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+;;; tramp-rpc-protocol.el --- JSON-RPC protocol for TRAMP-RPC -*- lexical-binding: t; -*-
+
+;; Copyright (C) 2026 Arthur Heymans <arthur@aheymans.xyz>
+
+;; Author: Arthur Heymans <arthur@aheymans.xyz>
+;; Keywords: comm, processes
+;; Package-Requires: ((emacs "30.1"))
+
+;; This file is part of tramp-rpc.
+
+;; tramp-rpc is free software: you can redistribute it and/or modify
+;; it under the terms of the GNU General Public License as published by
+;; the Free Software Foundation, either version 3 of the License, or
+;; (at your option) any later version.
+
+;;; Commentary:
+
+;; This file provides the JSON-RPC 2.0 protocol implementation for
+;; communicating with the tramp-rpc-server binary.
+
+;;; Code:
+
+(require 'json)
+
+(defvar tramp-rpc-protocol--request-id 0
+  "Counter for generating unique request IDs.")
+
+(defun tramp-rpc-protocol--next-id ()
+  "Generate the next request ID."
+  (cl-incf tramp-rpc-protocol--request-id))
+
+(defun tramp-rpc-protocol-encode-request (method params)
+  "Encode a JSON-RPC 2.0 request for METHOD with PARAMS.
+Returns a JSON string."
+  (let ((request `((jsonrpc . "2.0")
+                   (id . ,(tramp-rpc-protocol--next-id))
+                   (method . ,method)
+                   (params . ,params))))
+    (json-encode request)))
+
+(defun tramp-rpc-protocol-encode-request-with-id (method params)
+  "Encode a JSON-RPC 2.0 request for METHOD with PARAMS.
+Returns a cons cell (ID . JSON-STRING) for pipelining support."
+  (let* ((id (tramp-rpc-protocol--next-id))
+         (request `((jsonrpc . "2.0")
+                    (id . ,id)
+                    (method . ,method)
+                    (params . ,params))))
+    (cons id (json-encode request))))
+
+(defun tramp-rpc-protocol-decode-response (json-string)
+  "Decode a JSON-RPC 2.0 response from JSON-STRING.
+Returns a plist with :id, :result, and :error keys."
+  (let* ((response (json-parse-string json-string
+                                      :object-type 'alist
+                                      :false-object nil
+                                      :null-object nil))
+         (id (alist-get 'id response))
+         (result (alist-get 'result response))
+         (error-obj (alist-get 'error response)))
+    (list :id id
+          :result result
+          :error (when error-obj
+                   (list :code (alist-get 'code error-obj)
+                         :message (alist-get 'message error-obj)
+                         :data (alist-get 'data error-obj))))))
+
+(defun tramp-rpc-protocol-error-p (response)
+  "Return non-nil if RESPONSE contains an error."
+  (plist-get response :error))
+
+(defun tramp-rpc-protocol-error-message (response)
+  "Extract the error message from RESPONSE."
+  (plist-get (plist-get response :error) :message))
+
+(defun tramp-rpc-protocol-error-code (response)
+  "Extract the error code from RESPONSE."
+  (plist-get (plist-get response :error) :code))
+
+;; Error codes
+(defconst tramp-rpc-protocol-error-parse -32700)
+(defconst tramp-rpc-protocol-error-invalid-request -32600)
+(defconst tramp-rpc-protocol-error-method-not-found -32601)
+(defconst tramp-rpc-protocol-error-invalid-params -32602)
+(defconst tramp-rpc-protocol-error-internal -32603)
+(defconst tramp-rpc-protocol-error-file-not-found -32001)
+(defconst tramp-rpc-protocol-error-permission-denied -32002)
+(defconst tramp-rpc-protocol-error-io -32003)
+(defconst tramp-rpc-protocol-error-process -32004)
+
+;; ============================================================================
+;; Batch request support
+;; ============================================================================
+
+(defun tramp-rpc-protocol-encode-batch-request (requests)
+  "Encode a batch request containing multiple REQUESTS.
+REQUESTS is a list of (METHOD . PARAMS) cons cells.
+Returns a JSON string for a single RPC call to the batch method."
+  (let ((batch-requests
+         (mapcar (lambda (req)
+                   `((method . ,(car req))
+                     (params . ,(cdr req))))
+                 requests)))
+    (tramp-rpc-protocol-encode-request
+     "batch"
+     `((requests . ,(vconcat batch-requests))))))
+
+(defun tramp-rpc-protocol-encode-batch-request-with-id (requests)
+  "Encode a batch request containing multiple REQUESTS.
+REQUESTS is a list of (METHOD . PARAMS) cons cells.
+Returns a cons cell (ID . JSON-STRING) for ID tracking."
+  (let ((batch-requests
+         (mapcar (lambda (req)
+                   `((method . ,(car req))
+                     (params . ,(cdr req))))
+                 requests)))
+    (tramp-rpc-protocol-encode-request-with-id
+     "batch"
+     `((requests . ,(vconcat batch-requests))))))
+
+(defun tramp-rpc-protocol-decode-batch-response (response)
+  "Decode a batch response into a list of individual results.
+RESPONSE is the decoded response plist from
+`tramp-rpc-protocol-decode-response'.
+Returns a list where each element is either:
+  - The result value (if successful)
+  - A plist (:error CODE :message MSG) if that sub-request failed."
+  (let ((results-array (alist-get 'results (plist-get response :result))))
+    (mapcar (lambda (result-obj)
+              (if-let ((error-obj (alist-get 'error result-obj)))
+                  (list :error (alist-get 'code error-obj)
+                        :message (alist-get 'message error-obj))
+                (alist-get 'result result-obj)))
+            results-array)))
+
+(provide 'tramp-rpc-protocol)
+;;; tramp-rpc-protocol.el ends here