aisdecode.R

gistfile1.txt

library(vikingr)
library(dplyr)
library(purrr)
library(stringr)

log <- read_ais_log(vikingr_example("vikingr-visby-2019-ais-2"))

log_tail <- 
  log$message %>%
  str_replace("(.*?,){5}(.*?)", "\\2")

log_head <- 
  log$message %>%
  str_extract("((.*?,){5})") %>% 
  str_remove(",$") %>%
  readr::read_csv(col_names = letters[1:5])

log_clean <- bind_cols(
  timestamp = log$timestamp, 
  log_head, 
  message = log_tail
)

to_payload <- function(message) {
  message %>% utf8ToInt() - 48 %>%
  map_dbl(function(x) if_else(x > 40, x - 8, x))
}

to_bits <- function(x, n_bits = 8) {
  x %>% map(~ intToBits(.)[n_bits:1] %>% as.logical) %>% unlist
}

to_decimal <- function(bits) {
  # using as.integer means bits should be <= 32 
  stopifnot(bits <= 32)
  bits %*% 2^((length(bits) - 1):0) %>% as.integer
}

decode_bits <- function(bits) {
  list(
    msg_type = bits[1:6],
    repeat_indicator = bits[7:8],
    mmsi = bits[9:38],
    nav_stat = bits[39:42],
    rot = bits[43:50],
    sog = bits[51:60],
    pos_acc = bits[61],
    lon = bits[62:89],
    lat = bits[90:116],
    cog = bits[117:128],
    hdg = bits[118:137]
  ) %>% 
    map_df(to_decimal) %>%
    mutate_at(vars(lon, lat), ~ . / 600000)
}

decode_ais <- function(message) {
  
  decode_message <- function(x) 
    x %>% to_payload() %>% to_bits() %>% decode_bits()
  
  message %>% map_df(decode_message)
}

# this is the resulting df after decoding the messages

df <- bind_cols(
  log_clean,
  decode_ais(log_clean$message)
)

decode_sixbits <- function(payload) {
  six_bit <- c('@', LETTERS, '[', '\\', ']', 
    '^', '_', ' ', '!', '"', '#', '$', '%', 
    '&', '\'', '(', ')', '*', '+', ',', '-', 
    '.', '/', 0:9, ':', ';', '<', '=', '>', '?') 
  six_bit %>% set_names(c(0:(2^6 - 1)))
  six_bit[payload]
}

# we seem not to have any type 5 messages so cannot test this
df %>% filter(msg_type == 5)