#!/usr/bin/env bash
# len2nl.sh — convert length-prefixed and/or MLLP-framed HL7 to newline-readable.
#
# Handles both common framing patterns:
#   1. Length-prefix:  `<digits>MSH|...`        →  strip leading digits before MSH
#   2. MLLP:           `\x0b...\x1c\x0d`        →  strip VT and FS+CR end-block
#   3. Segment CRs:    `<seg>\r<seg>\r...`     →  replace \r with \n
#
# After conversion, each segment is on its own line (LF), making the message
# greppable / pageable.
#
# Usage:
#   len2nl.sh [FILE]              # file or stdin
#   cat raw.hl7 | len2nl.sh
#   tcpdump ... | len2nl.sh
#
# This is a strict superset of the v1 `len2nl` behavior.
set -o pipefail

usage() { sed -n '2,18p' "$0"; exit 0; }
case "${1:-}" in -h|--help) usage ;; esac

INPUT="${1:-}"
if [ -n "$INPUT" ] && [ ! -f "$INPUT" ]; then
  echo "len2nl: no such file: $INPUT" >&2; exit 2
fi

if [ -n "$INPUT" ]; then
  CMD=(cat "$INPUT")
else
  CMD=(cat)
fi

"${CMD[@]}" \
  | tr -d '\013'              `# strip MLLP VT (0x0B = start-block)` \
  | sed 's/\x1c\x0d//g'        `# strip MLLP FS+CR (0x1C 0x0D = end-block)` \
  | sed -E 's/[0-9]+MSH\|/MSH|/g'  `# strip numeric length-prefix before MSH` \
  | tr '\r' '\n'               `# CR-separated segments → LF`