diff in oldish bash!

diff.sh

#!/bin/bash
#
# Implementation of https://en.wikipedia.org/wiki/Longest_common_subsequence_problem in old-ish BASH
# because my Android emulator doesn't have diff on it.
# Seriously bogs down on files >100 lines
# Could implement the common head and tail trimming efficiency.

XL=()
XC=()
while IFS= read -r line || [[ "$line" ]] ; do XL+=("$line") ; XC+=($(echo "$line" | cksum | tr ' ' -)) ; done < ${1?First filename required}
SXC=${#XC[@]}

YL=()
YC=()
while IFS= read -r line || [[ "$line" ]] ; do YL+=("$line") ; YC+=($(echo "$line" | cksum | tr ' ' -)); done < ${2?Second filename required}
SYC=${#YC[@]}

WW=$(($SYC + 1))

C=()

ix=0 ; while [ $ix -lt $SXC ] ; do
  C[$((ix + 0 * WW))]=$((0))
  ix=$((ix + 1))
done

iy=0 ; while [ $iy -lt $SYC ]; do
  C[$((0 + iy * WW))]=$((0))
  iy=$((iy + 1))
done

ix=1 ; while [ $ix -lt $SXC ] ; do
  iy=1 ; while [ $iy -lt $SYC ] ; do
    if [ ${XC[ix]} == ${YC[iy]} ] ; then
      d=$((${C[(ix - 1) + (iy - 1) * WW]} + 1))
    else
      d1=${C[ix + (iy - 1) * WW]} ; d1=${d1:--1}
      d2=${C[(ix - 1) + iy * WW]} ; d2=${d2:--1}
      [ $d1 -lt $d2 ] && d=$d2 || d=$d1
    fi
    C[$((ix + iy * WW))]=$((d))
    iy=$((iy + 1))
  done
  1>&2 echo -n .$((SXC - ix))
  ix=$((ix + 1))
done

echo

pdiff() {
  local ix=$(($1))
  local iy=$(($2))
  local up=${C[$((ix + (iy - 1) * WW))]} ; up=${up:--1}
  local left=${C[$((ix - 1 + iy * WW))]} ; left=${left:--1}
  if [ $ix -gt 0 ] && [ $iy -gt 0 ] && [ "${XC[$ix]}" = "${YC[$iy]}" ] ; then
    pdiff $((ix - 1)) $((iy - 1))
    echo "$ix=${XL[$ix]}"
  elif [ $iy -gt 0 ] && [ $ix -eq 0 -o $up -ge $left ]; then
    pdiff $ix $((iy - 1))
    echo "$iy>${YL[$iy]}"
  elif [ $ix -gt 0 ] && [ $iy -eq 0 -o $up -lt $left ]; then
    pdiff $((ix - 1)) $iy
    echo "$ix<${XL[$ix]}"
  fi
}

pdiff $SXC $SYC