Fix macOS build, where le32toh is not available

src/libfetchers/zip-input-accessor.cc

@@ -1,7 +1,7 @@
 #include "input-accessor.hh"
 
 #include <zip.h>
-#include <endian.h>
+#include <arpa/inet.h>
 
 namespace nix {
 
@@ -61,7 +61,7 @@ struct ZipInputAccessor : InputAccessor
                 zip_uint16_t id, len;
                 auto extra = zip_file_extra_field_get(zipFile, i, 0, &id, &len, ZIP_FL_CENTRAL);
                 if (id == 0x5455 && len >= 5)
-                    lastModified = std::max(lastModified, (time_t) le32toh(*((uint32_t *) (extra + 1))));
+                    lastModified = std::max(lastModified, readLittleEndian<time_t>((unsigned char *) extra + 1));
             }
 
             auto slash = strchr(sb.name, '/');

src/libutil/serialise.hh

@@ -331,17 +331,9 @@ T readNum(Source & source)
     unsigned char buf[8];
     source((char *) buf, sizeof(buf));
 
-    uint64_t n =
+    auto n = readLittleEndian<uint64_t>(buf);
-        ((uint64_t) buf[0]) |
-        ((uint64_t) buf[1] << 8) |
-        ((uint64_t) buf[2] << 16) |
-        ((uint64_t) buf[3] << 24) |
-        ((uint64_t) buf[4] << 32) |
-        ((uint64_t) buf[5] << 40) |
-        ((uint64_t) buf[6] << 48) |
-        ((uint64_t) buf[7] << 56);
 
-    if (n > (uint64_t)std::numeric_limits<T>::max())
+    if (n > (uint64_t) std::numeric_limits<T>::max())
         throw SerialisationError("serialised integer %d is too large for type '%s'", n, typeid(T).name());
 
     return (T) n;

src/libutil/util.hh

@@ -506,6 +506,17 @@ std::optional<N> string2Float(const std::string_view s)
 }
 
 
+/* Convert a little-endian integer to host order. */
+template<typename T>
+T readLittleEndian(unsigned char * p)
+{
+    T x = 0;
+    for (size_t i = 0; i < sizeof(x); ++i)
+        x |= *p++ << (i * 8);
+    return x;
+}
+
+
 /* Return true iff `s' starts with `prefix'. */
 bool hasPrefix(std::string_view s, std::string_view prefix);