1 files changed, 16 insertions(+), 29 deletions(-)

M 2023/10.lua

@@ 1,6 1,7 @@
-local dirs = { -- LRUD
-	[string.byte'|'] = 0x3, [string.byte'F'] = 0x5, [string.byte'L'] = 0x6,
-	[string.byte'7'] = 0x9, [string.byte'J'] = 0xa, [string.byte'-'] = 0xc,
+local L, R, U, D = 1, 2, 4, 8
+local dirs = {
+	[string.byte'|'] = U|D, [string.byte'F'] = R|D, [string.byte'L'] = R|U,
+	[string.byte'7'] = L|D, [string.byte'J'] = L|U, [string.byte'-'] = L|R,
 }
 local S, grid, y, sx, sy = string.byte'S', {}, 1
 for line in io.lines() do


@@ 13,33 14,19 @@ for line in io.lines() do
 	grid[y], y = row, y + 1
 end
 
-grid[sy][sx] = (grid[sy][sx - 1] & 0x4) << 1 | (grid[sy][sx + 1] & 0x8) >> 1
-             | (grid[sy - 1][sx] & 0x1) << 1 | (grid[sy + 1][sx] & 0x2) >> 1
-local x, y, dir, step = sx, sy, 0, 0
+local x, y, i, a, dir = sx, sy, 0, 0, D
+if     sx > 1          and grid[sy][sx - 1] & R ~= 0 then dir = L
+elseif sx < #grid[sy]  and grid[sy][sx + 1] & L ~= 0 then dir = R
+elseif sy > 1          and grid[sy - 1][sx] & D ~= 0 then dir = U
+else assert(sy < #grid and grid[sy + 1][sx] & U ~= 0) dir = D
+end
 repeat
-	local tile = grid[y][x]
-	grid[y][x], dir = tile | 0x10, tile & ~dir
-	if dir & 0x1 ~= 0 then
-		y = y + 1
-	elseif dir & 0x2 ~= 0 then
-		y = y - 1
-	elseif dir & 0x4 ~= 0 then
-		x = x + 1
-	elseif dir & 0x8 ~= 0 then
-		x = x - 1
+	if     dir == L then  x, a = x - 1, a + y
+	elseif dir == R then  x, a = x + 1, a - y
+	elseif dir == U then  y, a = y - 1, a - x
+	else assert(dir == D) y, a = y + 1, a + x
 	end
-	step, dir = step + 1, (dir & 0x5) << 1 | (dir & 0xa) >> 1
+	i, dir = i + 1, grid[y][x] & ~((dir & 0x5) << 1 | (dir & 0xa) >> 1)
 until x == sx and y == sy
-
-local ans1, ans2 = step // 2, 0
-for y, row in ipairs(grid) do
-	local inside
-	for x, tile in ipairs(row) do
-		if tile & 0x10 ~= 0 then
-			if tile & 1 ~= 0 then inside = not inside end
-		elseif inside then
-			ans2 = ans2 + 1
-		end
-	end
-end
+local ans1, ans2 = i // 2, (math.abs(a) - i) // 2 + 1
 print(ans1, ans2)