dump1090/dump1090.c

/* Mode1090, a Mode S messages decoder for RTLSDR devices.
 *
 * Copyright (C) 2012 by Salvatore Sanfilippo <antirez@gmail.com>
 *
 * All rights reserved.
 * 
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions are
 * met:
 * 
 *  *  Redistributions of source code must retain the above copyright
 *     notice, this list of conditions and the following disclaimer.
 *
 *  *  Redistributions in binary form must reproduce the above copyright
 *     notice, this list of conditions and the following disclaimer in the
 *     documentation and/or other materials provided with the distribution.
 * 
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
 * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
 * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
 * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
 * HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
 * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
 * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
 * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
 * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <pthread.h>
#include <stdint.h>
#include <errno.h>
#include <unistd.h>
#include <math.h>
#include <sys/time.h>
#include <fcntl.h>
#include <ctype.h>
#include "rtl-sdr.h"

#define MODES_DEFAULT_RATE         2000000
#define MODES_DEFAULT_FREQ         1090000000
#define MODES_DEFAULT_WIDTH        1000
#define MODES_DEFAULT_HEIGHT       700
#define MODES_ASYNC_BUF_NUMBER     12
#define MODES_DATA_LEN             (16*16384)   /* 256k */
#define MODES_AUTO_GAIN            -100         /* Use automatic gain. */
#define MODES_MAX_GAIN             999999       /* Use max available gain. */

#define MODES_PREAMBLE_US 8       /* microseconds */
#define MODES_LONG_MSG_BITS 112
#define MODES_SHORT_MSG_BITS 56
#define MODES_FULL_LEN (MODES_PREAMBLE_US+MODES_LONG_MSG_BITS)
#define MODES_LONG_MSG_BYTES (112/8)
#define MODES_SHORT_MSG_BYTES (56/8)

#define MODES_ICAO_CACHE_LEN 32
#define MODES_UNIT_FEET 0
#define MODES_UNIT_METERS 1

#define MODES_DEBUG_DEMOD 1
#define MODES_DEBUG_DEMODERR 2
#define MODES_DEBUG_BADCRC 3
#define MODES_DEBUG_GOODCRC 4
#define MODES_DEBUG_NOPREAMBLE 5

/* When debug is set to MODES_DEBUG_NOPREAMBLE, the first sample must be
 * at least greater than a given level for us to dump the signal. */
#define MODES_DEBUG_NOPREAMBLE_LEVEL 25

#define MODES_INTERACTIVE_REFRESH_TIME 250      /* Milliseconds */
#define MODES_INTERACTIVE_ROWS 15

#define MODES_NOTUSED(V) ((void) V)

/* Structure used to describe an aircraft in iteractive mode. */
struct aircraft {
    uint32_t addr;      /* ICAO address */
    char hexaddr[7];    /* Printable ICAO address */
    char flight[9];     /* Flight number */
    int altitude;       /* Altitude */
    int speed;          /* Velocity computed from EW and NS components. */
    time_t seen;        /* Time at which the last packet was received. */
    long messages;      /* Number of Mode S messages received. */
    struct aircraft *next; /* Next aircraft in our linked list. */
};

/* Program global state. */
struct {
    /* Internal state */
    pthread_t reader_thread;
    pthread_mutex_t data_mutex;     /* Mutex to synchronize buffer access. */
    pthread_cond_t data_cond;       /* Conditional variable associated. */
    unsigned char *data;            /* Raw IQ samples buffer */
    uint16_t *magnitude;            /* Magnitude vector */
    uint32_t data_len;              /* Buffer length. */
    int fd;                         /* --ifile option file descriptor. */
    int data_ready;                 /* Data ready to be processed. */
    uint32_t icao_cache[MODES_ICAO_CACHE_LEN];/* Recently seen ICAO addresses */
    int icao_cache_idx;             /* icao_cache circular buf idx. */
    uint16_t *maglut;               /* I/Q -> Magnitude lookup table. */
    int exit;                       /* Exit from the main loop when true. */

    /* RTLSDR */
    int dev_index;
    int gain;
    int enable_agc;
    rtlsdr_dev_t *dev;
    int freq;

    /* Configuration */
    char *filename;                 /* Input form file, --ifile option. */
    int fix_errors;                 /* Single bit error correction if true. */
    int check_crc;                  /* Only display messages with good CRC. */
    int raw;                        /* Raw output format */
    int debug;                      /* Debugging mode */
    int interactive;                /* Interactive mode */
    int interactive_rows;           /* Interactive mode: max number of rows */
    int stats;                      /* Print stats at exit in --ifile mode. */
    int onlyaddr;                   /* Print only ICAO addresses. */

    /* Interactive mode */
    struct aircraft *aircrafts;
    long long interactive_last_update;  /* Last screen update in milliseconds */

    /* Statistics */
    long long stat_valid_preamble;
    long long stat_demodulated;
    long long stat_goodcrc;
    long long stat_badcrc;
    long long stat_fixed;
} Modes;

/* The struct we use to store information about a decoded message. */
struct modesMessage {
    /* Generic fields */
    unsigned char msg[MODES_LONG_MSG_BYTES]; /* Binary message. */
    int msgbits;                /* Number of bits in message */
    int msgtype;                /* Downlink format # */
    int crcok;                  /* True if CRC was valid */
    uint32_t crc;               /* Message CRC */
    int errorbit;               /* Bit corrected. -1 if no bit corrected. */
    int aa1, aa2, aa3;          /* ICAO Address bytes 1 2 and 3 */

    /* DF 11 */
    int ca;                     /* Responder capabilities. */

    /* DF 17 */
    int metype;                 /* Extended squitter message type. */
    int mesub;                  /* Extended squitter message subtype. */
    int heading_is_valid;
    int heading;
    int aircraft_type;
    int fflag;                  /* Odd or Even CPR message? */
    int tflag;                  /* UTC synchronized? */
    int raw_latitude;           /* Non decoded latitude */
    int raw_longitude;          /* Non decoded longitude */
    char flight[9];             /* 8 chars flight number. */
    int ew_dir;                 /* E/W direction. */
    int ew_velocity;            /* E/W velocity. */
    int ns_dir;                 /* N/S direction. */
    int ns_velocity;            /* N/S velocity. */
    int vert_rate_source;       /* Vertical rate source. */
    int vert_rate_sign;         /* Vertical rate sign. */
    int vert_rate;              /* Vertical rate. */
    int velocity;               /* Computed from EW and NS velocity. */

    /* DF4, DF5, DF20, DF21 */
    int fs;                     /* Flight status for DF4,5,20,21 */
    int dr;                     /* Request extraction of downlink request. */
    int um;                     /* Request extraction of downlink request. */
    int identity;               /* 13 bits identity (Squawk). */

    /* Fields used by multiple message types. */
    int altitude, unit;
};

void interactiveShowData(void);
void interactiveReceiveData(struct modesMessage *mm);

/* ============================= Utility functions ========================== */

static long long mstime(void) {
    struct timeval tv;
    long long mst;

    gettimeofday(&tv, NULL);
    mst = ((long long)tv.tv_sec)*1000;
    mst += tv.tv_usec/1000;
    return mst;
}

/* =============================== Initialization =========================== */

void modesInitConfig(void) {
    Modes.gain = MODES_MAX_GAIN;
    Modes.dev_index = 0;
    Modes.enable_agc = 0;
    Modes.freq = MODES_DEFAULT_FREQ;
    Modes.filename = NULL;
    Modes.fix_errors = 1;
    Modes.check_crc = 1;
    Modes.raw = 0;
    Modes.onlyaddr = 0;
    Modes.debug = 0;
    Modes.interactive = 0;
    Modes.interactive_rows = MODES_INTERACTIVE_ROWS;
}

void modesInit(void) {
    int i, q;

    pthread_mutex_init(&Modes.data_mutex,NULL);
    pthread_cond_init(&Modes.data_cond,NULL);
    Modes.data_len = MODES_DATA_LEN;
    Modes.data_ready = 0;
    memset(Modes.icao_cache,0,sizeof(Modes.icao_cache));
    Modes.icao_cache_idx = 0;
    Modes.aircrafts = NULL;
    Modes.interactive_last_update = 0;
    if ((Modes.data = malloc(Modes.data_len)) == NULL ||
        (Modes.magnitude = malloc(Modes.data_len*2)) == NULL) {
        fprintf(stderr, "Out of memory allocating data buffer.\n");
        exit(1);
    }

    /* Populate the I/Q -> Magnitude lookup table. It is used because
     * sqrt or round may be expensive and may vary a lot depending on
     * the libc used.
     *
     * We scale to 0-255 range multiplying by 1.4 in order to ensure that
     * every different I/Q pair will result in a different magnitude value,
     * not losing any resolution. */
    Modes.maglut = malloc(129*129*2);
    for (i = 0; i <= 128; i++) {
        for (q = 0; q <= 128; q++) {
            Modes.maglut[i*129+q] = round(sqrt(i*i+q*q)*360);
        }
    }

    /* Statistics */
    Modes.stat_valid_preamble = 0;
    Modes.stat_demodulated = 0;
    Modes.stat_goodcrc = 0;
    Modes.stat_badcrc = 0;
    Modes.stat_fixed = 0;
    Modes.exit = 0;
}

/* =============================== RTLSDR handling ========================== */

void modesInitRTLSDR(void) {
    int j;
    int device_count;
    int ppm_error = 0;
    char vendor[256], product[256], serial[256];

    device_count = rtlsdr_get_device_count();
    if (!device_count) {
        fprintf(stderr, "No supported RTLSDR devices found.\n");
        exit(1);
    }

    fprintf(stderr, "Found %d device(s):\n", device_count);
    for (j = 0; j < device_count; j++) {
        rtlsdr_get_device_usb_strings(j, vendor, product, serial);
        fprintf(stderr, "%d: %s, %s, SN: %s %s\n", j, vendor, product, serial,
            (j == Modes.dev_index) ? "(currently selected)" : "");
    }

    if (rtlsdr_open(&Modes.dev, Modes.dev_index) < 0) {
        fprintf(stderr, "Error opening the RTLSDR device: %s\n",
            strerror(errno));
        exit(1);
    }

    /* Set gain, frequency, sample rate, and reset the device. */
    rtlsdr_set_tuner_gain_mode(Modes.dev,
        (Modes.gain == MODES_AUTO_GAIN) ? 0 : 1);
    if (Modes.gain != MODES_AUTO_GAIN) {
        if (Modes.gain == MODES_MAX_GAIN) {
            /* Find the maximum gain available. */
            int numgains;
            int gains[100];

            numgains = rtlsdr_get_tuner_gains(Modes.dev, gains);
            Modes.gain = gains[numgains-1];
            fprintf(stderr, "Max available gain is: %.2f\n", Modes.gain/10.0);
        }
        rtlsdr_set_tuner_gain(Modes.dev, Modes.gain);
        fprintf(stderr, "Setting gain to: %.2f\n", Modes.gain/10.0);
    } else {
        fprintf(stderr, "Using automatic gain control.\n");
    }
    rtlsdr_set_freq_correction(Modes.dev, ppm_error);
    if (Modes.enable_agc) rtlsdr_set_agc_mode(Modes.dev, 1);
    rtlsdr_set_center_freq(Modes.dev, Modes.freq);
    rtlsdr_set_sample_rate(Modes.dev, MODES_DEFAULT_RATE);
    rtlsdr_reset_buffer(Modes.dev);
    fprintf(stderr, "Gain reported by device: %.2f\n",
        rtlsdr_get_tuner_gain(Modes.dev)/10.0);
}

/* We use a thread reading data in background, while the main thread
 * handles decoding and visualization of data to the user.
 *
 * The reading thread calls the RTLSDR API to read data asynchronously, and
 * uses a callback to populate the data buffer.
 * A Mutex is used to avoid races with the decoding thread. */
void rtlsdrCallback(unsigned char *buf, uint32_t len, void *ctx) {
    MODES_NOTUSED(ctx);

    pthread_mutex_lock(&Modes.data_mutex);
    if (len > Modes.data_len) len = Modes.data_len;
    memcpy(Modes.data, buf, len);
    Modes.data_ready = 1;
    /* Signal to the other thread that new data is ready */
    pthread_cond_signal(&Modes.data_cond);
    pthread_mutex_unlock(&Modes.data_mutex);
}

/* This is used when --ifile is specified in order to read data from file
 * instead of using an RTLSDR device. */
void readDataFromFile(void) {
    pthread_mutex_lock(&Modes.data_mutex);
    while(1) {
        ssize_t nread, toread;
        unsigned char *p;

        if (Modes.data_ready) {
            pthread_cond_wait(&Modes.data_cond,&Modes.data_mutex);
            continue;
        }

        toread = Modes.data_len;
        p = Modes.data;
        while(toread) {
            nread = read(Modes.fd, p, toread);
            if (nread <= 0) {
                Modes.exit = 1; /* Signal the other thread to exit. */
                break;
            }
            p += nread;
            toread -= nread;
        }
        if (toread) {
            /* Not enough data on file to fill the buffer? Pad with
             * no signal. */
            memset(p,127,toread);
        }
        Modes.data_ready = 1;
        /* Signal to the other thread that new data is ready */
        pthread_cond_signal(&Modes.data_cond);
    }
}

/* We read data using a thread, so the main thread only handles decoding
 * without caring about data acquisition. */
void *readerThreadEntryPoint(void *arg) {
    MODES_NOTUSED(arg);

    if (Modes.filename == NULL) {
        rtlsdr_read_async(Modes.dev, rtlsdrCallback, NULL,
                              MODES_ASYNC_BUF_NUMBER,
                              Modes.data_len);
    } else {
        readDataFromFile();
    }
    return NULL;
}

/* ============================== Debugging ================================= */

/* Helper function for dumpMagnitudeVector().
 * It prints a single bar used to display raw signals.
 *
 * Since every magnitude sample is between 0-255, the function uses
 * up to 63 characters for every bar. Every character represents
 * a length of 4, 3, 2, 1, specifically:
 *
 * "O" is 4
 * "o" is 3
 * "-" is 2
 * "." is 1
 */
void dumpMagnitudeBar(int index, int magnitude) {
    char *set = ".-o";
    char buf[256];
    int div = magnitude / 4;
    int rem = magnitude % 4;

    memset(buf,'O',div);
    buf[div] = set[rem];
    buf[div+1] = '\0';

    if (index >= 0)
        printf("[%.3d] |%-69s %d\n", index, buf, magnitude);
    else
        printf("[%.2d] |%-69s %d\n", index, buf, magnitude);
}

/* Display an ASCII-art alike graphical representation of the undecoded
 * message as a magnitude signal.
 *
 * The message starts at the specified offset in the "m" buffer.
 * The function will display enough data to cover a short 56 bit message.
 *
 * If possible a few samples before the start of the messsage are included
 * for context. */

void dumpMagnitudeVector(uint16_t *m, uint32_t offset) {
    uint32_t padding = 5; /* Show 5 samples before the actual start. */
    uint32_t start = (offset < padding) ? 0 : offset-padding;
    uint32_t end = offset + (MODES_PREAMBLE_US*2)+(MODES_SHORT_MSG_BITS*2) - 1;
    uint32_t j;

    for (j = start; j <= end; j++) {
        /* Scale magnitude to 0-255 from 0-65535 before printing. */
        dumpMagnitudeBar(j-offset, m[j]/256);
    }
}

/* This is a wrapper for dumpMagnitudeVector() that also show the message
 * in hex format with an additional description.
 *
 * descr  is the additional message to show to describe the dump.
 * msg    points to the decoded message
 * m      is the original magnitude vector
 * offset is the offset where the message starts
 */
void dumpRawMessage(char *descr, unsigned char *msg,
                    uint16_t *m, uint32_t offset)
{
    int j;

    printf("\n--- %s\n    ", descr);
    for (j = 0; j < MODES_LONG_MSG_BYTES; j++) {
        printf("%02x",msg[j]);
        if (j == MODES_SHORT_MSG_BYTES-1) printf(" ... ");
    }
    printf("\n");
    dumpMagnitudeVector(m,offset);
    printf("---\n\n");
}

/* ===================== Mode S detection and decoding  ===================== */

/* Parity table for MODE S Messages.
 * The table contains 112 elements, every element corresponds to a bit set
 * in the message, starting from the first bit of actual data after the
 * preamble.
 *
 * For messages of 112 bit, the whole table is used.
 * For messages of 56 bits only the last 56 elements are used.
 *
 * The algorithm is as simple as xoring all the elements in this table
 * for which the corresponding bit on the message is set to 1.
 *
 * The latest 24 elements in this table are set to 0 as the checksum at the
 * end of the message should not affect the computation.
 *
 * Note: this function can be used with DF11 and DF17, other modes have
 * the CRC xored with the sender address as they are reply to interrogations,
 * but a casual listener can't split the address from the checksum.
 */
uint32_t modes_checksum_table[112] = {
0x3935ea, 0x1c9af5, 0xf1b77e, 0x78dbbf, 0xc397db, 0x9e31e9, 0xb0e2f0, 0x587178,
0x2c38bc, 0x161c5e, 0x0b0e2f, 0xfa7d13, 0x82c48d, 0xbe9842, 0x5f4c21, 0xd05c14,
0x682e0a, 0x341705, 0xe5f186, 0x72f8c3, 0xc68665, 0x9cb936, 0x4e5c9b, 0xd8d449,
0x939020, 0x49c810, 0x24e408, 0x127204, 0x093902, 0x049c81, 0xfdb444, 0x7eda22,
0x3f6d11, 0xe04c8c, 0x702646, 0x381323, 0xe3f395, 0x8e03ce, 0x4701e7, 0xdc7af7,
0x91c77f, 0xb719bb, 0xa476d9, 0xadc168, 0x56e0b4, 0x2b705a, 0x15b82d, 0xf52612,
0x7a9309, 0xc2b380, 0x6159c0, 0x30ace0, 0x185670, 0x0c2b38, 0x06159c, 0x030ace,
0x018567, 0xff38b7, 0x80665f, 0xbfc92b, 0xa01e91, 0xaff54c, 0x57faa6, 0x2bfd53,
0xea04ad, 0x8af852, 0x457c29, 0xdd4410, 0x6ea208, 0x375104, 0x1ba882, 0x0dd441,
0xf91024, 0x7c8812, 0x3e4409, 0xe0d800, 0x706c00, 0x383600, 0x1c1b00, 0x0e0d80,
0x0706c0, 0x038360, 0x01c1b0, 0x00e0d8, 0x00706c, 0x003836, 0x001c1b, 0xfff409,
0x000000, 0x000000, 0x000000, 0x000000, 0x000000, 0x000000, 0x000000, 0x000000,
0x000000, 0x000000, 0x000000, 0x000000, 0x000000, 0x000000, 0x000000, 0x000000,
0x000000, 0x000000, 0x000000, 0x000000, 0x000000, 0x000000, 0x000000, 0x000000
};

uint32_t modesChecksum(unsigned char *msg, int bits) {
    uint32_t crc = 0;
    int offset = (bits == 112) ? 0 : (112-56);
    int j;

    for(j = 0; j < bits; j++) {
        int byte = j/8;
        int bit = j%8;
        int bitmask = 1 << (7-bit);

        /* If bit is set, xor with corresponding table entry. */
        if (msg[byte] & bitmask)
            crc ^= modes_checksum_table[j+offset];
    }
    return crc; /* 24 bit checksum. */
}

/* Given the Downlink Format (DF) of the message, return the message length
 * in bits. */
int modesMessageLenByType(int type) {
    if (type == 16 || type == 17 ||
        type == 19 || type == 20 ||
        type == 21)
        return MODES_LONG_MSG_BITS;
    else
        return MODES_SHORT_MSG_BITS;
}

/* Try to fix single bit errors using the checksum. On success modifies
 * the original buffer with the fixed version, and returns the position
 * of the error bit. Otherwise if fixing failed -1 is returned. */
int fixSingleBitErrors(unsigned char *msg, int bits) {
    int j;
    unsigned char aux[MODES_LONG_MSG_BITS/8];

    for (j = 0; j < bits; j++) {
        int byte = j/8;
        int bitmask = 1 << (7-(j%8));
        uint32_t crc1, crc2;

        memcpy(aux,msg,bits/8);
        aux[byte] ^= bitmask; /* Flip j-th bit. */

        crc1 = ((uint32_t)aux[(bits/8)-3] << 16) |
               ((uint32_t)aux[(bits/8)-2] << 8) |
                (uint32_t)aux[(bits/8)-1];
        crc2 = modesChecksum(aux,bits);

        if (crc1 == crc2) {
            /* The error is fixed. Overwrite the original buffer with
             * the corrected sequence, and returns the error bit
             * position. */
            memcpy(msg,aux,bits/8);
            return j;
        }
    }
    return -1;
}

/* Add the specified entry to the list of recently seen ICAO addresses.
 * We use the array as a circular buffer. */
void addRecentlySeenICAOAddr(uint32_t addr) {
    Modes.icao_cache[Modes.icao_cache_idx] = addr;
    Modes.icao_cache_idx = (Modes.icao_cache_idx+1) % MODES_ICAO_CACHE_LEN;
}

/* If the message type appears to be about a DF that has the checksum xored
 * with the ICAO address, try to brute force it using a list of recently
 * seen ICAO addresses.
 *
 * Do this in a brute-force fashion xoring the addresses we know with the
 * message checksum, and latest testing if the message verifies.
 *
 * On success the input buffer is modified to remove the xored checksum
 * from the packet, so that the last three bytes will contain the
 * plain ICAO address.
 *
 * If the function successfully recovers a message with a correct checksum
 * it returns 1. Otherwise 0 is returned. */
int bruteForceAP(unsigned char *msg, int msgbits) {
    int j;
    unsigned char aux[MODES_LONG_MSG_BITS/8];
    int msgtype = msg[0]>>3;

    if (msgtype == 0 ||         /* Short air surveillance */
        msgtype == 4 ||         /* Surveillance, altitude reply */
        msgtype == 5 ||         /* Surveillance, identity reply */
        msgtype == 16 ||        /* Long Air-Air survillance */
        msgtype == 20 ||        /* Comm-A, altitude request */
        msgtype == 21 ||        /* Comm-A, identity request */
        msgtype == 24)          /* Comm-C ELM */
    {
        for (j = 0; j < MODES_ICAO_CACHE_LEN; j++) {
            uint32_t addr = Modes.icao_cache[j];
            uint32_t crc1, crc2;
            int lastbyte = (msgbits/8)-1;

            if (addr == 0) continue; /* Empty field. */
            memcpy(aux,msg,msgbits/8);
            /* Xor with the address, so that if we picked the right address
             * what remains is just the checksum. */
            aux[lastbyte] ^= addr & 0xff;
            aux[lastbyte-1] ^= (addr >> 8) & 0xff;
            aux[lastbyte-2] ^= (addr >> 16) & 0xff;
            crc1 = aux[lastbyte-2] << 16 |
                   aux[lastbyte-1] << 8 |
                   aux[lastbyte];
            crc2 = modesChecksum(aux,msgbits);
            if (crc1 == crc2) {
                /* Restore the address as last three bytes. */
                msg[lastbyte] = addr & 0xff;
                msg[lastbyte-1] = (addr >> 8) & 0xff;
                msg[lastbyte-2] = (addr >> 16) & 0xff;
                return 1;
            }
        }
    }
    return 0;
}

/* Decode the 13 bit AC altitude field (in DF 20 and others).
 * Returns the altitude, and set 'unit' to either MODES_UNIT_METERS
 * or MDOES_UNIT_FEETS. */
int decodeAC13Field(unsigned char *msg, int *unit) {
    int m_bit = msg[3] & (1<<6);
    int q_bit = msg[3] & (1<<4);

    if (!m_bit) {
        *unit = MODES_UNIT_FEET;
        if (q_bit) {
            /* N is the 11 bit integer resulting from the removal of bit
             * Q and M */
            int n = ((msg[2]&31)<<6) |
                    ((msg[3]&0x80)>>2) |
                    ((msg[3]&0x20)>>1) |
                     (msg[3]&15);
            /* The final altitude is due to the resulting number multiplied
             * by 25, minus 1000. */
            return n*25-1000;
        } else {
            /* TODO: Implement altitude where Q=0 and M=0 */
        }
    } else {
        *unit = MODES_UNIT_METERS;
        /* TODO: Implement altitude when meter unit is selected. */
    }
    return 0;
}

/* Decode the 12 bit AC altitude field (in DF 17 and others).
 * Returns the altitude or 0 if it can't be decoded. */
int decodeAC12Field(unsigned char *msg, int *unit) {
    int q_bit = msg[5] & 1;

    if (q_bit) {
        /* N is the 11 bit integer resulting from the removal of bit
         * Q */
        *unit = MODES_UNIT_FEET;
        int n = ((msg[5]>>1)<<4) | ((msg[6]&0xF0) >> 4);
        /* The final altitude is due to the resulting number multiplied
         * by 25, minus 1000. */
        return n*25-1000;
    } else {
        return 0;
    }
}

/* Capability table. */
char *ca_str[8] = {
    /* 0 */ "Level 1 (Survillance Only)",
    /* 1 */ "Level 2 (DF0,4,5,11)",
    /* 2 */ "Level 3 (DF0,4,5,11,20,21)",
    /* 3 */ "Level 4 (DF0,4,5,11,20,21,24)",
    /* 4 */ "Level 2+3+4 (DF0,4,5,11,20,21,24,code7 - is on ground)",
    /* 5 */ "Level 2+3+4 (DF0,4,5,11,20,21,24,code7 - is on airborne)",
    /* 6 */ "Level 2+3+4 (DF0,4,5,11,20,21,24,code7)",
    /* 7 */ "Level 7 ???"
};

/* Flight status table. */
char *fs_str[8] = {
    /* 0 */ "Normal, Airborne",
    /* 1 */ "Normal, On the ground",
    /* 2 */ "ALERT,  Airborne",
    /* 3 */ "ALERT,  On the ground",
    /* 4 */ "ALERT & Special Position Identification. Airborne or Ground",
    /* 5 */ "Special Position Identification. Airborne or Ground",
    /* 6 */ "Value 6 is not assigned",
    /* 7 */ "Value 7 is not assigned"
};

/* ME message type to description table. */
char *me_str[] = {
};

char *getMEDescription(int metype, int mesub) {
    char *mename = "Unknown";

    if (metype >= 1 && metype <= 4)
        mename = "Aircraft Identification and Category";
    else if (metype >= 5 && metype <= 8)
        mename = "Surface Position";
    else if (metype >= 9 && metype <= 18)
        mename = "Airborne Position (Baro Altitude)";
    else if (metype == 19 && mesub >=1 && mesub <= 4)
        mename = "Airborne Velocity";
    else if (metype >= 20 && metype <= 22)
        mename = "Airborne Position (GNSS Height)";
    else if (metype == 23 && mesub == 0)
        mename = "Test Message";
    else if (metype == 24 && mesub == 1)
        mename = "Surface System Status";
    else if (metype == 28 && mesub == 1)
        mename = "Extended Squitter Aircraft Status (Emergency)";
    else if (metype == 28 && mesub == 2)
        mename = "Extended Squitter Aircraft Status (1090ES TCAS RA)";
    else if (metype == 29 && (mesub == 0 || mesub == 1))
        mename = "Target State and Status Message";
    else if (metype == 31 && (mesub == 0 || mesub == 1))
        mename = "Aircraft Operational Status Message";
    return mename;
}

/* Decode a raw Mode S message demodulated as a stream of bytes by
 * detectModeS(), and split it into fields populating a modesMessage
 * structure. */
void decodeModesMessage(struct modesMessage *mm, unsigned char *msg) {
    uint32_t crc2;   /* Computed CRC, used to verify the message CRC. */
    char *ais_charset = "?ABCDEFGHIJKLMNOPQRSTUVWXYZ????? ???????????????0123456789??????";

    /* Work on our local copy */
    memcpy(mm->msg,msg,MODES_LONG_MSG_BYTES);
    msg = mm->msg;

    /* Get the message type ASAP as other operations depend on this */
    mm->msgtype = msg[0]>>3;    /* Downlink Format */
    mm->msgbits = modesMessageLenByType(mm->msgtype);

    /* CRC is always the last three bytes. */
    mm->crc = ((uint32_t)msg[(mm->msgbits/8)-3] << 16) |
              ((uint32_t)msg[(mm->msgbits/8)-2] << 8) |
               (uint32_t)msg[(mm->msgbits/8)-1];
    crc2 = modesChecksum(msg,mm->msgbits);

    /* Check CRC and fix single bit errors using the CRC when
     * possible (DF 11 and 17). */
    mm->errorbit = -1;  /* No error */
    mm->crcok = (mm->crc == crc2);

    if (!mm->crcok && Modes.fix_errors &&
        (mm->msgtype == 11 || mm->msgtype == 17))
    {
        if ((mm->errorbit = fixSingleBitErrors(msg,mm->msgbits)) != -1) {
            mm->crc = modesChecksum(msg,mm->msgbits);
            mm->crcok = 1;
        }
    }

    /* Note that most of the other computation happens *after* we fix
     * the single bit errors, otherwise we would need to recompute the
     * fields again. */
    mm->ca = msg[0] & 7;        /* Responder capabilities. */

    /* ICAO address */
    mm->aa1 = msg[1];
    mm->aa2 = msg[2];
    mm->aa3 = msg[3];

    /* DF 17 type (assuming this is a DF17, otherwise not used) */
    mm->metype = msg[4] >> 3;   /* Extended squitter message type. */
    mm->mesub = msg[4] & 7;     /* Extended squitter message subtype. */

    /* Fields for DF4,5,20,21 */
    mm->fs = msg[0] & 7;        /* Flight status for DF4,5,20,21 */
    mm->dr = msg[1] >> 3 & 31;  /* Request extraction of downlink request. */
    mm->um = ((msg[1] & 7)<<3)| /* Request extraction of downlink request. */
              msg[2]>>5;
    mm->identity = (msg[2]&31 << 8) | msg[3]; /* 13 bits identity. */

    /* Check if we can check the checksum for the Downlink Formats where
     * the checksum is xored with the aircraft ICAO address. We try to
     * brute force it using a list of recently seen aircraft addresses. */
    if (mm->msgtype != 11 && mm->msgtype != 17) {
        /* Return to the caller now if we can't resolve the API field and
         * we need to check the checksum. */
        if (bruteForceAP(msg,mm->msgbits)) {
            /* We recovered the message!
             * Populate the AA fields with the right information. */
            mm->aa3 = msg[mm->msgbits/8-1];
            mm->aa2 = msg[mm->msgbits/8-2];
            mm->aa1 = msg[mm->msgbits/8-3];
            mm->crcok = 1;
        } else {
            mm->crcok = 0;
        }
    }
    /* If this is DF 11 or DF 17 and the checksum was ok,
     * we can add this address to the list of recently seen
     * addresses. */
    if (mm->crcok && mm->errorbit == -1) {
        uint32_t addr = (mm->aa1 << 16) | (mm->aa2 << 8) | mm->aa3;
        addRecentlySeenICAOAddr(addr);
    }

    /* Decode 13 bit altitude for DF0, DF4, DF16, DF20 */
    if (mm->msgtype == 0 || mm->msgtype == 4 ||
        mm->msgtype == 16 || mm->msgtype == 20) {
        mm->altitude = decodeAC13Field(msg, &mm->unit);
    }

    /* Decode extended squitter specific stuff. */
    if (mm->msgtype == 17) {
        /* Decode the extended squitter message. */

        if (mm->metype >= 1 && mm->metype <= 4) {
            /* Aircraft Identification and Category */
            mm->aircraft_type = mm->metype-1;
            mm->flight[0] = ais_charset[msg[5]>>2];
            mm->flight[1] = ais_charset[((msg[5]&3)<<4)|(msg[6]>>4)];
            mm->flight[2] = ais_charset[((msg[6]&15)<<2)|(msg[7]>>6)];
            mm->flight[3] = ais_charset[msg[7]&63];
            mm->flight[4] = ais_charset[msg[8]>>2];
            mm->flight[5] = ais_charset[((msg[8]&3)<<4)|(msg[9]>>4)];
            mm->flight[6] = ais_charset[((msg[9]&15)<<2)|(msg[10]>>6)];
            mm->flight[7] = ais_charset[msg[10]&63];
            mm->flight[8] = '\0';
        } else if (mm->metype >= 9 && mm->metype <= 18) {
            /* Airborne position Message */
            mm->fflag = msg[6] & (1<<2);
            mm->tflag = msg[6] & (1<<3);
            mm->altitude = decodeAC12Field(msg,&mm->unit);
            mm->raw_latitude = ((msg[6] & 3) << 15) |
                                (msg[7] << 7) |
                                (msg[8] >> 1);
            mm->raw_longitude = ((msg[8]&1) << 16) |
                                 (msg[9] << 8) |
                                 msg[10];
        } else if (mm->metype == 19 && mm->mesub >= 1 && mm->mesub <= 4) {
            /* Airborne Velocity Message */
            if (mm->mesub == 1 || mm->mesub == 2) {
                mm->ew_dir = (msg[5]&4) >> 2;
                mm->ew_velocity = ((msg[5]&3) << 8) | msg[6];
                mm->ns_dir = (msg[7]&0x80) >> 7;
                mm->ns_velocity = ((msg[7]&0x7f) << 3) | ((msg[8]&0xe0) >> 5);
                mm->vert_rate_source = (msg[8]&0x10) >> 4;
                mm->vert_rate_sign = (msg[8]&0x8) >> 5;
                mm->vert_rate = ((msg[8]&7) << 6) | ((msg[9]&0xfc) >> 2);
                mm->velocity = sqrt(mm->ns_velocity*mm->ns_velocity+
                                    mm->ew_velocity*mm->ew_velocity);
            } else if (mm->mesub == 3 || mm->mesub == 4) {
                mm->heading_is_valid = msg[5] & (1<<2);
                mm->heading = (360.0/128) * (((msg[5] & 3) << 5) |
                                              (msg[6] >> 3));
            }
        }
    }
}

/* This function gets a decoded Mode S Message and prints it on the screen
 * in a human readable format. */
void displayModesMessage(struct modesMessage *mm) {
    int j;

    /* Handle only addresses mode first. */
    if (Modes.onlyaddr) {
        printf("%02x%02x%02x\n", mm->aa1, mm->aa2, mm->aa3);
        return;
    }

    /* Show the raw message. */
    printf("*");
    for (j = 0; j < mm->msgbits/8; j++) printf("%02x", mm->msg[j]);
    printf(";\n");

    if (Modes.raw) return; /* Enough for --raw mode */

    printf("CRC: %06x (%s)\n", (int)mm->crc, mm->crcok ? "ok" : "wrong");
    if (mm->errorbit != -1)
        printf("Single bit error fixed, bit %d\n", mm->errorbit);

    if (mm->msgtype == 0) {
        /* DF 0 */
        printf("DF 0: Short Air-Air Surveillance.\n");
        printf("  Altitude       : %d %s\n", mm->altitude,
            (mm->unit == MODES_UNIT_METERS) ? "meters" : "feet");
        printf("  ICAO Address   : %02x%02x%02x\n", mm->aa1, mm->aa2, mm->aa3);
    } else if (mm->msgtype == 4 || mm->msgtype == 20) {
        printf("DF %d: %s, Altitude Reply.\n", mm->msgtype,
            (mm->msgtype == 4) ? "Surveillance" : "Comm-B");
        printf("  Flight Status  : %s\n", fs_str[mm->fs]);
        printf("  DR             : %d\n", mm->dr);
        printf("  UM             : %d\n", mm->um);
        printf("  Altitude       : %d %s\n", mm->altitude,
            (mm->unit == MODES_UNIT_METERS) ? "meters" : "feet");
        printf("  ICAO Address   : %02x%02x%02x\n", mm->aa1, mm->aa2, mm->aa3);

        if (mm->msgtype == 20) {
            /* TODO: 56 bits DF20 MB additional field. */
        }
    } else if (mm->msgtype == 5 || mm->msgtype == 21) {
        printf("DF %d: %s, Identity Reply.\n", mm->msgtype,
            (mm->msgtype == 5) ? "Surveillance" : "Comm-B");
        printf("  Flight Status  : %s\n", fs_str[mm->fs]);
        printf("  DR             : %d\n", mm->dr);
        printf("  UM             : %d\n", mm->um);
        printf("  Squawk         : %o (octal)\n", mm->identity);
        printf("  ICAO Address   : %02x%02x%02x\n", mm->aa1, mm->aa2, mm->aa3);

        if (mm->msgtype == 21) {
            /* TODO: 56 bits DF21 MB additional field. */
        }
    } else if (mm->msgtype == 11) {
        /* DF 11 */
        printf("DF 11: All Call Reply.\n");
        printf("  Capability  : %s\n", ca_str[mm->ca]);
        printf("  ICAO Address: %02x%02x%02x\n", mm->aa1, mm->aa2, mm->aa3);
    } else if (mm->msgtype == 17) {
        /* DF 17 */
        printf("DF 17: ADS-B message.\n");
        printf("  Capability     : %d (%s)\n", mm->ca, ca_str[mm->ca]);
        printf("  ICAO Address   : %02x%02x%02x\n", mm->aa1, mm->aa2, mm->aa3);
        printf("  Extended Squitter  Type: %d\n", mm->metype);
        printf("  Extended Squitter  Sub : %d\n", mm->mesub);
        printf("  Extended Squitter  Name: %s\n",
            getMEDescription(mm->metype,mm->mesub));

        /* Decode the extended squitter message. */
        if (mm->metype >= 1 && mm->metype <= 4) {
            /* Aircraft identification. */
            char *ac_type_str[4] = {
                "Aircraft Type D",
                "Aircraft Type C",
                "Aircraft Type B",
                "Aircraft Type A"
            };

            printf("    Aircraft Type  : %s\n", ac_type_str[mm->aircraft_type]);
            printf("    Identification : %s\n", mm->flight);
        } else if (mm->metype >= 9 && mm->metype <= 18) {
            printf("    F flag   : %s\n", mm->fflag ? "odd" : "even");
            printf("    T flag   : %s\n", mm->tflag ? "UTC" : "non-UTC");
            printf("    Altitude : %d feet\n", mm->altitude);
            printf("    Latitude : %d (not decoded)\n", mm->raw_latitude);
            printf("    Longitude: %d (not decoded)\n", mm->raw_longitude);
        } else if (mm->metype == 19 && mm->mesub >= 1 && mm->mesub <= 4) {
            if (mm->mesub == 1 || mm->mesub == 2) {
                /* Velocity */
                printf("    EW direction      : %d\n", mm->ew_dir);
                printf("    EW velocity       : %d\n", mm->ew_velocity);
                printf("    NS direction      : %d\n", mm->ns_dir);
                printf("    NS velocity       : %d\n", mm->ns_velocity);
                printf("    Vertical rate src : %d\n", mm->vert_rate_source);
                printf("    Vertical rate sign: %d\n", mm->vert_rate_sign);
                printf("    Vertical rate     : %d\n", mm->vert_rate);
            } else if (mm->mesub == 3 || mm->mesub == 4) {
                printf("    Heading status: %d", mm->heading_is_valid);
                printf("    Heading: %d", mm->heading);
            }
        } else {
            printf("    Unrecognized ME type: %d subtype: %d\n", 
                mm->metype, mm->mesub);
        }
    } else {
        if (Modes.check_crc)
            printf("DF %d with good CRC received "
                   "(decoding still not implemented).\n",
                mm->msgtype);
    }
}

/* Turn I/Q samples pointed by Modes.data into the magnitude vector
 * pointed by Modes.magnitude. */
void computeMagnitudeVector(void) {
    uint16_t *m = Modes.magnitude;
    unsigned char *p = Modes.data;
    uint32_t j;

    /* Compute the magnitudo vector. It's just SQRT(I^2 + Q^2), but
     * we rescale to the 0-255 range to exploit the full resolution. */
    for (j = 0; j < Modes.data_len; j += 2) {
        int i = p[j]-127;
        int q = p[j+1]-127;

        if (i < 0) i = -i;
        if (q < 0) q = -q;
        m[j/2] = Modes.maglut[i*129+q];
    }
}

/* Detect a Mode S messages inside the magnitude buffer pointed by 'm' and of
 * size 'mlen' bytes. Every detected Mode S message is convert it into a
 * stream of bits and passed to the function to display it. */
void detectModeS(uint16_t *m, uint32_t mlen) {
    unsigned char bits[MODES_LONG_MSG_BITS];
    unsigned char msg[MODES_LONG_MSG_BITS/2];
    uint32_t j;

    /* The Mode S preamble is made of impulses of 0.5 microseconds at
     * the following time offsets:
     *
     * 0   - 0.5 usec: first impulse.
     * 1.0 - 1.5 usec: second impulse.
     * 3.5 - 4   usec: third impulse.
     * 4.5 - 5   usec: last impulse.
     * 
     * Since we are sampling at 2 Mhz every sample in our magnitude vector
     * is 0.5 usec, so the preamble will look like this, assuming there is
     * an impulse at offset 0 in the array:
     *
     * 0   -----------------
     * 1   -
     * 2   ------------------
     * 3   --
     * 4   -
     * 5   --
     * 6   -
     * 7   ------------------
     * 8   --
     * 9   -------------------
     */
    for (j = 0; j < mlen - MODES_FULL_LEN*2; j++) {
        int low, high, i, errors;

        /* First check of relations between the first 10 samples
         * representing a valid preamble. We don't even investigate further
         * if this simple test is not passed. */
        if (!(m[j] > m[j+1] &&
            m[j+1] < m[j+2] &&
            m[j+2] > m[j+3] &&
            m[j+3] < m[j] &&
            m[j+4] < m[j] &&
            m[j+5] < m[j] &&
            m[j+6] < m[j] &&
            m[j+7] > m[j+8] &&
            m[j+8] < m[j+9] &&
            m[j+9] > m[j+6]))
        {
            if (Modes.debug == MODES_DEBUG_NOPREAMBLE &&
                m[j] > MODES_DEBUG_NOPREAMBLE_LEVEL)
                dumpRawMessage("Unexpected ratio among first 10 samples",
                    msg, m, j);
            continue;
        }

        /* The samples between the two spikes must be < than the average
         * of the high spikes level. */
        high = (m[j]+m[j+2]+m[j+7]+m[j+9])/4;
        if (m[j+3] >= high ||
            m[j+4] >= high ||
            m[j+5] >= high ||
            m[j+6] >= high)
        {
            if (Modes.debug == MODES_DEBUG_NOPREAMBLE &&
                m[j] > MODES_DEBUG_NOPREAMBLE_LEVEL)
                dumpRawMessage("Too high level in samples between 3 and 6",
                    msg, m, j);
            continue;
        }

        /* Similarly samples in the range 10-15 must be low, as it is the
         * space between the preamble and real data. */
        if (m[j+10] >= high ||
            m[j+11] >= high ||
            m[j+12] >= high ||
            m[j+13] >= high ||
            m[j+14] >= high ||
            m[j+15] >= high)
        {
            if (Modes.debug == MODES_DEBUG_NOPREAMBLE &&
                m[j] > MODES_DEBUG_NOPREAMBLE_LEVEL)
                dumpRawMessage("Too high level in samples between 10 and 15",
                    msg, m, j);
            continue;
        }
        Modes.stat_valid_preamble++;

        /* Decode all the next 112 bits, regardless of the actual message
         * size. We'll check the actual message type later. */
        errors = 0;
        for (i = 0; i < MODES_LONG_MSG_BITS*2; i += 2) {
            low = m[j+i+MODES_PREAMBLE_US*2];
            high = m[j+i+MODES_PREAMBLE_US*2+1];
            if (low == high) {
                /* Checking if two adiacent samples have the same magnitude
                 * is an effective way to detect if it's just random noise
                 * that was detected as a valid preamble. */
                bits[i/2] = 2; /* error */
                if (i < MODES_SHORT_MSG_BITS*2) errors++;
            } else if (low > high) {
                bits[i/2] = 1;
            } else {
                /* (low < high) for exclusion  */
                bits[i/2] = 0;
            }
        }

        /* Pack bits into bytes */
        for (i = 0; i < MODES_LONG_MSG_BITS; i += 8) {
            msg[i/8] =
                bits[i]<<7 | 
                bits[i+1]<<6 | 
                bits[i+2]<<5 | 
                bits[i+3]<<4 | 
                bits[i+4]<<3 | 
                bits[i+5]<<2 | 
                bits[i+6]<<1 | 
                bits[i+7];
        }

        int msgtype = msg[0]>>3;
        int msglen = modesMessageLenByType(msgtype)/8;

        /* Last check, high and low bits are different enough in magnitude
         * to mark this as real message and not just noise? */
        int delta = 0;
        for (i = 0; i < msglen*8*2; i += 2) {
            delta += abs(m[j+i+MODES_PREAMBLE_US*2]-
                         m[j+i+MODES_PREAMBLE_US*2+1]);
        }
        delta /= msglen*4;

        /* Filter for an average delta of three is small enough to let almost
         * every kind of message to pass, but high enough to filter some
         * random noise. */
        if (delta < 10*255) continue;

        /* If we reached this point, and error is zero, we are very likely
         * with a Mode S message in our hands, but it may still be broken
         * and CRC may not be correct. This is handled by the next layer. */
        if (errors == 0) {
            struct modesMessage mm;

            /* Decode the received message and update statistics */
            decodeModesMessage(&mm,msg);
            Modes.stat_demodulated++;
            if (mm.errorbit == -1) {
                if (mm.crcok)
                    Modes.stat_goodcrc++;
                else
                    Modes.stat_badcrc++;
            } else {
                Modes.stat_badcrc++;
                Modes.stat_fixed++;
            }

            /* Output debug mode info if needed. */
            if (Modes.debug == MODES_DEBUG_DEMOD)
                dumpRawMessage("Demodulated with 0 errors", msg, m, j);
            else if (Modes.debug == MODES_DEBUG_BADCRC && !mm.crcok)
                dumpRawMessage("Decoded with bad CRC", msg, m, j);
            else if (Modes.debug == MODES_DEBUG_GOODCRC && mm.crcok &&
                     mm.errorbit == -1)
                dumpRawMessage("Decoded with good CRC", msg, m, j);

            /* Pass data to the next layer */
            if (!Modes.stats && (Modes.check_crc == 0 || mm.crcok)) {
                if (Modes.interactive) {
                    interactiveReceiveData(&mm);
                } else {
                    displayModesMessage(&mm);
                    if (!Modes.raw && !Modes.onlyaddr) printf("\n");
                }
            }

            /* Skip this message if we are sure it's fine. */
            if (mm.crcok) j += (MODES_PREAMBLE_US+(msglen*8))*2;
        } else {
            if (Modes.debug == MODES_DEBUG_DEMODERR) {
                printf("The following message has %d demod errors\n", errors);
                dumpRawMessage("Demodulated with errors", msg, m, j);
            }
        }
    }
}

/* ========================= Interactive mode =============================== */

/* Return a new aircraft structure for the interactive mode linked list
 * of aircrafts. */
struct aircraft *interactiveCreateAircraft(uint32_t addr) {
    struct aircraft *a = malloc(sizeof(*a));

    a->addr = addr;
    snprintf(a->hexaddr,sizeof(a->hexaddr),"%06x",(int)addr);
    a->flight[0] = '\0';
    a->altitude = 0;
    a->seen = time(NULL);
    a->messages = 0;
    a->next = NULL;
    return a;
}

/* Return the aircraft with the specified address, or NULL if no aircraft
 * exists with this address. */
struct aircraft *interactiveFindAircraft(uint32_t addr) {
    struct aircraft *a = Modes.aircrafts;

    while(a) {
        if (a->addr == addr) return a;
        a = a->next;
    }
    return NULL;
}

/* Receive new messages and populate the interactive mode with more info. */
void interactiveReceiveData(struct modesMessage *mm) {
    uint32_t addr;
    struct aircraft *a, *aux;

    if (Modes.check_crc && mm->crcok == 0) return;
    addr = (mm->aa1 << 16) | (mm->aa2 << 8) | mm->aa3;

    /* Loookup our aircraft or create a new one. */
    a = interactiveFindAircraft(addr);
    if (!a) {
        a = interactiveCreateAircraft(addr);
        a->next = Modes.aircrafts;
        Modes.aircrafts = a;
    } else {
        /* If it is an already known aircraft, move it on head
         * so we keep aircrafts ordered by received message time.
         *
         * However move it on head only if at least one second elapsed
         * since the aircraft that is currently on head sent a message,
         * othewise with multiple aircrafts at the same time we have an
         * useless shuffle of positions on the screen. */
        if (Modes.aircrafts != a && (time(NULL) - a->seen) >= 1) {
            aux = Modes.aircrafts;
            while(aux->next != a) aux = aux->next;
            /* Now we are a node before the aircraft to remove. */
            aux->next = aux->next->next; /* removed. */
            /* Add on head */
            a->next = Modes.aircrafts;
            Modes.aircrafts = a;
        }
    }

    a->seen = time(NULL);
    a->messages++;

    if (mm->msgtype == 0 || mm->msgtype == 4 || mm->msgtype == 20) {
        a->altitude = mm->altitude;
    } else if (mm->msgtype == 17) {
        if (mm->metype >= 1 && mm->metype <= 4) {
            memcpy(a->flight, mm->flight, sizeof(a->flight));
        } else if (mm->metype >= 9 && mm->metype <= 18) {
            a->altitude = mm->altitude;
        } else if (mm->metype == 19) {
            if (mm->mesub == 1 || mm->mesub == 2) {
                a->speed = mm->velocity;
            }
        }
    }
}

/* Show the currently captured interactive data on screen. */
void interactiveShowData(void) {
    struct aircraft *a = Modes.aircrafts;
    time_t now = time(NULL);
    char progress[4];
    int count = 0;

    memset(progress,' ',3);
    progress[time(NULL)%3] = '.';
    progress[3] = '\0';

    printf("\x1b[H\x1b[2J");    /* Clear the screen */
    printf("Hex    Flight   Altitude  Speed     Messages  Seen    %s\n",
            progress);
    printf("----------------------------------------------------\n");
    while(a && count < Modes.interactive_rows) {
        printf("%-6s %-8s %-9d %-9d %-9ld %d sec ago\n",
            a->hexaddr, a->flight, a->altitude, a->speed, a->messages,
            (int)(now - a->seen));
        a = a->next;
        count++;
    }
}

/* ============================== Snip mode ================================= */

/* Get raw IQ samples and filter everything is < than the specified level
 * for more than 256 samples in order to reduce example file size. */
void snipMode(int level) {
    int i, q;
    long long c = 0;

    while ((i = getchar()) != EOF && (q = getchar()) != EOF) {
        if (abs(i-127) < level && abs(q-127) < level) {
            c++;
            if (c > MODES_PREAMBLE_US*4) continue;
        } else {
            c = 0;
        }
        putchar(i);
        putchar(q);
    }
}

/* ================================ Main ==================================== */

void showHelp(void) {
    printf(
"--device-index <index>   Select RTL device (default: 0).\n"
"--gain <db>              Set gain (default: max gain. Use -100 for auto-gain).\n"
"--enable-agc <db>        Enable the Automatic Gain Control (default: off).\n"
"--freq <hz>              Set frequency (default: 1090 Mhz).\n"
"--ifile <filename>       Read data from file (use '-' for stdin).\n"
"--interactive            Interactive mode refreshing data on screen.\n"
"--interactive-rows <num> Max number of rows in interactive mode (default: 15).\n"
"--raw                    Show only messages hex values.\n"
"--no-fix                 Disable single-bits error correction using CRC.\n"
"--no-crc-check           Disable messages with broken CRC (discouraged).\n"
"--stats                  With --ifile print stats at exit. No other output.\n"
"--onlyaddr               Show only ICAO addresses (testing purposes).\n"
"--snip <level>           Strip IQ file removing samples < level.\n"
"--debug <level>          Debug mode, see README for more information.\n"
"--help                   Show this help.\n"
    );
}

int main(int argc, char **argv) {
    int j;

    /* Set sane defaults. */
    modesInitConfig();

    /* Parse the command line options */
    for (j = 1; j < argc; j++) {
        int more = j+1 < argc; /* There are more arguments. */

        if (!strcmp(argv[j],"--device-index") && more) {
            Modes.dev_index = atoi(argv[++j]);
        } else if (!strcmp(argv[j],"--gain") && more) {
            Modes.gain = atof(argv[++j])*10; /* Gain is in tens of DBs */
        } else if (!strcmp(argv[j],"--enable-agc")) {
            Modes.enable_agc++;
        } else if (!strcmp(argv[j],"--freq") && more) {
            Modes.freq = strtoll(argv[++j],NULL,10);
        } else if (!strcmp(argv[j],"--ifile") && more) {
            Modes.filename = strdup(argv[++j]);
        } else if (!strcmp(argv[j],"--no-fix")) {
            Modes.fix_errors = 0;
        } else if (!strcmp(argv[j],"--no-crc-check")) {
            Modes.check_crc = 0;
        } else if (!strcmp(argv[j],"--raw")) {
            Modes.raw = 1;
        } else if (!strcmp(argv[j],"--onlyaddr")) {
            Modes.onlyaddr = 1;
        } else if (!strcmp(argv[j],"--interactive")) {
            Modes.interactive = 1;
        } else if (!strcmp(argv[j],"--interactive-rows")) {
            Modes.interactive_rows = atoi(argv[++j]);
        } else if (!strcmp(argv[j],"--debug") && more) {
            Modes.debug = atoi(argv[++j]);
        } else if (!strcmp(argv[j],"--stats")) {
            Modes.stats = 1;
        } else if (!strcmp(argv[j],"--snip") && more) {
            snipMode(atoi(argv[++j]));
            exit(0);
        } else if (!strcmp(argv[j],"--help")) {
            showHelp();
            exit(0);
        } else {
            fprintf(stderr,
                "Unknown or not enough arguments for option '%s'.\n\n",
                argv[j]);
            showHelp();
            exit(1);
        }
    }

    modesInit();
    if (Modes.filename == NULL) {
        modesInitRTLSDR();
    } else {
        if (Modes.filename[0] == '-' && Modes.filename[1] == '\0') {
            Modes.fd = STDIN_FILENO;
        } else if ((Modes.fd = open(Modes.filename,O_RDONLY)) == -1) {
            perror("Opening data file");
            exit(1);
        }
    }

    /* Create the thread that will read the data from the device. */
    pthread_create(&Modes.reader_thread, NULL, readerThreadEntryPoint, NULL);

    pthread_mutex_lock(&Modes.data_mutex);
    while(1) {
        if (!Modes.data_ready) {
            pthread_cond_wait(&Modes.data_cond,&Modes.data_mutex);
            continue;
        }
        computeMagnitudeVector();

        /* Signal to the other thread that we processed the available data
         * and we want more (useful for --ifile). */
        Modes.data_ready = 0;
        pthread_cond_signal(&Modes.data_cond);

        /* Process data after releasing the lock, so that the capturing
         * thread can read data while we perform computationally expensive
         * stuff * at the same time. (This should only be useful with very
         * slow processors). */
        pthread_mutex_unlock(&Modes.data_mutex);
        detectModeS(Modes.magnitude, Modes.data_len/2);

        /* Refresh screen when in interactive mode. */
        if (Modes.interactive &&
            (mstime() - Modes.interactive_last_update) >
            MODES_INTERACTIVE_REFRESH_TIME)
        {
            interactiveShowData();
            Modes.interactive_last_update = mstime();
        }
        pthread_mutex_lock(&Modes.data_mutex);
        if (Modes.exit) break;
    }

    /* If --ifile and --stats were given, print statistics. */
    if (Modes.stats && Modes.filename) {
        printf("%lld valid preambles\n", Modes.stat_valid_preamble);
        printf("%lld demodulated with zero errors\n", Modes.stat_demodulated);
        printf("%lld with good crc\n", Modes.stat_goodcrc);
        printf("%lld with bad crc\n", Modes.stat_badcrc);
        printf("%lld single bit errors corrected\n", Modes.stat_fixed);
    }

    rtlsdr_close(Modes.dev);
    return 0;
}