utf8: add function to detect a missing UTF-16/32 BOM

If the endianness is not defined in the encoding name, then let's
be strict and require a BOM to avoid any encoding confusion. The
is_missing_required_utf_bom() function returns true if a required BOM
is missing.

The Unicode standard instructs to assume big-endian if there in no BOM
for UTF-16/32 [1][2]. However, the W3C/WHATWG encoding standard used
in HTML5 recommends to assume little-endian to "deal with deployed
content" [3]. Strictly requiring a BOM seems to be the safest option
for content in Git.

This function is used in a subsequent commit.

[1] http://unicode.org/faq/utf_bom.html#gen6
[2] http://www.unicode.org/versions/Unicode10.0.0/ch03.pdf
     Section 3.10, D98, page 132
[3] https://encoding.spec.whatwg.org/#utf-16le

Signed-off-by: Lars Schneider <larsxschneider@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 19 insertions, 0 deletions

diff --git a/utf8.h b/utf8.h
index 0db1db4519..cce654a64a 100644
--- a/utf8.h
+++ b/utf8.h
@@ -79,4 +79,23 @@ void strbuf_utf8_align(struct strbuf *buf, align_type position, unsigned int wid
  */
 int has_prohibited_utf_bom(const char *enc, const char *data, size_t len);
 
+/*
+ * If the endianness is not defined in the encoding name, then we
+ * require a BOM. The function returns true if a required BOM is missing.
+ *
+ * The Unicode standard instructs to assume big-endian if there in no
+ * BOM for UTF-16/32 [1][2]. However, the W3C/WHATWG encoding standard
+ * used in HTML5 recommends to assume little-endian to "deal with
+ * deployed content" [3].
+ *
+ * Therefore, strictly requiring a BOM seems to be the safest option for
+ * content in Git.
+ *
+ * [1] http://unicode.org/faq/utf_bom.html#gen6
+ * [2] http://www.unicode.org/versions/Unicode10.0.0/ch03.pdf
+ *     Section 3.10, D98, page 132
+ * [3] https://encoding.spec.whatwg.org/#utf-16le
+ */
+int is_missing_required_utf_bom(const char *enc, const char *data, size_t len);
+
 #endif