From e12bbf00c1b42de724bb7ee4acf4079919536c00 Mon Sep 17 00:00:00 2001
From: Johannes Schindelin <johannes.schindelin@gmx.de>
Date: Wed, 13 Apr 2016 17:43:05 +0200
Subject: [PATCH 1/9] rebase -i: generate the script via rebase--helper

The first step of an interactive rebase is to generate the so-called "todo
script", to be stored in the state directory as "git-rebase-todo" and to
be edited by the user.

Originally, we adjusted the output of `git log <options>` using a simple
sed script. Over the course of the years, the code became more
complicated. We now use shell scripting to edit the output of `git log`
conditionally, depending whether to keep "empty" commits (i.e. commits
that do not change any files).

On platforms where shell scripting is not native, this can be a serious
drag. And it opens the door for incompatibilities between platforms when
it comes to shell scripting or to Unix-y commands.

Let's just re-implement the todo script generation in plain C, using the
revision machinery directly.

This is substantially faster, improving the speed relative to the
shell script version of the interactive rebase from 2x to 3x on Windows.

Note that the rearrange_squash() function in git-rebase--interactive
relied on the fact that we set the "format" variable to the config setting
rebase.instructionFormat. Relying on a side effect like this is no good,
hence we explicitly perform that assignment (possibly again) in
rearrange_squash().

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
---
 builtin/rebase--helper.c   |  8 ++++++-
 git-rebase--interactive.sh | 42 +++++++++++++++++++-----------------
 sequencer.c                | 44 ++++++++++++++++++++++++++++++++++++++
 sequencer.h                |  3 +++
 4 files changed, 76 insertions(+), 21 deletions(-)

diff --git a/builtin/rebase--helper.c b/builtin/rebase--helper.c
index ca1ebb2fa1..821058d452 100644
--- a/builtin/rebase--helper.c
+++ b/builtin/rebase--helper.c
@@ -11,15 +11,19 @@ static const char * const builtin_rebase_helper_usage[] = {
 int cmd_rebase__helper(int argc, const char **argv, const char *prefix)
 {
 	struct replay_opts opts = REPLAY_OPTS_INIT;
+	int keep_empty = 0;
 	enum {
-		CONTINUE = 1, ABORT
+		CONTINUE = 1, ABORT, MAKE_SCRIPT
 	} command = 0;
 	struct option options[] = {
 		OPT_BOOL(0, "ff", &opts.allow_ff, N_("allow fast-forward")),
+		OPT_BOOL(0, "keep-empty", &keep_empty, N_("keep empty commits")),
 		OPT_CMDMODE(0, "continue", &command, N_("continue rebase"),
 				CONTINUE),
 		OPT_CMDMODE(0, "abort", &command, N_("abort rebase"),
 				ABORT),
+		OPT_CMDMODE(0, "make-script", &command,
+			N_("make rebase script"), MAKE_SCRIPT),
 		OPT_END()
 	};
 
@@ -36,5 +40,7 @@ int cmd_rebase__helper(int argc, const char **argv, const char *prefix)
 		return !!sequencer_continue(&opts);
 	if (command == ABORT && argc == 1)
 		return !!sequencer_remove_state(&opts);
+	if (command == MAKE_SCRIPT && argc > 1)
+		return !!sequencer_make_script(keep_empty, stdout, argc, argv);
 	usage_with_options(builtin_rebase_helper_usage, options);
 }
diff --git a/git-rebase--interactive.sh b/git-rebase--interactive.sh
index 2c9c0165b5..609e150d38 100644
--- a/git-rebase--interactive.sh
+++ b/git-rebase--interactive.sh
@@ -785,6 +785,7 @@ collapse_todo_ids() {
 # each log message will be re-retrieved in order to normalize the
 # autosquash arrangement
 rearrange_squash () {
+	format=$(git config --get rebase.instructionFormat)
 	# extract fixup!/squash! lines and resolve any referenced sha1's
 	while read -r pick sha1 message
 	do
@@ -1210,26 +1211,27 @@ else
 	revisions=$onto...$orig_head
 	shortrevisions=$shorthead
 fi
-format=$(git config --get rebase.instructionFormat)
-# the 'rev-list .. | sed' requires %m to parse; the instruction requires %H to parse
-git rev-list $merges_option --format="%m%H ${format:-%s}" \
-	--reverse --left-right --topo-order \
-	$revisions ${restrict_revision+^$restrict_revision} | \
-	sed -n "s/^>//p" |
-while read -r sha1 rest
-do
+if test t != "$preserve_merges"
+then
+	git rebase--helper --make-script ${keep_empty:+--keep-empty} \
+		$revisions ${restrict_revision+^$restrict_revision} >"$todo"
+else
+	format=$(git config --get rebase.instructionFormat)
+	# the 'rev-list .. | sed' requires %m to parse; the instruction requires %H to parse
+	git rev-list $merges_option --format="%m%H ${format:-%s}" \
+		--reverse --left-right --topo-order \
+		$revisions ${restrict_revision+^$restrict_revision} | \
+		sed -n "s/^>//p" |
+	while read -r sha1 rest
+	do
 
-	if test -z "$keep_empty" && is_empty_commit $sha1 && ! is_merge_commit $sha1
-	then
-		comment_out="$comment_char "
-	else
-		comment_out=
-	fi
+		if test -z "$keep_empty" && is_empty_commit $sha1 && ! is_merge_commit $sha1
+		then
+			comment_out="$comment_char "
+		else
+			comment_out=
+		fi
 
-	if test t != "$preserve_merges"
-	then
-		printf '%s\n' "${comment_out}pick $sha1 $rest" >>"$todo"
-	else
 		if test -z "$rebase_root"
 		then
 			preserve=t
@@ -1248,8 +1250,8 @@ do
 			touch "$rewritten"/$sha1
 			printf '%s\n' "${comment_out}pick $sha1 $rest" >>"$todo"
 		fi
-	fi
-done
+	done
+fi
 
 # Watch for commits that been dropped by --cherry-pick
 if test t = "$preserve_merges"
diff --git a/sequencer.c b/sequencer.c
index 1f729b053b..46a72ed05a 100644
--- a/sequencer.c
+++ b/sequencer.c
@@ -2379,3 +2379,47 @@ void append_signoff(struct strbuf *msgbuf, int ignore_footer, unsigned flag)
 
 	strbuf_release(&sob);
 }
+
+int sequencer_make_script(int keep_empty, FILE *out,
+		int argc, const char **argv)
+{
+	char *format = "%s";
+	struct pretty_print_context pp = {0};
+	struct strbuf buf = STRBUF_INIT;
+	struct rev_info revs;
+	struct commit *commit;
+
+	init_revisions(&revs, NULL);
+	revs.verbose_header = 1;
+	revs.max_parents = 1;
+	revs.cherry_pick = 1;
+	revs.limited = 1;
+	revs.reverse = 1;
+	revs.right_only = 1;
+	revs.sort_order = REV_SORT_IN_GRAPH_ORDER;
+	revs.topo_order = 1;
+
+	revs.pretty_given = 1;
+	git_config_get_string("rebase.instructionFormat", &format);
+	get_commit_format(format, &revs);
+	pp.fmt = revs.commit_format;
+	pp.output_encoding = get_log_output_encoding();
+
+	if (setup_revisions(argc, argv, &revs, NULL) > 1)
+		return error(_("make_script: unhandled options"));
+
+	if (prepare_revision_walk(&revs) < 0)
+		return error(_("make_script: error preparing revisions"));
+
+	while ((commit = get_revision(&revs))) {
+		strbuf_reset(&buf);
+		if (!keep_empty && is_original_commit_empty(commit))
+			strbuf_addf(&buf, "%c ", comment_line_char);
+		strbuf_addf(&buf, "pick %s ", oid_to_hex(&commit->object.oid));
+		pretty_print_commit(&pp, commit, &buf);
+		strbuf_addch(&buf, '\n');
+		fputs(buf.buf, out);
+	}
+	strbuf_release(&buf);
+	return 0;
+}
diff --git a/sequencer.h b/sequencer.h
index f885b68395..83f2943b7a 100644
--- a/sequencer.h
+++ b/sequencer.h
@@ -45,6 +45,9 @@ int sequencer_continue(struct replay_opts *opts);
 int sequencer_rollback(struct replay_opts *opts);
 int sequencer_remove_state(struct replay_opts *opts);
 
+int sequencer_make_script(int keep_empty, FILE *out,
+		int argc, const char **argv);
+
 extern const char sign_off_header[];
 
 void append_signoff(struct strbuf *msgbuf, int ignore_footer, unsigned flag);

From 92ff6ffe33a70d4f51c101d8a73127d864a77a2c Mon Sep 17 00:00:00 2001
From: Johannes Schindelin <johannes.schindelin@gmx.de>
Date: Fri, 15 Apr 2016 10:00:39 +0200
Subject: [PATCH 2/9] rebase -i: remove useless indentation

The commands used to be indented, and it is nice to look at, but when we
transform the SHA-1s, the indentation is removed. So let's do away with it.

For the moment, at least: when we will use the upcoming rebase--helper
to transform the SHA-1s, we *will* keep the indentation and can
reintroduce it. Yet, to be able to validate the rebase--helper against
the output of the current shell script version, we need to remove the
extra indentation.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
---
 git-rebase--interactive.sh | 14 +++++++-------
 1 file changed, 7 insertions(+), 7 deletions(-)

diff --git a/git-rebase--interactive.sh b/git-rebase--interactive.sh
index 609e150d38..c40b1fd1d2 100644
--- a/git-rebase--interactive.sh
+++ b/git-rebase--interactive.sh
@@ -155,13 +155,13 @@ reschedule_last_action () {
 append_todo_help () {
 	gettext "
 Commands:
- p, pick = use commit
- r, reword = use commit, but edit the commit message
- e, edit = use commit, but stop for amending
- s, squash = use commit, but meld into previous commit
- f, fixup = like \"squash\", but discard this commit's log message
- x, exec = run command (the rest of the line) using shell
- d, drop = remove commit
+p, pick = use commit
+r, reword = use commit, but edit the commit message
+e, edit = use commit, but stop for amending
+s, squash = use commit, but meld into previous commit
+f, fixup = like \"squash\", but discard this commit's log message
+x, exec = run command (the rest of the line) using shell
+d, drop = remove commit
 
 These lines can be re-ordered; they are executed from top to bottom.
 " | git stripspace --comment-lines >>"$todo"

From 8304be88bdb5dd43ef75dec6708466f66376f1ac Mon Sep 17 00:00:00 2001
From: Johannes Schindelin <johannes.schindelin@gmx.de>
Date: Sun, 17 Apr 2016 08:48:44 +0200
Subject: [PATCH 3/9] rebase -i: do not invent onelines when
 expanding/collapsing SHA-1s

To avoid problems with short SHA-1s that become non-unique during the
rebase, we rewrite the todo script with short/long SHA-1s before and
after letting the user edit the script. Since SHA-1s are not intuitive
for humans, rebase -i also provides the onelines (commit message
subjects) in the script, purely for the user's convenience.

It is very possible to generate a todo script via different means than
rebase -i and then to let rebase -i run with it; In this case, these
onelines are not required.

And this is where the expand/collapse machinery has a bug: it *expects*
that oneline, and failing to find one reuses the previous SHA-1 as
"oneline".

It was most likely an oversight, and made implementation in the (quite
limiting) shell script language less convoluted. However, we are about
to reimplement performance-critical parts in C (and due to spawning a
git.exe process for every single line of the todo script, the
expansion/collapsing of the SHA-1s *is* performance-hampering on
Windows), therefore let's fix this bug to make cross-validation with the
C version of that functionality possible.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
---
 git-rebase--interactive.sh | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/git-rebase--interactive.sh b/git-rebase--interactive.sh
index c40b1fd1d2..214af0372b 100644
--- a/git-rebase--interactive.sh
+++ b/git-rebase--interactive.sh
@@ -760,7 +760,12 @@ transform_todo_ids () {
 			;;
 		*)
 			sha1=$(git rev-parse --verify --quiet "$@" ${rest%%[	 ]*}) &&
-			rest="$sha1 ${rest#*[	 ]}"
+			if test "a$rest" = "a${rest#*[	 ]}"
+			then
+				rest=$sha1
+			else
+				rest="$sha1 ${rest#*[	 ]}"
+			fi
 			;;
 		esac
 		printf '%s\n' "$command${rest:+ }$rest"

From c3d92bf1e96f89b2897baac887f9b4eff2836e9c Mon Sep 17 00:00:00 2001
From: Johannes Schindelin <johannes.schindelin@gmx.de>
Date: Wed, 13 Apr 2016 18:17:19 +0200
Subject: [PATCH 4/9] rebase -i: also expand/collapse the SHA-1s via the
 rebase--helper

This is crucial to improve performance on Windows, as the speed is now
mostly dominated by the SHA-1 transformation (because it spawns a new
rev-parse process for *every* line, and spawning processes is pretty
slow from Git for Windows' MSYS2 Bash).

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
---
 builtin/rebase--helper.c   | 10 ++++++-
 git-rebase--interactive.sh |  4 +--
 sequencer.c                | 59 ++++++++++++++++++++++++++++++++++++++
 sequencer.h                |  2 ++
 4 files changed, 72 insertions(+), 3 deletions(-)

diff --git a/builtin/rebase--helper.c b/builtin/rebase--helper.c
index 821058d452..9444c8d6c6 100644
--- a/builtin/rebase--helper.c
+++ b/builtin/rebase--helper.c
@@ -13,7 +13,7 @@ int cmd_rebase__helper(int argc, const char **argv, const char *prefix)
 	struct replay_opts opts = REPLAY_OPTS_INIT;
 	int keep_empty = 0;
 	enum {
-		CONTINUE = 1, ABORT, MAKE_SCRIPT
+		CONTINUE = 1, ABORT, MAKE_SCRIPT, SHORTEN_SHA1S, EXPAND_SHA1S
 	} command = 0;
 	struct option options[] = {
 		OPT_BOOL(0, "ff", &opts.allow_ff, N_("allow fast-forward")),
@@ -24,6 +24,10 @@ int cmd_rebase__helper(int argc, const char **argv, const char *prefix)
 				ABORT),
 		OPT_CMDMODE(0, "make-script", &command,
 			N_("make rebase script"), MAKE_SCRIPT),
+		OPT_CMDMODE(0, "shorten-sha1s", &command,
+			N_("shorten SHA-1s in the todo list"), SHORTEN_SHA1S),
+		OPT_CMDMODE(0, "expand-sha1s", &command,
+			N_("expand SHA-1s in the todo list"), EXPAND_SHA1S),
 		OPT_END()
 	};
 
@@ -42,5 +46,9 @@ int cmd_rebase__helper(int argc, const char **argv, const char *prefix)
 		return !!sequencer_remove_state(&opts);
 	if (command == MAKE_SCRIPT && argc > 1)
 		return !!sequencer_make_script(keep_empty, stdout, argc, argv);
+	if (command == SHORTEN_SHA1S && argc == 1)
+		return !!transform_todo_ids(1);
+	if (command == EXPAND_SHA1S && argc == 1)
+		return !!transform_todo_ids(0);
 	usage_with_options(builtin_rebase_helper_usage, options);
 }
diff --git a/git-rebase--interactive.sh b/git-rebase--interactive.sh
index 214af0372b..52a19e0bdb 100644
--- a/git-rebase--interactive.sh
+++ b/git-rebase--interactive.sh
@@ -774,11 +774,11 @@ transform_todo_ids () {
 }
 
 expand_todo_ids() {
-	transform_todo_ids
+	git rebase--helper --expand-sha1s
 }
 
 collapse_todo_ids() {
-	transform_todo_ids --short
+	git rebase--helper --shorten-sha1s
 }
 
 # Rearrange the todo list that has both "pick sha1 msg" and
diff --git a/sequencer.c b/sequencer.c
index 46a72ed05a..695572c41e 100644
--- a/sequencer.c
+++ b/sequencer.c
@@ -2423,3 +2423,62 @@ int sequencer_make_script(int keep_empty, FILE *out,
 	strbuf_release(&buf);
 	return 0;
 }
+
+
+int transform_todo_ids(int shorten_sha1s)
+{
+	const char *todo_file = rebase_path_todo();
+	struct todo_list todo_list = TODO_LIST_INIT;
+	int fd, res, i;
+	FILE *out;
+
+	strbuf_reset(&todo_list.buf);
+	fd = open(todo_file, O_RDONLY);
+	if (fd < 0)
+		return error_errno(_("could not open '%s'"), todo_file);
+	if (strbuf_read(&todo_list.buf, fd, 0) < 0) {
+		close(fd);
+		return error(_("could not read '%s'."), todo_file);
+	}
+	close(fd);
+
+	res = parse_insn_buffer(todo_list.buf.buf, &todo_list);
+	if (res) {
+		todo_list_release(&todo_list);
+		return error(_("unusable instruction sheet: '%s'"), todo_file);
+	}
+
+	out = fopen(todo_file, "w");
+	if (!out) {
+		todo_list_release(&todo_list);
+		return error(_("unable to open '%s' for writing"), todo_file);
+	}
+	for (i = 0; i < todo_list.nr; i++) {
+		struct todo_item *item = todo_list.items + i;
+		int bol = item->offset_in_buf;
+		const char *p = todo_list.buf.buf + bol;
+		int eol = i + 1 < todo_list.nr ?
+			todo_list.items[i + 1].offset_in_buf :
+			todo_list.buf.len;
+
+		if (item->command >= TODO_EXEC && item->command != TODO_DROP)
+			fwrite(p, eol - bol, 1, out);
+		else {
+			int eoc = strcspn(p, " \t");
+			const char *sha1 = shorten_sha1s ?
+				short_commit_name(item->commit) :
+				oid_to_hex(&item->commit->object.oid);
+
+			if (!eoc) {
+				p += strspn(p, " \t");
+				eoc = strcspn(p, " \t");
+			}
+
+			fprintf(out, "%.*s %s %.*s\n",
+				eoc, p, sha1, item->arg_len, item->arg);
+		}
+	}
+	fclose(out);
+	todo_list_release(&todo_list);
+	return 0;
+}
diff --git a/sequencer.h b/sequencer.h
index 83f2943b7a..47a81034e7 100644
--- a/sequencer.h
+++ b/sequencer.h
@@ -48,6 +48,8 @@ int sequencer_remove_state(struct replay_opts *opts);
 int sequencer_make_script(int keep_empty, FILE *out,
 		int argc, const char **argv);
 
+int transform_todo_ids(int shorten_sha1s);
+
 extern const char sign_off_header[];
 
 void append_signoff(struct strbuf *msgbuf, int ignore_footer, unsigned flag);

From 1855fb963656701d89c79390b42063f8b49ddcd1 Mon Sep 17 00:00:00 2001
From: Johannes Schindelin <johannes.schindelin@gmx.de>
Date: Tue, 14 Jun 2016 16:54:43 +0200
Subject: [PATCH 5/9] t3404: relax rebase.missingCommitsCheck tests

These tests were a bit anal about the *exact* warning/error message
printed by git rebase. But those messages are intended for the *end
user*, therefore it does not make sense to test so rigidly for the
*exact* wording.

In the following, we will reimplement the missing commits check in
the sequencer, with slightly different words.

So let's just test for the parts in the warning/error message that
we *really* care about, nothing more, nothing less.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
---
 t/t3404-rebase-interactive.sh | 22 ++++------------------
 1 file changed, 4 insertions(+), 18 deletions(-)

diff --git a/t/t3404-rebase-interactive.sh b/t/t3404-rebase-interactive.sh
index 33d392ba11..61113be08a 100755
--- a/t/t3404-rebase-interactive.sh
+++ b/t/t3404-rebase-interactive.sh
@@ -1242,20 +1242,13 @@ test_expect_success 'rebase -i respects rebase.missingCommitsCheck = error' '
 	test B = $(git cat-file commit HEAD^ | sed -ne \$p)
 '
 
-cat >expect <<EOF
-Warning: the command isn't recognized in the following line:
- - badcmd $(git rev-list --oneline -1 master~1)
-
-You can fix this with 'git rebase --edit-todo' and then run 'git rebase --continue'.
-Or you can abort the rebase with 'git rebase --abort'.
-EOF
-
 test_expect_success 'static check of bad command' '
 	rebase_setup_and_clean bad-cmd &&
 	set_fake_editor &&
 	test_must_fail env FAKE_LINES="1 2 3 bad 4 5" \
 		git rebase -i --root 2>actual &&
-	test_i18ncmp expect actual &&
+	test_i18ngrep "badcmd $(git rev-list --oneline -1 master~1)" actual &&
+	test_i18ngrep "You can fix this with .git rebase --edit-todo.." actual &&
 	FAKE_LINES="1 2 3 drop 4 5" git rebase --edit-todo &&
 	git rebase --continue &&
 	test E = $(git cat-file commit HEAD | sed -ne \$p) &&
@@ -1277,20 +1270,13 @@ test_expect_success 'tabs and spaces are accepted in the todolist' '
 	test E = $(git cat-file commit HEAD | sed -ne \$p)
 '
 
-cat >expect <<EOF
-Warning: the SHA-1 is missing or isn't a commit in the following line:
- - edit XXXXXXX False commit
-
-You can fix this with 'git rebase --edit-todo' and then run 'git rebase --continue'.
-Or you can abort the rebase with 'git rebase --abort'.
-EOF
-
 test_expect_success 'static check of bad SHA-1' '
 	rebase_setup_and_clean bad-sha &&
 	set_fake_editor &&
 	test_must_fail env FAKE_LINES="1 2 edit fakesha 3 4 5 #" \
 		git rebase -i --root 2>actual &&
-	test_i18ncmp expect actual &&
+	test_i18ngrep "edit XXXXXXX False commit" actual &&
+	test_i18ngrep "You can fix this with .git rebase --edit-todo.." actual &&
 	FAKE_LINES="1 2 4 5 6" git rebase --edit-todo &&
 	git rebase --continue &&
 	test E = $(git cat-file commit HEAD | sed -ne \$p)

From e7024fbe7cef9ac5c3f2629715822c462ef8cc90 Mon Sep 17 00:00:00 2001
From: Johannes Schindelin <johannes.schindelin@gmx.de>
Date: Tue, 14 Jun 2016 16:58:59 +0200
Subject: [PATCH 6/9] rebase -i: check for missing commits in the
 rebase--helper

In particular on Windows, where shell scripts are even more expensive
than on MacOSX or Linux, it makes sense to move a loop that forks
Git at least once for every line in the todo list into a builtin.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
---
 builtin/rebase--helper.c   |   7 +-
 git-rebase--interactive.sh | 164 ++-----------------------------------
 sequencer.c                | 125 ++++++++++++++++++++++++++++
 sequencer.h                |   1 +
 4 files changed, 137 insertions(+), 160 deletions(-)

diff --git a/builtin/rebase--helper.c b/builtin/rebase--helper.c
index 9444c8d6c6..e706eac710 100644
--- a/builtin/rebase--helper.c
+++ b/builtin/rebase--helper.c
@@ -13,7 +13,8 @@ int cmd_rebase__helper(int argc, const char **argv, const char *prefix)
 	struct replay_opts opts = REPLAY_OPTS_INIT;
 	int keep_empty = 0;
 	enum {
-		CONTINUE = 1, ABORT, MAKE_SCRIPT, SHORTEN_SHA1S, EXPAND_SHA1S
+		CONTINUE = 1, ABORT, MAKE_SCRIPT, SHORTEN_SHA1S, EXPAND_SHA1S,
+		CHECK_TODO_LIST
 	} command = 0;
 	struct option options[] = {
 		OPT_BOOL(0, "ff", &opts.allow_ff, N_("allow fast-forward")),
@@ -28,6 +29,8 @@ int cmd_rebase__helper(int argc, const char **argv, const char *prefix)
 			N_("shorten SHA-1s in the todo list"), SHORTEN_SHA1S),
 		OPT_CMDMODE(0, "expand-sha1s", &command,
 			N_("expand SHA-1s in the todo list"), EXPAND_SHA1S),
+		OPT_CMDMODE(0, "check-todo-list", &command,
+			N_("check the todo list"), CHECK_TODO_LIST),
 		OPT_END()
 	};
 
@@ -50,5 +53,7 @@ int cmd_rebase__helper(int argc, const char **argv, const char *prefix)
 		return !!transform_todo_ids(1);
 	if (command == EXPAND_SHA1S && argc == 1)
 		return !!transform_todo_ids(0);
+	if (command == CHECK_TODO_LIST && argc == 1)
+		return !!check_todo_list();
 	usage_with_options(builtin_rebase_helper_usage, options);
 }
diff --git a/git-rebase--interactive.sh b/git-rebase--interactive.sh
index 52a19e0bdb..1649506e1e 100644
--- a/git-rebase--interactive.sh
+++ b/git-rebase--interactive.sh
@@ -890,96 +890,6 @@ add_exec_commands () {
 	mv "$1.new" "$1"
 }
 
-# Check if the SHA-1 passed as an argument is a
-# correct one, if not then print $2 in "$todo".badsha
-# $1: the SHA-1 to test
-# $2: the line number of the input
-# $3: the input filename
-check_commit_sha () {
-	badsha=0
-	if test -z "$1"
-	then
-		badsha=1
-	else
-		sha1_verif="$(git rev-parse --verify --quiet $1^{commit})"
-		if test -z "$sha1_verif"
-		then
-			badsha=1
-		fi
-	fi
-
-	if test $badsha -ne 0
-	then
-		line="$(sed -n -e "${2}p" "$3")"
-		warn "$(eval_gettext "\
-Warning: the SHA-1 is missing or isn't a commit in the following line:
- - \$line")"
-		warn
-	fi
-
-	return $badsha
-}
-
-# prints the bad commits and bad commands
-# from the todolist in stdin
-check_bad_cmd_and_sha () {
-	retval=0
-	lineno=0
-	while read -r command rest
-	do
-		lineno=$(( $lineno + 1 ))
-		case $command in
-		"$comment_char"*|''|noop|x|exec)
-			# Doesn't expect a SHA-1
-			;;
-		"$cr")
-			# Work around CR left by "read" (e.g. with Git for
-			# Windows' Bash).
-			;;
-		pick|p|drop|d|reword|r|edit|e|squash|s|fixup|f)
-			if ! check_commit_sha "${rest%%[ 	]*}" "$lineno" "$1"
-			then
-				retval=1
-			fi
-			;;
-		*)
-			line="$(sed -n -e "${lineno}p" "$1")"
-			warn "$(eval_gettext "\
-Warning: the command isn't recognized in the following line:
- - \$line")"
-			warn
-			retval=1
-			;;
-		esac
-	done <"$1"
-	return $retval
-}
-
-# Print the list of the SHA-1 of the commits
-# from stdin to stdout
-todo_list_to_sha_list () {
-	git stripspace --strip-comments |
-	while read -r command sha1 rest
-	do
-		case $command in
-		"$comment_char"*|''|noop|x|"exec")
-			;;
-		*)
-			long_sha=$(git rev-list --no-walk "$sha1" 2>/dev/null)
-			printf "%s\n" "$long_sha"
-			;;
-		esac
-	done
-}
-
-# Use warn for each line in stdin
-warn_lines () {
-	while read -r line
-	do
-		warn " - $line"
-	done
-}
-
 # Switch to the branch in $into and notify it in the reflog
 checkout_onto () {
 	GIT_REFLOG_ACTION="$GIT_REFLOG_ACTION: checkout $onto_name"
@@ -994,74 +904,6 @@ get_missing_commit_check_level () {
 	printf '%s' "$check_level" | tr 'A-Z' 'a-z'
 }
 
-# Check if the user dropped some commits by mistake
-# Behaviour determined by rebase.missingCommitsCheck.
-# Check if there is an unrecognized command or a
-# bad SHA-1 in a command.
-check_todo_list () {
-	raise_error=f
-
-	check_level=$(get_missing_commit_check_level)
-
-	case "$check_level" in
-	warn|error)
-		# Get the SHA-1 of the commits
-		todo_list_to_sha_list <"$todo".backup >"$todo".oldsha1
-		todo_list_to_sha_list <"$todo" >"$todo".newsha1
-
-		# Sort the SHA-1 and compare them
-		sort -u "$todo".oldsha1 >"$todo".oldsha1+
-		mv "$todo".oldsha1+ "$todo".oldsha1
-		sort -u "$todo".newsha1 >"$todo".newsha1+
-		mv "$todo".newsha1+ "$todo".newsha1
-		comm -2 -3 "$todo".oldsha1 "$todo".newsha1 >"$todo".miss
-
-		# Warn about missing commits
-		if test -s "$todo".miss
-		then
-			test "$check_level" = error && raise_error=t
-
-			warn "$(gettext "\
-Warning: some commits may have been dropped accidentally.
-Dropped commits (newer to older):")"
-
-			# Make the list user-friendly and display
-			opt="--no-walk=sorted --format=oneline --abbrev-commit --stdin"
-			git rev-list $opt <"$todo".miss | warn_lines
-
-			warn "$(gettext "\
-To avoid this message, use \"drop\" to explicitly remove a commit.
-
-Use 'git config rebase.missingCommitsCheck' to change the level of warnings.
-The possible behaviours are: ignore, warn, error.")"
-			warn
-		fi
-		;;
-	ignore)
-		;;
-	*)
-		warn "$(eval_gettext "Unrecognized setting \$check_level for option rebase.missingCommitsCheck. Ignoring.")"
-		;;
-	esac
-
-	if ! check_bad_cmd_and_sha "$todo"
-	then
-		raise_error=t
-	fi
-
-	if test $raise_error = t
-	then
-		# Checkout before the first commit of the
-		# rebase: this way git rebase --continue
-		# will work correctly as it expects HEAD to be
-		# placed before the commit of the next action
-		checkout_onto
-
-		warn "$(gettext "You can fix this with 'git rebase --edit-todo' and then run 'git rebase --continue'.")"
-		die "$(gettext "Or you can abort the rebase with 'git rebase --abort'.")"
-	fi
-}
-
 # The whole contents of this file is run by dot-sourcing it from
 # inside a shell function.  It used to be that "return"s we see
 # below were not inside any function, and expected to return
@@ -1322,7 +1164,11 @@ git_sequence_editor "$todo" ||
 has_action "$todo" ||
 	return 2
 
-check_todo_list
+git rebase--helper --check-todo-list || {
+	ret=$?
+	checkout_onto
+	exit $ret
+}
 
 expand_todo_ids
 
diff --git a/sequencer.c b/sequencer.c
index 695572c41e..d1fb9ebd03 100644
--- a/sequencer.c
+++ b/sequencer.c
@@ -2482,3 +2482,128 @@ int transform_todo_ids(int shorten_sha1s)
 	todo_list_release(&todo_list);
 	return 0;
 }
+
+enum check_level {
+	CHECK_IGNORE = 0, CHECK_WARN, CHECK_ERROR
+};
+
+static enum check_level get_missing_commit_check_level(void)
+{
+	const char *value;
+
+	if (git_config_get_value("rebase.missingcommitscheck", &value) ||
+			!strcasecmp("ignore", value))
+		return CHECK_IGNORE;
+	if (!strcasecmp("warn", value))
+		return CHECK_WARN;
+	if (!strcasecmp("error", value))
+		return CHECK_ERROR;
+	warning(_("unrecognized setting %s for option"
+		  "rebase.missingCommitsCheck. Ignoring."), value);
+	return CHECK_IGNORE;
+}
+
+/*
+ * Check if the user dropped some commits by mistake
+ * Behaviour determined by rebase.missingCommitsCheck.
+ * Check if there is an unrecognized command or a
+ * bad SHA-1 in a command.
+ */
+int check_todo_list(void)
+{
+	enum check_level check_level = get_missing_commit_check_level();
+	struct strbuf todo_file = STRBUF_INIT;
+	struct todo_list todo_list = TODO_LIST_INIT;
+	struct commit_list *missing = NULL;
+	int raise_error = 0, res = 0, fd, i;
+
+	strbuf_addstr(&todo_file, rebase_path_todo());
+	fd = open(todo_file.buf, O_RDONLY);
+	if (fd < 0) {
+		res = error_errno(_("could not open '%s'"), todo_file.buf);
+		goto leave_check;
+	}
+	if (strbuf_read(&todo_list.buf, fd, 0) < 0) {
+		close(fd);
+		res = error(_("could not read '%s'."), todo_file.buf);
+		goto leave_check;
+	}
+	close(fd);
+	raise_error = res =
+		parse_insn_buffer(todo_list.buf.buf, &todo_list);
+
+	if (check_level == CHECK_IGNORE)
+		goto leave_check;
+
+	/* Get the SHA-1 of the commits */
+	for (i = 0; i < todo_list.nr; i++) {
+		struct commit *commit = todo_list.items[i].commit;
+		if (commit)
+			commit->util = todo_list.items + i;
+	}
+
+	todo_list_release(&todo_list);
+	strbuf_addstr(&todo_file, ".backup");
+	fd = open(todo_file.buf, O_RDONLY);
+	if (fd < 0) {
+		res = error_errno(_("could not open '%s'"), todo_file.buf);
+		goto leave_check;
+	}
+	if (strbuf_read(&todo_list.buf, fd, 0) < 0) {
+		close(fd);
+		res = error(_("could not read '%s'."), todo_file.buf);
+		goto leave_check;
+	}
+	close(fd);
+	strbuf_release(&todo_file);
+	res = !!parse_insn_buffer(todo_list.buf.buf, &todo_list);
+
+	/* Find commits that are missing after editing */
+	for (i = 0; i < todo_list.nr; i++) {
+		struct commit *commit = todo_list.items[i].commit;
+		if (commit && !commit->util) {
+			commit_list_insert(commit, &missing);
+			commit->util = todo_list.items + i;
+		}
+	}
+
+	/* Warn about missing commits */
+	if (!missing)
+		goto leave_check;
+
+	if (check_level == CHECK_ERROR)
+		raise_error = res = 1;
+
+	fprintf(stderr,
+		_("Warning: some commits may have been dropped accidentally.\n"
+		"Dropped commits (newer to older):\n"));
+
+	/* Make the list user-friendly and display */
+	while (missing) {
+		struct commit *commit = pop_commit(&missing);
+		struct todo_item *item = commit->util;
+
+		fprintf(stderr, " - %s %.*s\n", short_commit_name(commit),
+			item->arg_len, item->arg);
+	}
+	free_commit_list(missing);
+
+	fprintf(stderr, _("To avoid this message, use \"drop\" to "
+		"explicitly remove a commit.\n\n"
+		"Use 'git config rebase.missingCommitsCheck' to change "
+		"the level of warnings.\n"
+		"The possible behaviours are: ignore, warn, error.\n\n"));
+
+leave_check:
+	strbuf_release(&todo_file);
+	todo_list_release(&todo_list);
+
+	if (raise_error)
+		fprintf(stderr,
+			_("You can fix this with 'git rebase --edit-todo' "
+			  "and then run 'git rebase --continue'.\n"
+			  "Or you can abort the rebase with 'git rebase"
+			  " --abort'.\n"));
+
+	return res;
+}
diff --git a/sequencer.h b/sequencer.h
index 47a81034e7..4978a61b83 100644
--- a/sequencer.h
+++ b/sequencer.h
@@ -49,6 +49,7 @@ int sequencer_make_script(int keep_empty, FILE *out,
 		int argc, const char **argv);
 
 int transform_todo_ids(int shorten_sha1s);
+int check_todo_list(void);
 
 extern const char sign_off_header[];
 

From 3940b2aa7db1fba10627d8461fe3536b6c146c4e Mon Sep 17 00:00:00 2001
From: Johannes Schindelin <johannes.schindelin@gmx.de>
Date: Wed, 15 Jun 2016 09:09:09 +0200
Subject: [PATCH 7/9] rebase -i: skip unnecessary picks using the
 rebase--helper

In particular on Windows, where shell scripts are even more expensive
than on MacOSX or Linux, it makes sense to move a loop that forks
Git at least once for every line in the todo list into a builtin.

Note: The original code did not try to skip unnecessary picks of root
commits but punts instead (probably --root was not considered common
enough of a use case to bother optimizing). We do the same, for now.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
---
 builtin/rebase--helper.c   |  6 ++-
 git-rebase--interactive.sh | 41 ++---------------
 sequencer.c                | 90 ++++++++++++++++++++++++++++++++++++++
 sequencer.h                |  1 +
 4 files changed, 99 insertions(+), 39 deletions(-)

diff --git a/builtin/rebase--helper.c b/builtin/rebase--helper.c
index e706eac710..de3ccd9bfb 100644
--- a/builtin/rebase--helper.c
+++ b/builtin/rebase--helper.c
@@ -14,7 +14,7 @@ int cmd_rebase__helper(int argc, const char **argv, const char *prefix)
 	int keep_empty = 0;
 	enum {
 		CONTINUE = 1, ABORT, MAKE_SCRIPT, SHORTEN_SHA1S, EXPAND_SHA1S,
-		CHECK_TODO_LIST
+		CHECK_TODO_LIST, SKIP_UNNECESSARY_PICKS
 	} command = 0;
 	struct option options[] = {
 		OPT_BOOL(0, "ff", &opts.allow_ff, N_("allow fast-forward")),
@@ -31,6 +31,8 @@ int cmd_rebase__helper(int argc, const char **argv, const char *prefix)
 			N_("expand SHA-1s in the todo list"), EXPAND_SHA1S),
 		OPT_CMDMODE(0, "check-todo-list", &command,
 			N_("check the todo list"), CHECK_TODO_LIST),
+		OPT_CMDMODE(0, "skip-unnecessary-picks", &command,
+			N_("skip unnecessary picks"), SKIP_UNNECESSARY_PICKS),
 		OPT_END()
 	};
 
@@ -55,5 +57,7 @@ int cmd_rebase__helper(int argc, const char **argv, const char *prefix)
 		return !!transform_todo_ids(0);
 	if (command == CHECK_TODO_LIST && argc == 1)
 		return !!check_todo_list();
+	if (command == SKIP_UNNECESSARY_PICKS && argc == 1)
+		return !!skip_unnecessary_picks();
 	usage_with_options(builtin_rebase_helper_usage, options);
 }
diff --git a/git-rebase--interactive.sh b/git-rebase--interactive.sh
index 1649506e1e..931bc09e0c 100644
--- a/git-rebase--interactive.sh
+++ b/git-rebase--interactive.sh
@@ -713,43 +713,6 @@ do_rest () {
 	done
 }
 
-# skip picking commits whose parents are unchanged
-skip_unnecessary_picks () {
-	fd=3
-	while read -r command rest
-	do
-		# fd=3 means we skip the command
-		case "$fd,$command" in
-		3,pick|3,p)
-			# pick a commit whose parent is current $onto -> skip
-			sha1=${rest%% *}
-			case "$(git rev-parse --verify --quiet "$sha1"^)" in
-			"$onto"*)
-				onto=$sha1
-				;;
-			*)
-				fd=1
-				;;
-			esac
-			;;
-		3,"$comment_char"*|3,)
-			# copy comments
-			;;
-		*)
-			fd=1
-			;;
-		esac
-		printf '%s\n' "$command${rest:+ }$rest" >&$fd
-	done <"$todo" >"$todo.new" 3>>"$done" &&
-	mv -f "$todo".new "$todo" &&
-	case "$(peek_next_command)" in
-	squash|s|fixup|f)
-		record_in_rewritten "$onto"
-		;;
-	esac ||
-		die "$(gettext "Could not skip unnecessary pick commands")"
-}
-
 transform_todo_ids () {
 	while read -r command rest
 	do
@@ -1172,7 +1135,9 @@ git rebase--helper --check-todo-list || {
 
 expand_todo_ids
 
-test -d "$rewritten" || test -n "$force_rebase" || skip_unnecessary_picks
+test -d "$rewritten" || test -n "$force_rebase" ||
+onto="$(git rebase--helper --skip-unnecessary-picks)" ||
+die "Could not skip unnecessary pick commands"
 
 checkout_onto
 if test -z "$rebase_root" && test ! -d "$rewritten"
diff --git a/sequencer.c b/sequencer.c
index d1fb9ebd03..8b972c4ab9 100644
--- a/sequencer.c
+++ b/sequencer.c
@@ -2607,3 +2607,93 @@ leave_check:
 
 	return res;
 }
+
+/* skip picking commits whose parents are unchanged */
+int skip_unnecessary_picks(void)
+{
+	const char *todo_file = rebase_path_todo();
+	struct strbuf buf = STRBUF_INIT;
+	struct todo_list todo_list = TODO_LIST_INIT;
+	struct object_id onto_oid, *oid = &onto_oid, *parent_oid;
+	int fd, i;
+
+	if (!read_oneliner(&buf, rebase_path_onto(), 0))
+		return error(_("could not read 'onto'"));
+	if (get_sha1(buf.buf, onto_oid.hash)) {
+		strbuf_release(&buf);
+		return error(_("need a HEAD to fixup"));
+	}
+	strbuf_release(&buf);
+
+	fd = open(todo_file, O_RDONLY);
+	if (fd < 0) {
+		return error_errno(_("could not open '%s'"), todo_file);
+	}
+	if (strbuf_read(&todo_list.buf, fd, 0) < 0) {
+		close(fd);
+		return error(_("could not read '%s'."), todo_file);
+	}
+	close(fd);
+	if (parse_insn_buffer(todo_list.buf.buf, &todo_list) < 0) {
+		todo_list_release(&todo_list);
+		return -1;
+	}
+
+	for (i = 0; i < todo_list.nr; i++) {
+		struct todo_item *item = todo_list.items + i;
+
+		if (item->command >= TODO_NOOP)
+			continue;
+		if (item->command != TODO_PICK)
+			break;
+		if (parse_commit(item->commit)) {
+			todo_list_release(&todo_list);
+			return error(_("could not parse commit '%s'"),
+				oid_to_hex(&item->commit->object.oid));
+		}
+		if (!item->commit->parents)
+			break; /* root commit */
+		if (item->commit->parents->next)
+			break; /* merge commit */
+		parent_oid = &item->commit->parents->item->object.oid;
+		if (hashcmp(parent_oid->hash, oid->hash))
+			break;
+		oid = &item->commit->object.oid;
+	}
+	if (i > 0) {
+		int offset = i < todo_list.nr ?
+			todo_list.items[i].offset_in_buf : todo_list.buf.len;
+		const char *done_path = rebase_path_done();
+
+		fd = open(done_path, O_CREAT | O_WRONLY | O_APPEND, 0666);
+		if (write_in_full(fd, todo_list.buf.buf, offset) < 0) {
+			todo_list_release(&todo_list);
+			return error_errno(_("could not write to '%s'"),
+				done_path);
+		}
+		close(fd);
+
+		fd = open(rebase_path_todo(), O_WRONLY, 0666);
+		if (write_in_full(fd, todo_list.buf.buf + offset,
+				todo_list.buf.len - offset) < 0) {
+			todo_list_release(&todo_list);
+			return error_errno(_("could not write to '%s'"),
+				rebase_path_todo());
+		}
+		if (ftruncate(fd, todo_list.buf.len - offset) < 0) {
+			todo_list_release(&todo_list);
+			return error_errno(_("could not truncate '%s'"),
+				rebase_path_todo());
+		}
+		close(fd);
+
+		todo_list.current = i;
+		if (is_fixup(peek_command(&todo_list, 0)))
+			record_in_rewritten(oid, peek_command(&todo_list, 0));
+	}
+
+	todo_list_release(&todo_list);
+	printf("%s\n", oid_to_hex(oid));
+
+	return 0;
+}
diff --git a/sequencer.h b/sequencer.h
index 4978a61b83..28e1fc1e9b 100644
--- a/sequencer.h
+++ b/sequencer.h
@@ -50,6 +50,7 @@ int sequencer_make_script(int keep_empty, FILE *out,
 
 int transform_todo_ids(int shorten_sha1s);
 int check_todo_list(void);
+int skip_unnecessary_picks(void);
 
 extern const char sign_off_header[];
 

From b1f209d5620447baaa0224782b7f7deb19737d95 Mon Sep 17 00:00:00 2001
From: Johannes Schindelin <johannes.schindelin@gmx.de>
Date: Sun, 24 Jul 2016 10:22:13 +0200
Subject: [PATCH 8/9] t3415: test fixup with wrapped oneline

The `git commit --fixup` command unwraps wrapped onelines when
constructing the commit message, without wrapping the result.

We need to make sure that `git rebase --autosquash` keeps handling such
cases correctly, in particular since we are about to move the autosquash
handling into the rebase--helper.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
---
 t/t3415-rebase-autosquash.sh | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/t/t3415-rebase-autosquash.sh b/t/t3415-rebase-autosquash.sh
index 48346f1cc0..9fd629a6e2 100755
--- a/t/t3415-rebase-autosquash.sh
+++ b/t/t3415-rebase-autosquash.sh
@@ -304,4 +304,18 @@ test_expect_success 'extra spaces after fixup!' '
 	test $base = $parent
 '
 
+test_expect_success 'wrapped original subject' '
+	if test -d .git/rebase-merge; then git rebase --abort; fi &&
+	base=$(git rev-parse HEAD) &&
+	echo "wrapped subject" >wrapped &&
+	git add wrapped &&
+	test_tick &&
+	git commit --allow-empty -m "$(printf "To\nfixup")" &&
+	test_tick &&
+	git commit --allow-empty -m "fixup! To fixup" &&
+	git rebase -i --autosquash --keep-empty HEAD~2 &&
+	parent=$(git rev-parse HEAD^) &&
+	test $base = $parent
+'
+
 test_done

From a2383709dd9d9617b288920a9ca562299fcb928d Mon Sep 17 00:00:00 2001
From: Johannes Schindelin <johannes.schindelin@gmx.de>
Date: Thu, 16 Jun 2016 15:37:45 +0200
Subject: [PATCH 9/9] rebase -i: rearrange fixup/squash lines using the
 rebase--helper

This operation has quadratic complexity, which is especially painful
on Windows, where shell scripts are *already* slow (mainly due to the
overhead of the POSIX emulation layer).

Let's reimplement this with linear complexity (using a hash map to
match the commits' subject lines) for the common case; Sadly, the
fixup/squash feature's design neglected performance considerations,
allowing arbitrary prefixes (read: `fixup! hell` will match the
commit subject `hello world`), which means that we are stuck with
quadratic performance in the worst case.

The reimplemented logic also happens to fix a bug where commented-out
lines (representing empty patches) were dropped by the previous code.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
---
 builtin/rebase--helper.c     |   6 +-
 git-rebase--interactive.sh   |  90 +---------------
 sequencer.c                  | 197 +++++++++++++++++++++++++++++++++++
 sequencer.h                  |   1 +
 t/t3415-rebase-autosquash.sh |   2 +-
 5 files changed, 205 insertions(+), 91 deletions(-)

diff --git a/builtin/rebase--helper.c b/builtin/rebase--helper.c
index de3ccd9bfb..e6591f0111 100644
--- a/builtin/rebase--helper.c
+++ b/builtin/rebase--helper.c
@@ -14,7 +14,7 @@ int cmd_rebase__helper(int argc, const char **argv, const char *prefix)
 	int keep_empty = 0;
 	enum {
 		CONTINUE = 1, ABORT, MAKE_SCRIPT, SHORTEN_SHA1S, EXPAND_SHA1S,
-		CHECK_TODO_LIST, SKIP_UNNECESSARY_PICKS
+		CHECK_TODO_LIST, SKIP_UNNECESSARY_PICKS, REARRANGE_SQUASH
 	} command = 0;
 	struct option options[] = {
 		OPT_BOOL(0, "ff", &opts.allow_ff, N_("allow fast-forward")),
@@ -33,6 +33,8 @@ int cmd_rebase__helper(int argc, const char **argv, const char *prefix)
 			N_("check the todo list"), CHECK_TODO_LIST),
 		OPT_CMDMODE(0, "skip-unnecessary-picks", &command,
 			N_("skip unnecessary picks"), SKIP_UNNECESSARY_PICKS),
+		OPT_CMDMODE(0, "rearrange-squash", &command,
+			N_("rearrange fixup/squash lines"), REARRANGE_SQUASH),
 		OPT_END()
 	};
 
@@ -59,5 +61,7 @@ int cmd_rebase__helper(int argc, const char **argv, const char *prefix)
 		return !!check_todo_list();
 	if (command == SKIP_UNNECESSARY_PICKS && argc == 1)
 		return !!skip_unnecessary_picks();
+	if (command == REARRANGE_SQUASH && argc == 1)
+		return !!rearrange_squash();
 	usage_with_options(builtin_rebase_helper_usage, options);
 }
diff --git a/git-rebase--interactive.sh b/git-rebase--interactive.sh
index 931bc09e0c..d39fe4f5fb 100644
--- a/git-rebase--interactive.sh
+++ b/git-rebase--interactive.sh
@@ -744,94 +744,6 @@ collapse_todo_ids() {
 	git rebase--helper --shorten-sha1s
 }
 
-# Rearrange the todo list that has both "pick sha1 msg" and
-# "pick sha1 fixup!/squash! msg" appears in it so that the latter
-# comes immediately after the former, and change "pick" to
-# "fixup"/"squash".
-#
-# Note that if the config has specified a custom instruction format
-# each log message will be re-retrieved in order to normalize the
-# autosquash arrangement
-rearrange_squash () {
-	format=$(git config --get rebase.instructionFormat)
-	# extract fixup!/squash! lines and resolve any referenced sha1's
-	while read -r pick sha1 message
-	do
-		test -z "${format}" || message=$(git log -n 1 --format="%s" ${sha1})
-		case "$message" in
-		"squash! "*|"fixup! "*)
-			action="${message%%!*}"
-			rest=$message
-			prefix=
-			# skip all squash! or fixup! (but save for later)
-			while :
-			do
-				case "$rest" in
-				"squash! "*|"fixup! "*)
-					prefix="$prefix${rest%%!*},"
-					rest="${rest#*! }"
-					;;
-				*)
-					break
-					;;
-				esac
-			done
-			printf '%s %s %s %s\n' "$sha1" "$action" "$prefix" "$rest"
-			# if it's a single word, try to resolve to a full sha1 and
-			# emit a second copy. This allows us to match on both message
-			# and on sha1 prefix
-			if test "${rest#* }" = "$rest"; then
-				fullsha="$(git rev-parse -q --verify "$rest" 2>/dev/null)"
-				if test -n "$fullsha"; then
-					# prefix the action to uniquely identify this line as
-					# intended for full sha1 match
-					echo "$sha1 +$action $prefix $fullsha"
-				fi
-			fi
-		esac
-	done >"$1.sq" <"$1"
-	test -s "$1.sq" || return
-
-	used=
-	while read -r pick sha1 message
-	do
-		case " $used" in
-		*" $sha1 "*) continue ;;
-		esac
-		printf '%s\n' "$pick $sha1 $message"
-		test -z "${format}" || message=$(git log -n 1 --format="%s" ${sha1})
-		used="$used$sha1 "
-		while read -r squash action msg_prefix msg_content
-		do
-			case " $used" in
-			*" $squash "*) continue ;;
-			esac
-			emit=0
-			case "$action" in
-			+*)
-				action="${action#+}"
-				# full sha1 prefix test
-				case "$msg_content" in "$sha1"*) emit=1;; esac ;;
-			*)
-				# message prefix test
-				case "$message" in "$msg_content"*) emit=1;; esac ;;
-			esac
-			if test $emit = 1; then
-				if test -n "${format}"
-				then
-					msg_content=$(git log -n 1 --format="${format}" ${squash})
-				else
-					msg_content="$(echo "$msg_prefix" | sed "s/,/! /g")$msg_content"
-				fi
-				printf '%s\n' "$action $squash $msg_content"
-				used="$used$squash "
-			fi
-		done <"$1.sq"
-	done >"$1.rearranged" <"$1"
-	cat "$1.rearranged" >"$1"
-	rm -f "$1.sq" "$1.rearranged"
-}
-
 # Add commands after a pick or after a squash/fixup serie
 # in the todo list.
 add_exec_commands () {
@@ -1091,7 +1003,7 @@ then
 fi
 
 test -s "$todo" || echo noop >> "$todo"
-test -n "$autosquash" && rearrange_squash "$todo"
+test -z "$autosquash" || git rebase--helper --rearrange-squash || exit
 test -n "$cmd" && add_exec_commands "$todo"
 
 todocount=$(git stripspace --strip-comments <"$todo" | wc -l)
diff --git a/sequencer.c b/sequencer.c
index 8b972c4ab9..e25a4e1180 100644
--- a/sequencer.c
+++ b/sequencer.c
@@ -19,6 +19,7 @@
 #include "trailer.h"
 #include "log-tree.h"
 #include "wt-status.h"
+#include "hashmap.h"
 
 #define GIT_REFLOG_ACTION "GIT_REFLOG_ACTION"
 
@@ -2697,3 +2698,199 @@ int skip_unnecessary_picks(void)
 
 	return 0;
 }
+
+struct subject2item_entry {
+	struct hashmap_entry entry;
+	int i;
+	char subject[FLEX_ARRAY];
+};
+
+static int subject2item_cmp(const struct subject2item_entry *a,
+	const struct subject2item_entry *b, const void *key)
+{
+	return key ? strcmp(a->subject, key) : strcmp(a->subject, b->subject);
+}
+
+/*
+ * Rearrange the todo list that has both "pick sha1 msg" and "pick sha1
+ * fixup!/squash! msg" in it so that the latter is put immediately after the
+ * former, and change "pick" to "fixup"/"squash".
+ *
+ * Note that if the config has specified a custom instruction format, each log
+ * message will have to be retrieved from the commit (as the oneline in the
+ * script cannot be trusted) in order to normalize the autosquash arrangement.
+ */
+int rearrange_squash(void)
+{
+	const char *todo_file = rebase_path_todo();
+	struct todo_list todo_list = TODO_LIST_INIT;
+	struct hashmap subject2item;
+	int res = 0, rearranged = 0, *next, *tail, fd, i;
+	char **subjects;
+
+	fd = open(todo_file, O_RDONLY);
+	if (fd < 0)
+		return error_errno(_("could not open '%s'"), todo_file);
+	if (strbuf_read(&todo_list.buf, fd, 0) < 0) {
+		close(fd);
+		return error(_("could not read '%s'."), todo_file);
+	}
+	close(fd);
+	if (parse_insn_buffer(todo_list.buf.buf, &todo_list) < 0) {
+		todo_list_release(&todo_list);
+		return -1;
+	}
+
+	/*
+	 * The hashmap maps onelines to the respective todo list index.
+	 *
+	 * If any items need to be rearranged, the next[i] value will indicate
+	 * which item was moved directly after the i'th.
+	 *
+	 * In that case, last[i] will indicate the index of the latest item to
+	 * be moved to appear after the i'th.
+	 */
+	hashmap_init(&subject2item, (hashmap_cmp_fn) subject2item_cmp,
+		     todo_list.nr);
+	ALLOC_ARRAY(next, todo_list.nr);
+	ALLOC_ARRAY(tail, todo_list.nr);
+	ALLOC_ARRAY(subjects, todo_list.nr);
+	for (i = 0; i < todo_list.nr; i++) {
+		struct strbuf buf = STRBUF_INIT;
+		struct todo_item *item = todo_list.items + i;
+		const char *commit_buffer, *subject, *p;
+		int i2 = -1;
+		struct subject2item_entry *entry;
+
+		next[i] = tail[i] = -1;
+		if (item->command >= TODO_EXEC) {
+			subjects[i] = NULL;
+			continue;
+		}
+
+		if (is_fixup(item->command)) {
+			todo_list_release(&todo_list);
+			return error(_("the script was already rearranged."));
+		}
+
+		item->commit->util = item;
+
+		parse_commit(item->commit);
+		commit_buffer = get_commit_buffer(item->commit, NULL);
+		find_commit_subject(commit_buffer, &subject);
+		format_subject(&buf, subject, " ");
+		subject = subjects[i] = buf.buf;
+		unuse_commit_buffer(item->commit, commit_buffer);
+		if ((skip_prefix(subject, "fixup! ", &p) ||
+		     skip_prefix(subject, "squash! ", &p))) {
+			struct commit *commit2;
+
+			for (;;) {
+				while (isspace(*p))
+					p++;
+				if (!skip_prefix(p, "fixup! ", &p) &&
+				    !skip_prefix(p, "squash! ", &p))
+					break;
+			}
+
+			if ((entry = hashmap_get_from_hash(&subject2item,
+							   strhash(p), p)))
+				/* found by title */
+				i2 = entry->i;
+			else if (!strchr(p, ' ') &&
+				 (commit2 =
+				  lookup_commit_reference_by_name(p)) &&
+				 commit2->util)
+				/* found by commit name */
+				i2 = (struct todo_item *)commit2->util
+					- todo_list.items;
+			else {
+				/* copy can be a prefix of the commit subject */
+				for (i2 = 0; i2 < i; i2++)
+					if (subjects[i2] &&
+					    starts_with(subjects[i2], p))
+						break;
+				if (i2 == i)
+					i2 = -1;
+			}
+		}
+		if (i2 >= 0) {
+			rearranged = 1;
+			todo_list.items[i].command =
+				starts_with(subject, "fixup!") ?
+				TODO_FIXUP : TODO_SQUASH;
+			if (next[i2] < 0)
+				next[i2] = i;
+			else
+				next[tail[i2]] = i;
+			tail[i2] = i;
+		} else if (!hashmap_get_from_hash(&subject2item,
+						strhash(subject), subject)) {
+			FLEX_ALLOC_MEM(entry, subject, buf.buf, buf.len);
+			entry->i = i;
+			hashmap_entry_init(entry, strhash(entry->subject));
+			hashmap_put(&subject2item, entry);
+		}
+		strbuf_detach(&buf, NULL);
+	}
+
+	if (rearranged) {
+		struct strbuf buf = STRBUF_INIT;
+		char *format = NULL;
+
+		git_config_get_string("rebase.instructionFormat", &format);
+		for (i = 0; i < todo_list.nr; i++) {
+			enum todo_command command = todo_list.items[i].command;
+			int cur = i;
+
+			/*
+			 * Initially, all commands are 'pick's. If it is a
+			 * fixup or a squash now, we have rearranged it.
+			 */
+			if (is_fixup(command))
+				continue;
+
+			while (cur >= 0) {
+				int offset = todo_list.items[cur].offset_in_buf;
+				int end_offset = cur + 1 < todo_list.nr ?
+					todo_list.items[cur + 1].offset_in_buf :
+					todo_list.buf.len;
+				char *bol = todo_list.buf.buf + offset;
+				char *eol = todo_list.buf.buf + end_offset;
+
+				/* replace 'pick', by 'fixup' or 'squash' */
+				command = todo_list.items[cur].command;
+				if (is_fixup(command)) {
+					strbuf_addstr(&buf,
+						todo_command_info[command].str);
+					bol += strcspn(bol, " \t");
+				}
+
+				strbuf_add(&buf, bol, eol - bol);
+
+				cur = next[cur];
+			}
+		}
+
+		fd = open(todo_file, O_WRONLY);
+		if (fd < 0)
+			res = error_errno(_("could not open '%s'"), todo_file);
+		else if (write(fd, buf.buf, buf.len) < 0)
+			res = error_errno(_("could not read '%s'."), todo_file);
+		else if (ftruncate(fd, buf.len) < 0)
+			res = error_errno(_("could not finish '%s'"),
+					   todo_file);
+		close(fd);
+		strbuf_release(&buf);
+	}
+
+	free(next);
+	free(tail);
+	for (i = 0; i < todo_list.nr; i++)
+		free(subjects[i]);
+	free(subjects);
+	hashmap_free(&subject2item, 1);
+	todo_list_release(&todo_list);
+
+	return res;
+}
diff --git a/sequencer.h b/sequencer.h
index 28e1fc1e9b..1c94bec762 100644
--- a/sequencer.h
+++ b/sequencer.h
@@ -51,6 +51,7 @@ int sequencer_make_script(int keep_empty, FILE *out,
 int transform_todo_ids(int shorten_sha1s);
 int check_todo_list(void);
 int skip_unnecessary_picks(void);
+int rearrange_squash(void);
 
 extern const char sign_off_header[];
 
diff --git a/t/t3415-rebase-autosquash.sh b/t/t3415-rebase-autosquash.sh
index 9fd629a6e2..b9e26008a7 100755
--- a/t/t3415-rebase-autosquash.sh
+++ b/t/t3415-rebase-autosquash.sh
@@ -278,7 +278,7 @@ set_backup_editor () {
 	test_set_editor "$PWD/backup-editor.sh"
 }
 
-test_expect_failure 'autosquash with multiple empty patches' '
+test_expect_success 'autosquash with multiple empty patches' '
 	test_tick &&
 	git commit --allow-empty -m "empty" &&
 	test_tick &&