FileLoader.cpp: create hash and link

src/FileLoader.cpp

 #include <DD4hep/DetFactoryHelper.h>
+#include <DD4hep/Primitives.h>
 #include <DD4hep/Factories.h>
 #include <DD4hep/Printout.h>
 #include <XML/Utilities.h>
 
+#include <fmt/core.h>
+
 #include <filesystem>
 #include <iostream>
 #include <string>
@@ -11,12 +14,14 @@ namespace fs = std::filesystem;
 
 using namespace dd4hep;
 
-void usage(int argc, char** argv)    {
+void usage(int argc, char** argv) {
   std::cout <<
     "Usage: -plugin <name> -arg [-arg]                                                  \n"
     "     name:   factory name     FileLoader                                           \n"
+    "     cache:<string>           cache location (may be read-only)                    \n"
     "     file:<string>            file location                                        \n"
     "     url:<string>             url location                                         \n"
+    "     cmd:<string>             download command with {0} for url, {1} for output    \n"
     "\tArguments given: " << arguments(argc,argv) << std::endl;
   std::exit(EINVAL);
 }
@@ -27,24 +32,120 @@ long load_file(
     int argc,
     char** argv
 ) {
-  std::string file, url;
+  // argument parsing
+  std::string cache, file, url;
+  std::string cmd("curl --retry 5 -f {0} -o {1}");
   for (int i = 0; i < argc && argv[i]; ++i) {
-    if      (0 == std::strncmp("file:",argv[i], 5)) file = (argv[i] + 5);
-    else if (0 == std::strncmp("url:", argv[i], 4)) url  = (argv[i] + 4);
+    if      (0 == std::strncmp("cache:", argv[i], 6)) cache = (argv[i] + 6);
+    else if (0 == std::strncmp("file:", argv[i], 5)) file = (argv[i] + 5);
+    else if (0 == std::strncmp("url:", argv[i], 4)) url = (argv[i] + 4);
+    else if (0 == std::strncmp("cmd:", argv[i], 4)) cmd = (argv[i] + 4);
     else usage(argc, argv);
   }
-  std::cout << "Loading " << file << " from " << url << std::endl;
+  printout(DEBUG, "FileLoader", "arg cache: " + cache);
+  printout(DEBUG, "FileLoader", "arg file: " + file);
+  printout(DEBUG, "FileLoader", "arg url: " + url);
+  printout(DEBUG, "FileLoader", "arg cmd: " + cmd);
+
+  // if file or url is empty, do nothing
+  if (file.empty()) {
+    printout(WARNING, "FileLoader", "no file specified");
+    return 0;
+  }
+  if (url.empty()) {
+    printout(WARNING, "FileLoader", "no url specified");
+    return 0;
+  }
+
+  // create file path
+  fs::path file_path(file);
 
-  if (!fs::exists(fs::path(file))) {
-    std::string parent_path = fs::path(file).parent_path();
-    auto ret = std::system(("mkdir -p " + parent_path + " && "
-                            "curl --retry 5 -f " + url + " -o " + file).c_str());
-    if (!fs::exists(fs::path(file))) {
-      std::cerr << "ERROR: file, " << file << ", does not exist\n";
+  // create hash from url, hex of unsigned long long
+  std::string hash = fmt::format("{:016x}", dd4hep::detail::hash64(url)); // TODO: Use c++20 std::fmt
+
+  // create file parent path, if not exists
+  fs::path parent_path = file_path.parent_path();
+  if (!fs::exists(parent_path)) {
+    if (fs::create_directories(parent_path) == false) {
+      printout(ERROR, "FileLoader", "parent path " + parent_path.string() + " cannot be created");
+      printout(ERROR, "FileLoader", "check permissions and retry");
       std::quick_exit(1);
     }
   }
 
+  // if file exists and is symlink to correct hash
+  fs::path hash_path(parent_path / hash);
+  if (fs::exists(file_path)
+   && fs::equivalent(file_path, hash_path)) {
+    printout(INFO, "FileLoader", "Link " + file + " -> hash " + hash + " already exists");
+    return 0;
+  }
+
+  // if hash does not exist, we must retrieve file from cache or url
+  if (!fs::exists(hash_path)) {
+    fs::path cache_path(cache);
+    fs::path cache_hash_path(cache_path / hash);
+    if (fs::exists(cache_hash_path)) {
+      // if cache/hash exists
+      // symlink hash to cache/hash
+      printout(INFO, "FileLoader", "File " + file + " with hash " + hash + " found in " + cache);
+      try {
+        fs::create_symlink(cache_hash_path, hash_path);
+      } catch (const fs::filesystem_error&) {
+        printout(ERROR, "FileLoader", "unable to link from " + hash_path.string() + " to " + cache_hash_path.string());
+        printout(ERROR, "FileLoader", "check permissions and retry");
+        std::quick_exit(1);
+      }
+    } else {
+      // if cache/hash doesn't exists
+      cmd = fmt::format(cmd, url, hash_path.c_str()); // TODO: Use c++20 std::fmt
+      printout(INFO, "FileLoader", "Downloading " + file + " as hash " + hash + " with " + cmd);
+      // run cmd
+      auto ret = std::system(cmd.c_str());
+      if (!fs::exists(hash_path)) {
+        printout(ERROR, "FileLoader", "unable to run cmd " + cmd);
+        printout(ERROR, "FileLoader", "check command and retry");
+        std::quick_exit(1);
+      }
+    }
+  }
+  // hash_path now exists
+
+  // check if file already exists
+  if (fs::exists(file_path)) {
+    // file already exists
+    if (fs::is_symlink(file_path)) {
+      // file is symlink
+      if (fs::equivalent(hash_path, fs::read_symlink(file_path))) {
+        // link points to correct path
+        return 0;
+      } else {
+        // link points to incorrect path 
+        if (fs::remove(file_path) == false) {
+          printout(ERROR, "FileLoader", "unable to remove symlink " + file_path.string());
+          printout(ERROR, "FileLoader", "check permissions or remove manually");
+          std::quick_exit(1);
+        }
+      }
+    } else {
+      // file exists but not symlink
+      printout(ERROR, "FileLoader", "will not remove actual file " + file_path.string());
+      printout(ERROR, "FileLoader", "check content, remove manually, and retry");
+      std::quick_exit(1);
+    }
+  }
+  // file_path now does not exist
+
+  // symlink file_path to hash_path
+  try {
+    // use new path from hash so file link is local
+    fs::create_symlink(fs::path(hash), file_path);
+  } catch (const fs::filesystem_error&) {
+    printout(ERROR, "FileLoader", "unable to link from " + file_path.string() + " to " + hash_path.string());
+    printout(ERROR, "FileLoader", "check permissions and retry");
+    std::quick_exit(1);
+  }
+
   return 0;
 }