[JSON]: Implement support for optional operand

This commit enhances the JSON Backend to properly handle instructions with optional operands.
Previously, these operands were not handled correctly.

The implementation introduces two new fields:
`optional`, which indicates whether an operand is optional (boolean),
and `default`, which holds the default value if no value is specified.

for 'vle16.v':

Before:
```
{
  "operands": [
    ...
    { "name": "vm", "type": "bits(1)" }
    ...
  ]
}
```
After:
```
{
  "operands": [
    ...
    { "name": "vm", "type": "bits(1)", "optional": true, "default": "v0.t" }
    ...
  ]
}
```
Fixes: ThinkOpenly#2

src/sail_json_backend/json.ml

@@ -398,14 +398,102 @@ let remove_identity_funcs op =
       else op
   ) op identity_funcs
 
+let extract_captured_operand pattern s =
+  if Str.string_match pattern s 0 then
+    let captured_group = Str.matched_group 0 s in
+    let optional_operand_parts = String.split_on_char '(' captured_group in
+    let captured_op = List.hd optional_operand_parts in
+    debug_print ("Captured operand: " ^ captured_op);
+    Some captured_op
+  else
+    None
+
+let extract_opt_operands_fn pattern op_list =
+    (*
+       This extracts the optional operands function names from the captured regex:
+       eg: maybe_vmask(vm) -> maybe_vmask
+       This is further used as key to the mappings table to extract the optional
+       operands.
+    *)
+  let captured_operand = List.find_map (fun s -> extract_captured_operand pattern s) op_list in
+  match captured_operand with
+  | Some op -> Some true, op
+  | None -> None, ""
+
+let optional_operand k op =
+    (*
+        The current regex pattern targets a specific function name pattern
+        that starts with "maybe_*", such as "maybe_vmask", "maybe_lmul_flag", and "maybe_ta_flag".
+        Since those kinds are the most prevalent ones as of now.
+
+        TODO: come up with a more robust implementation, that doesn't involve
+        specifying function names explicitly.
+    *)
+  let pattern = Str.regexp_case_fold ("maybe[a-z0-9_]+\\(" ^ "(" ^ op ^ ")" ^ "\\)") in
+    match Hashtbl.find_opt assembly_clean k with
+    | Some op_list -> extract_opt_operands_fn pattern op_list
+    | None -> None, ""
+
+let find_operand zero_bit_pattern (l, r) =
+  match l, r with
+  | [], _ | _, [] -> None
+  | last_l :: _, last_r :: _ when zero_bit_pattern last_r ->
+      let default_value = List.hd (List.rev l) in
+      debug_print ("Found default operand: " ^ default_value);
+      Some default_value
+  | _ -> None
+
+(*
+    This examines zero-bit patterns like 0b0, 0b00, 0b000, etc.,
+    retrieved from the mappings Hashtbl to deduce the default operands.
+
+    The function expects mappings where a null value will be associated with an
+    all-zero bit string on the right side of the mapping.
+    The left most element of the list associated with the corresponding mappings is then
+    extracted to get the default operands.
+
+    Example mapping:
+    ```
+    mapping maybe_ta_flag : string <-> bits(1) = {
+      ""           <-> 0b0, (* tu by default *)
+      sep() ^ "ta" <-> 0b1,
+      sep() ^ "tu" <-> 0b0
+    }
+    ```
+    Sample hastable representation can be seen by printing the mappings table.
+    ```
+    maybe_ta_flag: sep,tu <-> 0b0
+    maybe_ta_flag: sep,ta <-> 0b1
+    ```
+    In the above case, "tu" is extracted.
+
+    TODO: Come up with a more robust implementation to parse the operands.
+*)
+let default_operand optional opt_operand =
+  let values = Hashtbl.find_all mappings opt_operand in
+  let zero_bit_pattern s = Str.string_match (Str.regexp "0b0+$") s 0 in
+  match optional with
+  | Some true -> List.find_map (find_operand zero_bit_pattern) values
+  | _ -> None
+
 let json_of_operand k op =
   debug_print ("json_of_operand " ^ k ^ ":" ^ op);
   let opmap = List.combine (Hashtbl.find operands k) (Hashtbl.find sigs k) in
-    let opplus = remove_identity_funcs op in
-      let opname = List.hd (String.split_on_char ',' opplus) in
-        match List.find_opt (fun (name, t) -> String.equal name opname) opmap with
-            Some (name, t) -> "{ \"name\": \"" ^ name ^ "\", \"type\": \"" ^ t ^ "\" }"
-          | None -> "{ \"name\": \"" ^ opname ^ "\", \"type\": \"unknown\" }"
+      let opplus = remove_identity_funcs op in
+          let opname = List.hd (String.split_on_char ',' opplus) in
+              let optional, opt_operand = optional_operand k op in
+                  let default = default_operand optional opt_operand in
+                      let optional_str = match optional with
+                        | Some b ->  string_of_bool b
+                        | None -> "false" in
+                        let name, t =
+                            match List.find_opt (fun (name, t) -> String.equal name opname) opmap with
+                            | Some (n, t) -> n, t
+                            | None -> opname, "unknown" in
+                        let default_str = match default with
+                            | Some d -> ", \"default\": \"" ^ d ^ "\""
+                            | None -> "" in
+                        "{ \"name\": \"" ^ name ^ "\", \"type\": \"" ^ t ^ "\", \"optional\": " ^ optional_str ^ default_str ^ " }"
 
 let json_of_operands k = match Hashtbl.find_opt operands k with
   | Some ops ->