Emulating non-rectangular arrays - haskell

Question

Often times you want the performance of arrays over linked lists while having not conforming to the requirement of having rectangular arrays.
As an example consider an hexagonal grid, here shown with the 1-distance neighbors of cell (3, 3) in medium gray and the 2-distance neighbors in light gray.
Say we want an array that contains, for each cell, the indices of every 1- and 2-distance neighbor for that cell. One slight issue is that cells have a different amount of X-distance neighbors -- cells on the grid border will have fewer neighbors than cells closer to the grid center.
(We want an array of neighbor indices --- instead of a function from cell coordinates to neighbor indices --- for performance reasons.)
We can work around this problem by keeping track of how many neighbors each cell has. Say you have an array
neighbors2 of size R x C x N x 2, where R is the number of grid rows, C for columns, and N is the maximum number of 2-distance neighbors for any cell in the grid.
Then, by keeping an additional array n_neighbors2 of size R x C, we can keep track of which indices in neighbors2 are populated and which are just zero padding. For example, to retrieve the the 2-distance neighbors of cell (2, 5), we simply index into the array as such:
someNeigh = neighbors2[2, 5, 0..n_neighbors2[2, 5], ..]
someNeigh will be a n_neighbors2[2, 5] x 2 array (or view) of indicies, where someNeigh[0, 0] yields the row of the first neighbor, and someNeigh[0, 1] yields the column of the first neighbor and so forth.
Note that the elements at the positions
neighbors2[2, 5, n_neighbors2[2, 5]+1.., ..]
are irrelevant; this space is just padding to keep the matrix rectangular.
Provided we have a function for finding the d-distance neighbors for any cell:
import Data.Bits (shift)
rows, cols = (7, 7)
type Cell = (Int, Int)
generateNeighs :: Int -> Cell -> [Cell]
generateNeighs d cell1 = [ (row2, col2)
| row2 <- [0..rows-1]
, col2 <- [0..cols-1]
, hexDistance cell1 (row2, col2) == d]
hexDistance :: Cell -> Cell -> Int
hexDistance (r1, c1) (r2, c2) = shift (abs rd + abs (rd + cd) + abs cd) (-1)
where
rd = r1 - r2
cd = c1 - c2
How can we create the aforementioned arrays neighbors2 and n_neighbors2? Assume we know the maximum amount of 2-distance neighbors N beforehand. Then it is possible to modify generateNeighs to always return lists of the same size, as we can fill up remaining entries with (0, 0). That leaves, as I see it, two problems:
We need a function to populate neighbors2 which operates not every individual index but on a slice, in our case it should fill one cell at a time.
n_neighbors2 should be populated simultaneously as neighbors2
A solution is welcome with either repa or accelerate arrays.

Answer 1

Here's you picture skewed 30 degrees to the right:
As you can see your array is actually perfectly rectangular.
The indices of a neighborhood's periphery are easily found as six straight pieces around the chosen center cell, e.g. (imagine n == 2 is the distance of the periphery from the center (i,j) == (3,3) in the picture):
periphery n (i,j) =
-- 2 (3,3)
let
((i1,j1):ps1) = reverse . take (n+1) . iterate (\(i,j)->(i,j+1)) $ (i-n,j)
-- ( 1, 3)
((i2,j2):ps2) = reverse . take (n+1) . iterate (\(i,j)->(i+1,j)) $ (i1,j1)
-- ( 1, 5)
.....
ps6 = ....... $ (i5,j5)
in filter isValid (ps6 ++ ... ++ ps2 ++ ps1)
The whole neighborhood is simply
neighborhood n (i,j) = (i,j) : concat [ periphery k (i,j) | k <- [1..n] ]
For each cell/distance combination, simply generate the neighborhood indices on the fly and access your array in O(1) time for each index pair.

Answer 2

Writing out the answer from #WillNess in full, and incorporating the proposal from #leftroundabout to store indecies in a 1D vector instead, and we get this:
import qualified Data.Array.Accelerate as A
import Data.Array.Accelerate (Acc, Array, DIM1, DIM2, DIM3, Z(..), (:.)(..), (!), fromList, use)
rows = 7
cols = 7
type Cell = (Int, Int)
(neighs, nNeighs) = generateNeighs
-- Return a vector of indices of cells at distance 'd' or less from the given cell
getNeighs :: Int -> Cell -> Acc (Array DIM1 Cell)
getNeighs d (r,c) = A.take n $ A.drop start neighs
where
start = nNeighs ! A.constant (Z :. r :. c :. 0)
n = nNeighs ! A.constant (Z :. r :. c :. d)
generateNeighs :: (Acc (Array DIM1 Cell), Acc (Array DIM3 Int))
generateNeighs = (neighsArr, nNeighsArr)
where
idxs = concat [[(r, c) | c <- [0..cols-1]] | r <- [0..rows-1]]
(neighsLi, nNeighsLi, n) = foldl inner ([], [], 0) idxs
neighsArr = use $ fromList (Z :. n) neighsLi
nNeighsArr = use $ fromList (Z :. rows :. cols :. 5) nNeighsLi
inner (neighs', nNeighs', n') idx = (neighs' ++ cellNeighs, nNeighs'', n'')
where
(cellNeighs, cellNNeighs) = neighborhood idx
n'' = n' + length cellNeighs
nNeighs'' = nNeighs' ++ n' : cellNNeighs
neighborhood :: Cell -> ([Cell], [Int])
neighborhood (r,c) = (neighs, nNeighs)
where
neighsO = [ periphery d (r,c) | d <- [1..4] ]
neighs = (r,c) : concat neighsO
nNeighs = tail $ scanl (+) 1 $ map length neighsO
periphery d (r,c) =
-- The set of d-distance neighbors form a hexagon shape. Traverse each of
-- the sides of this hexagon and gather up the cell indices.
let
ps1 = take d . iterate (\(r,c)->(r,c+1)) $ (r-d,c)
ps2 = take d . iterate (\(r,c)->(r+1,c)) $ (r-d,c+d)
ps3 = take d . iterate (\(r,c)->(r+1,c-1)) $ (r,c+d)
ps4 = take d . iterate (\(r,c)->(r,c-1)) $ (r+d,c)
ps5 = take d . iterate (\(r,c)->(r-1,c)) $ (r+d,c-d)
ps6 = take d . iterate (\(r,c)->(r-1,c+1)) $ (r,c-d)
in filter isValid (ps6 ++ ps5 ++ ps4 ++ ps3 ++ ps2 ++ ps1)
isValid :: Cell -> Bool
isValid (r, c)
| r < 0 || r >= rows = False
| c < 0 || c >= cols = False
| otherwise = True

Answer 3

This can be by using the permute function to fill the neighbors for 1 cell at a time.
import Data.Bits (shift)
import Data.Array.Accelerate as A
import qualified Prelude as P
import Prelude hiding ((++), (==))
rows = 7
cols = 7
channels = 70
type Cell = (Int, Int)
(neighs, nNeighs) = fillNeighs
getNeighs :: Cell -> Acc (Array DIM1 Cell)
getNeighs (r, c) = A.take (nNeighs ! sh1) $ slice neighs sh2
where
sh1 = constant (Z :. r :. c)
sh2 = constant (Z :. r :. c :. All)
fillNeighs :: (Acc (Array DIM3 Cell), Acc (Array DIM2 Int))
fillNeighs = (neighs2, nNeighs2)
where
sh = constant (Z :. rows :. cols :. 18) :: Exp DIM3
neighZeros = fill sh (lift (0 :: Int, 0 :: Int)) :: Acc (Array DIM3 Cell)
-- nNeighZeros = fill (constant (Z :. rows :. cols)) 0 :: Acc (Array DIM2 Int)
(neighs2, nNeighs2li) = foldr inner (neighZeros, []) indices
nNeighs2 = use $ fromList (Z :. rows :. cols) nNeighs2li
-- Generate indices by varying column fastest. This assures that fromList, which fills
-- the array in row-major order, gets nNeighs in the correct order.
indices = foldr (\r acc -> foldr (\c acc2 -> (r, c):acc2 ) acc [0..cols-1]) [] [0..rows-1]
inner :: Cell
-> (Acc (Array DIM3 Cell), [Int])
-> (Acc (Array DIM3 Cell), [Int])
inner cell (neighs, nNeighs) = (newNeighs, n : nNeighs)
where
(newNeighs, n) = fillCell cell neighs
-- Given an cell and a 3D array to contain cell neighbors,
-- fill in the neighbors for the given cell
-- and return the number of neighbors filled in
fillCell :: Cell -> Acc (Array DIM3 Cell) -> (Acc (Array DIM3 Cell), Int)
fillCell (r, c) arr = (permute const arr indcomb neighs2arr, nNeighs)
where
(ra, ca) = (lift r, lift c) :: (Exp Int, Exp Int)
neighs2li = generateNeighs 2 (r, c)
nNeighs = P.length neighs2li
neighs2arr = use $ fromList (Z :. nNeighs) neighs2li
-- Traverse the 3rd dimension of the given cell
indcomb :: Exp DIM1 -> Exp DIM3
indcomb nsh = index3 ra ca (unindex1 nsh)
generateNeighs :: Int -> Cell -> [Cell]
generateNeighs d cell1 = [ (row2, col2)
| row2 <- [0..rows]
, col2 <- [0..cols]
, hexDistance cell1 (row2, col2) P.== d]
-- Manhattan distance between two cells in an hexagonal grid with an axial coordinate system
hexDistance :: Cell -> Cell -> Int
hexDistance (r1, c1) (r2, c2) = shift (abs rd + abs (rd + cd) + abs cd) (-1)
where
rd = r1 - r2
cd = c1 - c2