(defmacro for [settings :rest body] ;; settings = variable, from, to, <step>
  (if (> (length body) 1)
    (macro-error "Warning: the body of the 'for' loop can only contain one expression")
    (let [variable (car settings)
          from (cadr settings)
          to (caddr settings)
          step (if (> (length settings) 3) (cadddr settings) 1)
          comp (if (> (length settings) 4)
                 (cadddr (cdr settings))
                 (if (< step (- step step)) '> '<))
          ]
      `(let [%variable %from]
         (while (%comp %variable %to)
           (do
             %(cond
                (= (length body) 0) ()
                (list? body) (car body)
                body)
             (set! %variable (+ %variable %step))))))))

(doc Array "is the indexable collection data structure.

Its literal uses brackets, so an array of integers would look like this:
`[1 2 3]`. It provides both a functional and an imperative API, and is one of
the core data structures of Carp. It is heap-allocated, for a stack-allocated
variant you might want to check out [`StaticArray`](./StaticArray.html).")
(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 &(fn [x y] (+ x @y)) 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 scan "Similar to `Array.reduce`, but instead returns an array with the starting element,
and then all intermediate values.

For example, a scan using `Int.+` over the array [1 1 1 1 1] (starting at 0) will return [0 1 2 3 4 5].")
  (defn scan [f x xs]
    (let [n (length xs)
          ys (allocate (inc n))]
      (do
        (aset-uninitialized! &ys 0 @&x)
        (for [i 1 (inc n)]
          (aset-uninitialized! &ys i (~f (unsafe-nth &ys (dec i)) (unsafe-nth xs (dec i)))))
        ys)))

  (doc endo-scan "Like `Array.scan`, but uses the first element of the array as the starting value.
Also does not create a new array, but reuses the initial one instead (by taking ownership over `xs`.)

For example, an endo-scan using `Int.+` over the array [1 1 1 1 1] will return [1 2 3 4 5]")
  (defn endo-scan [f xs]
    (let [n (length &xs)]
      (do
        (for [i 1 n]
          (aset! &xs i (~f (unsafe-nth &xs (dec i)) (unsafe-nth &xs i))))
        xs)))

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

  (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)))
   (implements = Array.=)

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

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

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

  (doc rest "gets all but the first element from the array.")
  (defn rest [xs]
    (suffix xs 1))

  (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 unsafe-nth-value "returns the value at index `i` of an array `a` (just like [unsafe-nth](#unsafe-nth)) but does not take its reference, and does *not* copy the value. Should only be used for optimizations and when you know what you're doing, circumvents the borrow checker!")
  (deftemplate unsafe-nth-value (Fn [(Ref (Array a)) Int] a)
    "$a $NAME(Array *a, int i)"
    "$DECL { return (($a*)a->data)[i]; }")

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

  (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-uninitialized! a i (~f (unsafe-nth-value 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)))

  (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 unreduce "creates an array by producing values using `step` until they
no longer satisfy `test`. The initial value is `start`.

Example:
```
; if we didn’t have Array.range, we could define it like this:
(defn range [start end step]
  (unreduce start &(fn [x] (< x (+ step end))) &(fn [x] (+ x step)))
)
```")
  (defn unreduce [start test step]
    (let-do [elem start
             acc []]
      (while-do (~test elem)
        (push-back! &acc elem)
        (set! elem (~step elem)))
      acc))

  (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 (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 [] [])
  (implements zero Array.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]
    (let-do [arr (allocate (length xs))]
      (for [i 0 (length xs)]
        (aset-uninitialized!
          &arr
          i
          (Pair.init-from-refs &i (unsafe-nth xs i))))
      arr))

  (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 [xs index]
    (if (and (>= index 0) (< index (length xs)))
       (Maybe.Just @(unsafe-nth xs index)) ; 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))

  (doc partition
    "Partitions an array `arr` into an array of arrays of length `n`
    sequentially filled with the `arr`'s original values.

    This function will fill partitions until `arr` is exhuasted.

    If `n` is greater than or equal to the length of `arr`, the result of this
    function is an array containing a single array of length `n`.

    For example:

    ```clojure
    (Array.partition &[1 2 3 4] 2)
    => [[1 2] [3 4]]
    (Array.partition &[1 2 3 4] 3)
    => [[1 2 3] [4]]
    (Array.partition &[1 2 3 4] 6)
    => [[1 2 3 4]]
    ```")
  (sig partition (Fn [(Ref (Array a) b) Int] (Array (Array a))))
  (defn partition [arr n]
      (let-do [x 0
               y 0
               a []]
        ;; We use while since we're doing custom incrementation of x
        ;; dealing with the extra increment implicitly called by for is messier
        (while (< x (Array.length arr))
          (do
            (set! y (+ x n))
            (when (> y (Array.length arr))
              (set! y (Array.length arr)))
            (set! a (push-back a (Array.slice arr x y)))
            (set! x y)))
        a))

  (doc from-static "Turns a `StaticArray` into an `Array`. Copies elements.")
  (defn from-static [sarr]
    (let-do [darr (allocate (StaticArray.length sarr))]
      (for [i 0 (StaticArray.length sarr)]
        (aset-uninitialized! &darr i @(StaticArray.unsafe-nth sarr i)))
      darr))

  (doc map-reduce "reduces an array `a` by invoking the function `f` on each
element, while keeping an accumulator and a list.

Returns a `Pair` where the first element is the mapped array and the second one
is the final accumulator.

The function `f` receives two arguments: the first one is the accumulator, and
the second one is the element. `f` must return `(Pair accumulator result)`.

Example:
```
(map-reduce &(fn [acc x] (Pair.init (+ @x @acc) (* @x 2))) 0 &[1 2 3])
; => (Pair 6 [2 4 6])
```")
  (defn map-reduce [f acc a]
    (reduce
      &(fn [a el]
        (let [l (Pair.b &a)
              acc (Pair.a &a)
              p (~f acc el)]
          (Pair.init @(Pair.a &p) (Array.push-back @l @(Pair.b &p)))))
      (Pair.init acc [])
      a))
)

(defmacro doall [f xs]
  `(for [i 0 (Array.length &%xs)]
     (%f (Array.unsafe-nth &%xs i))))

(defndynamic foreach-internal [var xs expr]
  (let [xsym (gensym-with 'xs)
        len (gensym-with 'len)
        i (gensym-with 'i)]
    `(let [%xsym %xs
           %len (Array.length %xsym)]
       (for [%i 0 %len]
         (let [%var (Array.unsafe-nth %xsym %i)]
           %expr)))))

(defmacro foreach [binding expr]
  (if (array? binding)
    (foreach-internal (car binding) (cadr binding) expr)
    (macro-error "Binding has to be an array.")))