Export and clean up net downloading script

Fixes https://github.com/official-stockfish/Stockfish/issues/5564

This patch extracts the net downloading script in Makefile into an
external script file. Also the script is moderately rewritten for
improved readability and speed.

* Use wget preferentially over curl, as curl is known to have slight
  overhead.
* Use command instead of hash to check if command exists. Reportedly,
  hash always returns zero in some POSIX shells even when the command
  fails.
* Command existence checks (wget/curl, sha256sum) are performed only
  once at the beginning.
* Each of common patterns is encapsulated in a function
  (get_nnue_filename, validate_network).
* Print out error/warning messages to stderr.

closes https://github.com/official-stockfish/Stockfish/pull/5563

No functional change

Co-authored-by: Disservin <disservin.social@gmail.com>

scripts/net.sh

@@ -0,0 +1,75 @@
+#!/bin/sh
+
+wget_or_curl=$( (command -v wget > /dev/null 2>&1 && echo "wget -q") || \
+                (command -v curl > /dev/null 2>&1 && echo "curl -L -s -k"))
+
+if [ -z "$wget_or_curl" ]; then
+  >&2 printf "%s\n" "Neither wget or curl is installed." \
+	         "Install one of these tools to download NNUE files automatically."
+  exit 1
+fi
+
+sha256sum=$( (command -v shasum > /dev/null 2>&1 && echo "shasum -a 256") || \
+             (command -v sha256sum > /dev/null 2>&1 && echo "sha256sum"))
+
+if [ -z "$sha256sum" ]; then
+  >&2 echo "sha256sum not found, NNUE files will be assumed valid."
+fi
+
+get_nnue_filename() {
+  grep "$1" evaluate.h | grep "#define" | sed "s/.*\(nn-[a-z0-9]\{12\}.nnue\).*/\1/"
+}
+
+validate_network() {
+  # If no sha256sum command is available, assume the file is always valid.
+  if [ -n "$sha256sum" ] && [ -f "$1" ]; then
+    if [ "$1" != "nn-$($sha256sum "$1" | cut -c 1-12).nnue" ]; then
+      rm -f "$1"
+      return 1
+    fi
+  fi
+}
+
+fetch_network() {
+  _filename="$(get_nnue_filename "$1")"
+
+  if [ -z "$_filename" ]; then
+    >&2 echo "NNUE file name not found for: $1"
+    return 1
+  fi
+
+  if [ -f "$_filename" ]; then
+    if validate_network "$_filename"; then
+      echo "Existing $_filename validated, skipping download"
+      return
+    else
+      echo "Removing invalid NNUE file: $_filename"
+    fi
+  fi
+
+  for url in \
+    "https://tests.stockfishchess.org/api/nn/$_filename" \
+    "https://github.com/official-stockfish/networks/raw/master/$_filename"; do
+    echo "Downloading from $url ..."
+    if $wget_or_curl "$url"; then
+      if validate_network "$_filename"; then
+        echo "Successfully validated $_filename"
+      else
+        echo "Downloaded $_filename is invalid"
+        continue
+      fi
+    else
+      echo "Failed to download from $url"
+    fi
+    if [ -f "$_filename" ]; then
+      return
+    fi
+  done
+
+  # Download was not successful in the loop, return false.
+  >&2 echo "Failed to download $_filename"
+  return 1
+}
+
+fetch_network EvalFileDefaultNameBig && \
+fetch_network EvalFileDefaultNameSmall