From 6ecf681d3a8227c39cb72859a982c6d1ca5ab110 Mon Sep 17 00:00:00 2001
From: Simon Forman <sforman@hushmail.com>
Date: Tue, 4 Oct 2022 08:32:04 -0700
Subject: [PATCH] BigInts in terms of Oberon Ints

Oberon RISC has Two's Complement 32-bit ints with carry.

So far so good.
---
 bigjoyints/big.py | 59 +++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 59 insertions(+)
 create mode 100644 bigjoyints/big.py

diff --git a/bigjoyints/big.py b/bigjoyints/big.py
new file mode 100644
index 0000000..a388aaa
--- /dev/null
+++ b/bigjoyints/big.py
@@ -0,0 +1,59 @@
+import ctypes
+
+
+class OberonInt:
+    '''
+    Let's model the Oberon RISC integers,
+    32-bit, two's complement.
+    '''
+
+    def __init__(self, initial=0):
+        assert -2**31 <= initial < 2**31
+        self.value = ctypes.c_int32(initial)
+        assert self.value.value == initial
+
+    def add(self, other):
+        '''
+        Return carry bit and new value.
+        '''
+        assert isinstance(other, OberonInt)
+        n = self.value.value + other.value.value
+        carry = not (-2**31 <= n < 2**31)
+        if carry:
+            n &= (2**31-1)
+        return carry, OberonInt(n)
+
+    def negate(self):
+        # Instead of binary ops, just cheat:
+        return OberonInt(-self.value.value)
+
+    def __repr__(self):
+        #b = bin(self.value.value & (2**32-1))
+        return f'OberonInt({self.value.value})'
+
+    def __eq__(self, other):
+        assert isinstance(other, OberonInt)
+        return self.value.value == other.value.value
+
+
+one = OberonInt(1)
+obmin, zero, obmax = map(OberonInt, (
+    -(2**31),
+    0,
+    2**31-1,
+    ))
+
+
+carry, z = obmax.add(one)
+assert carry
+assert z == zero
+
+negative_one = one.negate()
+carry, m = obmin.add(negative_one)
+assert carry
+assert m == obmax
+
+##        if initial >= 2**31:
+##            raise ValueError(f'too big: {initial!r}')
+##        if initial < -2**31:
+##            raise ValueError(f'too small: {initial!r}')