Field: More efficient conversion of into Bignum_bigint.t

src/lib/crypto/kimchi_backend/common/field.ml

@@ -179,20 +179,30 @@ module Make (F : Input_intf) :
             let of_sexpable = of_bigint
           end)
 
+      let zero = of_int 0
+
       let to_bignum_bigint n =
-        let rec go i two_to_the_i acc =
-          if Int.equal i size_in_bits then acc
-          else
-            let acc' =
-              if Bigint.test_bit n i then Bignum_bigint.(acc + two_to_the_i)
-              else acc
-            in
-            go (i + 1) Bignum_bigint.(two_to_the_i + two_to_the_i) acc'
-        in
-        go 0 Bignum_bigint.one Bignum_bigint.zero
-
-      let hash_fold_t s x =
-        Bignum_bigint.hash_fold_t s (to_bignum_bigint (to_bigint x))
+        (* For non-zero values, conversion is done by creating the bin_prot representation
+           of the [Bignum_bigit.t] value, and then parsing it with bin_prot. *)
+        match compare n zero with
+        | 0 ->
+            Bignum_bigint.zero
+        | c ->
+            (* Tag for positive values is 1, negative is 2:
+               https://github.com/janestreet/bignum/blob/6c63419787a4e209e85befd3d823fff2790677e0/bigint/src/bigint.ml#L27-L30 *)
+            let tag_byte = if c > 0 then '\x01' else '\x02' in
+            let bytes = to_bytes n in
+            let len = Bytes.length bytes in
+            let size_byte = Char.of_int_exn len in
+            let buf = Bytes.create (2 + len) in
+            (* First byte is the tag, second the amount of bytes *)
+            Bytes.unsafe_set buf 0 tag_byte ;
+            Bytes.unsafe_set buf 1 size_byte ;
+            (* Copy the bytes representation of the value, skip the tag and size bytes *)
+            Bytes.unsafe_blit ~src:bytes ~src_pos:0 ~dst_pos:2 ~len ~dst:buf ;
+            Bin_prot.Reader.of_bytes Bignum_bigint.Stable.V1.bin_reader_t buf
+
+      let hash_fold_t s x = Bignum_bigint.hash_fold_t s (to_bignum_bigint x)
 
       let hash = Hash.of_fold hash_fold_t