Day 04, return to Lua

After comparing the time I take with both languages, Lua seems more
practical.

1 files changed, 78 insertions, 0 deletions

diff --git a/04.lua b/04.lua
new file mode 100644
index 0000000..9b1d8f9
--- /dev/null
+++ b/04.lua
@@ -0,0 +1,78 @@
+local input = {}
+
+for line in io.lines() do
+    local row = {}
+    for i = 1,#line do
+        -- X,M,A,S --> 1,2,3,4
+        row[i] = string.find('XMAS', string.sub(line, i, i)) or 0
+    end
+    table.insert(input, row)
+end
+
+function compute_bounds (dx, size)
+    return math.max(0, -3*dx)+1, math.min(size, size-3*dx)
+end
+
+function input:_has_xmas (i, j, di, dj)
+    for x = 0,3 do
+        if self[i+x*di][j+x*dj] ~= x+1 then
+            return false
+        end
+    end
+    return true
+end
+
+function input:part1 ()
+    local m = #self
+    local n = 0
+    if m ~= 0 then n = #self[1] end
+    local sum = 0
+
+    for di = -1,1 do
+        local istart, iend = compute_bounds(di, m)
+        for dj = -1,1 do
+            if di ~= 0 or dj ~= 0 then
+                local jstart, jend = compute_bounds(dj, n)
+                for i = istart,iend do
+                    for j = jstart,jend do
+                        if self:_has_xmas(i, j, di, dj) then
+                            sum = sum + 1
+                        end
+                    end
+                end
+            end
+        end
+    end
+    print(sum)
+end
+
+function input:_has_x_mas (i, j)
+    if self[i][j] ~= 3 then return false end
+    local nm = 0
+    for ci = i-1,i+1,2 do
+        for cj = j-1,j+1,2 do
+            if self[ci][cj] == 2 then
+                nm = nm + 1
+                if nm > 2 then return false end
+            elseif self[ci][cj] ~= 4 then
+                return false
+            end
+        end
+    end
+    return nm == 2 and self[i-1][j-1] ~= self[i+1][j+1]
+end
+
+function input:part2 ()
+    local m = #self
+    local n = 0
+    if m ~= 0 then n = #self[1] end
+    local sum = 0
+    for i = 2,m-1 do
+        for j = 2,n-1 do
+            if self:_has_x_mas(i, j) then sum = sum + 1 end
+        end
+    end
+    print(sum)
+end
+
+return input