1 files changed, 41 insertions, 3 deletions

diff --git a/io.c b/io.c
index 893cabc248..0db981e39c 100644
--- a/io.c
+++ b/io.c
@@ -6170,20 +6170,23 @@ io_strip_bom(VALUE io)
     return 0;
 }
 
-static void
+static rb_encoding *
 io_set_encoding_by_bom(VALUE io)
 {
     int idx = io_strip_bom(io);
     rb_io_t *fptr;
+    rb_encoding *extenc = NULL;
 
     GetOpenFile(io, fptr);
     if (idx) {
-	io_encoding_set(fptr, rb_enc_from_encoding(rb_enc_from_index(idx)),
-		rb_io_internal_encoding(io), Qnil);
+        extenc = rb_enc_from_index(idx);
+        io_encoding_set(fptr, rb_enc_from_encoding(extenc),
+                        rb_io_internal_encoding(io), Qnil);
     }
     else {
 	fptr->encs.enc2 = NULL;
     }
+    return extenc;
 }
 
 static VALUE
@@ -8308,6 +8311,40 @@ rb_io_initialize(int argc, VALUE *argv, VALUE io)
 
 /*
  *  call-seq:
+ *     ios.set_encoding_by_bom   -> encoding or nil
+ *
+ *  Checks if +ios+ starts with a BOM, and then consumes it and sets
+ *  the external encoding.  Returns the result encoding if found, or
+ *  nil.  If +ios+ is not binmode or its encoding has been set
+ *  already, an exception will be raised.
+ *
+ *    File.write("bom.txt", "\u{FEFF}abc")
+ *    ios = File.open("bom.txt", "rb")
+ *    ios.set_encoding_by_bom    #=>  #<Encoding:UTF-8>
+ *
+ *    File.write("nobom.txt", "abc")
+ *    ios = File.open("nobom.txt", "rb")
+ *    ios.set_encoding_by_bom    #=>  nil
+ */
+
+static VALUE
+rb_io_set_encoding_by_bom(VALUE io)
+{
+    rb_io_t *fptr;
+
+    GetOpenFile(io, fptr);
+    if (!(fptr->mode & FMODE_BINMODE)) {
+        rb_raise(rb_eArgError, "ASCII incompatible encoding needs binmode");
+    }
+    if (fptr->encs.enc2) {
+        rb_raise(rb_eArgError, "encoding conversion is set");
+    }
+    if (!io_set_encoding_by_bom(io)) return Qnil;
+    return rb_enc_from_encoding(fptr->encs.enc);
+}
+
+/*
+ *  call-seq:
  *     File.new(filename, mode="r" [, opt])            -> file
  *     File.new(filename [, mode [, perm]] [, opt])    -> file
  *
@@ -13319,6 +13356,7 @@ Init_IO(void)
     rb_define_method(rb_cIO, "external_encoding", rb_io_external_encoding, 0);
     rb_define_method(rb_cIO, "internal_encoding", rb_io_internal_encoding, 0);
     rb_define_method(rb_cIO, "set_encoding", rb_io_set_encoding, -1);
+    rb_define_method(rb_cIO, "set_encoding_by_bom", rb_io_set_encoding_by_bom, 0);
 
     rb_define_method(rb_cIO, "autoclose?", rb_io_autoclose_p, 0);
     rb_define_method(rb_cIO, "autoclose=", rb_io_set_autoclose, 1);