#!/bin/bash
#
# showevic.sh -- decode and display live EVIC music-info text from
#                CAN-IHS message $328.  Passive read counterpart to
#                evic.sh (which writes the same display).
#
# Originally created:  jmccorm
# Last updated:        05.2026 (polish by magikh0e)
#
# WHAT IT DOES
#   Tails `candump` for $328 frames on CAN-IHS and reconstructs the
#   ASCII text the radio / head unit is currently displaying on the
#   instrument cluster's EVIC region.  Output is line-buffered so each
#   frame becomes visible immediately.  Runs until interrupted
#   (Ctrl+C clean-exits).
#
#   Typical uses:
#     - Watch what the radio is writing while testing evic.sh
#       writes from a second terminal
#     - Verify that a custom CAN-replay rig is broadcasting the
#       right bytes
#     - Debug EVIC text that "should" be displayed but isn't
#       (compare what the bus is carrying vs what the cluster shows)
#
# $328 FRAME FORMAT  (see bus-message-reference.html#id-328)
#
#   Each frame is 8 bytes: 07 NN cc cc cc cc cc cc
#
#     byte 0  = 0x07   marker / write-cluster command type
#     byte 1  =   line marker  if 0x41 / 0x42 / 0x43
#                  -> "Line 1: " / "Line 2: " / "Line 3: " emitted
#                sub-position byte for continuation frames otherwise
#                (0x0F / 0x1F / 0x2F etc.)
#     bytes 2-7 = 6 ASCII chars (or 0x00 padding)
#
#   An all-zero bytes-2-through-7 frame is the line terminator -- we
#   emit a newline.  A normal 6-char frame just gets printed as raw
#   ASCII using printf's \x escape.
#
# USAGE
#     ./showevic.sh              Stream live EVIC text
#     ./showevic.sh --help       Print this header
#
# OVERRIDABLE ENV VARS
#     IHS_BUS    CAN-IHS interface name    (default: can0)
#
# REQUIRES
#     - can-utils (candump)                apt install can-utils
#     - CAN-IHS interface up at 125 kbps:
#         ip link set $IHS_BUS up type can bitrate 125000
#     - Radio active / vehicle awake -- $328 only broadcasts when the
#       head unit has something to display.
#
# REVISION NOTES  (2026-05-18)
#     - jmccorm's exact decode logic preserved (line-marker detection,
#       terminator-frame newline, ASCII-byte printf reconstruction).
#     - FIXED: `[ "$e" -gt 39 ]` was bash arithmetic on a hex byte
#       string.  Works on 0x41/0x42/0x43 (the values it cares about),
#       but throws "integer expression expected" on 0x0F / 0x1F / 0x2F
#       (the sub-position bytes in continuation frames).  Without
#       set -e, the legacy silently swallowed the error and kept
#       going; functional, but loud in stderr and brittle if run
#       under set -e.  New version string-matches against literal
#       "4[123]" instead.
#     - Renamed the cryptic single-letter read variables (a..k) to
#       semantic names (iface, id, dlc, b0..b7) so a reader can
#       follow the decode without consulting candump's output format
#       in another window.
#     - Added shebang doc-block, --help, $IHS_BUS env var override,
#       SIGINT/SIGTERM trap for clean exit, pre-flight checks for
#       candump and the CAN interface.
#     - Original preserved as showevic.legacy.txt.
#

set -u

# ---------------------------------------------------------------------
# Configuration
# ---------------------------------------------------------------------

: "${IHS_BUS:=can0}"

# ---------------------------------------------------------------------
# CLI
# ---------------------------------------------------------------------

case "${1:-}" in
    -h|--help)
        awk '/^# showevic\.sh/,/^$/ { print }' "$0" | sed 's/^# \{0,1\}//'
        exit 0
        ;;
esac

# ---------------------------------------------------------------------
# Pre-flight
# ---------------------------------------------------------------------

if ! command -v candump >/dev/null 2>&1; then
    echo "ERROR: candump not found.  apt install can-utils" >&2
    exit 2
fi

if ! ip link show "$IHS_BUS" >/dev/null 2>&1; then
    echo "ERROR: CAN interface $IHS_BUS not found" >&2
    echo "       Bring it up: ip link set $IHS_BUS up type can bitrate 125000" >&2
    exit 2
fi

# ---------------------------------------------------------------------
# SIGINT cleanup
# ---------------------------------------------------------------------

# Ctrl+C should emit a trailing newline so the prompt comes back on a
# fresh line, then exit cleanly.  The candump subprocess in the pipe
# gets SIGPIPE from the shell teardown automatically.
cleanup() {
    echo
    exit 0
}
trap cleanup INT TERM

# ---------------------------------------------------------------------
# Main: stream $328 frames, decode each one inline.
# ---------------------------------------------------------------------

# candump default output for a $328 frame looks like:
#     can0  328   [8]  07 41 4D 61 67 69 6B 65
#
# Read splits into 11 fields:
#     iface=can0  id=328  dlc=[8]  b0..b7 = the 8 payload bytes
candump "${IHS_BUS},328:0fff" | while read -r iface id dlc b0 b1 b2 b3 b4 b5 b6 b7; do

    # Line marker frame: byte 1 is one of 0x41 / 0x42 / 0x43.
    # Emit "Line N: " using the second nibble (1, 2, or 3) of the
    # line marker.  String match instead of arithmetic so we don't
    # eat a parse error on continuation-frame bytes like 0x0F.
    if [[ "$b1" =~ ^4[123]$ ]]; then
        echo -n "Line ${b1:1:1}: "
    fi

    # Terminator frame: bytes 2-7 are all zero.  Emit a newline so
    # the next line of EVIC text starts cleanly.
    if [[ "$b2$b3$b4$b5$b6$b7" == "000000000000" ]]; then
        echo ""
        continue
    fi

    # Decode bytes 2-7 as ASCII characters.
    printf "%b" "\x${b2}\x${b3}\x${b4}\x${b5}\x${b6}\x${b7}"
done