Encoding&Decoding

example_connect.bin

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+CONNECT
+Hex:
+10 10 00 04 4d 51 54 54 05 02 00 3c 03 21 00 14
+00 00
+
+Bin:
+  TYPE      LEN        PROTOCOL NAME
+0001 0000 0001 0000 0000 0000 0000 0100 0100 1101 0101 0001
+    PROTOCOL NAME    VERSION   FLAGS     KEEP ALIVE
+0101 0100 0101 0100 0000 0101 0000 0010 0000 0000 0011 1100
+PROP LEN  .             PROPERTIES    .    CLIENT ID LEN
+0000 0011 0010 0001 0000 0000 0001 0100 0000 0000 0000 0000
+
+
+PUBLISH
+30 18 00 0a 74 65 73 74 2f 74 6f 70 69 63 00 68
+65 6c 6c 6f 20 77 6f 72 6c 64
+
+0011 0000 0001 1000 0000 0000 0000 1010 0111 0100 0110 0101
+0111 0011 0111 0100 0010 1111 0111 0100 0110 1111 0111 0000
+0110 1001 0110 0011 0000 0000 0110 1000 0110 0101 0110 1100
+0110 1100 0110 1111 0010 0000 0111 0111 0110 1111 0111 0010
+0110 1100 0110 0100
+
+Variable int
+- LS 7 bitu jsou data
+- MS bit urcuje, jestli nasleduje dalsi byte
+b3 01 = 179
+1001 0011 0000 0001
+
+00000000  1 0001 0011

src/encoding/mod.rs

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+pub mod variable_byte_integer;

src/encoding/variable_byte_integer.rs

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+#![crate_name = "doc"]
+/// VariableByteIntegerEncoder and VariableByteIntegerDecoder are implemented based on 
+/// pseudo code which is introduced in MQTT version 5.0 OASIS standard accesible from
+/// https://docs.oasis-open.org/mqtt/mqtt/v5.0/os/mqtt-v5.0-os.html#_Toc3901107
+
+/// Variable byte integer encoder structure is help structure which implements function used to
+/// encode integer into MQTT variable byte integer format. This format is mainly used to encode
+/// lenghts stored in a packet.
+pub struct VariableByteIntegerEncoder;
+/// Variable byte integers error enumeration is used by both encoder and decoder for
+/// error notification.
+pub enum VariableByteIntegerError {
+    EncodingError,
+    DecodingError
+}
+
+
+
+impl VariableByteIntegerEncoder {
+    /// Encode function takes as parameter integer as u32 type and encodes
+    /// this integer into maximal 4 Bytes. MSb of each Byte is controll bit.
+    /// This bit is saying if there is continuing Byte in stream or not, this way
+    /// we can effectively use 1 to 4 Bytes based in integer len.
+    pub fn encode(mut target: u32) -> Result<[u8; 4], VariableByteIntegerError> {
+        // General known informations from OASIS
+        const MAX_ENCODABLE: u32 = 268435455;
+        const MOD: u32 = 128;
+        if target > MAX_ENCODABLE {
+            log::error!("Maximal value of integer for encoding was exceeded");
+            return Err(VariableByteIntegerError::EncodingError);
+        }
+
+        let mut res: [u8; 4] = [0; 4];
+        let mut encoded_byte: u8;
+        let mut i: usize = 0;
+
+        loop {
+            encoded_byte = (target % MOD) as u8;
+            target = target / 128;
+            if target > 0 {
+                encoded_byte = encoded_byte | 128;
+            }
+            res[i] = encoded_byte;
+            i = i + 1;
+            if target <= 0 {
+                break;
+            }
+        }
+        return Ok(res); 
+    }
+}
+
+/// Variable byte integer decoder structure is help structure which implements function used to
+/// decode message lenghts in MQTT packet and other parts encoded into variable byte integer.
+pub struct VariableByteIntegerDecoder;
+
+
+impl VariableByteIntegerDecoder {
+    /// Decode function takes as paramater encoded integer represented
+    /// as array of 4 unsigned numbers of exactly 1 Byte each -> 4 Bytes maximal
+    /// same as maximal amount of bytes for variable byte encoding in MQTT.
+    pub fn decode(encoded: [u8; 4]) -> Result<u32, VariableByteIntegerError> {
+        let mut multiplier: u32 = 1;
+        let mut ret: u32 = 0;
+
+        let mut encoded_byte: u8;
+        let mut i: usize = 0;
+        
+        loop {
+            encoded_byte = encoded[i];
+            i = i + 1;
+            ret = ret + ((encoded_byte & 127) as u32 * multiplier) as u32;
+            if multiplier > 128 * 128 * 128 {
+                return Err(VariableByteIntegerError::DecodingError);
+            }
+            multiplier = multiplier * 128;
+            if (encoded_byte & 128) == 0 {
+                break;
+            }
+        }
+
+        return Ok(ret);
+    }
+}