Adds functionality for set-nth.

Fixes #578 and #581.

context.cpp

@@ -406,6 +406,7 @@ namespace Sass {
     // List Functions
     register_function(ctx, length_sig, length, env);
     register_function(ctx, nth_sig, nth, env);
+    register_function(ctx, set_nth_sig, set_nth, env);
     register_function(ctx, index_sig, index, env);
     register_function(ctx, join_sig, join, env);
     register_function(ctx, append_sig, append, env);

functions.cpp

@@ -1052,6 +1052,26 @@ namespace Sass {
       return l->value_at_index(index);
     }
 
+    Signature set_nth_sig = "set-nth($list, $n, $value)";
+    BUILT_IN(set_nth)
+    {
+      List* l = dynamic_cast<List*>(env["$list"]);
+      Number* n = ARG("$n", Number);
+      Expression* v = ARG("$value", Expression);
+      if (!l) {
+        l = new (ctx.mem) List(path, position, 1);
+        *l << ARG("$list", Expression);
+      }
+      if (l->empty()) error("argument `$list` of `" + string(sig) + "` must not be empty", path, position);
+      double index = std::floor(n->value() < 0 ? l->length() + n->value() : n->value() - 1);
+      if (index < 0 || index > l->length() - 1) error("index out of bounds for `" + string(sig) + "`", path, position);
+      List* result = new (ctx.mem) List(path, position, l->length(), l->separator());
+      for (size_t i = 0, L = l->length(); i < L; ++i) {
+        *result << ((i == index) ? v : (*l)[i]);
+      }
+      return result;
+    }
+
     Signature index_sig = "index($list, $value)";
     BUILT_IN(index)
     {

functions.hpp

@@ -104,6 +104,7 @@ namespace Sass {
     extern Signature map_values_sig;
     extern Signature map_has_key_sig;
     extern Signature keywords_sig;
+    extern Signature set_nth_sig;
 
     BUILT_IN(rgb);
     BUILT_IN(rgba_4);
@@ -174,6 +175,7 @@ namespace Sass {
     BUILT_IN(map_values);
     BUILT_IN(map_has_key);
     BUILT_IN(keywords);
+    BUILT_IN(set_nth);
 
   }
 }