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| (defn move-to-front [first others] | |
| "Moves an item to the front of the list" | |
| (cons first (remove #(identical? first %) others))) | |
| (defn first-level [list] | |
| "Returns a list of all first-level permutations." | |
| (map #(move-to-front % list) list)) | |
| (defn permute-deeper [depth list] | |
| "Takes a list and a depth. Ignores the first few items according to the depth, and returns a list of all permutations for the rest." | |
| (let [head-of-list (take depth list) | |
| rest-of-list (drop depth list) | |
| add-head #(reduce conj % (reverse head-of-list))] | |
| (map #(add-head (move-to-front % rest-of-list)) rest-of-list))) | |
| ;; (first-level [1 2 3]) | |
| ;; ((1 2 3) (2 1 3) (3 1 2)) | |
| ;; (map #(permute-deeper 1 %) (first-level [1 2 3])) | |
| ;; (((1 2 3) (1 3 2)) ((2 1 3) (2 3 1)) ((3 1 2) (3 2 1))) | |
| ;; goal: | |
| ;; (permute [1 2 3]) | |
| ;; ((1 2 3) (1 3 2) (2 1 3) (2 3 1) (3 1 2) (3 2 1)) | |
| (defn permute [coll] | |
| (reduce | |
| #(mapcat (partial permute-deeper %2) %) | |
| [coll] | |
| (range (count coll)))) |
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