Add function to parse jj diff output

lua/jj_mini_diff/init.lua

@@ -39,6 +39,48 @@ local function _get_jj_diff_for_buffer()
   return diff_output
 end
 
+-- Parse jj diff output
+local function _parse_diff_output(diff_output)
+  local added_lines = {}
+  local changed_lines = {}
+  local deleted_lines = {}
+
+  local current_line_num = 0
+  local lines = vim.split(diff_output, "\n", { plain = true })
+  local prev_line_was_deleted = false
+
+  for _, line in ipairs(lines) do
+    if line:match("^@@ .- +(%d+)") then
+      -- Extract new_start_line from hunk header
+      local _, _, new_start_line_str = line:find("^@@ .- +(%d+)")
+      current_line_num = tonumber(new_start_line_str) - 1
+      prev_line_was_deleted = false
+    elseif line:match("^[ ]") then
+      current_line_num = current_line_num + 1
+      prev_line_was_deleted = false
+    elseif line:match("^[+]") then
+      current_line_num = current_line_num + 1
+      if prev_line_was_deleted then
+        table.insert(changed_lines, current_line_num)
+      else
+        table.insert(added_lines, current_line_num)
+      end
+      prev_line_was_deleted = false
+    elseif line:match("^[-]") then
+      -- For deleted lines, we mark the line *before* the deletion as changed, 
+      -- or if it's the first line, we can't mark it.
+      -- This is a simplification for now.
+      if current_line_num > 0 then
+        table.insert(deleted_lines, current_line_num)
+      end
+      prev_line_was_deleted = true
+    end
+  end
+
+  return added_lines, changed_lines, deleted_lines
+end
+
+
 function M.setup(opts)
   opts = opts or {}
   _define_signs() -- Call to define signs