From f211934d6ff53388a806c5cc06483e9d843056e4 Mon Sep 17 00:00:00 2001
From: Boris Kolpackov <boris@codesynthesis.com>
Date: Wed, 27 Jan 2021 06:14:57 +0200
Subject: Make std::map prepend (=+) overriding (like insert_or_assign())

---
 libbuild2/variable.cxx |  6 ++++++
 libbuild2/variable.hxx | 24 +++++++++++++++++++-----
 libbuild2/variable.ixx | 36 ++++++++++++++++++++++++++++++++++++
 libbuild2/variable.txx | 31 ++++++++++++++++++++++++++++++-
 4 files changed, 91 insertions(+), 6 deletions(-)

diff --git a/libbuild2/variable.cxx b/libbuild2/variable.cxx
index 47703da..6f45e90 100644
--- a/libbuild2/variable.cxx
+++ b/libbuild2/variable.cxx
@@ -1838,6 +1838,9 @@ namespace build2
   value_traits<vector<pair<optional<string>, string>>>;
 
   template struct LIBBUILD2_DEFEXPORT
+  value_traits<vector<pair<string, optional<bool>>>>;
+
+  template struct LIBBUILD2_DEFEXPORT
   value_traits<std::map<string, string>>;
 
   template struct LIBBUILD2_DEFEXPORT
@@ -1847,5 +1850,8 @@ namespace build2
   value_traits<std::map<optional<string>, string>>;
 
   template struct LIBBUILD2_DEFEXPORT
+  value_traits<std::map<string, optional<bool>>>;
+
+  template struct LIBBUILD2_DEFEXPORT
   value_traits<std::map<project_name, dir_path>>;
 }
diff --git a/libbuild2/variable.hxx b/libbuild2/variable.hxx
index a9dde7f..a04e0a9 100644
--- a/libbuild2/variable.hxx
+++ b/libbuild2/variable.hxx
@@ -339,15 +339,17 @@ namespace build2
     // Assign/Append/Prepend.
     //
   public:
-    // Assign/append a typed value. For assign, LHS should be either of the
-    // same type or untyped. For append, LHS should be either of the same type
-    // or untyped and NULL.
+    // Assign/append/prepend a typed value. For assign, LHS should be either
+    // of the same type or untyped. For append, LHS should be either of the
+    // same type or untyped and NULL.
     //
     template <typename T> value& operator= (T);
     template <typename T> value& operator+= (T);
+    template <typename T> value& prepend (T);
 
     value& operator= (names);
     value& operator+= (names);
+    //value& prepend (names); // See below.
 
     template <typename T> value& operator= (T* v) {
       return v != nullptr ? *this = *v : *this = nullptr;}
@@ -355,8 +357,12 @@ namespace build2
     template <typename T> value& operator+= (T* v) {
       return v != nullptr ? *this += *v : *this;}
 
+    template <typename T> value& prepend (T* v) {
+      return v != nullptr ? prepend (*v) : *this;}
+
     value& operator= (const char* v) {return *this = string (v);}
     value& operator+= (const char* v) {return *this += string (v);}
+    value& prepend (const char* v) {return prepend (string (v));}
 
     // Assign/append/prepend raw data. Variable is optional and is only used
     // for diagnostics.
@@ -1113,6 +1119,9 @@ namespace build2
   //
   // Either K or V can be optional<T> making the key or value optional.
   //
+  // Note that append/+= is non-overriding (like insert()) while prepend/=+
+  // is (like insert_or_assign()).
+  //
   template <typename K, typename V>
   struct map_value_type;
 
@@ -1125,8 +1134,7 @@ namespace build2
 
     static void assign (value&, map<K, V>&&);
     static void append (value&, map<K, V>&&);
-    static void prepend (value& v, map<K, V>&& x) {
-      return append (v, move (x));}
+    static void prepend (value&, map<K, V>&&);
     static bool empty (const map<K, V>& x) {return x.empty ();}
 
     static const map<K, V> empty_instance;
@@ -1156,6 +1164,9 @@ namespace build2
   value_traits<vector<pair<optional<string>, string>>>;
 
   extern template struct LIBBUILD2_DECEXPORT
+  value_traits<vector<pair<string, optional<bool>>>>;
+
+  extern template struct LIBBUILD2_DECEXPORT
   value_traits<std::map<string, string>>;
 
   extern template struct LIBBUILD2_DECEXPORT
@@ -1165,6 +1176,9 @@ namespace build2
   value_traits<std::map<optional<string>, string>>;
 
   extern template struct LIBBUILD2_DECEXPORT
+  value_traits<std::map<string, optional<bool>>>;
+
+  extern template struct LIBBUILD2_DECEXPORT
   value_traits<std::map<project_name, dir_path>>; // var_subprojects
 
   // Project-wide (as opposed to global) variable overrides (see context ctor
diff --git a/libbuild2/variable.ixx b/libbuild2/variable.ixx
index 9b24b9d..fd78842 100644
--- a/libbuild2/variable.ixx
+++ b/libbuild2/variable.ixx
@@ -114,6 +114,22 @@ namespace build2
     return *this;
   }
 
+  template <typename T>
+  inline value& value::
+  prepend (T v)
+  {
+    assert (type == &value_traits<T>::value_type || (type == nullptr && null));
+
+    // Prepare the receiving value.
+    //
+    if (type == nullptr)
+      type = &value_traits<T>::value_type;
+
+    value_traits<T>::prepend (*this, move (v));
+    null = false;
+    return *this;
+  }
+
   inline value& value::
   operator= (names v)
   {
@@ -867,6 +883,26 @@ namespace build2
       new (&v.data_) map<K, V> (move (x));
   }
 
+  template <typename K, typename V>
+  inline void value_traits<std::map<K, V>>::
+  prepend (value& v, map<K, V>&& x)
+  {
+    if (v)
+    {
+      map<K, V>& m (v.as<map<K, V>> ());
+
+      m.swap (x);
+
+      // Note that this will only move values. Keys (being const) are still
+      // copied.
+      //
+      m.insert (make_move_iterator (x.begin ()),
+                make_move_iterator (x.end ()));
+    }
+    else
+      new (&v.data_) map<K, V> (move (x));
+  }
+
   // variable_pool
   //
   inline const variable& variable_pool::
diff --git a/libbuild2/variable.txx b/libbuild2/variable.txx
index 9c9c867..e1808fc 100644
--- a/libbuild2/variable.txx
+++ b/libbuild2/variable.txx
@@ -838,6 +838,35 @@ namespace build2
 
   template <typename K, typename V>
   void
+  map_prepend (value& v, names&& ns, const variable* var)
+  {
+    using std::map;
+
+    map<K, V>& p (v
+                  ? v.as<map<K, V>> ()
+                  : *new (&v.data_) map<K, V> ());
+
+    // Verify we have a sequence of pairs and convert each lhs/rhs to K/V.
+    //
+    for (auto i (ns.begin ()); i != ns.end (); ++i)
+    {
+      name& l (*i);
+      name* r (l.pair ? &*++i : nullptr);
+
+      pair<K, V> v (value_traits<pair<K, V>>::convert (
+                      move (l), r,
+                      value_traits<map<K, V>>::value_type.name,
+                      "element",
+                      var));
+
+      // Poor man's emplace_or_assign().
+      //
+      p.emplace (move (v.first), V ()).first->second = move (v.second);
+    }
+  }
+
+  template <typename K, typename V>
+  void
   map_assign (value& v, names&& ns, const variable* var)
   {
     using std::map;
@@ -961,7 +990,7 @@ namespace build2
     &default_copy_assign<map<K, V>>,
     &map_assign<K, V>,
     &map_append<K, V>,
-    &map_append<K, V>,   // Prepend is the same as append.
+    &map_prepend<K, V>,
     &map_reverse<K, V>,
     nullptr,             // No cast (cast data_ directly).
     &map_compare<K, V>,
-- 
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