2 files changed, 59 insertions, 32 deletions

diff --git a/Makefile b/Makefile
index fec3007..011a775 100644
--- a/Makefile
+++ b/Makefile
@@ -7,6 +7,7 @@ OBJ=$(SRC:.cpp=.o)
 all: $(PROJECTNAME)
 
 LDLIBS += -lfmt
+CXXFLAGS += -std=c++17
 
 $(PROJECTNAME): $(OBJ)
 	$(CXX) $(LDFLAGS) $^ $(LDLIBS) $(LIBS) -o $@
diff --git a/xmake.cpp b/xmake.cpp
index e0a972d..e5f85ac 100644
--- a/xmake.cpp
+++ b/xmake.cpp
@@ -27,33 +27,51 @@ namespace {
   std::cout << "Usage: xmake <target>" << std::endl;
  }
 
- std::string get_target(pt::ptree& ptree) {
+ fs::path get_target(const pt::ptree& ptree) {
   return ptree.get<std::string>("xmake.build.name");
  }
  
- std::vector<std::string> get_sources(pt::ptree& ptree) {
-  std::vector<std::string> sources;
-  for (pt::ptree::value_type &v: ptree.get_child("xmake.build")) {
+ std::vector<fs::path> get_sources(const pt::ptree& ptree) {
+  std::vector<fs::path> sources;
+  for (const pt::ptree::value_type &v: ptree.get_child("xmake.build")) {
    if (v.first == "source")
     sources.push_back(v.second.data());
   }
   return sources;
  }
 
- std::vector<std::string> get_objects(pt::ptree& ptree) {
-  std::vector<std::string> objects{get_sources(ptree)};
+ std::vector<fs::path> get_objects(const pt::ptree& ptree) {
+  std::vector<fs::path> objects{get_sources(ptree)};
   for (auto &i: objects) {
-   fs::path p{i};
-   p.replace_extension("o");
-   i = p.string();
+   i.replace_extension("o");
   }
   return objects;
  }
 
- void build(pt::ptree& ptree) {
-  std::string target{get_target(ptree)};
+ // both need to exist
+ bool is_older(const fs::path& p, const fs::path& other) {
+  auto t_p{fs::last_write_time(p)};
+  auto t_other{fs::last_write_time(other)};
+  return t_p < t_other;
+ }
+
+ bool is_outdated(const fs::path& p, const std::vector<fs::path> &dependencies) {
+  if (!fs::exists(p))
+   return true;
+
+  for (auto& dep: dependencies) {
+   if (!exists(dep) || is_older(p, dep)) {
+    return true;
+   }
+  }
 
-  std::vector<std::string> sources{get_sources(ptree)};
+  return false;
+ }
+
+ void build(const pt::ptree& ptree) {
+  fs::path target{get_target(ptree)};
+
+  std::vector<fs::path> sources{get_sources(ptree)};
 
   std::cout << "Target: " << target << std::endl;
 
@@ -64,44 +82,52 @@ namespace {
 
   std::vector<std::string> commands;
   // compile
-  for (auto &i: sources) {
-   fs::path p{i};
-   fs::path p_obj{i};
+  for (auto &p: sources) {
+   fs::path p_obj{p};
    p_obj.replace_extension("o");
-   commands.push_back(fmt::format("g++ -std=c++17 -c {} -o {}", p.string(), p_obj.string()));
+   if (is_outdated(p_obj, std::vector<fs::path>{p})) {
+    commands.push_back(fmt::format("g++ -std=c++17 -c {} -o {}", p.string(), p_obj.string()));
+   }
   }
   // link
-  std::string link_command{"g++"};
-  std::vector<std::string> objects{get_objects(ptree)};
-  for (auto &i: objects) {
-   link_command += fmt::format(" {}", i);
+  std::vector<fs::path> objects{get_objects(ptree)};
+  std::vector<fs::path> dependencies{sources};
+  if (is_outdated(target, dependencies)) {
+   std::string link_command{"g++"};
+   for (auto &i: objects) {
+    link_command += fmt::format(" {}", i.string());
+   }
+   link_command += " -lfmt";
+   link_command += fmt::format(" -o {}", target.string());
+   commands.push_back(link_command);
   }
-  link_command += " -lfmt";
-  link_command += fmt::format(" -o {}", target);
-  commands.push_back(link_command);
 
   std::cout << "Commands: " << std::endl;
   for (auto &i: commands) {
    std::cout << "  " << i << std::endl;
   }
 
-  std::cout << "Running commands: " << std::endl;
-  for (auto &i: commands) {
-   std::cout << i << std::endl;
-   int result{system(i.c_str())};
-   if (result != 0) {
-    throw std::runtime_error(fmt::format("Error {}", result));
+  if (commands.size() == 0) {
+   std::cout << "Everything up to date." << std::endl;
+  } else {
+   std::cout << "Running commands: " << std::endl;
+   for (auto &i: commands) {
+    std::cout << i << std::endl;
+    int result{system(i.c_str())};
+    if (result != 0) {
+     throw std::runtime_error(fmt::format("Error {}", result));
+    }
    }
   }
  }
 
- void clean(pt::ptree& ptree) {
-  std::vector<std::string> cleanlist{get_objects(ptree)};
+ void clean(const pt::ptree& ptree) {
+  std::vector<fs::path> cleanlist{get_objects(ptree)};
   cleanlist.push_back(get_target(ptree));
 
   std::vector<std::string> commands;
   for (auto &i: cleanlist) {
-   commands.push_back(fmt::format("rm -f {}", i));
+   commands.push_back(fmt::format("rm -f {}", i.string()));
   }
 
   std::cout << "Running commands: " << std::endl;