node/caches: Extract cache calculation logic

I strongly recommend reviewing with the following git-diff flags:
  --color-moved=dimmed_zebra --color-moved-ws=allow-indentation-change

[META] In a future commit, this function will be re-used in TestingSetup
       so that the behaviour matches across test and non-test init
       codepaths.

src/node/caches.cpp

@@ -0,0 +1,38 @@
+// Copyright (c) 2021 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#include <node/caches.h>
+
+#include <txdb.h>
+#include <util/system.h>
+#include <validation.h>
+
+CacheSizes CalculateCacheSizes(const ArgsManager& args, size_t n_indexes)
+{
+    int64_t nTotalCache = (args.GetIntArg("-dbcache", nDefaultDbCache) << 20);
+    nTotalCache = std::max(nTotalCache, nMinDbCache << 20); // total cache cannot be less than nMinDbCache
+    nTotalCache = std::min(nTotalCache, nMaxDbCache << 20); // total cache cannot be greater than nMaxDbcache
+    int64_t nBlockTreeDBCache = std::min(nTotalCache / 8, nMaxBlockDBCache << 20);
+    nTotalCache -= nBlockTreeDBCache;
+    int64_t nTxIndexCache = std::min(nTotalCache / 8, args.GetBoolArg("-txindex", DEFAULT_TXINDEX) ? nMaxTxIndexCache << 20 : 0);
+    nTotalCache -= nTxIndexCache;
+    int64_t filter_index_cache = 0;
+    if (n_indexes > 0) {
+        int64_t max_cache = std::min(nTotalCache / 8, max_filter_index_cache << 20);
+        filter_index_cache = max_cache / n_indexes;
+        nTotalCache -= filter_index_cache * n_indexes;
+    }
+    int64_t nCoinDBCache = std::min(nTotalCache / 2, (nTotalCache / 4) + (1 << 23)); // use 25%-50% of the remainder for disk cache
+    nCoinDBCache = std::min(nCoinDBCache, nMaxCoinsDBCache << 20); // cap total coins db cache
+    nTotalCache -= nCoinDBCache;
+    int64_t nCoinCacheUsage = nTotalCache; // the rest goes to in-memory cache
+
+    return {
+        nBlockTreeDBCache,
+        nCoinDBCache,
+        nCoinCacheUsage,
+        nTxIndexCache,
+        filter_index_cache,
+    };
+}