(defmodule Array

  (doc reduce "will reduce an array `xs` into a single value using a function `f` that takes the reduction thus far and the next value. The initial reduction value is `x`.

As an example, consider this definition of `sum` based on `reduce`:

```
(defn sum [x]
  (reduce + 0 x))
```

It will sum the previous sum with each new value, starting at `0`.")
  (defn reduce [f x xs]
    (let [total x]
      (do
        (for [i 0 (length xs)]
          (set! total (~f total (unsafe-nth xs i))))
        total)))

  (doc empty? "checks whether the array `a` is empty.")
  (defn empty? [a]
    (= (Array.length a) 0))

  (doc any? "checks whether any of the elements in `a` match the function `f`.")
  (defn any? [f a]
    (let-do [res false]
      (for [i 0 (length a)]
        (when (~f (unsafe-nth a i))
          (do
            (set! res true)
            (break))))
      res))

  (doc all? "checks whether all of the elements in `a` match the function `f`.")
  (defn all? [f a]
    (let-do [res true]
      (for [i 0 (length a)]
        (when (not (~f (unsafe-nth a i)))
          (do
            (set! res false)
            (break))))
      res))

  (doc find "finds an element in `a` that matches the function `f` and wraps it in a `Just`.

If it doesn’t find an element, `Nothing` will be returned.")
  (defn find [f a]
    (let-do [res (Maybe.Nothing)]
      (for [i 0 (length a)]
        (when (~f (unsafe-nth a i))
          (do
            (set! res (Maybe.Just @(unsafe-nth a i)))
            (break))))
      res))

  (doc find-index "finds the index of the first element in `a` that matches the function `f` and wraps it in a `Just`.

If it doesn’t find an index, `Nothing` will be returned.")
  (defn find-index [f a]
    (let-do [ret (Maybe.Nothing)]
      (for [i 0 (length a)]
        (when (~f (unsafe-nth a i))
          (do
            (set! ret (Maybe.Just i))
            (break))))
      ret))

  (doc unsafe-first "takes the first element of an array.

Generates a runtime error if the array is empty.")
  (defn unsafe-first [a]
    @(Array.unsafe-nth a 0))

  (doc first "takes the first element of an array and returns a `Just`.

Returns `Nothing` if the array is empty.")
  (defn first [a]
    (if (empty? a)
      (Maybe.Nothing)
      (Maybe.Just @(Array.unsafe-nth a 0))))

  (doc unsafe-last "takes the last element of an array.

Generates a runtime error if the array is empty.")
  (defn unsafe-last [a]
    @(Array.unsafe-nth a (Int.dec (Array.length a))))


  (doc last "takes the last element of an array and returns a `Just`.

Returns `Nothing` if the array is empty.")
  (defn last [a]
    (if (empty? a)
      (Maybe.Nothing)
      (Maybe.Just @(Array.unsafe-nth a (Int.dec (Array.length a))))))

  (doc = "compares two arrays.")
  (defn = [a b]
    (if (/= (length a) (length b))
      false
      (let-do [eq true]
        (for [i 0 (length a)]
          (when (/= (unsafe-nth a i) (unsafe-nth b i))
            (do
              (set! eq false)
              (break))))
        eq)))

    (doc maximum "gets the maximum in an array (elements must support `<`) and wraps it in a `Just`.

If the array is empty, it returns `Nothing`.")
  (defn maximum [xs]
    (if (empty? xs)
      (Maybe.Nothing)
      (let-do [result (unsafe-nth xs 0)
               n (length xs)]
        (for [i 1 n]
          (let [x (unsafe-nth xs i)]
            (when (< result x)
              (set! result x))))
        (Maybe.Just @result))))

  (doc minimum "gets the minimum in an array (elements must support `>`) and wraps it in a `Just`.

If the array is empty, returns `Nothing`")
  (defn minimum [xs]
    (if (empty? xs)
      (Maybe.Nothing)
      (let-do [result (unsafe-nth xs 0)
             n (length xs)]
        (for [i 1 n]
          (let [x (unsafe-nth xs i)]
            (when (> result x)
              (set! result x))))
        (Maybe.Just @result))))

  (doc sum "sums an array (elements must support `+` and `zero`).")
  (defn sum [xs]
    (Array.reduce &(fn [x y] (+ x @y)) (zero) xs))

  (doc subarray "gets a subarray from `start-index` to `end-index`.")
  (defn subarray [xs start-index end-index]
    (let-do [result []]
      (for [i start-index end-index]
        (set! result (push-back result @(unsafe-nth xs i))))
      result))

  (doc prefix-array "gets a prefix array to `end-index`.")
  (defn prefix-array [xs end-index]
    (subarray xs 0 end-index))

  (doc suffix-array "gets a suffix array from `start-index`.")
  (defn suffix-array [xs start-index]
    (subarray xs start-index (length xs)))

  (doc reverse "reverses an array.")
  (defn reverse [a]
    (let-do [i 0
             j (Int.dec (length &a))]
      (while (Int.< i j)
        (let-do [tmp @(unsafe-nth &a i)]
          (aset! &a i @(unsafe-nth &a j))
          (set! i (Int.inc i))
          (aset! &a j tmp)
          (set! j (Int.dec j))))
      a))

  (doc index-of "gets the index of element `e` in an array and wraps it on a `Just`.

If the element is not found, returns `Nothing`")
  (defn index-of [a e]
    (let-do [idx (Maybe.Nothing)]
      (for [i 0 (length a)]
        (when (= (unsafe-nth a i) e)
          (do
            (set! idx (Maybe.Just i))
            (break))))
      idx))

  (doc element-count "counts the occurrences of element `e` in an array.")
  (defn element-count [a e]
    (let-do [c 0]
      (for [i 0 (length a)]
        (when (= e (unsafe-nth a i)) (set! c (Int.inc c))))
      c))

  (doc predicate-count "counts the number of elements satisfying the predicate function `pred` in an array.")
  (defn predicate-count [a pred]
    (let-do [c 0]
      (for [i 0 (length a)]
        (when (~pred (unsafe-nth a i))
          (set! c (Int.inc c))))
      c))

  (doc aupdate "transmutes (i.e. updates) the element at index `i` of an array `a` using the function `f`.")
  (defn aupdate [a i f]
    (let [new-value (~f (unsafe-nth &a i))]
      (aset a i new-value)))

  (doc aupdate! "transmutes (i.e. updates) the element at index `i` of an array `a` using the function `f` in place.")
  (defn aupdate! [a i f]
    (aset! a i (~f (unsafe-nth a i))))

  (doc swap "swaps the indices `i` and `j` of an array `a`.")
  (defn swap [a i j]
    (let [x @(unsafe-nth &a i)
          y @(unsafe-nth &a j)]
      (aset (aset a i y) j x)))

  (doc swap! "swaps the indices `i` and `j` of an array `a` in place.")
  (defn swap! [a i j]
    (let-do [x @(unsafe-nth a i)
             y @(unsafe-nth a j)]
      (aset! a i y)
      (aset! a j x)))

  ; cannot use for, because we want also be able to go downwards
  (doc range "creates an array from `start` to `end` with `step` between them (the elements must support `<`, `<=`, `>=`, and `to-int`).")
  (defn range [start end step]
    (let-do [x (allocate (Int.inc (Int.abs (to-int (/ (- end start) step)))))
             e start
             i 0
             op (if (< start end) <= >=)]
      (while (op e end)
        (do
          (aset! &x i e)
          (set! i (Int.inc i))
          (set! e (+ e step))))
      x))

  (doc repeat "repeats the function `f` `n` times and stores the results in an array.")
  (defn repeat [n f]
    (let-do [a (allocate n)]
      (for [i 0 n] (aset-uninitialized! &a i (~f)))
      a))

  (doc repeat-indexed "repeats function `f` `n` times and stores the results in an array.

This is similar to [`repeat`](#repeat), but the function `f` will be supplied with the index of the element.")
  (defn repeat-indexed [n f]
    (let-do [a (allocate n)]
      (for [i 0 n] (aset-uninitialized! &a i (f i)))
      a))

  (doc replicate "repeats element `e` `n` times and stores the results in an array.")
  (defn replicate [n e]
    (let-do [a (allocate n)]
      (for [i 0 n] (aset-uninitialized! &a i @e))
      a))

  (doc copy-map "maps over an array `a` using the function `f`.

This function copies the array. If you don’t want that, use [`endo-map`](#endo-map).")
  (defn copy-map [f a]
    (let-do [na (allocate (length a))]
      (for [i 0 (length a)]
        (aset-uninitialized! &na i (~f (unsafe-nth a i))))
      na))

  (doc zip "maps over two arrays using a function `f` that takes two arguments. It will produces a new array with the length of the shorter input.

The trailing elements of the longer array will be discarded.")
  (defn zip [f a b]
    (let-do [l (Int.min (length a) (length b))
             na (allocate l)]
      (for [i 0 l]
        (aset-uninitialized! &na i (~f (unsafe-nth a i) (unsafe-nth b i))))
      na))

  (doc sum-length "returns the sum of lengths from a nested array `xs`.")
  (defn sum-length [xs]
    (let-do [sum 0
             lxs (Array.length xs)]
      (for [i 0 lxs]
        (set! sum (+ sum (Array.length (Array.unsafe-nth xs i)))))
      sum))

  (doc zero "returns the empty array.")
  (defn zero [] [])

  (doc concat "returns a new array which is the concatenation of the provided nested array `xs`.")
  (defn concat [xs]
    ;; This is using a StringBuilder pattern to only perform one allocation and
    ;; to only copy each of the incoming Array(s) once.
    ;; This currently performs wasted Array.length calls, as we call it for each
    ;; Array once here and once in sum-length.
    (let-do [j 0
             lxs (Array.length xs)
             result (Array.allocate (sum-length xs))]
      (for [i 0 lxs]
        (let-do [arr (Array.unsafe-nth xs i)
                 len (Array.length arr)]
          (for [k 0 len]
            (aset-uninitialized! &result (+ j k) @(Array.unsafe-nth arr k)))
          (set! j (+ j len))))
      result))

  (doc enumerated "creates a new array of `Pair`s where the first position is the index and the second position is the element from the original array `xs`.")
  (defn enumerated [xs]
    (zip &Pair.init-from-refs
         &(range 0 (length xs) 1) ; Inefficient, creates a temporary array.
         xs))

  (doc nth "gets a reference to the `n`th element from an array `arr` wrapped on a `Maybe`.

If the `index` is out of bounds, return `Maybe.Nothing`")
  (defn nth [arr index]
  (if (and (>= index 0) (< index (length arr)))
    (Maybe.Just @(unsafe-nth a i)) ; the copy will go away with lifetimes
    (Maybe.Nothing)))

  (doc remove "removes all occurrences of the element `el` in the array `arr`, in place.")
  (defn remove [el arr]
    (endo-filter &(fn [x] (not (= el x)))
                 arr))

  (doc remove-nth "removes element at index `idx` from the array `arr`.")
  (defn remove-nth [i arr]
    (do
      ;;(assert (<= 0 i))
      ;;(assert (< i (Array.length &arr)))
      (for [j i (Int.dec (Array.length &arr))]
        (aset! &arr j @(unsafe-nth &arr (inc j))))
      (pop-back arr)))

   (doc copy-filter "filters the elements in an array.

It will create a copy. If you want to avoid that, consider using [`endo-filter`](#endo-filter) instead.")
  (defn copy-filter [f a] (endo-filter f @a))

  (doc contains? "checks wether an element exists in the array.")
  (defn contains? [arr el]
    (let-do [result false]
      (for [i 0 (Array.length arr)]
        (when (= el (Array.unsafe-nth arr i))
          (do
            (set! result true)
            (break))))
      result))
)