#!/bin/sh
#
# Copyright (c) 2006 Johannes E. Schindelin

# SHORT DESCRIPTION
#
# This script makes it easy to fix up commits in the middle of a series,
# and rearrange commits.
#
# The original idea comes from Eric W. Biederman, in
# http://article.gmane.org/gmane.comp.version-control.git/22407

OPTIONS_KEEPDASHDASH=
OPTIONS_SPEC="\
git-rebase [-i] [options] [--] <upstream> [<branch>]
git-rebase [-i] (--continue | --abort | --skip)
--
 Available options are
v,verbose          display a diffstat of what changed upstream
onto=              rebase onto given branch instead of upstream
p,preserve-merges  try to recreate merges instead of ignoring them
s,strategy=        use the given merge strategy
m,merge            always used (no-op)
i,interactive      always used (no-op)
 Actions:
continue           continue rebasing process
abort              abort rebasing process and restore original branch
skip               skip current patch and continue rebasing process
no-verify          override pre-rebase hook from stopping the operation
root               rebase all reachable commmits up to the root(s)
autosquash         move commits that begin with squash!/fixup! under -i
"

. git-sh-setup
require_work_tree

DOTEST="$GIT_DIR/rebase-merge"

# The file containing rebase commands, comments, and empty lines.
# This file is created by "git rebase -i" then edited by the user.  As
# the lines are processed, they are removed from the front of this
# file and written to the tail of $DONE.
TODO="$DOTEST"/git-rebase-todo

# The rebase command lines that have already been processed.  A line
# is moved here when it is first handled, before any associated user
# actions.
DONE="$DOTEST"/done

# The commit message that is planned to be used for any changes that
# need to be committed following a user interaction.
MSG="$DOTEST"/message

# The file into which is accumulated the suggested commit message for
# squash/fixup commands.  When the first of a series of squash/fixups
# is seen, the file is created and the commit message from the
# previous commit and from the first squash/fixup commit are written
# to it.  The commit message for each subsequent squash/fixup commit
# is appended to the file as it is processed.
#
# The first line of the file is of the form
#     # This is a combination of $COUNT commits.
# where $COUNT is the number of commits whose messages have been
# written to the file so far (including the initial "pick" commit).
# Each time that a commit message is processed, this line is read and
# updated.  It is deleted just before the combined commit is made.
SQUASH_MSG="$DOTEST"/message-squash

# If the current series of squash/fixups has not yet included a squash
# command, then this file exists and holds the commit message of the
# original "pick" commit.  (If the series ends without a "squash"
# command, then this can be used as the commit message of the combined
# commit without opening the editor.)
FIXUP_MSG="$DOTEST"/message-fixup

# $REWRITTEN is the name of a directory containing files for each
# commit that is reachable by at least one merge base of $HEAD and
# $UPSTREAM. They are not necessarily rewritten, but their children
# might be.  This ensures that commits on merged, but otherwise
# unrelated side branches are left alone. (Think "X" in the man page's
# example.)
REWRITTEN="$DOTEST"/rewritten

DROPPED="$DOTEST"/dropped

# A script to set the GIT_AUTHOR_NAME, GIT_AUTHOR_EMAIL, and
# GIT_AUTHOR_DATE that will be used for the commit that is currently
# being rebased.
AUTHOR_SCRIPT="$DOTEST"/author-script

# When an "edit" rebase command is being processed, the SHA1 of the
# commit to be edited is recorded in this file.  When "git rebase
# --continue" is executed, if there are any staged changes then they
# will be amended to the HEAD commit, but only provided the HEAD
# commit is still the commit to be edited.  When any other rebase
# command is processed, this file is deleted.
AMEND="$DOTEST"/amend

PRESERVE_MERGES=
STRATEGY=
ONTO=
VERBOSE=
OK_TO_SKIP_PRE_REBASE=
REBASE_ROOT=
AUTOSQUASH=

GIT_CHERRY_PICK_HELP="  After resolving the conflicts,
mark the corrected paths with 'git add <paths>', and
run 'git rebase --continue'"
export GIT_CHERRY_PICK_HELP

warn () {
        echo "$*" >&2
}

output () {
        case "$VERBOSE" in
        '')
                output=$("$@" 2>&1 )
                status=$?
                test $status != 0 && printf "%s\n" "$output"
                return $status
                ;;
        *)
                "$@"
                ;;
        esac
}

# Output the commit message for the specified commit.
commit_message () {
        git cat-file commit "$1" | sed "1,/^$/d"
}

run_pre_rebase_hook () {
        if test -z "$OK_TO_SKIP_PRE_REBASE" &&
           test -x "$GIT_DIR/hooks/pre-rebase"
        then
                "$GIT_DIR/hooks/pre-rebase" ${1+"$@"} || {
                        echo >&2 "The pre-rebase hook refused to rebase."
                        exit 1
                }
        fi
}

require_clean_work_tree () {
        # test if working tree is dirty
        git rev-parse --verify HEAD > /dev/null &&
        git update-index --ignore-submodules --refresh &&
        git diff-files --quiet --ignore-submodules &&
        git diff-index --cached --quiet HEAD --ignore-submodules -- ||
        die "Working tree is dirty"
}

ORIG_REFLOG_ACTION="$GIT_REFLOG_ACTION"

comment_for_reflog () {
        case "$ORIG_REFLOG_ACTION" in
        ''|rebase*)
                GIT_REFLOG_ACTION="rebase -i ($1)"
                export GIT_REFLOG_ACTION
                ;;
        esac
}

last_count=
mark_action_done () {
        sed -e 1q < "$TODO" >> "$DONE"
        sed -e 1d < "$TODO" >> "$TODO".new
        mv -f "$TODO".new "$TODO"
        count=$(sane_grep -c '^[^#]' < "$DONE")
        total=$(($count+$(sane_grep -c '^[^#]' < "$TODO")))
        if test "$last_count" != "$count"
        then
                last_count=$count
                printf "Rebasing (%d/%d)\r" $count $total
                test -z "$VERBOSE" || echo
        fi
}

make_patch () {
        sha1_and_parents="$(git rev-list --parents -1 "$1")"
        case "$sha1_and_parents" in
        ?*' '?*' '?*)
                git diff --cc $sha1_and_parents
                ;;
        ?*' '?*)
                git diff-tree -p "$1^!"
                ;;
        *)
                echo "Root commit"
                ;;
        esac > "$DOTEST"/patch
        test -f "$MSG" ||
                commit_message "$1" > "$MSG"
        test -f "$AUTHOR_SCRIPT" ||
                get_author_ident_from_commit "$1" > "$AUTHOR_SCRIPT"
}

die_with_patch () {
        make_patch "$1"
        git rerere
        die "$2"
}

die_abort () {
        rm -rf "$DOTEST"
        die "$1"
}

has_action () {
        sane_grep '^[^#]' "$1" >/dev/null
}

# Run command with GIT_AUTHOR_NAME, GIT_AUTHOR_EMAIL, and
# GIT_AUTHOR_DATE exported from the current environment.
do_with_author () {
        (
                export GIT_AUTHOR_NAME GIT_AUTHOR_EMAIL GIT_AUTHOR_DATE
                "$@"
        )
}

pick_one () {
        no_ff=
        case "$1" in -n) sha1=$2; no_ff=t ;; *) sha1=$1 ;; esac
        output git rev-parse --verify $sha1 || die "Invalid commit name: $sha1"
        test -d "$REWRITTEN" &&
                pick_one_preserving_merges "$@" && return
        if test -n "$REBASE_ROOT"
        then
                output git cherry-pick "$@"
                return
        fi
        parent_sha1=$(git rev-parse --verify $sha1^) ||
                die "Could not get the parent of $sha1"
        current_sha1=$(git rev-parse --verify HEAD)
        if test -z "$no_ff" && test "$current_sha1" = "$parent_sha1"
        then
                output git reset --hard $sha1
                output warn Fast-forward to $(git rev-parse --short $sha1)
        else
                output git cherry-pick "$@"
        fi
}

pick_one_preserving_merges () {
        fast_forward=t
        case "$1" in
        -n)
                fast_forward=f
                sha1=$2
                ;;
        *)
                sha1=$1
                ;;
        esac
        sha1=$(git rev-parse $sha1)

        if test -f "$DOTEST"/current-commit
        then
                if test "$fast_forward" = t
                then
                        cat "$DOTEST"/current-commit | while read current_commit
                        do
                                git rev-parse HEAD > "$REWRITTEN"/$current_commit
                        done
                        rm "$DOTEST"/current-commit ||
                        die "Cannot write current commit's replacement sha1"
                fi
        fi

        echo $sha1 >> "$DOTEST"/current-commit

        # rewrite parents; if none were rewritten, we can fast-forward.
        new_parents=
        pend=" $(git rev-list --parents -1 $sha1 | cut -d' ' -s -f2-)"
        if test "$pend" = " "
        then
                pend=" root"
        fi
        while [ "$pend" != "" ]
        do
                p=$(expr "$pend" : ' \([^ ]*\)')
                pend="${pend# $p}"

                if test -f "$REWRITTEN"/$p
                then
                        new_p=$(cat "$REWRITTEN"/$p)

                        # If the todo reordered commits, and our parent is marked for
                        # rewriting, but hasn't been gotten to yet, assume the user meant to
                        # drop it on top of the current HEAD
                        if test -z "$new_p"
                        then
                                new_p=$(git rev-parse HEAD)
                        fi

                        test $p != $new_p && fast_forward=f
                        case "$new_parents" in
                        *$new_p*)
                                ;; # do nothing; that parent is already there
                        *)
                                new_parents="$new_parents $new_p"
                                ;;
                        esac
                else
                        if test -f "$DROPPED"/$p
                        then
                                fast_forward=f
                                replacement="$(cat "$DROPPED"/$p)"
                                test -z "$replacement" && replacement=root
                                pend=" $replacement$pend"
                        else
                                new_parents="$new_parents $p"
                        fi
                fi
        done
        case $fast_forward in
        t)
                output warn "Fast-forward to $sha1"
                output git reset --hard $sha1 ||
                        die "Cannot fast-forward to $sha1"
                ;;
        f)
                first_parent=$(expr "$new_parents" : ' \([^ ]*\)')

                if [ "$1" != "-n" ]
                then
                        # detach HEAD to current parent
                        output git checkout $first_parent 2> /dev/null ||
                                die "Cannot move HEAD to $first_parent"
                fi

                case "$new_parents" in
                ' '*' '*)
                        test "a$1" = a-n && die "Refusing to squash a merge: $sha1"

                        # redo merge
                        author_script=$(get_author_ident_from_commit $sha1)
                        eval "$author_script"
                        msg="$(commit_message $sha1)"
                        # No point in merging the first parent, that's HEAD
                        new_parents=${new_parents# $first_parent}
                        if ! do_with_author output \
                                git merge $STRATEGY -m "$msg" $new_parents
                        then
                                printf "%s\n" "$msg" > "$GIT_DIR"/MERGE_MSG
                                die_with_patch $sha1 "Error redoing merge $sha1"
                        fi
                        ;;
                *)
                        output git cherry-pick "$@" ||
                                die_with_patch $sha1 "Could not pick $sha1"
                        ;;
                esac
                ;;
        esac
}

nth_string () {
        case "$1" in
        *1[0-9]|*[04-9]) echo "$1"th;;
        *1) echo "$1"st;;
        *2) echo "$1"nd;;
        *3) echo "$1"rd;;
        esac
}

update_squash_messages () {
        if test -f "$SQUASH_MSG"; then
                mv "$SQUASH_MSG" "$SQUASH_MSG".bak || exit
                COUNT=$(($(sed -n \
                        -e "1s/^# This is a combination of \(.*\) commits\./\1/p" \
                        -e "q" < "$SQUASH_MSG".bak)+1))
                {
                        echo "# This is a combination of $COUNT commits."
                        sed -e 1d -e '2,/^./{
                                /^$/d
                        }' <"$SQUASH_MSG".bak
                } >"$SQUASH_MSG"
        else
                commit_message HEAD > "$FIXUP_MSG" || die "Cannot write $FIXUP_MSG"
                COUNT=2
                {
                        echo "# This is a combination of 2 commits."
                        echo "# The first commit's message is:"
                        echo
                        cat "$FIXUP_MSG"
                } >"$SQUASH_MSG"
        fi
        case $1 in
        squash)
                rm -f "$FIXUP_MSG"
                echo
                echo "# This is the $(nth_string $COUNT) commit message:"
                echo
                commit_message $2
                ;;
        fixup)
                echo
                echo "# The $(nth_string $COUNT) commit message will be skipped:"
                echo
                commit_message $2 | sed -e 's/^/#       /'
                ;;
        esac >>"$SQUASH_MSG"
}

peek_next_command () {
        sed -n -e "/^#/d" -e '/^$/d' -e "s/ .*//p" -e "q" < "$TODO"
}

# A squash/fixup has failed.  Prepare the long version of the squash
# commit message, then die_with_patch.  This code path requires the
# user to edit the combined commit message for all commits that have
# been squashed/fixedup so far.  So also erase the old squash
# messages, effectively causing the combined commit to be used as the
# new basis for any further squash/fixups.  Args: sha1 rest
die_failed_squash() {
        mv "$SQUASH_MSG" "$MSG" || exit
        rm -f "$FIXUP_MSG"
        cp "$MSG" "$GIT_DIR"/MERGE_MSG || exit
        warn
        warn "Could not apply $1... $2"
        die_with_patch $1 ""
}

do_next () {
        rm -f "$MSG" "$AUTHOR_SCRIPT" "$AMEND" || exit
        read command sha1 rest < "$TODO"
        case "$command" in
        '#'*|''|noop)
                mark_action_done
                ;;
        pick|p)
                comment_for_reflog pick

                mark_action_done
                pick_one $sha1 ||
                        die_with_patch $sha1 "Could not apply $sha1... $rest"
                ;;
        reword|r)
                comment_for_reflog reword

                mark_action_done
                pick_one $sha1 ||
                        die_with_patch $sha1 "Could not apply $sha1... $rest"
                git commit --amend
                ;;
        edit|e)
                comment_for_reflog edit

                mark_action_done
                pick_one $sha1 ||
                        die_with_patch $sha1 "Could not apply $sha1... $rest"
                make_patch $sha1
                git rev-parse --verify HEAD > "$AMEND"
                warn "Stopped at $sha1... $rest"
                warn "You can amend the commit now, with"
                warn
                warn "  git commit --amend"
                warn
                warn "Once you are satisfied with your changes, run"
                warn
                warn "  git rebase --continue"
                warn
                exit 0
                ;;
        squash|s|fixup|f)
                case "$command" in
                squash|s)
                        squash_style=squash
                        ;;
                fixup|f)
                        squash_style=fixup
                        ;;
                esac
                comment_for_reflog $squash_style

                test -f "$DONE" && has_action "$DONE" ||
                        die "Cannot '$squash_style' without a previous commit"

                mark_action_done
                update_squash_messages $squash_style $sha1
                author_script=$(get_author_ident_from_commit HEAD)
                echo "$author_script" > "$AUTHOR_SCRIPT"
                eval "$author_script"
                output git reset --soft HEAD^
                pick_one -n $sha1 || die_failed_squash $sha1 "$rest"
                case "$(peek_next_command)" in
                squash|s|fixup|f)
                        # This is an intermediate commit; its message will only be
                        # used in case of trouble.  So use the long version:
                        do_with_author output git commit --no-verify -F "$SQUASH_MSG" ||
                                die_failed_squash $sha1 "$rest"
                        ;;
                *)
                        # This is the final command of this squash/fixup group
                        if test -f "$FIXUP_MSG"
                        then
                                do_with_author git commit --no-verify -F "$FIXUP_MSG" ||
                                        die_failed_squash $sha1 "$rest"
                        else
                                cp "$SQUASH_MSG" "$GIT_DIR"/SQUASH_MSG || exit
                                rm -f "$GIT_DIR"/MERGE_MSG
                                do_with_author git commit --no-verify -e ||
                                        die_failed_squash $sha1 "$rest"
                        fi
                        rm -f "$SQUASH_MSG" "$FIXUP_MSG"
                        ;;
                esac
                ;;
        *)
                warn "Unknown command: $command $sha1 $rest"
                if git rev-parse --verify -q "$sha1" >/dev/null
                then
                        die_with_patch $sha1 "Please fix this in the file $TODO."
                else
                        die "Please fix this in the file $TODO."
                fi
                ;;
        esac
        test -s "$TODO" && return

        comment_for_reflog finish &&
        HEADNAME=$(cat "$DOTEST"/head-name) &&
        OLDHEAD=$(cat "$DOTEST"/head) &&
        SHORTONTO=$(git rev-parse --short $(cat "$DOTEST"/onto)) &&
        NEWHEAD=$(git rev-parse HEAD) &&
        case $HEADNAME in
        refs/*)
                message="$GIT_REFLOG_ACTION: $HEADNAME onto $SHORTONTO" &&
                git update-ref -m "$message" $HEADNAME $NEWHEAD $OLDHEAD &&
                git symbolic-ref HEAD $HEADNAME
                ;;
        esac && {
                test ! -f "$DOTEST"/verbose ||
                        git diff-tree --stat $(cat "$DOTEST"/head)..HEAD
        } &&
        rm -rf "$DOTEST" &&
        git gc --auto &&
        warn "Successfully rebased and updated $HEADNAME."

        exit
}

do_rest () {
        while :
        do
                do_next
        done
}

# skip picking commits whose parents are unchanged
skip_unnecessary_picks () {
        fd=3
        while read command sha1 rest
        do
                # fd=3 means we skip the command
                case "$fd,$command,$(git rev-parse --verify --quiet $sha1^)" in
                3,pick,"$ONTO"*|3,p,"$ONTO"*)
                        # pick a commit whose parent is current $ONTO -> skip
                        ONTO=$sha1
                        ;;
                3,#*|3,,*)
                        # copy comments
                        ;;
                *)
                        fd=1
                        ;;
                esac
                echo "$command${sha1:+ }$sha1${rest:+ }$rest" >&$fd
        done <"$TODO" >"$TODO.new" 3>>"$DONE" &&
        mv -f "$TODO".new "$TODO" ||
        die "Could not skip unnecessary pick commands"
}

# check if no other options are set
is_standalone () {
        test $# -eq 2 -a "$2" = '--' &&
        test -z "$ONTO" &&
        test -z "$PRESERVE_MERGES" &&
        test -z "$STRATEGY" &&
        test -z "$VERBOSE"
}

get_saved_options () {
        test -d "$REWRITTEN" && PRESERVE_MERGES=t
        test -f "$DOTEST"/strategy && STRATEGY="$(cat "$DOTEST"/strategy)"
        test -f "$DOTEST"/verbose && VERBOSE=t
        test -f "$DOTEST"/rebase-root && REBASE_ROOT=t
}

# 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".
rearrange_squash () {
        sed -n -e 's/^pick \([0-9a-f]*\) \(squash\)! /\1 \2 /p' \
                -e 's/^pick \([0-9a-f]*\) \(fixup\)! /\1 \2 /p' \
                "$1" >"$1.sq"
        test -s "$1.sq" || return

        used=
        while read pick sha1 message
        do
                case " $used" in
                *" $sha1 "*) continue ;;
                esac
                echo "$pick $sha1 $message"
                while read squash action msg
                do
                        case "$message" in
                        "$msg"*)
                                echo "$action $squash $action! $msg"
                                used="$used$squash "
                                ;;
                        esac
                done <"$1.sq"
        done >"$1.rearranged" <"$1"
        cat "$1.rearranged" >"$1"
        rm -f "$1.sq" "$1.rearranged"
}

LF='
'
parse_onto () {
        case "$1" in
        *...*)
                if      left=${1%...*} right=${1#*...} &&
                        onto=$(git merge-base --all ${left:-HEAD} ${right:-HEAD})
                then
                        case "$onto" in
                        ?*"$LF"?* | '')
                                exit 1 ;;
                        esac
                        echo "$onto"
                        exit 0
                fi
        esac
        git rev-parse --verify "$1^0"
}

while test $# != 0
do
        case "$1" in
        --no-verify)
                OK_TO_SKIP_PRE_REBASE=yes
                ;;
        --verify)
                ;;
        --continue)
                is_standalone "$@" || usage
                get_saved_options
                comment_for_reflog continue

                test -d "$DOTEST" || die "No interactive rebase running"

                # Sanity check
                git rev-parse --verify HEAD >/dev/null ||
                        die "Cannot read HEAD"
                git update-index --ignore-submodules --refresh &&
                        git diff-files --quiet --ignore-submodules ||
                        die "Working tree is dirty"

                # do we have anything to commit?
                if git diff-index --cached --quiet --ignore-submodules HEAD --
                then
                        : Nothing to commit -- skip this
                else
                        . "$AUTHOR_SCRIPT" ||
                                die "Cannot find the author identity"
                        amend=
                        if test -f "$AMEND"
                        then
                                amend=$(git rev-parse --verify HEAD)
                                test "$amend" = $(cat "$AMEND") ||
                                die "\
You have uncommitted changes in your working tree. Please, commit them
first and then run 'git rebase --continue' again."
                                git reset --soft HEAD^ ||
                                die "Cannot rewind the HEAD"
                        fi
                        do_with_author git commit --no-verify -F "$MSG" -e || {
                                test -n "$amend" && git reset --soft $amend
                                die "Could not commit staged changes."
                        }
                fi

                require_clean_work_tree
                do_rest
                ;;
        --abort)
                is_standalone "$@" || usage
                get_saved_options
                comment_for_reflog abort

                git rerere clear
                test -d "$DOTEST" || die "No interactive rebase running"

                HEADNAME=$(cat "$DOTEST"/head-name)
                HEAD=$(cat "$DOTEST"/head)
                case $HEADNAME in
                refs/*)
                        git symbolic-ref HEAD $HEADNAME
                        ;;
                esac &&
                output git reset --hard $HEAD &&
                rm -rf "$DOTEST"
                exit
                ;;
        --skip)
                is_standalone "$@" || usage
                get_saved_options
                comment_for_reflog skip

                git rerere clear
                test -d "$DOTEST" || die "No interactive rebase running"

                output git reset --hard && do_rest
                ;;
        -s)
                case "$#,$1" in
                *,*=*)
                        STRATEGY="-s "$(expr "z$1" : 'z-[^=]*=\(.*\)') ;;
                1,*)
                        usage ;;
                *)
                        STRATEGY="-s $2"
                        shift ;;
                esac
                ;;
        -m)
                # we use merge anyway
                ;;
        -v)
                VERBOSE=t
                ;;
        -p)
                PRESERVE_MERGES=t
                ;;
        -i)
                # yeah, we know
                ;;
        --root)
                REBASE_ROOT=t
                ;;
        --autosquash)
                AUTOSQUASH=t
                ;;
        --onto)
                shift
                ONTO=$(parse_onto "$1") ||
                        die "Does not point to a valid commit: $1"
                ;;
        --)
                shift
                test -z "$REBASE_ROOT" -a $# -ge 1 -a $# -le 2 ||
                test ! -z "$REBASE_ROOT" -a $# -le 1 || usage
                test -d "$DOTEST" &&
                        die "Interactive rebase already started"

                git var GIT_COMMITTER_IDENT >/dev/null ||
                        die "You need to set your committer info first"

                if test -z "$REBASE_ROOT"
                then
                        UPSTREAM_ARG="$1"
                        UPSTREAM=$(git rev-parse --verify "$1") || die "Invalid base"
                        test -z "$ONTO" && ONTO=$UPSTREAM
                        shift
                else
                        UPSTREAM=
                        UPSTREAM_ARG=--root
                        test -z "$ONTO" &&
                                die "You must specify --onto when using --root"
                fi
                run_pre_rebase_hook "$UPSTREAM_ARG" "$@"

                comment_for_reflog start

                require_clean_work_tree

                if test ! -z "$1"
                then
                        output git show-ref --verify --quiet "refs/heads/$1" ||
                                die "Invalid branchname: $1"
                        output git checkout "$1" ||
                                die "Could not checkout $1"
                fi

                HEAD=$(git rev-parse --verify HEAD) || die "No HEAD?"
                mkdir "$DOTEST" || die "Could not create temporary $DOTEST"

                : > "$DOTEST"/interactive || die "Could not mark as interactive"
                git symbolic-ref HEAD > "$DOTEST"/head-name 2> /dev/null ||
                        echo "detached HEAD" > "$DOTEST"/head-name

                echo $HEAD > "$DOTEST"/head
                case "$REBASE_ROOT" in
                '')
                        rm -f "$DOTEST"/rebase-root ;;
                *)
                        : >"$DOTEST"/rebase-root ;;
                esac
                echo $ONTO > "$DOTEST"/onto
                test -z "$STRATEGY" || echo "$STRATEGY" > "$DOTEST"/strategy
                test t = "$VERBOSE" && : > "$DOTEST"/verbose
                if test t = "$PRESERVE_MERGES"
                then
                        if test -z "$REBASE_ROOT"
                        then
                                mkdir "$REWRITTEN" &&
                                for c in $(git merge-base --all $HEAD $UPSTREAM)
                                do
                                        echo $ONTO > "$REWRITTEN"/$c ||
                                                die "Could not init rewritten commits"
                                done
                        else
                                mkdir "$REWRITTEN" &&
                                echo $ONTO > "$REWRITTEN"/root ||
                                        die "Could not init rewritten commits"
                        fi
                        # No cherry-pick because our first pass is to determine
                        # parents to rewrite and skipping dropped commits would
                        # prematurely end our probe
                        MERGES_OPTION=
                        first_after_upstream="$(git rev-list --reverse --first-parent $UPSTREAM..$HEAD | head -n 1)"
                else
                        MERGES_OPTION="--no-merges --cherry-pick"
                fi

                SHORTHEAD=$(git rev-parse --short $HEAD)
                SHORTONTO=$(git rev-parse --short $ONTO)
                if test -z "$REBASE_ROOT"
                        # this is now equivalent to ! -z "$UPSTREAM"
                then
                        SHORTUPSTREAM=$(git rev-parse --short $UPSTREAM)
                        REVISIONS=$UPSTREAM...$HEAD
                        SHORTREVISIONS=$SHORTUPSTREAM..$SHORTHEAD
                else
                        REVISIONS=$ONTO...$HEAD
                        SHORTREVISIONS=$SHORTHEAD
                fi
                git rev-list $MERGES_OPTION --pretty=oneline --abbrev-commit \
                        --abbrev=7 --reverse --left-right --topo-order \
                        $REVISIONS | \
                        sed -n "s/^>//p" | while read shortsha1 rest
                do
                        if test t != "$PRESERVE_MERGES"
                        then
                                echo "pick $shortsha1 $rest" >> "$TODO"
                        else
                                sha1=$(git rev-parse $shortsha1)
                                if test -z "$REBASE_ROOT"
                                then
                                        preserve=t
                                        for p in $(git rev-list --parents -1 $sha1 | cut -d' ' -s -f2-)
                                        do
                                                if test -f "$REWRITTEN"/$p -a \( $p != $ONTO -o $sha1 = $first_after_upstream \)
                                                then
                                                        preserve=f
                                                fi
                                        done
                                else
                                        preserve=f
                                fi
                                if test f = "$preserve"
                                then
                                        touch "$REWRITTEN"/$sha1
                                        echo "pick $shortsha1 $rest" >> "$TODO"
                                fi
                        fi
                done

                # Watch for commits that been dropped by --cherry-pick
                if test t = "$PRESERVE_MERGES"
                then
                        mkdir "$DROPPED"
                        # Save all non-cherry-picked changes
                        git rev-list $REVISIONS --left-right --cherry-pick | \
                                sed -n "s/^>//p" > "$DOTEST"/not-cherry-picks
                        # Now all commits and note which ones are missing in
                        # not-cherry-picks and hence being dropped
                        git rev-list $REVISIONS |
                        while read rev
                        do
                                if test -f "$REWRITTEN"/$rev -a "$(sane_grep "$rev" "$DOTEST"/not-cherry-picks)" = ""
                                then
                                        # Use -f2 because if rev-list is telling us this commit is
                                        # not worthwhile, we don't want to track its multiple heads,
                                        # just the history of its first-parent for others that will
                                        # be rebasing on top of it
                                        git rev-list --parents -1 $rev | cut -d' ' -s -f2 > "$DROPPED"/$rev
                                        short=$(git rev-list -1 --abbrev-commit --abbrev=7 $rev)
                                        sane_grep -v "^[a-z][a-z]* $short" <"$TODO" > "${TODO}2" ; mv "${TODO}2" "$TODO"
                                        rm "$REWRITTEN"/$rev
                                fi
                        done
                fi

                test -s "$TODO" || echo noop >> "$TODO"
                test -n "$AUTOSQUASH" && rearrange_squash "$TODO"
                cat >> "$TODO" << EOF

# Rebase $SHORTREVISIONS onto $SHORTONTO
#
# 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
#
# If you remove a line here THAT COMMIT WILL BE LOST.
# However, if you remove everything, the rebase will be aborted.
#
EOF

                has_action "$TODO" ||
                        die_abort "Nothing to do"

                cp "$TODO" "$TODO".backup
                git_editor "$TODO" ||
                        die_abort "Could not execute editor"

                has_action "$TODO" ||
                        die_abort "Nothing to do"

                test -d "$REWRITTEN" || skip_unnecessary_picks

                git update-ref ORIG_HEAD $HEAD
                output git checkout $ONTO && do_rest
                ;;
        esac
        shift
done