hid_parser: handle negative numbers in descriptors

hid_parser/__init__.py

@@ -66,6 +66,14 @@ class TagGlobal():
     REPORT_COUNT = 0b1001
     PUSH = 0b1010
     POP = 0b1011
+    # These tags have signed integer data
+    _signed_tags = (
+        LOGICAL_MINIMUM,
+        LOGICAL_MAXIMUM,
+        PHYSICAL_MINIMUM,
+        PHYSICAL_MAXIMUM,
+        UNIT_EXPONENT,
+    )
 
 
 class TagLocal():
@@ -81,6 +89,15 @@ class TagLocal():
     DELIMITER = 0b1010
 
 
+def _item_is_signed(tag: int, typ: int) -> bool:
+    if typ != Type.GLOBAL:
+        return False
+    if tag in TagGlobal._signed_tags:
+        return True
+    else:
+        return False
+
+
 def _data_bit_shift(data: Sequence[int], offset: int, length: int) -> Sequence[int]:
     if not length > 0:
         raise ValueError(f'Invalid specified length: {length}')
@@ -688,33 +705,36 @@ def _iterate_raw(self) -> Iterable[Tuple[int, int, Optional[int]]]:
         i = 0
         while i < len(self.data):
             prefix = self.data[i]
+            i += 1
             tag = (prefix & 0b11110000) >> 4
             typ = (prefix & 0b00001100) >> 2
             size = prefix & 0b00000011
 
-            if size == 3:  # 6.2.2.2
-                size = 4
-
             if size == 0:
-                data = None
-            elif size == 1:
-                if i + 1 >= len(self.data):
-                    raise InvalidReportDescriptor(f'Invalid size: expecting >={i + 1}, got {len(self.data)}')
-                data = self.data[i+1]
+                # 6.2.2.4 Main items with empty data have a value of zero
+                if typ == Type.MAIN:
+                    data = 0
+                else:
+                    data = None
+            elif size > 3:
+                raise ValueError(f'Invalid item size: {size}')
             else:
-                if i + 1 + size >= len(self.data):
-                    raise InvalidReportDescriptor(f'Invalid size: expecting >={i + 1 + size}, got {len(self.data)}')
-                if size == 2:
-                    pack_type = 'H'
-                elif size == 4:
-                    pack_type = 'L'
+                # Pick an unpack format letter by size and signedness
+                # Per 6.2.2.2, size of 0b11 means 4
+                if _item_is_signed(tag, typ):
+                    pack_type = '0bhl'[size]
                 else:
-                    raise ValueError(f'Invalid item size: {size}')
-                data = struct.unpack(f'<{pack_type}', bytes(self.data[i+1:i+1+size]))[0]
+                    pack_type = '0BHL'[size]
+                fmt = f'<{pack_type}'
+                size = struct.calcsize(fmt)
+                if i + size > len(self.data):
+                    raise InvalidReportDescriptor(f'Invalid size: expecting >={i + size}, got {len(self.data)}')
+
+                data, = struct.unpack(fmt, bytes(self.data[i:i+size]))
 
             yield typ, tag, data
 
-            i += size + 1
+            i += size
 
     def _append_item(
         self,

tests/linux-hidpp-print.txt

@@ -46,14 +46,14 @@ Collection (Application)
   Report Size (1)
   Input (Data, Variable, Absolute, No Wrap, Linear, Preferred State, No Null position, Bit Field)
   Usage Page (Generic Desktop Controls)
-  Logical Minimum (32769)
+  Logical Minimum (-32767)
   Logical Maximum (32767)
   Report Size (16)
   Report Count (2)
   Usage (X)
   Usage (Y)
   Input (Data, Variable, Relative, No Wrap, Linear, Preferred State, No Null position, Bit Field)
-  Logical Minimum (129)
+  Logical Minimum (-127)
   Logical Maximum (127)
   Report Size (8)
   Report Count (1)

tests/simple-mouse-print.txt

@@ -19,7 +19,7 @@ Collection (Application)
    Usage Page (Generic Desktop Controls)
    Usage (X)
    Usage (Y)
-   Logical Minimum (129)
+   Logical Minimum (-127)
    Logical Maximum (127)
    Report Size (8)
    Report Count (2)

tests/test_parse.py

@@ -224,12 +224,16 @@ def test_simple_mouse_items():
     assert int(items[4].size) == 8
     assert items[4].usage.page == hid_parser.data.UsagePages.GENERIC_DESKTOP_CONTROLS_PAGE
     assert items[4].usage.usage == hid_parser.data.GenericDesktopControls.X
+    assert items[4].logical_min == -127
+    assert items[4].logical_max == 127
 
     assert isinstance(items[5], hid_parser.VariableItem)
     assert int(items[5].offset) == 8*2
     assert int(items[5].size) == 8
     assert items[5].usage.page == hid_parser.data.UsagePages.GENERIC_DESKTOP_CONTROLS_PAGE
     assert items[5].usage.usage == hid_parser.data.GenericDesktopControls.Y
+    assert items[5].logical_min == -127
+    assert items[5].logical_max == 127
 
 
 def test_linux_hidpp_items():