debug/gdb: add printers for endian::value

debug/gdb/printers.py

@@ -1,5 +1,8 @@
 import re
 import gdb
+import sys
+import struct
+
 
 class CoordPrinter(object):
     def __init__(self, val):
@@ -64,13 +67,55 @@ class ViewPrinter(object):
             self.val["m_end"]
         )
 
+class EndianValuePrinter(object):
+    val: gdb.Type
+
+    def __init__(self, val: gdb.Value):
+        self.val = val
+        # The type should be something like:
+        #     "cruft::endian::value<cruft::endian::BIG,u64>"
+        order_name = str(val.type.template_argument(0))
+
+        # Fuck knows how to use the GDB API. It's easier to just search for a
+        # differentiator that's unlikely to be present in anything else.
+        if '::BIG' in order_name:
+            self.order = '>'
+        elif '::LITTLE' in order_name:
+            self.order = '<'
+        else:
+            raise RuntimeError("Unknown endianness")
+
+        # Find a format specifier for the size of the underlying value
+        self.size = {
+            1: 'B',
+            2: 'H',
+            4: 'I',
+            8: 'Q',
+        }[self.val.type.sizeof]
+
+    def to_string (self):
+        # Construct format specifiers for converting to bytes, then from
+        # bytes, so that we can perform the byte reversal.
+        to_bytes = f"{self.order}{self.size}"
+        from_bytes = f"={self.size}"
+
+        raw = int(self.val["raw"])
+        native = struct.unpack(from_bytes, struct.pack(to_bytes, raw))[0]
+
+        # Print the value and some annotations so that the user isn't misled
+        # about what value they're looking at.
+        return f"{native} (0x{native:x}, from 0x{raw:x})"
+
+
 def build_cruft_dict():
     pretty_printers_dict[re.compile('^cruft::point.*$') ] = lambda val: CoordPrinter(val)
     pretty_printers_dict[re.compile('^cruft::vector.*$')] = lambda val: CoordPrinter(val)
     pretty_printers_dict[re.compile('^cruft::extent.*$')] = lambda val: CoordPrinter(val)
     pretty_printers_dict[re.compile('^cruft::colour.*$')] = lambda val: CoordPrinter(val)
 
-    pretty_printers_dict[re.compile('^cruft::view')] = lambda title, val: ViewPrinter(title, val)
+    pretty_printers_dict[re.compile('^cruft::view')] = lambda val: ViewPrinter(val)
+
+    pretty_printers_dict[re.compile('^cruft::endian::value')] = lambda val: EndianValuePrinter(val)
 
 
 def lookup(val):