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//! Big5 encoding.
// Note that we need to recreate Encoder/Decoders in these functions due to this note in the
// encoding_rs documentation:
//
// > Once the stream has ended, the Decoder object must not be used anymore. That is, you need to
// > create another one to process another stream.
use encoding_rs::{DecoderResult, EncoderResult, BIG5};
pub fn unicode_to_big5(u: char) -> Option<u16> {
let mut encoder = BIG5.new_encoder();
let src: &mut [u8] = &mut [0, 0, 0, 0];
let mut dst = [0, 0];
let (res, _read, written) =
encoder.encode_from_utf8_without_replacement(u.encode_utf8(src), &mut dst, true);
match res {
EncoderResult::InputEmpty => {
match written {
1 => Some(u16::from(dst[0])),
2 => Some(u16::from_be_bytes(dst)),
_ => None, // should not happen
}
}
EncoderResult::OutputFull => None, // should not happen
EncoderResult::Unmappable(_) => None,
}
}
/// Decodes a Big5 character into a Unicode char
pub fn big5_to_unicode(ch: u16) -> Option<char> {
// Strictly speaking, the Big5 encoding contains only double-byte character set characters.
// However, in practice, the Big5 codes are always used together with an unspecified,
// system-dependent single-byte character set (ASCII, or an 8-bit character set such as code
// page 437), so that you will find a mix of DBCS characters and single-byte characters in
// Big5-encoded text. Bytes in the range 0x00 to 0x7f that are not part of a double-byte
// character are assumed to be single-byte characters.
//
// — https://en.wikipedia.org/wiki/Big5
//
// encoding_rs::Decoder for Big5 returns 0 for single-byte codes, so we handle ASCII manually.
if ch < 128 {
return Some(ch as u8 as char);
}
let mut decoder = BIG5.new_decoder_without_bom_handling();
let src = ch.to_be_bytes();
let mut dst = [0, 0, 0, 0];
let (res, _read, written) = decoder.decode_to_utf8_without_replacement(&src, &mut dst, true);
match res {
DecoderResult::InputEmpty if written > 0 => {
// Safety: It is assumed that decode_to_utf8_without_replacement will yield valid utf-8
// output in dst. Additionally because written is > 1 we assume chars will yield a char
// thus it should always be valid utf-8.
let s = unsafe { std::str::from_utf8_unchecked(&dst[..written]) }
.chars()
.next()
.unwrap();
Some(s)
}
DecoderResult::InputEmpty |
DecoderResult::OutputFull | // should not happen
DecoderResult::Malformed(_, _) => None,
}
}
#[cfg(test)]
mod tests {
use super::*;
#[test]
fn ascii_to_big5() {
for i in 0..128 {
let u = char::from(i);
assert_eq!(unicode_to_big5(u), Some(u16::from(i)));
}
}
#[test]
fn chinese_to_big5() {
assert_eq!(unicode_to_big5('好'), Some(0xA66E));
}
#[test]
fn greek_to_big5() {
assert_eq!(unicode_to_big5('ε'), Some(0xA360));
}
#[test]
fn hindi_to_big5() {
assert_eq!(unicode_to_big5('म'), None);
}
#[test]
fn big5_to_ascii() {
for i in 0..128 {
assert_eq!(big5_to_unicode(u16::from(i)), Some(char::from(i)));
}
}
#[test]
fn chinese_big5_to_unicode() {
assert_eq!(big5_to_unicode(0xA66E), Some('好'));
}
#[test]
fn greek_big5_to_unicode() {
assert_eq!(big5_to_unicode(0xA360), Some('ε'));
}
#[test]
fn graphical_big5_to_unicode() {
assert_eq!(big5_to_unicode(0xA143), Some('。'));
}
}