(Coding System Basics): Describe about rondtrip

identity of coding systems.

1 files changed, 22 insertions, 0 deletions

diff --git a/lispref/nonascii.texi b/lispref/nonascii.texi
index 70e77e0a837..91a47ea50f9 100644
--- a/lispref/nonascii.texi
+++ b/lispref/nonascii.texi
@@ -628,6 +628,28 @@ characters; for example, there are three coding systems for the Cyrillic
 conversion, but some of them leave the choice unspecified---to be chosen
 heuristically for each file, based on the data.
 
+In general, a coding system doesn't guarantee a roundtrip identity,
+i.e. decoding followed by encoding in the same coding system can
+result in the different byte sequence.  But there are several coding
+systems that go guarantee that the result will be the same as what you
+originally decoded.  They are:
+
+@quotation
+chinese-big5 chinese-iso-8bit cyrillic-iso-8bit emacs-mule
+greek-iso-8bit hebrew-iso-8bit iso-latin-1 iso-latin-2 iso-latin-3
+iso-latin-4 iso-latin-5 iso-latin-8 iso-latin-9 iso-safe
+japanese-iso-8bit japanese-shift-jis korean-iso-8bit raw-text
+@end quotation
+
+Likewise, a coding systme doesn't guarantee the other way of roundtrip
+identity, i.e. encoding buffer text into a coding system followed by
+decoding again with the same coding system will produce the different
+buffer text.  For instance, when you encode Latin-2 characters by
+@code{utf-8} and decode it back by the same coding system, you'll get
+Unicode charactes (of charset @code{mule-unicode-0100-24ff}), and when
+you encode Unicode characters by @code{iso-latin-2} and decode it back
+by the same coding system, you'll get Latin-2 characters.
+
 @cindex end of line conversion
   @dfn{End of line conversion} handles three different conventions used
 on various systems for representing end of line in files.  The Unix