From 54b9bb6f25b703cded0a8552a8dfed1d53673b8f Mon Sep 17 00:00:00 2001
From: Tony Garnock-Jones <tonyg@leastfixedpoint.com>
Date: Fri, 23 Jun 2023 22:34:38 +0200
Subject: [PATCH] Repair machineword reads

---
 .../cpp/preserves_binary_reader.hpp           | 23 +++++++++++--------
 1 file changed, 14 insertions(+), 9 deletions(-)

diff --git a/implementations/cpp/preserves_binary_reader.hpp b/implementations/cpp/preserves_binary_reader.hpp
index 4176e2c..5ca5bb4 100644
--- a/implementations/cpp/preserves_binary_reader.hpp
+++ b/implementations/cpp/preserves_binary_reader.hpp
@@ -61,14 +61,18 @@ namespace Preserves {
             return d;
         }
 
-        boost::optional<uint64_t> next_unsigned(size_t n) {
+        boost::optional<Value<T>> next_machineword(size_t n, bool always_unsigned) {
             uint8_t buf[n];
             if (!next_chunk(buf, n)) return boost::none;
-            uint64_t v = 0;
-            for (size_t j = 0; j < n; j++) {
-                v = (v << 8) | buf[j];
+            if ((buf[0] & 0x80) && !always_unsigned) {
+                int64_t v = -1;
+                for (size_t j = 0; j < n; j++) v = (v << 8) | buf[j];
+                return Value<T>::from_int(v);
+            } else {
+                uint64_t v = 0;
+                for (size_t j = 0; j < n; j++) v = (v << 8) | buf[j];
+                return Value<T>::from_int(v);
             }
-            return v;
         }
 
         boost::optional<Value<T>> next_bignum(size_t n) {
@@ -115,12 +119,13 @@ namespace Preserves {
                 }
                 case BinaryTag::MediumInteger_lo ... BinaryTag::MediumInteger_hi: {
                     int n = int(tag) - int(BinaryTag::MediumInteger_lo) + 1;
-                    if (n < 9) {
-                        return next_unsigned(n).map([](uint64_t v) { return Value<T>::from_int(int64_t(v)); });
-                    }
+                    if (n < 9) return next_machineword(n, false);
                     if (n == 9) {
                         // We can handle this with uint64_t if it's unsigned and the first byte is 0.
-                        if (i.get() == 0) return next_unsigned(8).map(Value<T>::from_unsigned);
+                        if (i.peek() == 0) {
+                            i.get();
+                            return next_machineword(8, true);
+                        }
                     }
                     return next_bignum(n);
                 }