#!/bin/sh

# public domain by Nathan Wagner <nw@hydaspes.if.org>

# Assume N bytes total.
# Assume L is the lowest number of files allowable.
# H is the highest number of files allowable.
# F is the actual number of files used
# B is the minimum bytes per file
# R is the remaining bytes if all files are of size B
# K is the maximum number of files allowed to be one byte larger than the
# minimum, K < F
# 
# So, you need to determine if there is some L <= F <= H such that R <= K.
# 
# For a given candidate F:
# B = floor(N / F)
# R = N % B
# if R <= K then the candidate F is allowable, F files will be used,
# R of them will be of size B+1 and F-R of them will be of size B.

N=
L=
H=
K=
ECHO=
VERBOSE=0

set -e

while getopts l:h:k:n:ev flag ; do
	case $flag in
		l) L="$OPTARG" ;;
		h) H="$OPTARG" ;;
		k) K="$OPTARG" ;;
		n) N="$OPTARG" ;;
		e) ECHO=echo ;;
		v) VERBOSE=1 ;;
		?) printf "Usage: evensplit [-lhknev] <file>\n" 1>&2; exit 2 ;;
	esac
done
shift $(($OPTIND - 1))

INPUT=$1

N=$(stat -c '%s' $INPUT)
: ${L:=$((N/65536))} # default to at least 65536 bytes per split
if [ $L -lt 1 ]; then L=1; fi

: ${H:=$N} # use one byte per file if needed
if [ $H -lt $L ]; then H=$L; fi

: ${K:=$((H-1))} # by default K can by any number of the files

status=1

if [ $VERBOSE -ne 0 ]; then
	echo '#' F files: $L '<= F <=' $H, at most $K files one byte larger 1>&2
fi

for F in $(seq $L $H); do
	B=$(($N / $F))
	R=$(($N % $B))
	if [ $R -le $K ]; then
		if [ $VERBOSE -ne 0 ]; then
			printf "# Usable: %u files, size %u" $((F-R)) $B 1>&2
			if [ $R -gt 0 ]; then
				printf ", %u size %u", $R $((B+1)) 1>&2
			fi
			printf "\n" 1>&2
		fi
		status=0;
		break;
	fi
done
if [ $? -ne 0 ]; then
	echo "no usable splits found" 1>&2;
	exit $status
fi

DD="dd if=$INPUT count=1"

# ok, at this point we want F-R files of size $B
for ext in $(seq -w 1 $F); do
	x=$(echo $ext | sed -e 's/^0\+//')
	if [ $x -gt $((F-R)) ]; then
	${ECHO} $DD of=${INPUT}.$ext bs=$((B+1)) skip=$(((F-R) * B)) iflag=skip_bytes
	else
	${ECHO} $DD of=${INPUT}.$ext bs=$B skip=$((x-1))
	fi
done