abacus: bignum: use span

Once again, the code looks cleaner that way.

CMakeLists.txt

@@ -4,7 +4,7 @@ enable_testing()
 
 add_library(common_options INTERFACE)
 target_compile_features(common_options INTERFACE
-  cxx_std_17
+  cxx_std_20
 )
 target_compile_options(common_options INTERFACE
   -Wall

src/bignum/bignum.cc

@@ -3,6 +3,7 @@
 #include <algorithm>
 #include <iostream>
 #include <iterator>
+#include <span>
 
 #include <cassert>
 #include <cmath>
@@ -82,18 +83,17 @@ digits_type do_addition(digits_type const& lhs, digits_type const& rhs) {
         ++it2;
     }
 
-    auto it = it1;
-    auto end = end1;
-    if (it1 == end1) {
-        it = it2;
-        end = end2;
-    }
+    auto leftover = [=]() {
+        if (it1 != end1) {
+            return std::span(it1, end1);
+        }
+        return std::span(it2, end2);
+    }();
 
-    while (it != end) {
-        int addition = *it + carry;
+    for (auto value : leftover) {
+        int addition = value + carry;
         carry = addition / BASE;
         res.push_back(addition % BASE);
-        ++it;
     }
 
     if (carry != 0) {