507.txt

[SEE[ [[ISO/IEC 2022]] ]]

* ISO/IEC 2022 Decoder

[1] A [DFN[state]] is:

[FIG(list members)[

: [F[C0 designated]] : A [[character set]].
Initially, the [[initial C0 set]].
: [F[2/0 designated]] : [CODE(charname)@en[SP]] or [CODE(charname)@en[MSP]].
Initially, [CODE(charname)@en[SP]].
: [F[7/15 designated]] : [CODE(charname)@en[DEL]].
: [F[10/0 designated]] : [CODE(charname)@en[MSP]] or [CODE[null]].
Initially, [CODE[null]].
: [F[15/15 designated]] : [CODE[null]].
: [F[decoder state]] : One of [[decoder states][decoder state]].
Initially, [[initial state]].

]FIG]

[4] 
To [DFN[emit a character]] [VAR[char]], run these steps:

[FIG(steps)[
= 
[5] 
If [VAR[char]] is [CODE[null]]:
==
@@ Error
==
[6] 
@@ [CODE[U+FFFD]]?
=
[7] 
Otherwise:
==
@@

]FIG]

@@
[FIG(steps)[
= [8] Let [VAR[state]] be a new [[state]].
= [9] 
@@ Initialization steps
= [10] 
@@
Process the input stream,
a sequence of zero or more [[bit combinations][bit combination]]
]FIG]


@@

** Decoder states

[2] There are the following [DFN[decoder states][decoder state]]:

*** Initial state

[3] The [DFN[initial state]] switches by the [[bit combination]] [VAR[bc]]:

[FIG(switch)[

: [ [N[0/0]], [N[1/15]] ] :
[FIG(steps)[

= [11] Let [VAR[set]] be [VAR[state]]'s [F[C0 designated]].
= [12] Let [VAR[char]] be [VAR[set]] [ [VAR[bc]] ].
= [13] [[Emit a character][emit a character]] [VAR[char]].

]FIG]
: [N[2/0]] :
[[Emit a character][emit a character]] [VAR[state]]'s [F[2/0 designated]].
: [N[7/15]] :
[[Emit a character][emit a character]] [VAR[state]]'s [F[7/15 designated]].
: [N[10/0]] :
[[Emit a character][emit a character]] [VAR[state]]'s [F[10/0 designated]].
: [N[15/15]] :
[[Emit a character][emit a character]] [VAR[state]]'s [F[15/15 designated]].
: Otherwise :
@@

]FIG]

* Definitions

[14] 
This document depends on the [CITE[Infra Standard]].

[15] 
The terms
[[byte]],
[[byte sequence]],
[[ordered map]],
[[key]],
and
[[value]]
are defined by the [CITE[Infra Standard]].

[16] 
A [DFN[bit combination]] is a [[byte]].
A [[bit combination]] is represented as "[VAR[X]]/[VAR[Y]]",
where [VAR[X]] and [VAR[Y]] are integers in [ [N[0]], [N[15]] ],
which represents a [[byte]] [VAR[X]] × [N[16]] + [VAR[Y]].

** Character sets

[17] 
A [DFN[character set]] is an [[ordered map]],
where [[keys][key]] are [[byte sequences][byte sequence]] and
[[values][value]] are a [[character]].


* Notes