Styled text with message entities

Telegram supports styled text using message entities.

A client that wants to send styled messages would simply have to integrate a Markdown/HTML parser, and generate an array of message entities by iterating through the parsed tags.

Nested entities are supported.

Entity length

Special care must be taken to consider the length of strings when generating message entities as the number of UTF-16 code units, even if the message itself must be encoded using UTF-8.

Example implementations: tdlib, MadelineProto.

Unicode codepoints and encoding

A Unicode code point is a number ranging from 0x0 to 0x10FFFF, usually represented using U+0000 to U+10FFFF syntax.
Unicode defines a codespace of 1,112,064 assignable code points within the U+0000 to U+10FFFF range.
Each of the assignable codepoints, once assigned by the Unicode consortium, maps to a specific character, emoji or control symbol.

The Unicode codespace is further subdivided into 17 planes:

  • Plane 1: U+0000 to U+FFFF: Basic Multilingual Plane (BMP)
  • Planes 2-17: U+00000 to U+10FFFF: Multiple supplementary planes as specified by the Unicode standard

Since storing a 21-bit number for each letter would result in a waste of space, the Unicode consortium defines multiple encodings that allow storing a code point into a smaller code unit:

UTF-8

UTF-8 » is a Unicode encoding that allows storing a 21-bit Unicode code point into code units as small as 8 bits.
UTF-8 is used by the MTProto and Bot API when transmitting and receiving fields of type string.

UTF-16

UTF-16 » is a Unicode encoding that allows storing a 21-bit Unicode code point into one or two 16-bit code units.

UTF-16 is used when computing the length and offsets of entities in the MTProto and bot APIs, by counting the number of UTF-16 code units (not code points).

Computing entity length

  • Code points in the BMP (U+0000 to U+FFFF) count as 1, because they are encoded into a single UTF-16 code units
  • Code points in all other planes count as 2, because they are encoded into two UTF-16 code units (also called surrogate pairs)

A simple, but not very efficient way of computing the entity length is converting the text to UTF-16, and then taking the byte length divided by 2 (=number of UTF-16 code units).

However, since UTF-8 encodes codepoints in non-BMP planes as a 32-bit code unit starting with 0b11110, a more efficient way to compute the entity length without converting the message to UTF-16 is the following:

  • If the byte marks the beginning of a 32-bit UTF-8 code unit (all bytes starting with 0b11110) increment the count by 2, otherwise
  • If the byte marks the beginning of a UTF-8 code unit (all bytes not starting with 0b10) increment the count by 1.

Example:

length := 0
for byte in text {
    if (byte & 0xc0) != 0x80 {
        length += 1 + (byte >= 0xf0)
    }
}

Note: the length of an entity must not include the length of trailing newlines or whitespaces, rtrim entities before computing their length: however, the next offset must include the length of newlines or whitespaces that precede it.

Example implementations: tdlib, MadelineProto.

Allowed entities

For example the following HTML/Markdown aliases for message entities can be used:

```c++
code
```

The following entities can also be used to mention users:

A number of other entities are also available, see the type page for the full list ».