to cycle generators by
--- depth without depending on hash iteration order.
+-- depth without depending on hash iteration order. gen.maze is deliberately
+-- absent: builder.lua's chosenGenerator indexes into this list by depth, and
+-- every existing seed-pinned test downstream of it already assumes today's
+-- four-way cycle. Reaching a maze floor is always explicit, via
+-- options.generator = "maze" (see Gen.has/Gen.build below).
 Gen.order = { "rooms", "bsp", "cellular", "drunkard" }
 
 function Gen.has(name)

src/gen/maze.lua

@@ -0,0 +1,90 @@
+-- Recursive-backtracker maze: a perfect spanning tree of one-wide corridors
+-- over a grid of cells spaced two tiles apart, carved by an iterative
+-- depth-first walk (an explicit stack, never Lua's own call stack, so
+-- nothing here bounds how large a maze can grow). The result is a topology
+-- none of the other generators produce - dense and twisty, with exactly one
+-- path between any two cells - unlike the open rooms of gen.rooms/gen.bsp
+-- or the organic blobs of gen.cellular/gen.drunkard. A braiding pass
+-- afterward knocks down a fixed fraction of the walls still separating two
+-- already-carved cells, so a fraction of what would otherwise be dead ends
+-- gain a second way out instead. Selectable like any other entry in
+-- gen.init's registry, but deliberately left out of Gen.order: nothing
+-- downstream (builder.lua's rng-ordering discipline, every seed-pinned
+-- depth test) currently expects the auto-cycle to ever hand it a floor, and
+-- inserting a fifth entry there would reshuffle which of the existing four
+-- generators every depth from five onward gets, and with it every rng draw
+-- a builder places afterward. A maze floor is instead reached deliberately,
+-- via options.generator = "maze".
+
+local Map = require("map")
+local Tiles = require("tiles")
+
+local Maze = {}
+
+Maze.BRAID_NUMERATOR = 1
+Maze.BRAID_DENOMINATOR = 5
+
+local DIRS = { { 2, 0 }, { -2, 0 }, { 0, 2 }, { 0, -2 } }
+
+local function shuffledDirs(rng)
+  local dirs = { DIRS[1], DIRS[2], DIRS[3], DIRS[4] }
+  for i = 4, 2, -1 do
+    local j = rng:range(1, i)
+    dirs[i], dirs[j] = dirs[j], dirs[i]
+  end
+  return dirs
+end
+
+function Maze.generate(width, height, rng, options)
+  options = options or {}
+  local braidNumerator = options.braidNumerator or Maze.BRAID_NUMERATOR
+  local braidDenominator = options.braidDenominator or Maze.BRAID_DENOMINATOR
+  local map = Map.new(width, height)
+
+  local function key(x, y) return y * width + x end
+  local visited = {}
+
+  local startX, startY = 2, 2
+  visited[key(startX, startY)] = true
+  map:set(startX, startY, Tiles.FLOOR)
+  local stack = { { startX, startY } }
+
+  while #stack > 0 do
+    local top = stack[#stack]
+    local cx, cy = top[1], top[2]
+    local advanced = false
+    for _, dir in ipairs(shuffledDirs(rng)) do
+      local nx, ny = cx + dir[1], cy + dir[2]
+      if nx >= 2 and ny >= 2 and nx <= width - 1 and ny <= height - 1 and not visited[key(nx, ny)] then
+        local wx, wy = cx + dir[1] // 2, cy + dir[2] // 2
+        map:set(wx, wy, Tiles.FLOOR)
+        map:set(nx, ny, Tiles.FLOOR)
+        visited[key(nx, ny)] = true
+        stack[#stack + 1] = { nx, ny }
+        advanced = true
+        break
+      end
+    end
+    if not advanced then stack[#stack] = nil end
+  end
+
+  -- Braiding: a wall cell with floor on exactly one opposing pair of sides
+  -- (east-west or north-south) sits between two cells the backtracker above
+  -- already connected some longer way around; knocking a fixed fraction of
+  -- these down trims some, but never all, of the maze's dead ends.
+  for y = 3, height - 2 do
+    for x = 3, width - 2 do
+      if map:get(x, y) == Tiles.WALL then
+        local linksEastWest = map:get(x - 1, y) == Tiles.FLOOR and map:get(x + 1, y) == Tiles.FLOOR
+        local linksNorthSouth = map:get(x, y - 1) == Tiles.FLOOR and map:get(x, y + 1) == Tiles.FLOOR
+        if (linksEastWest or linksNorthSouth) and rng:chance(braidNumerator, braidDenominator) then
+          map:set(x, y, Tiles.FLOOR)
+        end
+      end
+    end
+  end
+
+  return map
+end
+
+return Maze