go-utf8

Package utf8 implements encoding and decoding of UTF-8, for the Go programming language.

This package is meant to be a replacement for Go's built-in "unicode/utf8" package.

Documention

Online documentation, which includes examples, can be found at: http://godoc.org/sourcecode.social/reiver/go-utf8

Reading a Single UTF-8 Character

This is the simplest way of reading a single UTF-8 character.

var reader io.Reader

// ...

r, n, err := utf8.ReadRune(reader)

Write a Single UTF-8 Character

This is the simplest way of writing a single UTF-8 character.

var writer io.Writer

// ...

var r rune

// ...

n, err := utf8.WriteRune(w, r)

io.RuneReader

This is how you can create an io.RuneReader:

var reader io.Reader

// ...

var runeReader io.RuneReader = utf8.RuneReaderWrap(reader)

// ...

r, n, err := runeReader.ReadRune()

io.RuneScanner

This is how you can create an io.RuneScanner:

var reader io.Reader

// ...

var runeScanner io.RuneScanner := utf8.RuneScannerWrap(reader)

// ...

r, n, err := runeScanner.ReadRune()

// ...

err = runeScanner.UnreadRune()

UTF-8

UTF-8 is a variable length encoding of Unicode. An encoding of a single Unicode code point can be from 1 to 4 bytes longs.

Some examples of UTF-8 encoding of Unicode code points are:

UTF-8 encoding				value	code point	decimal	binary	name
byte 1	byte 2	byte 3	byte 4
0b0,1000001				A	U+0041	65	0b0000,0000,0100,0001	LATIN CAPITAL LETTER A
0b0,1110010				r	U+0072	114	0b0000,0000,0111,0010	LATIN SMALL LETTER R
0b110,00010	0b10,100001
0b110,11011	0b10,110101

┏━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━┳━━━━━━━┳━━━━━━━━━━━━┳━━━━━━━━━┳━━━━━━━━━━━━━━━━━━━━━━━━━━━━┳━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━┓
|                    UTF-8 encoding                     |       |            |         |                            |                                  |
┣━━━━━━━━━━━━━┳━━━━━━━━━━━━━┳━━━━━━━━━━━━━┳━━━━━━━━━━━━━┫       |            |         |                            |                                  |
|    byte 1   ┋    byte 2   ┋    byte 3   ┋    byte 4   | value | code point | decimal |           binary           |               name               |
┡━━━━━━━━━━━━━╇━━━━━━━━━━━━━╇━━━━━━━━━━━━━╇━━━━━━━━━━━━━╇━━━━━━━╇━━━━━━━━━━━━╇━━━━━━━━━╇━━━━━━━━━━━━━━━━━━━━━━━━━━━━╇━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━┩
│ 0b0,1000001 ┊             ┊             ┊             │   A   │     U+0041 │      65 │      0b0000,0000,0100,0001 │ LATIN CAPITAL LETTER A           │
├-------------┼-------------┼-------------┼-------------┼-------┼------------┼---------┼----------------------------┼----------------------------------┤
│ 0b0,1110010 ┊             ┊             ┊             │   r   │     U+0072 │     114 │      0b0000,0000,0111,0010 │ LATIN SMALL LETTER R             │
├-------------┼-------------┼-------------┼-------------┼-------┼------------┼---------┼----------------------------┼----------------------------------┤
│ 0b110,00010 ┊ 0b10,100001 ┊             ┊             │   ¡   │     U+00A1 │     161 │      0b0000,0000,1010,0001 │ INVERTED EXCLAMATION MARK        │
├-------------┼-------------┼-------------┼-------------┼-------┼------------┼---------┼----------------------------┼----------------------------------┤
│ 0b110,11011 ┊ 0b10,110101 ┊             ┊             │   ۵   │     U+06F5 │    1781 │      0b0000,0110,1111,0101 │ EXTENDED ARABIC-INDIC DIGIT FIVE │
├-------------┼-------------┼-------------┼-------------┼-------┼------------┼---------┼----------------------------┼----------------------------------┤
│ 0b1110,0010 ┊ 0b10,000000 ┊ 0b10,110001 ┊             │   ‱   │     U+2031 │    8241 │      0b0010,0000,0011,0001 │ PER TEN THOUSAND SIGN            │
├-------------┼-------------┼-------------┼-------------┼-------┼------------┼---------┼----------------------------┼----------------------------------┤
│ 0b1110,0010 ┊ 0b10,001001 ┊ 0b10,100001 ┊             │   ≡   │     U+2261 │    8801 │      0b0010,0010,0110,0001 │ IDENTICAL TO                     │
├-------------┼-------------┼-------------┼-------------┼-------┼------------┼---------┼----------------------------┼----------------------------------┤
│ 0b11110,000 ┊ 0b10,010000 ┊ 0b10,001111 ┊ 0b10,010101 │   𐏕   │ U+000103D5 │   66517 │ 0b0001,0000,0011,1101,0101 │ OLD PERSIAN NUMBER HUNDRED       │
├-------------┼-------------┼-------------┼-------------┼-------┼------------┼---------┼----------------------------┼----------------------------------┤
│ 0b11110,000 ┊ 0b10,011111 ┊ 0b10,011001 ┊ 0b10,000010 │   🙂   │ U+0001F642 │  128578 │ 0b0001,1111,0110,0100,0010 │ SLIGHTLY SMILING FACE            │
└-------------┴-------------┴-------------┴-------------┴-------┴------------┴---------┴----------------------------┴----------------------------------┘

UTF-8 Versus ASCII

UTF-8 was (partially) designed to be backwards compatible with 7-bit ASCII.

Thus, all 7-bit ASCII is valid UTF-8.

UTF-8 Encoding

Since, at least as of 2003, Unicode fits into 21 bits, and thus UTF-8 was designed to support at most 21 bits of information.

This is done as described in the following table:

# of bytes	# bits for code point	1st code point	last code point	byte 1	byte 2	byte 3	byte 4
1	7	U+000000	U+00007F	0xxxxxxx
2	11	U+000080	U+0007FF	110xxxxx	10xxxxxx
3	16	U+000800	U+00FFFF	1110xxxx	10xxxxxx	10xxxxxx
4	21	U+010000	U+10FFFF	11110xxx	10xxxxxx	10xxxxxx	10xxxxxx