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Contents
module Spiral (spiral) where
-- | Create a square matrix of natural numbers in a inward, clockwise spiral.
spiral :: Int -> [[Int]]
spiral l = matrix (spiralIndex (l, l)) (l, l)
where
-- If an index isn't in the first row of a matrix, it is in the
-- 90 degress-rotated, complementary matrix.
spiralIndex :: Integral a => (a, a) -> (a, a) -> a
spiralIndex _ (1, j) = j
spiralIndex (h, w) (i, j) = w + spiralIndex (w, h - 1) (w - j + 1, i - 1)
-- Build a lazy, list-based matrix based on a formula.
matrix :: ((Int, Int) -> a) -> (Int, Int) -> [[a]]
matrix f (rows, columns) = [[ f (i, j)
| j <- [1..columns] ]
| i <- [1..rows ] ]
Version data entries
193 entries across 193 versions & 1 rubygems