GSM: Add basic decoding support to byte array

1 files changed, 58 insertions, 0 deletions

diff --git a/GSMEncoding.st b/GSMEncoding.st
index f82575a..03cb44f 100644
--- a/GSMEncoding.st
+++ b/GSMEncoding.st
@@ -16,6 +16,13 @@
  along with this program.  If not, see <http://www.gnu.org/licenses/>.
 "
 
+ByteArray extend [
+    decodeGSM7Bit [
+        <category: '*-osmo-gsm'>
+        ^ GSMDecoding decode: self.
+    ]
+]
+
 String extend [
     asGSM7Bit [
         "I convert a string into a 7bit encoded string. I should
@@ -33,6 +40,57 @@ String extend [
     ]
 ]
 
+Object subclass: GSMDecoding [
+    <category: 'osmo-gsm'>
+    <comment: 'I am the base class for GSM Decoding as of GSM 03.38. I
+can be subclassed to deal with specifics for USSD and other systems.'>
+
+    GSMDecoding class >> decode: aByteArray [
+        | bits bytes |
+        bits := self convertFromBytes: aByteArray.
+        bytes := self convertToBytes: bits.
+        ^ self handleBytes: bytes from: bits
+    ]
+
+    GSMDecoding class >> handleBytes: bytes from: bits [
+        ^ bytes asString
+    ]
+
+    GSMDecoding class >> convertFromBytes: aByteArray [
+        | bits |
+        "We convert the stream into single bits. It is the
+        easiest to do it like this."
+
+        bits := OrderedCollection new.
+        aByteArray do: [:each |
+            1 to: 8 do: [:pos | bits add: (each bitAt: pos)]
+        ].
+
+        ^ bits
+    ]
+
+    GSMDecoding class >> convertToBytes: bits [
+        | bytes |
+
+        bytes := ByteArray new: (bits size // 7).
+        1 to: bits size by: 7 do: [:pos |
+            (pos + 6 <= bits size) ifTrue: [ | byte |
+                byte := 0.
+                byte := (byte bitShift: 1) bitOr: (bits at: pos + 6).
+                byte := (byte bitShift: 1) bitOr: (bits at: pos + 5).
+                byte := (byte bitShift: 1) bitOr: (bits at: pos + 4).
+                byte := (byte bitShift: 1) bitOr: (bits at: pos + 3).
+                byte := (byte bitShift: 1) bitOr: (bits at: pos + 2).
+                byte := (byte bitShift: 1) bitOr: (bits at: pos + 1).
+                byte := (byte bitShift: 1) bitOr: (bits at: pos + 0).
+                bytes at: (pos // 7) + 1 put: byte.
+            ].
+        ].
+        
+        ^ bytes
+    ]
+]
+
 Object subclass: GSMEncoding [
     <category: 'osmo-gsm'>
     <comment: 'I am the base class for GSM Encoding as of GSM 03.38. I