(ns Enum
  "Elixir Enum module — eager operations on enumerables.

   In CljElixir: (Enum/map coll f), (Enum/reduce coll acc f), etc.
   Works on lists, maps, ranges, and any Enumerable.")

;; --- Mapping & Transformation ---

(defn map
  "Returns a list with `f` applied to each element.
   (Enum/map [1 2 3] (fn [x] (* x 2))) ;=> [2 4 6]"
  [enumerable f])

(defn map-every
  "Applies `f` to every `nth` element, starting with the first.
   (Enum/map-every [1 2 3 4 5] 2 (fn [x] (* x 2))) ;=> [2 2 6 4 10]"
  [enumerable nth f])

(defn flat-map
  "Maps `f` over `enumerable` and flattens the result.
   (Enum/flat-map [[1 2] [3 4]] (fn [x] x)) ;=> [1 2 3 4]"
  [enumerable f])

(defn map-reduce
  "Maps and reduces in one pass. Returns {mapped_list, acc}.
   (Enum/map-reduce [1 2 3] 0 (fn [x acc] {(* x 2) (+ acc x)})) ;=> {[2 4 6] 6}"
  [enumerable acc f])

(defn scan
  "Returns a list of successive reduced values from the left.
   (Enum/scan [1 2 3 4] (fn [x acc] (+ x acc))) ;=> [1 3 6 10]"
  ([enumerable f])
  ([enumerable acc f]))

;; --- Filtering ---

(defn filter
  "Returns elements for which `f` returns a truthy value.
   (Enum/filter [1 2 3 4] (fn [x] (> x 2))) ;=> [3 4]"
  [enumerable f])

(defn reject
  "Returns elements for which `f` returns a falsy value.
   (Enum/reject [1 2 3 4] (fn [x] (> x 2))) ;=> [1 2]"
  [enumerable f])

(defn uniq
  "Returns unique elements, preserving order.
   (Enum/uniq [1 2 1 3 2]) ;=> [1 2 3]"
  [enumerable])

(defn uniq-by
  "Returns elements unique by the result of `f`.
   (Enum/uniq-by [{:x 1 :y 2} {:x 1 :y 3}] (fn [m] (:x m))) ;=> [{:x 1 :y 2}]"
  [enumerable f])

(defn dedup
  "Removes consecutive duplicate elements.
   (Enum/dedup [1 1 2 2 3 1]) ;=> [1 2 3 1]"
  [enumerable])

(defn dedup-by
  "Removes consecutive elements where `f` returns the same value."
  [enumerable f])

;; --- Reducing ---

(defn reduce
  "Invokes `f` for each element with the accumulator.
   (Enum/reduce [1 2 3] 0 (fn [x acc] (+ acc x))) ;=> 6
   Note: Elixir callback order is (element, acc) not (acc, element)."
  ([enumerable f])
  ([enumerable acc f]))

(defn reduce-while
  "Reduces while `f` returns {:cont, acc}. Halts on {:halt, acc}.
   (Enum/reduce-while [1 2 3 4] 0 (fn [x acc] (if (< acc 5) {:cont (+ acc x)} {:halt acc})))"
  [enumerable acc f])

(defn map-join
  "Maps `f` over `enumerable` then joins results with `joiner`.
   (Enum/map-join [1 2 3] \", \" (fn [x] (str x))) ;=> \"1, 2, 3\""
  ([enumerable f])
  ([enumerable joiner f]))

;; --- Sorting ---

(defn sort
  "Sorts the enumerable. Uses Erlang term ordering or a custom comparator.
   (Enum/sort [3 1 2]) ;=> [1 2 3]
   (Enum/sort [3 1 2] (fn [a b] (> a b))) ;=> [3 2 1]"
  ([enumerable])
  ([enumerable sorter]))

(defn sort-by
  "Sorts by the result of applying `mapper` to each element.
   (Enum/sort-by [{:name \"b\"} {:name \"a\"}] (fn [x] (:name x))) ;=> [{:name \"a\"} {:name \"b\"}]"
  ([enumerable mapper])
  ([enumerable mapper sorter]))

;; --- Grouping & Partitioning ---

(defn group-by
  "Groups elements by the result of `f`.
   (Enum/group-by [\"ant\" \"bee\" \"ape\"] (fn [s] (String/at s 0)))
   ;=> %{\"a\" => [\"ant\" \"ape\"], \"b\" => [\"bee\"]}"
  [enumerable f])

(defn chunk-by
  "Splits into chunks where consecutive elements return the same value for `f`.
   (Enum/chunk-by [1 1 2 2 3] (fn [x] x)) ;=> [[1 1] [2 2] [3]]"
  [enumerable f])

(defn chunk-every
  "Splits into chunks of `count` elements.
   (Enum/chunk-every [1 2 3 4 5] 2) ;=> [[1 2] [3 4] [5]]"
  ([enumerable count])
  ([enumerable count step])
  ([enumerable count step leftover]))

(defn frequencies
  "Returns a map with keys as unique elements and values as counts.
   (Enum/frequencies [\"a\" \"b\" \"a\" \"c\" \"b\" \"a\"]) ;=> %{\"a\" => 3, \"b\" => 2, \"c\" => 1}"
  [enumerable])

(defn split-with
  "Splits into two lists: elements satisfying `f` and the rest.
   (Enum/split-with [1 2 3 4] (fn [x] (< x 3))) ;=> {[1 2] [3 4]}"
  [enumerable f])

(defn partition-by
  "Splits when `f` returns a new value.
   (Enum/partition-by [1 1 2 2 3] (fn [x] x)) ;=> [[1 1] [2 2] [3]]"
  [enumerable f])

;; --- Lookup & Search ---

(defn find
  "Returns the first element for which `f` returns truthy.
   (Enum/find [1 2 3 4] (fn [x] (> x 2))) ;=> 3"
  ([enumerable f])
  ([enumerable default f]))

(defn find-index
  "Returns the index of the first element for which `f` returns truthy.
   (Enum/find-index [1 2 3] (fn [x] (= x 2))) ;=> 1"
  [enumerable f])

(defn find-value
  "Returns the first truthy return value of `f`.
   (Enum/find-value [1 2 3] (fn [x] (if (> x 2) (* x 10) nil))) ;=> 30"
  ([enumerable f])
  ([enumerable default f]))

(defn member?
  "Returns true if `element` exists in the enumerable.
   (Enum/member? [1 2 3] 2) ;=> true"
  [enumerable element])

(defn any?
  "Returns true if any element satisfies `f` (or if any element is truthy).
   (Enum/any? [false nil true]) ;=> true
   (Enum/any? [1 2 3] (fn [x] (> x 2))) ;=> true"
  ([enumerable])
  ([enumerable f]))

(defn all?
  "Returns true if all elements satisfy `f` (or all are truthy).
   (Enum/all? [1 2 3] (fn [x] (> x 0))) ;=> true"
  ([enumerable])
  ([enumerable f]))

(defn count
  "Returns the count of elements, optionally only those satisfying `f`.
   (Enum/count [1 2 3]) ;=> 3
   (Enum/count [1 2 3] (fn [x] (> x 1))) ;=> 2"
  ([enumerable])
  ([enumerable f]))

(defn empty?
  "Returns true if the enumerable is empty.
   (Enum/empty? []) ;=> true"
  [enumerable])

;; --- Subsequences ---

(defn take
  "Takes the first `amount` elements.
   (Enum/take [1 2 3 4 5] 3) ;=> [1 2 3]"
  [enumerable amount])

(defn take-while
  "Takes elements while `f` returns truthy.
   (Enum/take-while [1 2 3 4] (fn [x] (< x 3))) ;=> [1 2]"
  [enumerable f])

(defn take-every
  "Takes every `nth` element (0-indexed).
   (Enum/take-every [1 2 3 4 5 6] 2) ;=> [1 3 5]"
  [enumerable nth])

(defn drop
  "Drops the first `amount` elements.
   (Enum/drop [1 2 3 4 5] 2) ;=> [3 4 5]"
  [enumerable amount])

(defn drop-while
  "Drops elements while `f` returns truthy.
   (Enum/drop-while [1 2 3 4] (fn [x] (< x 3))) ;=> [3 4]"
  [enumerable f])

(defn slice
  "Returns a subset of the enumerable.
   (Enum/slice [1 2 3 4 5] 1 3) ;=> [2 3 4]
   (Enum/slice [1 2 3 4 5] 1..3) ;=> [2 3 4]"
  ([enumerable index-range])
  ([enumerable start amount]))

(defn reverse
  "Reverses the enumerable.
   (Enum/reverse [1 2 3]) ;=> [3 2 1]"
  ([enumerable])
  ([enumerable tail]))

(defn shuffle
  "Returns a list with elements in random order.
   (Enum/shuffle [1 2 3 4 5]) ;=> [3 1 5 2 4]"
  [enumerable])

;; --- Aggregation ---

(defn sum
  "Returns the sum of all elements.
   (Enum/sum [1 2 3]) ;=> 6"
  [enumerable])

(defn product
  "Returns the product of all elements.
   (Enum/product [1 2 3 4]) ;=> 24"
  [enumerable])

(defn min
  "Returns the minimum element.
   (Enum/min [3 1 2]) ;=> 1"
  ([enumerable])
  ([enumerable empty-fallback]))

(defn max
  "Returns the maximum element.
   (Enum/max [3 1 2]) ;=> 3"
  ([enumerable])
  ([enumerable empty-fallback]))

(defn min-by
  "Returns the element for which `f` returns the smallest value.
   (Enum/min-by [\"aaa\" \"b\" \"cc\"] (fn [s] (String/length s))) ;=> \"b\""
  ([enumerable f])
  ([enumerable f sorter]))

(defn max-by
  "Returns the element for which `f` returns the largest value.
   (Enum/max-by [\"aaa\" \"b\" \"cc\"] (fn [s] (String/length s))) ;=> \"aaa\""
  ([enumerable f])
  ([enumerable f sorter]))

(defn min-max
  "Returns a tuple with the minimum and maximum elements.
   (Enum/min-max [3 1 2]) ;=> {1 3}"
  ([enumerable])
  ([enumerable empty-fallback]))

(defn min-max-by
  "Returns a tuple with the min/max elements by `f`."
  ([enumerable f])
  ([enumerable f sorter-or-empty]))

;; --- Joining & Conversion ---

(defn join
  "Joins elements into a string with an optional separator.
   (Enum/join [1 2 3] \", \") ;=> \"1, 2, 3\"
   (Enum/join [1 2 3]) ;=> \"123\""
  ([enumerable])
  ([enumerable joiner]))

(defn into
  "Inserts each element into a collectable.
   (Enum/into [1 2 3] []) ;=> [1 2 3]
   (Enum/into %{a: 1} %{b: 2}) ;=> %{a: 1, b: 2}
   (Enum/into [1 2 3] [] (fn [x] (* x 2))) ;=> [2 4 6]"
  ([enumerable collectable])
  ([enumerable collectable transform]))

(defn to-list
  "Converts an enumerable to a list.
   (Enum/to-list (1..5)) ;=> [1 2 3 4 5]"
  [enumerable])

(defn zip
  "Zips corresponding elements from a finite collection of enumerables.
   (Enum/zip [1 2 3] [:a :b :c]) ;=> [{1 :a} {2 :b} {3 :c}]"
  ([enumerables])
  ([enum1 enum2]))

(defn zip-with
  "Zips with a merge function.
   (Enum/zip-with [1 2 3] [4 5 6] (fn [a b] (+ a b))) ;=> [5 7 9]"
  ([enumerables zip-fun])
  ([enum1 enum2 zip-fun]))

(defn unzip
  "Opposite of zip. Takes a list of two-element tuples and returns two lists.
   (Enum/unzip [{1 :a} {2 :b}]) ;=> {[1 2] [:a :b]}"
  [list])

(defn with-index
  "Wraps each element in a tuple with its index.
   (Enum/with-index [:a :b :c]) ;=> [{:a 0} {:b 1} {:c 2}]"
  ([enumerable])
  ([enumerable fun-or-offset]))

(defn flat-map-reduce
  "Maps and reduces, emitting lists that get flattened. Returns {[flat_mapped], acc}."
  [enumerable acc f])

;; --- Element Access ---

(defn at
  "Returns the element at `index`. Returns `default` if out of bounds.
   (Enum/at [1 2 3] 1) ;=> 2
   (Enum/at [1 2 3] 5 :none) ;=> :none"
  ([enumerable index])
  ([enumerable index default]))

(defn fetch
  "Returns {:ok element} or :error for index lookup.
   (Enum/fetch [1 2 3] 1) ;=> {:ok 2}
   (Enum/fetch [1 2 3] 5) ;=> :error"
  [enumerable index])

(defn fetch!
  "Returns the element at `index`. Raises if out of bounds.
   (Enum/fetch! [1 2 3] 1) ;=> 2"
  [enumerable index])

(defn random
  "Returns a random element.
   (Enum/random [1 2 3 4 5]) ;=> 3"
  ([enumerable])
  ([enumerable count]))

;; --- List Operations ---

(defn concat
  "Concatenates enumerables.
   (Enum/concat [1 2] [3 4]) ;=> [1 2 3 4]
   (Enum/concat [[1 2] [3 4]]) ;=> [1 2 3 4]"
  ([enumerables])
  ([left right]))

(defn intersperse
  "Inserts `separator` between each element.
   (Enum/intersperse [1 2 3] 0) ;=> [1 0 2 0 3]"
  [enumerable separator])

(defn each
  "Invokes `f` for each element (side effects). Returns :ok.
   (Enum/each [1 2 3] (fn [x] (IO/puts x)))"
  [enumerable f])

(defn map-intersperse
  "Maps and intersperses in one pass."
  [enumerable separator mapper])

(defn split
  "Splits into two lists at `count`.
   (Enum/split [1 2 3 4 5] 3) ;=> {[1 2 3] [4 5]}"
  [enumerable count])