Ensure transpose function works for any bigarray kind.

lib/codecs/array_to_array.ml

@@ -1,6 +1,5 @@
 open Codecs_intf
 
-module Ndarray = Owl.Dense.Ndarray.Generic
 
 (* https://zarr-specs.readthedocs.io/en/latest/v3/codecs/transpose/v1.0.html *)
 module TransposeCodec = struct
@@ -29,7 +28,6 @@ module TransposeCodec = struct
           the decoded representation dimensionality." in
         Result.error @@ `Transpose_order (t, msg)
 
-
   let parse_order o =
     if Array.length o = 0 then
       let msg = "transpose order cannot be empty." in
@@ -66,14 +64,24 @@ module TransposeCodec = struct
     else
       Ok ()
 
-  let encode o x =
-    try Ok (Ndarray.transpose ~axis:o x) with
-    | Failure s -> Error (`Transpose_order (o, s))
+  (* NOTE: See https://github.com/owlbarn/owl/issues/671#issuecomment-2241761001 *)
+  let transpose ?axis x =
+    let module A = Owl.Dense.Ndarray.Any in
+    let module N = Owl.Dense.Ndarray.Generic in
+    try
+      let y = A.transpose ?axis @@ A.init_nd (N.shape x) @@ N.get x in
+      Result.ok @@ N.init_nd (N.kind x) (A.shape y) @@ A.get y
+    with
+    | Assert_failure _ ->
+      Result.error @@
+      `Transpose_order (Option.get axis, "Invalid transpose order.")
+
+  let encode o x = transpose ~axis:o x
 
   let decode o x =
     let inv_order = Array.(make (length o) 0) in
     Array.iteri (fun i x -> inv_order.(x) <- i) o;
-    Ok (Ndarray.transpose ~axis:inv_order x)
+    transpose ~axis:inv_order x
 
   let to_yojson order =
     let o = 

test/test_codecs.ml

@@ -40,10 +40,10 @@ let bytes_encode_decode
 let tests = [
 "test codec chain" >:: (fun _ ->
   let decoded_repr
-    : (float, Bigarray.float32_elt) array_repr =
+    : (int, Bigarray.int16_signed_elt) array_repr =
     {shape = [|10; 15; 10|]
-    ;kind = Bigarray.Float32
-    ;fill_value = (-10.)}
+    ;kind = Bigarray.Int16_signed
+    ;fill_value = 10}
   in
   let shard_cfg =
     {chunk_shape = [|2; 5; 5|]
@@ -323,7 +323,7 @@ let tests = [
   let cfg =
     {chunk_shape = [|3; 5; 5|]
     ;index_location = Start
-    ;index_codecs = [`Bytes LE; `Crc32c]
+    ;index_codecs = [`Transpose [|0; 3; 1; 2|]; `Bytes LE; `Crc32c]
     ;codecs = [`Bytes BE]}
   in
   let chain = [`ShardingIndexed cfg] in
@@ -362,20 +362,6 @@ let tests = [
     assert_failure
       "Successfully encoded array should decode without fail");
 
-  (* test if including a transpose codec for index_codec chain results in
-    a failure. *)
-  let chain' =
-   [`ShardingIndexed {cfg with
-      chunk_shape = [|5; 3; 5|]
-      ;index_codecs = `Transpose [|0; 3; 1; 2|] :: cfg.index_codecs}]
-  in
-  let cc = Chain.create decoded_repr chain' |> Result.get_ok in
-  assert_bool
-    "shard index chain can't be encoded since Owl does not support transposing
-    Int64 types.  See:
-    https://github.com/owlbarn/owl/issues/671#issuecomment-2211303040" @@
-    Result.is_error @@ Chain.encode cc arr;
-
   (* test correctness of decoding nested sharding codecs.*)
   let str =
     {|[