hid_parser: handle negative numbers in descriptors (#20)

Some Global descriptor items can have negative data values. Parse these
correctly. This also causes the printed output to match the comments in
the test descriptors.

Also, correctly parse report items that are signed (either Logical
Minium or Logical Maximum is negative).

Adjust tests accordingly.

Author: tlyu

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():