[feat](trx-rs): add ft8 decoder

Co-authored-by: Codex <codex@openai.com> Signed-off-by: Stanislaw Grams <stanislawgrams@gmail.com>
2026-02-09 21:19:56 +01:00
parent 7bd1a70607
commit 1199ab85e9
206 changed files with 9613 additions and 5 deletions
@@ -0,0 +1,63 @@
+#include "crc.h"
+#include "constants.h"
+
+#define TOPBIT (1u << (FT8_CRC_WIDTH - 1))
+
+// Compute 14-bit CRC for a sequence of given number of bits
+// Adapted from https://barrgroup.com/Embedded-Systems/How-To/CRC-Calculation-C-Code
+// [IN] message  - byte sequence (MSB first)
+// [IN] num_bits - number of bits in the sequence
+uint16_t ftx_compute_crc(const uint8_t message[], int num_bits)
+{
+    uint16_t remainder = 0;
+    int idx_byte = 0;
+
+    // Perform modulo-2 division, a bit at a time.
+    for (int idx_bit = 0; idx_bit < num_bits; ++idx_bit)
+    {
+        if (idx_bit % 8 == 0)
+        {
+            // Bring the next byte into the remainder.
+            remainder ^= (message[idx_byte] << (FT8_CRC_WIDTH - 8));
+            ++idx_byte;
+        }
+
+        // Try to divide the current data bit.
+        if (remainder & TOPBIT)
+        {
+            remainder = (remainder << 1) ^ FT8_CRC_POLYNOMIAL;
+        }
+        else
+        {
+            remainder = (remainder << 1);
+        }
+    }
+
+    return remainder & ((TOPBIT << 1) - 1u);
+}
+
+uint16_t ftx_extract_crc(const uint8_t a91[])
+{
+    uint16_t chksum = ((a91[9] & 0x07) << 11) | (a91[10] << 3) | (a91[11] >> 5);
+    return chksum;
+}
+
+void ftx_add_crc(const uint8_t payload[], uint8_t a91[])
+{
+    // Copy 77 bits of payload data
+    for (int i = 0; i < 10; i++)
+        a91[i] = payload[i];
+
+    // Clear 3 bits after the payload to make 82 bits
+    a91[9] &= 0xF8u;
+    a91[10] = 0;
+
+    // Calculate CRC of 82 bits (77 + 5 zeros)
+    // 'The CRC is calculated on the source-encoded message, zero-extended from 77 to 82 bits'
+    uint16_t checksum = ftx_compute_crc(a91, 96 - 14);
+
+    // Store the CRC at the end of 77 bit message
+    a91[9] |= (uint8_t)(checksum >> 11);
+    a91[10] = (uint8_t)(checksum >> 3);
+    a91[11] = (uint8_t)(checksum << 5);
+}