perf: optimize OpamString.split

this cuts the time spent on parsing `pacman -Si` significantly down

Co-authored-by: Kate <[email protected]>

master_changes.md

@@ -200,6 +200,7 @@ users)
   * `OpamFile.Repos_config.t`: change the type to not allow repositories without an URL [#6249 @kit-ty-kate]
 
 ## opam-core
+  * `OpamStd.List.split`: Improve performance [#6210 @kit-ty-kate]
   * `OpamStd.Sys.{get_terminal_columns,uname,getconf,guess_shell_compat}`: Harden the process calls to account for failures [#6230 @kit-ty-kate - fix #6215]
   * `OpamStd.Sys.getconf`: was removed, replaced by `get_long_bit` [#6217 @kit-ty-kate]
   * `OpamStd.Sys.get_long_bit`: was added, which returns the output of the `getconf LONG_BIT` command [#6217 @kit-ty-kate]

src/core/opamStd.ml

@@ -692,9 +692,25 @@ module OpamString = struct
   let rcut_at = cut_at_aux String.rindex
 
   let split s c =
-    (* old compat version (Re 1.2.0)
-       {[Re_str.split (Re_str.regexp (Printf.sprintf "[%c]+" c)) s]} *)
-    Re.(split (compile (rep1 (char c)))) s
+    let rec loop acc i slice_start len s c =
+      if (i : int) < (len : int) then
+        if s.[i] = (c : char) then
+          let acc =
+            if (slice_start : int) < (i : int) then
+              String.sub s slice_start (i - slice_start) :: acc
+            else
+              acc
+          in
+          let i = i+1 in
+          loop acc i i len s c
+        else
+          loop acc (i+1) slice_start len s c
+      else if (i : int) = (slice_start : int) then
+        acc
+      else
+        String.sub s slice_start (len - slice_start) :: acc
+    in
+    List.rev (loop [] 0 0 (String.length s) s c)
 
   let split_delim s c =
     let tokens = Re.(split_full (compile (char c)) s) in
@@ -1160,9 +1176,9 @@ module OpamSys = struct
     | SH_fish ->
       Some (List.fold_left Filename.concat (home ".config") ["fish"; "config.fish"])
     | SH_zsh  ->
-      let zsh_home f = 
+      let zsh_home f =
         try Filename.concat (Env.get "ZDOTDIR") f
-        with Not_found -> home f in 
+        with Not_found -> home f in
       Some (zsh_home ".zshrc")
     | SH_bash ->
       let shell =