fix(treesitter): correctly calculate bytes for text sources (#23655)

Fixes #20419

runtime/lua/vim/treesitter/_range.lua

@@ -143,6 +143,29 @@ function M.contains(r1, r2)
   return true
 end
 
+--- @param source integer|string
+--- @param index integer
+--- @return integer
+local function get_offset(source, index)
+  if index == 0 then
+    return 0
+  end
+
+  if type(source) == 'number' then
+    return api.nvim_buf_get_offset(source, index)
+  end
+
+  local byte = 0
+  local next_offset = source:gmatch('()\n')
+  local line = 1
+  while line <= index do
+    byte = next_offset() --[[@as integer]]
+    line = line + 1
+  end
+
+  return byte
+end
+
 ---@private
 ---@param source integer|string
 ---@param range Range
@@ -152,19 +175,10 @@ function M.add_bytes(source, range)
     return range --[[@as Range6]]
   end
 
-  local start_row, start_col, end_row, end_col = range[1], range[2], range[3], range[4]
-  local start_byte = 0
-  local end_byte = 0
+  local start_row, start_col, end_row, end_col = M.unpack4(range)
   -- TODO(vigoux): proper byte computation here, and account for EOL ?
-  if type(source) == 'number' then
-    -- Easy case, this is a buffer parser
-    start_byte = api.nvim_buf_get_offset(source, start_row) + start_col
-    end_byte = api.nvim_buf_get_offset(source, end_row) + end_col
-  elseif type(source) == 'string' then
-    -- string parser, single `\n` delimited string
-    start_byte = vim.fn.byteidx(source, start_col)
-    end_byte = vim.fn.byteidx(source, end_col)
-  end
+  local start_byte = get_offset(source, start_row) + start_col
+  local end_byte = get_offset(source, end_row) + end_col
 
   return { start_row, start_col, start_byte, end_row, end_col, end_byte }
 end