#!/bin/sh -eu
#
# SPDX-FileCopyrightText: 2021-2023, 2025 Alyssa Ross <hi@alyssa.is>
# SPDX-FileCopyrightText: 2022 Unikie
# SPDX-License-Identifier: EUPL-1.2+
#
# usage: make-gpt.sh GPT_PATH PATH:PARTTYPE[:PARTUUID[:PARTLABEL[:PARTMiB]]]...

ONE_MiB=1048576

# Prints the number of 1MiB blocks required to store the file named
# $1.  We use 1MiB blocks because that's what sfdisk uses for
# alignment.  It would be possible to get a slightly smaller image
# using actual normal-sized 512-byte blocks, but it's probably not
# worth it to configure sfdisk to do that.
sizeMiB() {
	wc -c "$1" | awk -v ONE_MiB=$ONE_MiB \
		'{printf "%d\n", ($1 + ONE_MiB - 1) / ONE_MiB}'
}

# Copies from path $3 into partition number $2 in partition table $1.
fillPartition() {
	start="$(sfdisk -J "$1" | jq -r --argjson index "$2" \
		'.partitiontable.partitions[$index].start * 512')"

	# GNU cat will use copy_file_range(2) if possible, whereas dd
	# will always do a userspace copy, which is significantly slower.
	lseek -S 1 "$start" cat "$3" 1<>"$1"
}

scriptsDir="$(dirname "$0")"

out="$1"
shift

table=$(for partition; do
	awk -f "$scriptsDir/sfdisk-field.awk" \
		-v partition="$partition" \
		-v size="$(sizeMiB "${partition%%:*}")"
done)

# Keep 1MiB free at the start, and 1MiB free at the end.
gptMiB=2
while read -r partition; do
	# Here we rely on sfdisk-field.awk always putting size last.
	: $((gptMiB += ${partition##*=}))
done <<EOF
$table
EOF

rm -f "$out"
truncate -s "${gptMiB}M" "$out"

sfdisk --no-reread --no-tell-kernel "$out" <<EOF
label: gpt
sector-size: $ONE_MiB
$table
EOF

n=0
for partition; do
	fillPartition "$out" "$n" "${partition%%:*}"
	n="$((n + 1))"
done