osmo-st-gsm/GSMEncoding.st

"
 (C) 2011 by Holger Hans Peter Freyther
 All Rights Reserved

 This program is free software: you can redistribute it and/or modify
 it under the terms of the GNU Affero General Public License as
 published by the Free Software Foundation, either version 3 of the
 License, or (at your option) any later version.

 This program is distributed in the hope that it will be useful,
 but WITHOUT ANY WARRANTY; without even the implied warranty of
 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 GNU Affero General Public License for more details.

 You should have received a copy of the GNU Affero General Public License
 along with this program.  If not, see <http://www.gnu.org/licenses/>.
"

String extend [
    asGSM7Bit [
        "I convert a string into a 7bit encoded string. I should
         also be in UnicodeString but I am not. This impl. is just
         basic and does not deal with difficult bits."
        <category: '*-osmo-gsm'>
        ^ GSMEncoding encode: self.
    ]

    asUSSD7Bit [
        "I convert a string into a 7bit encoded string. I know about
        padding rules for USSD messages."
        <category: '*-osmo-gsm'>
        ^ GSMUSSDEncoding encode: self.
    ]
]

Object subclass: GSMEncoding [
    <category: 'osmo-gsm'>
    <comment: 'I am the base class for GSM Encoding as of GSM 03.38. I
can be subclassed to deal with specifics for USSD and other systems.'>

    GSMEncoding class >> encode: aString [
        | bits |
        bits := self convertToBits: aString.
        self padBits: bits on: aString.
        ^ self convertToBytes: bits.
    ]

    GSMEncoding class >> padBits: bits on: aString [
        "I have to make sure that bits can be divided by/8"
        | rest |
        rest := 8 - (bits size \\ 8).
        rest to: 1 by: -1 do: [:each | bits add: 0].
    ]

    GSMEncoding class >> convertToBits: aString [
        | bits |
        bits := OrderedCollection new: (aString size * 7) // 8.

        "Split it into bits"
        aString do: [:char | | val |
            val := char value.
            1 to: 7 do: [:digit | bits add: (val bitAt: digit)].
        ].

        ^ bits
    ]

    GSMEncoding class >> convertToBytes: bits [
        | bytes |

        bytes := ByteArray new: bits size // 8.
        1 to: bits size by: 8 do: [:each | | byte |
            byte := 0.
            byte := (byte bitShift: 1) bitOr: (bits at: each + 7).
            byte := (byte bitShift: 1) bitOr: (bits at: each + 6).
            byte := (byte bitShift: 1) bitOr: (bits at: each + 5).
            byte := (byte bitShift: 1) bitOr: (bits at: each + 4).
            byte := (byte bitShift: 1) bitOr: (bits at: each + 3).
            byte := (byte bitShift: 1) bitOr: (bits at: each + 2).
            byte := (byte bitShift: 1) bitOr: (bits at: each + 1).
            byte := (byte bitShift: 1) bitOr: (bits at: each + 0).

            bytes at: (each // 8) + 1 put: byte.
        ].

        ^ bytes
    ]
]

GSMEncoding subclass: GSMUSSDEncoding [
    <category: 'osmo-gsm'>
    <comment: 'I know the funky stuff for USSD encoding/padding'>

    GSMUSSDEncoding class >> padBits: bits on: aString [
        | rest nl |

        "Check if we are on a byte boundary and a CR."
        ((aString last = Character cr) and: [(bits size \\ 8) = 0]) ifTrue: [ | cr |
            cr := Character cr value.
            1 to: 7 do: [:each | bits add: (cr bitAt: each) ].
            bits add: 0.
            ^ self
        ].

        "Check if we need to handle this, if not continue."
        aString size \\ 8 = 7
            ifFalse: [^ super padBits: bits on: aString].

        "Now add the padding."
        rest := 8 - (bits size \\ 8).
        rest = 7 ifFalse: [^self error: 'The rest should be 7 bits but were %1' % rest].

        nl := Character cr value.
        1 to: 7 do: [:each | bits add: (nl bitAt: each)].
    ]
]