Momentum-Apps/signal.c

/* Copyright (C) 2022-2023 Salvatore Sanfilippo -- All Rights Reserved
 * See the LICENSE file for information about the license. */

#include "app.h"

void decode_signal(RawSamplesBuffer *s, uint64_t len);

/* =============================================================================
 * Raw signal detection
 * ===========================================================================*/

/* Return the time difference between a and b, always >= 0 since
 * the absolute value is returned. */
uint32_t duration_delta(uint32_t a, uint32_t b) {
    return a > b ? a - b : b - a;
}

/* This function starts scanning samples at offset idx looking for the
 * longest run of pulses, either high or low, that are not much different
 * from each other, for a maximum of three duration classes.
 * So for instance 50 successive pulses that are roughly long 340us or 670us
 * will be sensed as a coherent signal (example: 312, 361, 700, 334, 667, ...)
 *
 * The classes are counted separtely for high and low signals (RF on / off)
 * because many devices tend to have different pulse lenghts depending on
 * the level of the pulse.
 *
 * For instance Oregon2 sensors, in the case of protocol 2.1 will send
 * pulses of ~400us (RF on) VS ~580us (RF off). */
#define SEARCH_CLASSES 3
uint32_t search_coherent_signal(RawSamplesBuffer *s, uint32_t idx) {
    struct {
        uint32_t dur[2];     /* dur[0] = low, dur[1] = high */
        uint32_t count[2];   /* Associated observed frequency. */
    } classes[SEARCH_CLASSES];

    memset(classes,0,sizeof(classes));
    uint32_t minlen = 40, maxlen = 4000; /* Depends on data rate, here we
                                            allow for high and low. */
    uint32_t len = 0; /* Observed len of coherent samples. */
    s->short_pulse_dur = 0;
    for (uint32_t j = idx; j < idx+500; j++) {
        bool level;
        uint32_t dur;
        raw_samples_get(s, j, &level, &dur);
        if (dur < minlen || dur > maxlen) break; /* return. */

        /* Let's see if it matches a class we already have or if we
         * can populate a new (yet empty) class. */
        uint32_t k;
        for (k = 0; k < SEARCH_CLASSES; k++) {
            if (classes[k].count[level] == 0) {
                classes[k].dur[level] = dur;
                classes[k].count[level] = 1;
                break; /* Sample accepted. */
            } else {
                uint32_t classavg = classes[k].dur[level];
                uint32_t count = classes[k].count[level];
                uint32_t delta = duration_delta(dur,classavg);
                /* Is the difference in duration between this signal and
                 * the class we are inspecting less than a given percentage?
                 * If so, accept this signal. */
                if (delta < classavg/8) { /* 100%/8 = 12%. */
                    /* It is useful to compute the average of the class
                     * we are observing. We know how many samples we got so
                     * far, so we can recompute the average easily.
                     * By always having a better estimate of the pulse len
                     * we can avoid missing next samples in case the first
                     * observed samples are too off. */
                    classavg = ((classavg * count) + dur) / (count+1);
                    classes[k].dur[level] = classavg;
                    classes[k].count[level]++;
                    break; /* Sample accepted. */
                }
            }
        }

        if (k == SEARCH_CLASSES) break; /* No match, return. */

        /* If we are here, we accepted this sample. Try with the next
         * one. */
        len++;
    }

    /* Update the buffer setting the shortest pulse we found
     * among the three classes. This will be used when scaling
     * for visualization. */
    uint32_t short_dur[2] = {0,0};
    for (int j = 0; j < SEARCH_CLASSES; j++) {
        for (int level = 0; level < 2; level++) {
            if (classes[j].dur[level] == 0) continue;
            if (classes[j].count[level] < 3) continue;
            if (short_dur[level] == 0 ||
                short_dur[level] > classes[j].dur[level])
            {
                short_dur[level] = classes[j].dur[level];
            }
        }
    }

    /* Use the average between high and low short pulses duration.
     * Often they are a bit different, and using the average is more robust
     * when we do decoding sampling at short_pulse_dur intervals. */
    if (short_dur[0] == 0) short_dur[0] = short_dur[1];
    if (short_dur[1] == 0) short_dur[1] = short_dur[0];
    s->short_pulse_dur = (short_dur[0]+short_dur[1])/2;

    return len;
}

/* Search the buffer with the stored signal (last N samples received)
 * in order to find a coherent signal. If a signal that does not appear to
 * be just noise is found, it is set in DetectedSamples global signal
 * buffer, that is what is rendered on the screen. */
void scan_for_signal(ProtoViewApp *app) {
    /* We need to work on a copy: the RawSamples buffer is populated
     * by the background thread receiving data. */
    RawSamplesBuffer *copy = raw_samples_alloc();
    raw_samples_copy(copy,RawSamples);

    /* Try to seek on data that looks to have a regular high low high low
     * pattern. */
    uint32_t minlen = 13;           /* Min run of coherent samples. Up to
                                       12 samples it's very easy to mistake
                                       noise for signal. */

    uint32_t i = 0;
    while (i < copy->total-1) {
        uint32_t thislen = search_coherent_signal(copy,i);
        if (thislen > minlen && thislen > app->signal_bestlen) {
            app->signal_bestlen = thislen;
            raw_samples_copy(DetectedSamples,copy);
            DetectedSamples->idx = (DetectedSamples->idx+i)%
                                   DetectedSamples->total;
            FURI_LOG_E(TAG, "Displayed sample updated (%d samples %lu us)",
                (int)thislen, DetectedSamples->short_pulse_dur);
            decode_signal(DetectedSamples,thislen);
        }
        i += thislen ? thislen : 1;
    }
    raw_samples_free(copy);
}

/* =============================================================================
 * Decoding
 *
 * The following code will translates the raw singals as received by
 * the CC1101 into logical signals: a bitmap of 0s and 1s sampled at
 * the detected data clock interval.
 *
 * Then the converted signal is passed to the protocols decoders, that look
 * for protocol-specific information. We stop at the first decoder that is
 * able to decode the data, so protocols here should be registered in
 * order of complexity and specificity, with the generic ones at the end.
 * ===========================================================================*/

/* Set the 'bitpos' bit to value 'val', in the specified bitmap
 * 'b' of len 'blen'.
 * Out of range bits will silently be discarded. */
void bitmap_set(uint8_t *b, uint32_t blen, uint32_t bitpos, bool val) {
    uint32_t byte = bitpos/8;
    uint32_t bit = bitpos&7;
    if (byte >= blen) return;
    if (val)
        b[byte] |= 1<<bit;
    else
        b[byte] &= ~(1<<bit);
}

/* Get the bit 'bitpos' of the bitmap 'b' of 'blen' bytes.
 * Out of range bits return false (not bit set). */
bool bitmap_get(uint8_t *b, uint32_t blen, uint32_t bitpos) {
    uint32_t byte = bitpos/8;
    uint32_t bit = bitpos&7;
    if (byte >= blen) return 0;
    return (b[byte] & (1<<bit)) != 0;
}

/* Return true if the specified sequence of bits, provided as a string in the
 * form "11010110..." is found in the 'b' bitmap of 'blen' bits at 'bitpos'
 * position. */
bool bitmap_match_bits(uint8_t *b, uint32_t blen, uint32_t bitpos, const char *bits) {
    size_t l = strlen(bits);
    for (size_t j = 0; j < l; j++) {
        bool expected = (bits[j] == '1') ? true : false;
        if (bitmap_get(b,blen,bitpos+j) != expected) return false;
    }
    return true;
}

/* Search for the specified bit sequence (see bitmap_match_bits() for details)
 * in the bitmap 'b' of 'blen' bytes. Returns the offset (in bits) of the
 * match, or BITMAP_SEEK_NOT_FOUND if not found.
 *
 * Note: there are better algorithms, such as Boyer-Moore. Here we hope that
 * for the kind of patterns we search we'll have a lot of early stops so
 * we use a vanilla approach. */
uint32_t bitmap_seek_bits(uint8_t *b, uint32_t blen, uint32_t startpos, const char *bits) {
    uint32_t endpos = blen*8;
    for (uint32_t j = startpos; j < endpos; j++)
        if (bitmap_match_bits(b,blen,j,bits)) return j;
    return BITMAP_SEEK_NOT_FOUND;
}

/* Take the raw signal and turn it into a sequence of bits inside the
 * buffer 'b'. Note that such 0s and 1s are NOT the actual data in the
 * signal, but is just a low level representation of the line code. Basically
 * if the short pulse we find in the signal is 320us, we convert high and
 * low levels in the raw sample in this way:
 *
 * If for instance we see a high level lasting ~600 us, we will add
 * two 1s bit. If then the signal goes down for 330us, we will add one zero,
 * and so forth. So for each period of high and low we find the closest
 * multiple and set the relevant number of bits.
 * 
 * In case of a short pulse of 320us detected, 320*2 is the closest to a
 * high pulse of 600us, so 2 bits will be set.
 *
 * In other terms what this function does is sampling the signal at
 * fixed 'rate' intervals.
 *
 * This representation makes it simple to decode the signal at a higher
 * level later, translating it from Marshal coding or other line codes
 * to the actual bits/bytes.
 *
 * The 'idx' argument marks the detected signal start index into the
 * raw samples buffer. The 'count' tells the function how many raw
 * samples to convert into bits. The function returns the number of
 * bits set into the buffer 'b'. The 'rate' argument, in microseconds, is
 * the detected short-pulse duration. We expect the line code to be
 * meaningful when interpreted at multiples of 'rate'. */
uint32_t convert_signal_to_bits(uint8_t *b, uint32_t blen, RawSamplesBuffer *s, uint32_t idx, uint32_t count, uint32_t rate) {
    if (rate == 0) return 0; /* We can't perform the conversion. */
    uint32_t bitpos = 0;
    for (uint32_t j = 0; j < count; j++) {
        uint32_t dur;
        bool level;
        raw_samples_get(s, j+idx, &level, &dur);

        uint32_t numbits = dur / rate; /* full bits that surely fit. */
        uint32_t rest = dur % rate;    /* How much we are left with. */
        if (rest > rate/2) numbits++;  /* There is another one. */

        FURI_LOG_E(TAG, "%lu converted into %lu (%d) bits", dur,numbits,(int)level);

        /* If the signal is too short, let's claim it an interference
         * and ignore it completely. */
        if (numbits == 0) continue;

        while(numbits--) bitmap_set(b,blen,bitpos++,level);
    }
    return bitpos;
}

/* This function converts the line code used to the final data representation.
 * The representation is put inside 'buf', for up to 'buflen' bytes of total
 * data. For instance in order to convert manchester I can use "10" and "01"
 * as zero and one patterns. It is possible to use "?" inside patterns in
 * order to skip certain bits. For instance certain devices encode data twice,
 * with each bit encoded in manchester encoding and then in its reversed
 * representation. In such a case I could use "10??" and "01??".
 *
 * The function returns the number of bits converted. It will stop as soon
 * as it finds a pattern that does not match zero or one patterns. The
 * decoding starts at the specified offset 'off'. */
uint32_t convert_from_line_code(uint8_t *buf, uint64_t buflen, uint8_t *bits, uint32_t len, uint32_t off, const char *zero_pattern, const char *one_pattern)
{
    uint32_t decoded = 0; /* Number of bits extracted. */
    while(off < len) {
        bool level;
        if (bitmap_match_bits(bits,len,off,zero_pattern)) {
            level = true;
            off += strlen(zero_pattern);
        } else if (bitmap_match_bits(bits,len,off,one_pattern)) {
            level = false;
            off += strlen(zero_pattern);
        } else {
            break;
        }
        bitmap_set(buf,buflen,decoded++,level);
        if (decoded/8 == buflen) break; /* No space left on target buffer. */
    }
    return decoded;
}

/* Supported protocols go here, with the relevant implementation inside
 * protocols/<name>.c */

extern ProtoViewDecoder Oregon2Decoder;
extern ProtoViewDecoder B4B1Decoder;

ProtoViewDecoder *Decoders[] = {
    &Oregon2Decoder,        /* Oregon sensors v2.1 protocol. */
    &B4B1Decoder,           /* PT, SC, ... 24 bits remotes. */
    NULL
};

/* Reset the message info structure before passing it to the decoding
 * functions. */
void initialize_msg_info(ProtoViewMsgInfo *i) {
    memset(i,0,sizeof(ProtoViewMsgInfo));
}

/* This function is called when a new signal is detected. It converts it
 * to a bitstream, and the calls the protocol specific functions for
 * decoding. */
void decode_signal(RawSamplesBuffer *s, uint64_t len) {
    uint32_t bitmap_bits_size = 4096*8;
    uint32_t bitmap_size = bitmap_bits_size/8;

    /* We call the decoders with an offset a few bits before the actual
     * signal detected and for a len of a few bits after its end. */
    uint32_t before_after_bits = 2;

    uint8_t *bitmap = malloc(bitmap_size);
    uint32_t bits = convert_signal_to_bits(bitmap,bitmap_size,s,-before_after_bits,len+before_after_bits*2,s->short_pulse_dur);

    if (DEBUG_MSG) { /* Useful for debugging purposes. Don't remove. */
        char *str = malloc(1024);
        uint32_t j;
        for (j = 0; j < bits && j < 1023; j++) {
            str[j] = bitmap_get(bitmap,bitmap_size,j) ? '1' : '0';
        }
        str[j] = 0;
        FURI_LOG_E(TAG, "%lu bits decoded: %s", bits, str);
        free(str);
    }

    /* Try all the decoders available. */
    int j = 0;

    ProtoViewMsgInfo info;
    initialize_msg_info(&info);
    while(Decoders[j]) {
        FURI_LOG_E(TAG, "Calling decoder %s", Decoders[j]->name);
        ProtoViewMsgInfo info;
        if (Decoders[j]->decode(bitmap,bits,&info)) {
            FURI_LOG_E(TAG, "Message detected by %s", Decoders[j]->name);
            break;
        }
        j++;
    }
    if (Decoders[j] == NULL) {
        FURI_LOG_E(TAG, "No decoding possible");
    } else {
        FURI_LOG_E(TAG, "Decoded %s, raw=%s",
            info.name, info.raw);
    }
    free(bitmap);
}