go-utf8/README.md

# 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

[![GoDoc](https://godoc.org/sourcecode.social/reiver/go-utf8?status.svg)](https://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.

```go
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.

```go
var writer io.Writer

// ...

var r rune

// ...

n, err := utf8.WriteRune(w, r)
```
## io.RuneReader

This is how you can create an `io.RuneReader`:

```go
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`:

```go
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                     ┃       ┃            ┃         ┃                            ┃                                  ┃
┣━━━━━━━━━━━━━┳━━━━━━━━━━━━━┳━━━━━━━━━━━━━┳━━━━━━━━━━━━━┫       ┃            ┃         ┃                            ┃                                  ┃
┃    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 │
└────────────┴───────────────────────┴────────────────┴──────────────────┴──────────┴──────────┴──────────┴──────────┘
```