diff --git a/src/fov.lua b/src/fov.lua
new file mode 100644
index 0000000..598d69b
--- /dev/null
+++ b/src/fov.lua
@@ -0,0 +1,62 @@
+-- Recursive shadowcasting over eight octants. Returns a set of visible map
+-- indices for a given origin and radius.
+
+local Fov = {}
+
+local mult = {
+  { 1, 0, 0, -1, -1, 0, 0, 1 },
+  { 0, 1, -1, 0, 0, -1, 1, 0 },
+  { 0, 1, 1, 0, 0, -1, -1, 0 },
+  { 1, 0, 0, 1, -1, 0, 0, -1 },
+}
+
+local function castLight(map, cx, cy, row, start, finish, radius, xx, xy, yx, yy, visible)
+  if start < finish then return end
+  local radiusSq = radius * radius
+  local newStart = 0.0
+  for j = row, radius do
+    local dx, dy = -j - 1, -j
+    local blocked = false
+    while dx <= 0 do
+      dx = dx + 1
+      local mapX = cx + dx * xx + dy * xy
+      local mapY = cy + dx * yx + dy * yy
+      local leftSlope = (dx - 0.5) / (dy + 0.5)
+      local rightSlope = (dx + 0.5) / (dy - 0.5)
+      if start < rightSlope then
+        -- cell not yet inside the current view; keep scanning
+      elseif finish > leftSlope then
+        break
+      else
+        if dx * dx + dy * dy <= radiusSq and map:inBounds(mapX, mapY) then
+          visible[map:index(mapX, mapY)] = true
+        end
+        if blocked then
+          if map:blocksSight(mapX, mapY) then
+            newStart = rightSlope
+          else
+            blocked = false
+            start = newStart
+          end
+        elseif map:blocksSight(mapX, mapY) and j < radius then
+          blocked = true
+          castLight(map, cx, cy, j + 1, start, leftSlope, radius, xx, xy, yx, yy, visible)
+          newStart = rightSlope
+        end
+      end
+    end
+    if blocked then break end
+  end
+end
+
+function Fov.compute(map, originX, originY, radius)
+  local visible = {}
+  visible[map:index(originX, originY)] = true
+  for octant = 1, 8 do
+    castLight(map, originX, originY, 1, 1.0, 0.0, radius,
+      mult[1][octant], mult[2][octant], mult[3][octant], mult[4][octant], visible)
+  end
+  return visible
+end
+
+return Fov