feat: solve day 4 puzzle 2

src/day04/puzzle2.lua

@@ -1,7 +1,106 @@
+local utils = require "utils"
+
 local puzzle2 = {}
 
+function puzzle2.inGrid(x, y, w, h)
+    return 1 <= x and x <= w and 1 <= y and y <= h
+end
+
+function puzzle2.countNeighbors(lines, w, h, x, y)
+    local neighbors = 0
+    for dy=-1, 1 do
+        for dx=-1, 1 do
+            if dx ~=0 or dy ~= 0 then
+                local x2 = x + dx
+                local y2 = y + dy
+                if puzzle2.inGrid(x2, y2, w, h) then
+                    if lines[y2]:sub(x2, x2) == "@" then
+                        neighbors = neighbors + 1
+                    end
+                end
+            end
+        end
+    end
+    return neighbors
+end
+
+function puzzle2.copyGrid(grid)
+    local grid2 = {}
+    for y, row in ipairs(grid) do
+        local row2 = {}
+        for x, n in ipairs(row) do
+            row2[x] = n
+        end
+        grid2[y] = row2
+    end
+    return grid2
+end
+
+function puzzle2.removeRoll(grid, x, y, w, h)
+    for dy=-1, 1 do
+        for dx=-1, 1 do
+            if dx ~= 0 or dy ~= 0 then
+                local x2 = x + dx
+                local y2 = y + dy
+                if puzzle2.inGrid(x2, y2, w, h) then
+                    if grid[y2][x2] > 0 then
+                        grid[y2][x2] = grid[y2][x2] - 1
+                    end
+                end
+            end
+        end
+    end
+    grid[y][x] = -1
+end
+
+function puzzle2.removeRolls(grid, w, h)
+    local removed = 0
+    local grid2 = puzzle2.copyGrid(grid)
+
+    for y, row in ipairs(grid) do
+        for x, n in ipairs(row) do
+            if 0 <= n and n < 4 then
+                puzzle2.removeRoll(grid2, x, y, w, h)
+                removed = removed + 1
+            end
+        end
+    end
+
+    return removed, grid2
+end
+
 function puzzle2.solve(input)
-    return 0
+    local lines = utils.splitLines(input)
+
+    local h = #lines
+    local w = #lines[1]
+
+    local grid = {}
+
+    for y, line in ipairs(lines) do
+        local row = {}
+        for x=1, #line do
+            local neighbors = -1
+            if line:sub(x, x) == "@" then
+                neighbors = puzzle2.countNeighbors(lines, w, h, x, y)
+            end
+            row[x] = neighbors
+        end
+        grid[y] = row
+    end
+
+    local totalRemoved = 0
+
+    local removed = 0
+    while true do
+        removed, grid = puzzle2.removeRolls(grid, w, h)
+        if removed == 0 then
+            break
+        end
+        totalRemoved = totalRemoved + removed
+    end
+
+    return totalRemoved
 end
 
 return puzzle2