Add reduceWithIndex for Arrays

Author: kutyel

jscomp/others/belt_Array.ml

@@ -362,6 +362,16 @@ let reduceReverse2U a b x f =
 let reduceReverse2 a b x f =
   reduceReverse2U a b x (fun [@bs] a b c -> f a b c)
 
+let rec reduceWithIndexAuxU a acc f i =
+  match a with
+  | [] -> acc
+  | x::xs -> reduceWithIndexAuxU xs ((f acc x i)[@bs]) f (i + 1)
+
+let reduceWithIndexU a acc f = reduceWithIndexAuxU a acc f 0
+
+let reduceWithIndex a acc f =
+  reduceWithIndexU a acc (fun [@bs] acc x i -> f acc x i)
+
 let rec everyAux arr i b len =
   if i = len then true
   else if b (getUnsafe arr i) [@bs] then

jscomp/others/belt_Array.mli

@@ -488,6 +488,19 @@ val reduceReverse2:
    ]}
 *)
 
+val reduceWithIndexU:  'a array -> 'b -> ('b -> 'a -> int -> 'b [@bs]) -> 'b
+val reduceWithIndex:  'a array -> 'b -> ('b -> 'a -> int -> 'b) -> 'b
+(** [reduceWithIndex xs f]
+
+    Applies [f] to each element of [xs] from beginning to end. Function [f] has three parameters: the item
+    from the array and an “accumulator”, which starts with a value of [init] and the index of each element. [reduceWithIndex]
+    returns the final value of the accumulator.
+    
+    @example {[
+      reduceWithIndex [|1;2;3;4|] 0 (fun acc x i -> acc + x + i) = 16;
+    ]}
+*)
+
 val someU: 'a array -> ('a -> bool [@bs]) -> bool
 val some: 'a array -> ('a -> bool) -> bool
 (** [some xs p]

jscomp/test/bs_array_test.ml

@@ -99,6 +99,7 @@ let () =
   eq __LOC__ (A.reduceReverse [||] 100 (-)) 100;
   eq __LOC__ (A.reduceReverse [|1;2|] 100 (-)) 97;
   eq __LOC__ (A.reduceReverse [|1;2;3;4|] 100 (-) ) 90;
+  eq __LOC__  (A.reduceWithIndex [|1;2;3;4|] 0 (fun acc x i -> acc + x + i) = 16);
   b __LOC__
     (A.reduceReverse2 [|1;2;3|] [|1;2|] 0 (fun acc x y -> acc + x + y) = 6)
 let addone = fun [@bs] x -> x + 1