Complete VIDEO-001 through VIDEO-004 software video studies

protocol/video004_viterbi.py

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+"""CPU JIT для той же 64-состояний решётки Витерби VIDEO-004.
+
+Режим выбирается вызывающим кодом до приёма пакета. При отсутствии Numba
+этот модуль не подменяет результат медленным декодером после ошибки CRC.
+"""
+
+from __future__ import annotations
+
+import numpy as np
+
+from protocol import convolutional_fec, punctured_fec
+
+try:
+    from numba import njit
+except ImportError:
+    njit = None
+
+
+PREDECESSORS = convolutional_fec._PREDECESSORS
+BRANCH_SYMBOLS = convolutional_fec._BRANCH_SYMBOLS
+
+
+def _trellis(soft_values: np.ndarray, information_bit_count: int,
+             predecessors: np.ndarray, branch_symbols: np.ndarray) -> np.ndarray:
+    """Те же метрики, порядок сложения и выбор ветви 0 при равенстве."""
+    step_count = information_bit_count + convolutional_fec.TAIL_BIT_COUNT
+    received = soft_values.reshape(step_count, 2)
+    path_metric = np.full(convolutional_fec.STATE_COUNT, 1e18, dtype=np.float64)
+    path_metric[0] = 0.0
+    next_metric = np.empty(convolutional_fec.STATE_COUNT, dtype=np.float64)
+    choices = np.empty((step_count, convolutional_fec.STATE_COUNT), dtype=np.uint8)
+    for step in range(step_count):
+        received_0 = received[step, 0]
+        received_1 = received[step, 1]
+        for state in range(convolutional_fec.STATE_COUNT):
+            difference_00 = received_0 - branch_symbols[state, 0, 0]
+            difference_01 = received_1 - branch_symbols[state, 0, 1]
+            difference_10 = received_0 - branch_symbols[state, 1, 0]
+            difference_11 = received_1 - branch_symbols[state, 1, 1]
+            first = (path_metric[predecessors[state, 0]]
+                     + difference_00 ** 2 + difference_01 ** 2)
+            second = (path_metric[predecessors[state, 1]]
+                      + difference_10 ** 2 + difference_11 ** 2)
+            take_second = second < first
+            choices[step, state] = take_second
+            next_metric[state] = second if take_second else first
+        path_metric, next_metric = next_metric, path_metric
+    decoded = np.empty(step_count, dtype=np.uint8)
+    current_state = 0
+    for step in range(step_count - 1, -1, -1):
+        branch = choices[step, current_state]
+        decoded[step] = current_state >> (convolutional_fec.MEMORY - 1)
+        current_state = predecessors[current_state, branch]
+    return decoded[:information_bit_count]
+
+
+COMPILED_TRELLIS = (njit(cache=False, nogil=True, fastmath=False)(_trellis)
+                    if njit is not None else None)
+
+
+def decode_soft(soft_values: np.ndarray, information_bit_count: int,
+                rate: str) -> np.ndarray:
+    """Проверить вход и декодировать проколотое слово без запасного алгоритма."""
+    if COMPILED_TRELLIS is None:
+        raise RuntimeError("Numba недоступна для VIDEO-004")
+    restored = punctured_fec.depuncture_soft(soft_values, information_bit_count, rate)
+    return COMPILED_TRELLIS(restored, int(information_bit_count),
+                            PREDECESSORS, BRANCH_SYMBOLS)