#!/usr/bin/env bash
#
# @license Apache-2.0
#
# Copyright (c) 2017 The Stdlib Authors.
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#    http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.

# Verify the checksum of one or more files.
#
# Usage: checksum <filepath> <hash> <filepath> <hash> ...
#
# Arguments:
#
#   filepath           File to verify.
#   hash               Expected checksum.
#

# shellcheck disable=SC2181


# VARIABLES #

# Checksum type:
checksum_type='sha256'

# Program used to compute a checksum:
checksum_program=

# Command to compute a checksum:
cmd=


# FUNCTIONS #

# Prints usage information.
usage() {
	echo '' >&2
	echo 'Usage: checksum <filepath> <hash> <filepath> <hash> ...' >&2
	echo '' >&2
}

# Defines an error handler.
#
# $1 - error status
on_error() {
	echo 'ERROR: An error was encountered during execution.' >&2
	cleanup
	exit "$1"
}

# Runs clean-up tasks.
cleanup() {
	echo '' >&2
}

# Prints a success message.
print_success() {
	echo 'Success.' >&2
}

# Prints a checksum.
#
# $1 - checksum
print_checksum() {
	local num_lines
	local width
	local lines
	local str
	local len
	local pos

	width=64 # characters

	len="${#1}"
	if [[ "${len}" -gt "${width}" ]]; then
		str="$1"
		num_lines=$((len / width)) # floored
		lines=$(seq 0 1 "${num_lines}")      # total_lines = num_lines + 1
		for i in ${lines}; do
			pos=$((i * width))
			echo "    ${str:${pos}:${width}}"
		done
	else
		echo "    $1" >&2
	fi
}

# Prints a checksum error.
#
# $1 - file
# $2 - checksum type
# $3 - checksum program
# $4 - expected checksum
# $5 - actual checksum
print_checksum_error() {
	echo '' >&2
	echo "ERROR: $2 checksum failure for $1. Expected:" >&2
	echo '' >&2
	print_checksum "$4"
	echo '' >&2
	echo "But \`$3\` results in:" >&2
	echo '' >&2
	print_checksum "$5"
	echo '' >&2
	echo "This may be due to bad downloads or network proxies. Check your network proxy/" >&2
	echo "firewall settings and try downloading and verifying again." >&2
	echo '' >&2
}

# Verifies a checksum.
#
# $1 - file
# $2 - expected checksum
verify() {
	local curr_checksum

	curr_checksum=$(eval "${cmd} $1 | awk '{ print \$1; }'")
	if [[ "${curr_checksum}" != "$2" ]]; then
		print_checksum_error "$1" "${checksum_type}" "${checksum_program}" "$2" "${curr_checksum}"
		return 1
	fi
	return 0
}

# Main execution sequence.
main() {
	local args
	local len

	args=("$@")
	len="${#args[@]}"

	for (( i=0; i<${len}; i+=2 )) ; do
	    verify "${args[i]}" "${args[i+1]}"
	    if [[ "$?" -ne 0 ]]; then
			on_error 1
		fi
	done
	print_success
	cleanup
	exit 0
}


# MAIN #

# Handle arguments...
if [[ "$#" -eq 0 ]]; then
	usage
	exit 0
elif [[ $(($#%2)) -eq 1 ]]; then
	echo 'ERROR: invalid arguments. Each file to verify must be followed by an expected checksum.' >&2
	exit 1
fi

# Find a program for computing a SHA256 checksum...
if [[ -n "$(command -v sha256sum)" ]]; then
	checksum_program="sha256sum"
	cmd='sha256sum'
elif [[ -n "$(command -v shasum)" ]]; then
	checksum_program="shasum"
	cmd='shasum -a 256'
fi

# Run main:
main "$@"