#!/bin/sh
#
# Build two documentation trees and diff the resulting formatted output.
# Compared to a source diff, this can reveal mistakes in the formatting.
# For example:
#
#   ./doc-diff origin/master HEAD
#
# would show the differences introduced by a branch based on master.

OPTIONS_SPEC="\
doc-diff [options] <from> <to> [-- <diff-options>]
--
j=n	parallel argument to pass to make
f	force rebuild; do not rely on cached results
"
SUBDIRECTORY_OK=1
. "$(git --exec-path)/git-sh-setup"

parallel=
force=
while test $# -gt 0
do
	case "$1" in
	-j)
		parallel=$2; shift ;;
	-f)
		force=t ;;
	--)
		shift; break ;;
	*)
		usage ;;
	esac
	shift
done

if test -z "$parallel"
then
	parallel=$(getconf _NPROCESSORS_ONLN 2>/dev/null)
	if test $? != 0 || test -z "$parallel"
	then
		parallel=1
	fi
fi

test $# -gt 1 || usage
from=$1; shift
to=$1; shift

from_oid=$(git rev-parse --verify "$from") || exit 1
to_oid=$(git rev-parse --verify "$to") || exit 1

cd_to_toplevel
tmp=Documentation/tmp-doc-diff

if test -n "$force"
then
	rm -rf "$tmp"
fi

# We'll do both builds in a single worktree, which lets "make" reuse
# results that don't differ between the two trees.
if ! test -d "$tmp/worktree"
then
	git worktree add --detach "$tmp/worktree" "$from" &&
	dots=$(echo "$tmp/worktree" | sed 's#[^/]*#..#g') &&
	ln -s "$dots/config.mak" "$tmp/worktree/config.mak"
fi

# generate_render_makefile <srcdir> <dstdir>
generate_render_makefile () {
	find "$1" -type f |
	while read src
	do
		dst=$2/${src#$1/}
		printf 'all:: %s\n' "$dst"
		printf '%s: %s\n' "$dst" "$src"
		printf '\t@echo >&2 "  RENDER $(notdir $@)" && \\\n'
		printf '\tmkdir -p $(dir $@) && \\\n'
		printf '\tMANWIDTH=80 man -l $< >$@+ && \\\n'
		printf '\tmv $@+ $@\n'
	done
}

# render_tree <committish_oid>
render_tree () {
	# Skip install-man entirely if we already have an installed directory.
	# We can't rely on make here, since "install-man" unconditionally
	# copies the files (spending effort, but also updating timestamps that
	# we then can't rely on during the render step). We use "mv" to make
	# sure we don't get confused by a previous run that failed partway
	# through.
	if ! test -d "$tmp/installed/$1"
	then
		git -C "$tmp/worktree" checkout --detach "$1" &&
		make -j$parallel -C "$tmp/worktree" \
			GIT_VERSION=omitted \
			SOURCE_DATE_EPOCH=0 \
			DESTDIR="$PWD/$tmp/installed/$1+" \
			install-man &&
		mv "$tmp/installed/$1+" "$tmp/installed/$1"
	fi &&

	# As with "installed" above, we skip the render if it's already been
	# done.  So using make here is primarily just about running in
	# parallel.
	if ! test -d "$tmp/rendered/$1"
	then
		generate_render_makefile "$tmp/installed/$1" "$tmp/rendered/$1+" |
		make -j$parallel -f - &&
		mv "$tmp/rendered/$1+" "$tmp/rendered/$1"
	fi
}

render_tree $from_oid &&
render_tree $to_oid &&
git -C $tmp/rendered diff --no-index "$@" $from_oid $to_oid