# ============================================================================
# grid.ludic — grid geometry and pathfinding over the tilemap, in Ludic.
#
# The Grid.* / Path.* namespaces (see emit_call.ludic) operate on the tilemap
# that Map.size/Map.row set up (rt_map / rt_tile in core.ludic). A cell is
# passable unless it is out of bounds or holds the caller's `wall` tile — a
# single char code, e.g. '#', so any impassable glyph works. Everything is
# integer and deterministic: same map + same query -> same path, every run.
#
# ludicc splices this file into a game via core.ludic (it needs the tilemap), so
# it links only where the tilemap does. Returned cell lists are ordinary Ludic
# slices — index them with `len` / `[i]` (Ludic's `for` iterates ranges, not
# collections).
# ============================================================================

property Cell { x: int = 0, y: int = 0 }

function grid_abs(v: int) -> int { if v < 0 { return -v };  return v }
function grid_in_bounds(x: int, y: int) -> bool { return x >= 0 and y >= 0 and x < rt_mapw and y < rt_maph }
# a cell blocks movement if it is out of bounds or holds the `wall` tile.
function grid_blocked(x: int, y: int, wall: int) -> bool {
  if not grid_in_bounds(x, y) { return true }
  return rt_tile(x, y) == wall
}

# Bresenham line from (x0,y0) to (x1,y1), inclusive — every cell it crosses.
function grid_line(x0: int, y0: int, x1: int, y1: int) -> []Cell {
  let out = new []Cell
  var x = x0
  var y = y0
  let dx = grid_abs(x1 - x0)
  let dy = grid_abs(y1 - y0)
  var sx = -1
  if x0 < x1 { sx = 1 }
  var sy = -1
  if y0 < y1 { sy = 1 }
  var err = dx - dy
  while true {
    let c = new Cell
    c.x = x;  c.y = y
    push(out, c)
    if x == x1 and y == y1 { break }
    let e2 = 2 * err
    if e2 > -dy { err -= dy;  x += sx }
    if e2 < dx { err += dx;  y += sy }
  }
  return out
}

# line of sight: true if the straight line hits no `wall` cell (endpoints incl).
function grid_line_of_sight(x0: int, y0: int, x1: int, y1: int, wall: int) -> bool {
  let cells = grid_line(x0, y0, x1, y1)
  var i = 0
  while i < len(cells) {
    if grid_blocked(cells[i].x, cells[i].y, wall) { return false }
    i += 1
  }
  return true
}

# 4-connected flood fill: every passable cell reachable from (sx,sy), BFS order.
function grid_flood(sx: int, sy: int, wall: int) -> []Cell {
  let out = new []Cell
  if grid_blocked(sx, sy, wall) { return out }
  let w = rt_mapw
  let n = w * rt_maph
  let seen = bytes(n)
  var i = 0
  while i < n { seen[i] = 0;  i += 1 }
  let qx = words(n)
  let qy = words(n)
  var head = 0
  var tail = 0
  qx[tail] = sx;  qy[tail] = sy;  tail += 1
  seen[sy * w + sx] = 1
  while head < tail {
    let cx = qx[head]
    let cy = qy[head]
    head += 1
    let c = new Cell
    c.x = cx;  c.y = cy
    push(out, c)
    var dir = 0
    while dir < 4 {
      var nx = cx
      var ny = cy
      if dir == 0 { nx = cx + 1 }
      if dir == 1 { nx = cx - 1 }
      if dir == 2 { ny = cy + 1 }
      if dir == 3 { ny = cy - 1 }
      if grid_in_bounds(nx, ny) and seen[ny * w + nx] == 0 and not grid_blocked(nx, ny, wall) {
        seen[ny * w + nx] = 1
        qx[tail] = nx;  qy[tail] = ny;  tail += 1
      }
      dir += 1
    }
  }
  return out
}

# A* shortest path over the 4-connected grid, uniform step cost, Manhattan
# heuristic. Returns the path start..goal inclusive, or an empty list if the
# goal is unreachable (or start/goal is a wall). The open set is a linear scan —
# ample for a tilemap (<= 96x64), and the heuristic keeps it near-optimal work.
function path_a_star(x0: int, y0: int, x1: int, y1: int, wall: int) -> []Cell {
  let out = new []Cell
  if grid_blocked(x0, y0, wall) or grid_blocked(x1, y1, wall) { return out }
  let w = rt_mapw
  let n = w * rt_maph
  let INF = 1000000000
  let g = words(n)             # cost from start (INF = unvisited)
  let came = words(n)          # parent cell index (-1 = none)
  let inopen = bytes(n)
  let closed = bytes(n)
  var i = 0
  while i < n { g[i] = INF;  came[i] = -1;  inopen[i] = 0;  closed[i] = 0;  i += 1 }
  let start = y0 * w + x0
  let goal = y1 * w + x1
  g[start] = 0
  inopen[start] = 1
  var found = false
  while true {
    var best = -1
    var bestf = INF
    i = 0
    while i < n {
      if inopen[i] == 1 {
        let cx = i - (i / w) * w
        let cy = i / w
        let f = g[i] + grid_abs(cx - x1) + grid_abs(cy - y1)
        if f < bestf { bestf = f;  best = i }
      }
      i += 1
    }
    if best < 0 { break }
    if best == goal { found = true;  break }
    inopen[best] = 0
    closed[best] = 1
    let cx = best - (best / w) * w
    let cy = best / w
    var dir = 0
    while dir < 4 {
      var nx = cx
      var ny = cy
      if dir == 0 { nx = cx + 1 }
      if dir == 1 { nx = cx - 1 }
      if dir == 2 { ny = cy + 1 }
      if dir == 3 { ny = cy - 1 }
      if grid_in_bounds(nx, ny) and not grid_blocked(nx, ny, wall) {
        let ni = ny * w + nx
        if closed[ni] == 0 {
          let ng = g[best] + 1
          if ng < g[ni] { g[ni] = ng;  came[ni] = best;  inopen[ni] = 1 }
        }
      }
      dir += 1
    }
  }
  if not found { return out }
  # reconstruct goal..start, then reverse into out
  let rev = new []Cell
  var cur = goal
  while cur >= 0 {
    let c = new Cell
    c.x = cur - (cur / w) * w
    c.y = cur / w
    push(rev, c)
    if cur == start { break }
    cur = came[cur]
  }
  var k = len(rev) - 1
  while k >= 0 { push(out, rev[k]);  k -= 1 }
  return out
}