Year 2024 Day 4

2024/day04.rb

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+input = STDIN.read
+mat = input.split("\n").map(&:chars)
+
+# Part 1
+def count_for(input)
+  (input + "\n" + input.reverse).scan('XMAS').size
+end
+
+def make_input_diag(mat)
+  input_diag = []
+  0.upto(mat.first.size - 4) do |col|
+    i = 0
+    j = col
+    while i < mat.size && j < mat.first.size
+      input_diag << mat[i][j]
+      i += 1
+      j += 1
+    end
+    input_diag << '|'
+  end
+  1.upto(mat.size - 4) do |row|
+    i = row
+    j = 0
+    while i < mat.size && j < mat.first.size
+      input_diag << mat[i][j]
+      i += 1
+      j += 1
+    end
+    input_diag << '|'
+  end
+  input_diag.join
+end
+
+input_transposed = mat.transpose.map(&:join).join("\n")
+
+horiz_count = count_for(input)
+vert_count = count_for(input_transposed)
+diag_count = count_for(make_input_diag(mat)) + count_for(make_input_diag(mat.reverse))
+
+puts horiz_count + vert_count + diag_count
+
+# Part 2
+count = 0
+1.upto(mat.size - 2).each do |i|
+  1.upto(mat.first.size - 2).each do |j|
+    next unless mat[i][j] == 'A'
+
+    primary = %w[MS SM].include?([mat[i-1][j-1], mat[i+1][j+1]].join)
+    secondary = %w[MS SM].include?([mat[i+1][j-1], mat[i-1][j+1]].join)
+    count += 1 if primary && secondary
+  end
+end
+puts count

2024/examples/day04.txt

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+MMMSXXMASM
+MSAMXMSMSA
+AMXSXMAAMM
+MSAMASMSMX
+XMASAMXAMM
+XXAMMXXAMA
+SMSMSASXSS
+SAXAMASAAA
+MAMMMXMMMM
+MXMXAXMASX