Guess if .r is R or REBOL

.r is the default extension used widely for both, R and REBOL. This
patch attempts to guess if .r is a REBOL file by looking for the
following very common REBOL code fragments:

- `REBOL`
- `: func [`
- `make object! [`
- `context [`

If any of those is found in a .r file, it is identified as a REBOL file.
Otherwise the language for a .r file is R.

Signed-off-by: Andreas Bolka <a@bolka.at>

lib/linguist/blob_helper.rb

@@ -259,6 +259,9 @@ module Linguist
       # If its a header file (.h) try to guess the language
       header_language ||
 
+        # If it's a .r file, try to guess the language
+        r_language ||
+
         # See if there is a Language for the extension
         pathname.language ||
 
@@ -288,6 +291,19 @@ module Linguist
       end
     end
 
+    # Internal: Guess language of .r files.
+    #
+    # Returns a Language.
+    def r_language
+      return unless extname == '.r'
+
+      if lines.grep(/(rebol|(:\s+func|make\s+object!|^\s*context)\s*\[)/i).any?
+        Language['Rebol']
+      else
+        Language['R']
+      end
+    end
+
     # Internal: Extract the script name from the shebang line
     #
     # Requires Blob#data

test/fixtures/hello-r.R

@@ -0,0 +1,4 @@
+hello <- function() {
+    print("hello, world!")
+}
+hello()

test/fixtures/hello-rebol.r

@@ -0,0 +1,5 @@
+REBOL []
+hello: func [] [
+    print "hello, world!"
+]
+hello

test/test_blob.rb

@@ -224,6 +224,10 @@ class TestBlob < Test::Unit::TestCase
     assert_equal Language['Ruby'],        blob("wrong_shebang.rb").language
     assert_nil blob("octocat.png").language
 
+    # .r disambiguation
+    assert_equal Language['R'],           blob("hello-r.R").language
+    assert_equal Language['Rebol'],       blob("hello-rebol.r").language
+
     # ML
     assert_equal Language['OCaml'],       blob("Foo.ml").language
     assert_equal Language['Standard ML'], blob("Foo.sig").language