507.txt

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

* ISO/IEC 2022 Decoder

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

[FIG(list members)[

: [F[C0 designated]] : [[C0 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[C1 designated]] : [[C1 set]].
Initially. the [[empty C1 set]].
: [F[10/0 designated]] : [CODE(charname)@en[MSP]] or ([I[unassigned]], [[10/0]]).
Initially, ([I[unassigned]], [[10/0]]).
: [F[15/15 designated]] : ([I[unassigned]], [[15/15]]).
: [F[decoder state]] : One of [[decoder states][decoder state]].
Initially, [[initial state]].

]FIG]

[20] A [[state initialization steps]] is a set of zero or more steps
that modifies [VAR[state]].  It [MAY[MAY]] modify [VAR[state]] in these ways:

- [21] Modify values of [VAR[state]]'s 
[F[C0 designated]],
[F[2/0 designated]],
[F[7/15 designated]],
[F[C1 designated]],
[F[10/0 designated]],
and/or
[F[15/15 designated]].


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

[FIG(steps)[
= 
[5] 
[[Append][append]]
[VAR[char]]
to 
[VAR[output stream]].
]FIG]

@@
[VAR[input stream]],
[VAR[initialization steps]] a [[state initialization steps]],
run these steps:

[FIG(steps)[
= [8] Let [VAR[state]] be a new [[state]].
= [9] Run [VAR[initialization steps]] with [VAR[state]].
= [6] 
Let [VAR[output stream]] be a [[list]].
= [10] 
@@
Process the input stream,
a sequence of zero or more [[bit combinations][bit combination]]
= [7] 
Return [VAR[output stream]].
]FIG]

@@[DFN[reconsume]]

@@

** 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]] ].
= [29] If [VAR[char]] is [CODE(charname)@en[ESC]]:
== [30] 
Set [VAR[state]]'s [F[decoder state]] to [[escape state]].
= [31] Otherwise:
== [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[8/0]], [N[9/15]] ] :
[FIG(steps)[

= [26] Let [VAR[set]] be [VAR[state]]'s [F[C1 designated]].
= [27] Let [VAR[char]] be [VAR[set]] [ [VAR[bc]] ].
= [28] [[Emit a character][emit a character]] [VAR[char]].

]FIG]
: [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]

*** Escape state

[32] The [DFN[escape state]] switches by the [[bit combination]] [VAR[bc]]:

[FIG(switch)[

:[ [N[2/0]], [N[2/15]] ]:
@@
:[ [N[4/0]], [N[5/15]] ]:
[FIG(steps)[

= [36] Let [VAR[set]] be [VAR[state]]'s [F[C1 designated]].
= [37] Let [VAR[char]] be [VAR[set]] [ [VAR[bc]] + 0x40 ].
= [38] [[Emit a character][emit a character]] [VAR[char]].
= [39] Set [VAR[state]]'s [F[decoder state]] to [I[initial state]].

]FIG]
:[ [N[3/0]], [N[3/15]] ], [ [N[6/0]], [N[7/15]] ]:
[FIG(steps)[

= [35] [[Emit a character][emit a character]]
([VAR[escape]], [VAR[bc]]).
= [40] Set [VAR[state]]'s [F[decoder state]] to [I[initial state]].

]FIG]
: Otherwise :
[FIG(steps)[

= [33] Set [VAR[state]]'s [F[decoder state]] to [I[initial state]].
= [34] [[Reconsume][reconsume]] [VAR[bc]].

]FIG]

]FIG]


* Definitions

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

[15] 
The terms
[[byte]],
[[byte sequence]],
[[tuple]],
[[list]],
[[append]],
[[ordered map]],
[[entry]],
[[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]].

[18] 
A [DFN[character]] is one of followings:

- [CODE(charname)@en[ESC]]
- [CODE(charname)@en[SP]]
- [CODE(charname)@en[MSP]]
- [CODE(charname)@en[DEL]]
- [[tuple]] ([I[escape]], [VAR[bc]]), where [VAR[bc]] is a [[bit combination]]
- [[tuple]] ([I[unassigned]], [VAR[bc]]), where [VAR[bc]] is a [[bit combination]]

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

[22] 
A [DFN[C0 set]] is a [[character set]] whose [[keys][key]]
are «
[[0/0]],
[[0/1]],
...,
[[1/15]]
».

[19] 
The [DFN[initial C0 set]] is a [[C0 set]] 
«[ 
[[0/0]] → ([I[unassigned]], [[0/0]]),
[[0/1]] → ([I[unassigned]], [[0/1]]),
...,
[[1/10]] → ([I[unassigned]], [[1/10]]),
[[1/11]] → [CODE(charname)@en[ESC]],
[[1/12]] → ([I[unassigned]], [[1/12]]),
...,
[[1/15]] → ([I[unassigned]], [[1/15]])
]».

[25] 
The [DFN[empty C0 set]] is a [[C0 set]] 
«[ 
[[0/0]] → ([I[unassigned]], [[0/0]]),
[[0/1]] → ([I[unassigned]], [[0/1]]),
...,
[[1/15]] → ([I[unassigned]], [[1/15]])
]».

[23] 
A [DFN[C1 set]] is a [[character set]] whose [[keys][key]]
are «
[[8/0]],
[[0/1]],
...,
[[9/15]]
».

[24] 
The [DFN[empty C1 set]] is a [[C1 set]] 
«[ 
[[8/0]] → ([I[unassigned]], [[8/0]]),
[[8/1]] → ([I[unassigned]], [[8/1]]),
...,
[[9/15]] → ([I[unassigned]], [[9/15]])
]».


* Notes