% Generated by roxygen2: do not edit by hand
% Please edit documentation in R/encoding.R
\name{about_encoding}
\alias{about_encoding}
\alias{stringi-encoding}
\alias{encoding}
\title{Character Encodings and \pkg{stringi}}
\description{
This manual page explains how \pkg{stringi} deals with character
strings in various encodings.

In particular we should note that:
\itemize{
   \item \R lets strings in ASCII, UTF-8, and your platform's
   native encoding coexist. A character vector printed on the console
   by calling \code{\link{print}} or \code{\link{cat}} is
   silently re-encoded to the native encoding.
   \item Functions in \pkg{stringi} process each string internally in
   Unicode, the most universal character encoding ever.
   Even if a string is given in the native encoding, i.e., your platform's
   default one, it will be converted to Unicode (precisely: UTF-8 or UTF-16).
   \item Most \pkg{stringi} functions always return UTF-8 encoded strings,
   regardless of the input encoding. What is more, the functions have been
   optimized for UTF-8/ASCII input (they have competitive, if not better
   performance, especially when performing more complex operations like
   string comparison, sorting, and even concatenation). Thus, it is
   best to rely on cascading calls to \pkg{stringi} operations solely.
}
}
\details{
'Hundreds of encodings have been developed over the years, each for small
groups of languages and for special purposes. As a result,
the interpretation of text, input, sorting, display, and storage
depends on the knowledge of all the different types of character sets
and their encodings. Programs have been written to handle either
one single encoding at a time and switch between them, or to convert
between external and internal encodings.'

'Unicode provides a single character set that covers the major
languages of the world, and a small number of machine-friendly encoding
forms and schemes to fit the needs of existing applications and protocols.
It is designed for best interoperability with both ASCII and ISO-8859-1
(the most widely used character sets) to make it easier for Unicode to be
used in almost all applications and protocols' (see the ICU User Guide).

The Unicode Standard determines the way to map any possible character
to a numeric value -- a so-called code point. Such code points, however,
have to be stored somehow in computer's memory.
The Unicode Standard encodes characters in the range U+0000..U+10FFFF,
which amounts to a 21-bit code space. Depending on the encoding
form (UTF-8, UTF-16, or UTF-32), each character will
then be represented either as a sequence of one to four 8-bit bytes,
one or two 16-bit code units, or a single 32-bit integer
(compare the ICU FAQ).

Unicode can be thought of as a superset of the spectrum of characters
supported by any given code page.
}
\section{UTF-8 and UTF-16}{


For portability reasons, the UTF-8 encoding is the most natural choice
for representing Unicode character strings in \R. UTF-8 has ASCII as its
subset (code points 1--127 represent the same characters in both of them).
Code points larger than 127 are represented by multi-byte sequences
(from 2 to 4 bytes: Please note that not all sequences of bytes
are valid UTF-8, compare \code{\link{stri_enc_isutf8}}).

Most of the computations in \pkg{stringi} are performed internally
using either UTF-8 or UTF-16 encodings (this depends on type of service
you request: some \pkg{ICU} services are designed only to work with UTF-16).
Due to such a choice, with \pkg{stringi} you get the same result on
each platform, which is -- unfortunately -- not the case of base \R's
functions (for instance, it is known that performing a regular expression
search under Linux on some texts may give you a different result
to those obtained under Windows). We really had portability in our minds
while developing our package!

We have observed that \R correctly handles UTF-8 strings regardless of your
platform's native encoding (see below). Therefore, we decided that most
functions in \pkg{stringi} will output its results in UTF-8
-- this speeds ups computations on cascading calls to our functions:
the strings does not have to be re-encoded each time.

Note that some Unicode characters may have an ambiguous representation.
For example, ``a with ogonek'' (one character) and ``a''+``ogonek''
(two graphemes) are semantically the same. \pkg{stringi} provides functions
to normalize character sequences, see \code{\link{stri_trans_nfc}}
for discussion. However, it is observed that denormalized strings
do appear very rarely in typical string processing activities.

Additionally, do note that \pkg{stringi} silently removes byte order marks
(BOMs - they may incidentally appear in a string read from a text file)
from UTF8-encoded strings, see \code{\link{stri_enc_toutf8}}.
}

\section{Character Encodings in \R}{


Data in memory are just bytes (small integer
values) -- an en\emph{coding} is a way to represent characters with such
numbers, it is a semantic 'key' to understand a given byte sequence.
For example, in ISO-8859-2 (Central European), the value 177 represents
Polish ``a with ogonek'', and in ISO-8859-1 (Western European),
the same value denotes the ``plus-minus'' sign. Thus, a character encoding
is a translation scheme: we need to communicate with \R somehow,
relying on how it represents strings.

Basically, \R has a very simple encoding marking mechanism,
see \code{\link{stri_enc_mark}}. There is an implicit assumption
that your platform's default (native) encoding always extends
ASCII -- \pkg{stringi} checks that whenever your native encoding
is being detected automatically on \pkg{ICU}'s initialization and each time
when you change it manually by calling \code{\link{stri_enc_set}}.

Character strings in \R (internally) can be declared to be in:
\itemize{
\item \code{UTF-8};
\item \code{latin1}, i.e., either ISO-8859-1 (Western European on
Linux, OS X, and other Unixes) or WINDOWS-1252 (Windows);
\item \code{bytes} -- for strings that
    should be manipulated as sequences of bytes.
}
Moreover, there are two other cases:
\itemize{
\item ASCII -- for strings consisting only of byte codes
               not greater than 127;-
\item \code{native} (a.k.a. \code{unknown} in \code{\link{Encoding}};
quite a misleading name: no explicit encoding mark) -- for
strings that are assumed to be in your platform's native (default) encoding.
This can represent UTF-8 if you are an OS X user,
or some 8-bit Windows code page, for example.
The native encoding used by \R may be determined by examining
the LC_CTYPE category, see \code{\link{Sys.getlocale}}.
}

Intuitively, ``native'' strings result from reading
a string from stdin (e.g., keyboard input). This makes sense: your operating
system works in some encoding and provides \R with some data.

Each time when a \pkg{stringi} function encounters a string declared
in native encoding, it assumes that the input data should be translated
from the default encoding, i.e., the one returned by \code{\link{stri_enc_get}}
(unless you know what you are doing, the default encoding should only be
changed if the automatic encoding detection process fails on \pkg{stringi}
load).

Functions which allow \code{'bytes'} encoding markings are very rare in
\pkg{stringi}, and were carefully selected. These are:
\code{\link{stri_enc_toutf8}} (with argument \code{is_unknown_8bit=TRUE}),
\code{\link{stri_enc_toascii}}, and \code{\link{stri_encode}}.

Finally, note that \R lets strings in ASCII, UTF-8, and your platform's
native encoding coexist. A character vector printed with
\code{\link{print}}, \code{\link{cat}}, etc., is silently re-encoded
so that it can be properly shown, e.g., on the console.
}

\section{Encoding Conversion}{


Apart from automatic conversion from the native encoding,
you may re-encode a string manually, for example
when you read it from a file created on a different platform.
Call \code{\link{stri_enc_list}} for the list of
encodings supported by \pkg{ICU}.
Note that converter names are case-insensitive
and \pkg{ICU} tries to normalize the encoding specifiers.
Leading zeroes are ignored in sequences of digits (if further digits follow),
and all non-alphanumeric characters are ignored. Thus the strings
'UTF-8', 'utf_8', 'u*Tf08' and 'Utf 8' are equivalent.

The \code{\link{stri_encode}} function
allows you to convert between any given encodings
(in some cases you will obtain \code{bytes}-marked
strings, or even lists of raw vectors (i.e., for UTF-16).
There are also some useful more specialized functions,
like \code{\link{stri_enc_toutf32}} (converts a character vector to a list
of integers, where one code point is exactly one numeric value)
or \code{\link{stri_enc_toascii}} (substitutes all non-ASCII
bytes with the SUBSTITUTE CHARACTER,
which plays a similar role as \R's \code{NA} value).

There are also some routines for automated encoding detection,
see, e.g., \code{\link{stri_enc_detect}}.
}

\section{Encoding Detection}{


Given a text file, one has to know how to interpret (encode)
raw data in order to obtain meaningful information.

Encoding detection is always an imprecise operation and
needs a considerable amount of data. However, in case of some
encodings (like UTF-8, ASCII, or UTF-32) a ``false positive'' byte
sequence is quite rare (statistically speaking).

Check out \code{\link{stri_enc_detect}} (among others) for a useful
function in this category.
}

\references{
\emph{Unicode Basics} -- ICU User Guide,
\url{http://userguide.icu-project.org/unicode}

\emph{Conversion} -- ICU User Guide,
\url{http://userguide.icu-project.org/conversion}

\emph{Converters} -- ICU User Guide,
\url{http://userguide.icu-project.org/conversion/converters}  (technical details)

\emph{UTF-8, UTF-16, UTF-32 & BOM} -- ICU FAQ, \url{https://www.unicode.org/faq/utf_bom.html}
}
\seealso{
Other stringi_general_topics: 
\code{\link{about_arguments}},
\code{\link{about_locale}},
\code{\link{about_search_boundaries}},
\code{\link{about_search_charclass}},
\code{\link{about_search_coll}},
\code{\link{about_search_fixed}},
\code{\link{about_search_regex}},
\code{\link{about_search}},
\code{\link{about_stringi}}

Other encoding_management: 
\code{\link{stri_enc_info}()},
\code{\link{stri_enc_list}()},
\code{\link{stri_enc_mark}()},
\code{\link{stri_enc_set}()}

Other encoding_detection: 
\code{\link{stri_enc_detect2}()},
\code{\link{stri_enc_detect}()},
\code{\link{stri_enc_isascii}()},
\code{\link{stri_enc_isutf16be}()},
\code{\link{stri_enc_isutf8}()}

Other encoding_conversion: 
\code{\link{stri_enc_fromutf32}()},
\code{\link{stri_enc_toascii}()},
\code{\link{stri_enc_tonative}()},
\code{\link{stri_enc_toutf32}()},
\code{\link{stri_enc_toutf8}()},
\code{\link{stri_encode}()}
}
\concept{encoding_conversion}
\concept{encoding_detection}
\concept{encoding_management}
\concept{stringi_general_topics}
