Solution for task 02 day 07

Author: Emrys-Merlin

2025/haskell/app/Day07.hs

@@ -10,7 +10,15 @@ task01 content =
   in show $ splitBeams grid [start]
 
 task02 :: String -> String
-task02 content = "Apply task02 solution to content: " ++ take 100 content
+task02 content =
+  let grid = toGrid content
+  in case grid of
+    [] -> error "Empty grid."
+    (first_row:remaining_grid) ->
+      let start = findStart first_row
+          l = length first_row
+          counts = replicate start 0 ++ [1] ++ replicate (l - start - 1) 0
+      in show $ beamCount remaining_grid counts
 
 toGrid :: String -> [[Char]]
 toGrid = lines
@@ -27,6 +35,7 @@ splitBeams (row:rest) beams =
       new_beams = nub duplicated_beams
   in counter + splitBeams rest new_beams
 
+-- TODO: deduplicae entries directly
 split :: [Char] -> [Int] -> ([Int], Int)
 split _ [] = ([], 0)
 split row (beam:remaining_beams) =
@@ -36,3 +45,26 @@ split row (beam:remaining_beams) =
         | otherwise = ([beam], 0)
       (other_beams, other_counter) = split row remaining_beams
   in (added_beams ++ other_beams, counter + other_counter)
+
+beamCount :: [[Char]] -> [Int] -> Int
+beamCount [] counts = sum counts
+beamCount (row:grid) counts =
+  let
+    prev = ('.', 0)
+    rows_counts = zip (row ++ ".") (counts ++ [0])
+  in
+    case rows_counts of
+      [] -> error "This should not be reached."
+      curr:rest ->
+        let
+          new_counts = iterRow prev curr rest
+        in beamCount grid new_counts
+
+iterRow :: (Char, Int) -> (Char, Int) -> [(Char, Int)] -> [Int]
+iterRow _ _ [] = []
+iterRow (prev_char, prev_count) (curr_char, curr_count) ((next_char, next_count):rows_counts) = new_count:iterRow (curr_char, curr_count) (next_char, next_count) rows_counts
+  where
+    new_count =
+      if curr_char == '^'
+      then 0
+      else curr_count + (if prev_char == '^' then prev_count else 0) + (if next_char == '^' then next_count else 0)