From 70314b3303f712a0277b80b23bc3613744e6178e Mon Sep 17 00:00:00 2001
From: Karen Arutyunov <karen@codesynthesis.com>
Date: Mon, 1 Jun 2020 19:51:17 +0300
Subject: Add builtin weight

Also invent the notion of external builtin (builtin_info::function is NULL).
---
 libbutl/builtin.cxx      | 29 +++++++++++++++--------------
 libbutl/builtin.mxx      | 27 +++++++++++++++++++++++----
 libbutl/command.cxx      | 20 ++++++++++----------
 tests/builtin/driver.cxx | 12 +++++++++---
 4 files changed, 57 insertions(+), 31 deletions(-)

diff --git a/libbutl/builtin.cxx b/libbutl/builtin.cxx
index 9efa697..fe74b05 100644
--- a/libbutl/builtin.cxx
+++ b/libbutl/builtin.cxx
@@ -2109,19 +2109,20 @@ namespace butl
 
   const builtin_map builtins
   {
-    {"cat",   &async_impl<&cat>},
-    {"cp",    &sync_impl<&cp>},
-    {"echo",  &async_impl<&echo>},
-    {"false", &false_},
-    {"ln",    &sync_impl<&ln>},
-    {"mkdir", &sync_impl<&mkdir>},
-    {"mv",    &sync_impl<&mv>},
-    {"rm",    &sync_impl<&rm>},
-    {"rmdir", &sync_impl<&rmdir>},
-    {"sed",   &async_impl<&sed>},
-    {"sleep", &sync_impl<&sleep>},
-    {"test",  &sync_impl<&test>},
-    {"touch", &sync_impl<&touch>},
-    {"true",  &true_}
+    {"cat",   {&async_impl<&cat>,  2}},
+    {"cp",    {&sync_impl<&cp>,    2}},
+    {"diff",  {nullptr,            2}},
+    {"echo",  {&async_impl<&echo>, 2}},
+    {"false", {&false_,            0}},
+    {"ln",    {&sync_impl<&ln>,    2}},
+    {"mkdir", {&sync_impl<&mkdir>, 2}},
+    {"mv",    {&sync_impl<&mv>,    2}},
+    {"rm",    {&sync_impl<&rm>,    1}},
+    {"rmdir", {&sync_impl<&rmdir>, 1}},
+    {"sed",   {&async_impl<&sed>,  2}},
+    {"sleep", {&sync_impl<&sleep>, 1}},
+    {"test",  {&sync_impl<&test>,  1}},
+    {"touch", {&sync_impl<&touch>, 2}},
+    {"true",  {&true_,             0}}
   };
 }
diff --git a/libbutl/builtin.mxx b/libbutl/builtin.mxx
index 2be2f90..e4dd4f8 100644
--- a/libbutl/builtin.mxx
+++ b/libbutl/builtin.mxx
@@ -153,19 +153,38 @@ LIBBUTL_MODEXPORT namespace butl
                                     const dir_path& cwd,
                                     const builtin_callbacks&);
 
-  class builtin_map: public std::map<std::string, builtin_function*>
+  // Builtin function and weight.
+  //
+  // The weight between 0 and 2 reflects the builtin's contribution to the
+  // containing script semantics with 0 being the lowest/ignore. Current
+  // mapping is as follows:
+  //
+  // 0 - non-contributing (true, false)
+  // 1 - non-creative     (rm, rmdir, sleep, test)
+  // 2 - creative         (any builtin that may produce output)
+  //
+  // If the function is NULL, then the builtin has an external implementation
+  // and should be executed by running the program with this name.
+  //
+  struct builtin_info
+  {
+    builtin_function* function;
+    uint8_t           weight;
+  };
+
+  class builtin_map: public std::map<std::string, builtin_info>
   {
   public:
-    using base = std::map<std::string, builtin_function*>;
+    using base = std::map<std::string, builtin_info>;
     using base::base;
 
     // Return NULL if not a builtin.
     //
-    builtin_function*
+    const builtin_info*
     find (const std::string& n) const
     {
       auto i (base::find (n));
-      return i != end () ? i->second : nullptr;
+      return i != end () ? &i->second : nullptr;
     }
   };
 
diff --git a/libbutl/command.cxx b/libbutl/command.cxx
index bb5287c..fadd617 100644
--- a/libbutl/command.cxx
+++ b/libbutl/command.cxx
@@ -227,9 +227,9 @@ namespace butl
                                  msg.c_str ());
     }
 
-    builtin_function* bf (builtins.find (prog));
+    const builtin_info* bi (builtins.find (prog));
 
-    if (bf != nullptr) // Execute the builtin.
+    if (bi != nullptr && bi->function != nullptr) // Execute the builtin.
     {
       if (callback)
       {
@@ -259,17 +259,17 @@ namespace butl
       uint8_t r; // Storage.
       builtin_callbacks cb;
 
-      builtin b (bf (r,
-                     args,
-                     nullfd    /* stdin */,
-                     move (rd) /* stdout */,
-                     nullfd    /* stderr */,
-                     cwd,
-                     cb));
+      builtin b (bi->function (r,
+                               args,
+                               nullfd    /* stdin */,
+                               move (rd) /* stdout */,
+                               nullfd    /* stderr */,
+                               cwd,
+                               cb));
 
       return process_exit (b.wait ());
     }
-    else               // Execute the program.
+    else                                          // Execute the program.
     {
       // Strip the potential leading `^`, indicating that this is an external
       // program rather than a builtin. Consider only simple paths and don't
diff --git a/tests/builtin/driver.cxx b/tests/builtin/driver.cxx
index 2583ba2..9fb6d6f 100644
--- a/tests/builtin/driver.cxx
+++ b/tests/builtin/driver.cxx
@@ -144,15 +144,21 @@ main (int argc, char* argv[])
 
   // Execute the builtin.
   //
-  builtin_function* bf (builtins.find (name));
+  const builtin_info* bi (builtins.find (name));
 
-  if (bf == nullptr)
+  if (bi == nullptr)
   {
     cerr << "unknown builtin '" << name << "'" << endl;
     return 1;
   }
 
+  if (bi->function == nullptr)
+  {
+    cerr << "external builtin '" << name << "'" << endl;
+    return 1;
+  }
+
   uint8_t r; // Storage.
-  builtin b (bf (r, args, nullfd, nullfd, nullfd, cwd, callbacks));
+  builtin b (bi->function (r, args, nullfd, nullfd, nullfd, cwd, callbacks));
   return b.wait ();
 }
-- 
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