Merge bitcoin/bitcoin#33878: refactor, docs: Embedded ASMap [2/3]: Refactor asmap internals and add documentation

4fec726c4d refactor: Simplify Interpret asmap function (Fabian Jahr)
79e97d45c1 doc: Add more extensive docs to asmap implementation (Fabian Jahr)
cf4943fdcd refactor: Use span instead of vector for data in util/asmap (Fabian Jahr)
385c34a052 refactor: Unify asmap version calculation and naming (Fabian Jahr)
fa41fc6a1a refactor: Operate on bytes instead of bits in Asmap code (Fabian Jahr)

Pull request description:

  This is a second slice carved out of #28792. It contains the following changes that are crucial for the embedding of asmap data which is added the following PR in the series (probably this will remain in #28792).

  The changes are:
  - Modernizes and simplifies the asmap code by operating on `std::byte` instead of bits
  - Unifies asmap version calculation and naming (previously it was called version and checksum interchangeably)
  - Operate on a `span` rather than a vector in the asmap internal to prevent holding the asmap data in memory twice
  - Add more extensive documentation to the asmap implementation
  - Unify asmap casing in implemetation function names

  The first three commits were already part of #28792, the others are new.

  The documentation commit came out of feedback gathered at the latest CoreDev. The primary input for the documentation was the documentation that already existed in the Python implementation (`contrib/asmap/asmap.py`) but there are several other comments as well. Please note: I have also asked several LLMs to provide suggestions on how to explain pieces of the implementation and better demonstrate how the parts work together. I have copied bits and pieces that I liked but everything has been edited further by me and obviously all mistakes here are my own.

ACKs for top commit:
  hodlinator:
    re-ACK 4fec726c4d
  sipa:
    ACK 4fec726c4d
  sedited:
    Re-ACK 4fec726c4d

Tree-SHA512: 950a591c3fcc9ddb28fcfdc3164ad3fbd325fa5004533c4a8b670fbf8b956060a0daeedd1fc2fced1f761ac49cd992b79cabe12ef46bc60b2559a7a613d0e166

src/init.cpp

@@ -96,6 +96,7 @@
 #include <algorithm>
 #include <cerrno>
 #include <condition_variable>
+#include <cstddef>
 #include <cstdint>
 #include <cstdio>
 #include <fstream>
@@ -1559,9 +1560,9 @@ bool AppInitMain(NodeContext& node, interfaces::BlockAndHeaderTipInfo* tip_info)
     ApplyArgsManOptions(args, peerman_opts);
 
     {
-
-        // Read asmap file if configured
-        std::vector<bool> asmap;
+        // Read asmap file if configured and initialize
+        // Netgroupman with or without it
+        assert(!node.netgroupman);
         if (args.IsArgSet("-asmap") && !args.IsArgNegated("-asmap")) {
             fs::path asmap_path = args.GetPathArg("-asmap");
             if (asmap_path.empty()) {
@@ -1575,21 +1576,19 @@ bool AppInitMain(NodeContext& node, interfaces::BlockAndHeaderTipInfo* tip_info)
                 InitError(strprintf(_("Could not find asmap file %s"), fs::quoted(fs::PathToString(asmap_path))));
                 return false;
             }
-            asmap = DecodeAsmap(asmap_path);
+            std::vector<std::byte> asmap{DecodeAsmap(asmap_path)};
             if (asmap.size() == 0) {
                 InitError(strprintf(_("Could not parse asmap file %s"), fs::quoted(fs::PathToString(asmap_path))));
                 return false;
             }
-            const uint256 asmap_version = (HashWriter{} << asmap).GetHash();
+            const uint256 asmap_version = AsmapVersion(asmap);
+            node.netgroupman = std::make_unique<NetGroupManager>(NetGroupManager::WithLoadedAsmap(std::move(asmap)));
             LogInfo("Using asmap version %s for IP bucketing", asmap_version.ToString());
         } else {
+            node.netgroupman = std::make_unique<NetGroupManager>(NetGroupManager::NoAsmap());
             LogInfo("Using /16 prefix for IP bucketing");
         }
 
-        // Initialize netgroup manager
-        assert(!node.netgroupman);
-        node.netgroupman = std::make_unique<NetGroupManager>(std::move(asmap));
-
         // Initialize addrman
         assert(!node.addrman);
         uiInterface.InitMessage(_("Loading P2P addresses…"));