#!/bin/bash
# A simple helper tool to reflow the text from the standard input to a given width
#
# Usage: cat [file] | phlow.sh [width] [leading_spaces] [trailing_spaces]
#
# Created by Luxferre in 2023, released into public domain

TARGET_WIDTH="$1"
LSPACES="$2"
TSPACES="$3"

CRLF=$'\r\n'
SPC=$'\x20'

[[ -z "$TARGET_WIDTH" ]] && TARGET_WIDTH=67 # default page width
[[ -z "$LSPACES" ]] && LSPACES=0
[[ -z "$TSPACES" ]] && TSPACES=0

fmtstr="%-$(( LSPACES ))s%-${TARGET_WIDTH}s%-$(( TSPACES ))s${CRLF}" # params: smth, line, smth

while read -rs line; do # fully line-based operation
  line="${line%%$'\r'}" # remove a trailing CR if it is there
  llen="${#line}" # get effective line length
  if (( llen < TARGET_WIDTH )); then # no need to run the logic for smaller lines
    printf "$fmtstr" '' "$line" ''
    continue
  fi
  lastws=0 # variable to track last whitespace
  cpos=0 # variable to track current position within the page line
  pagepos=0 # variable to track the position of new line start
  outbuf='' # temporary output buffer
  for ((i=0;i<llen;i++,cpos++)); do # start iterating over characters
    c="${line:i:1}" # get the current one
    if (( cpos >= TARGET_WIDTH )); then # we already exceeded the page width
      (( lastws == 0 )) && lastws=$TARGET_WIDTH # no whitespace encountered here
      printf "$fmtstr" '' "${outbuf:0:$lastws}" '' # truncate the buffer
      outbuf=''
      pagepos=$(( pagepos + lastws ))
      cpos=0
      lastws=0
      i=$pagepos # update current iteration index from the last valid whitespace
    else # save the whitespace position if found
      [[ "$c" == "$SPC" ]] && lastws="$cpos"
      outbuf="${outbuf}${c}" # save the character itself
    fi
  done
  [[ ! -z "$outbuf" ]] && printf "$fmtstr" '' "$outbuf" '' # output the last unprocessed chunk
done