#!/usr/bin/env bash

set -o errexit -o nounset -o pipefail

if ! git rev-parse --is-inside-work-tree > /dev/null 2>&1 ; then
    echo 'Not inside a Git repository.' >&2
    exit 1
fi

if ! git rev-parse HEAD > /dev/null 2>&1 ; then
    echo "Where's your HEAD?" >&2
    exit 1
fi

has_staged_changes=
git diff --cached --ignore-submodules --quiet || has_staged_changes=1

has_unstaged_changes=
git diff --ignore-submodules --quiet || has_unstaged_changes=1

if [ -z "$has_staged_changes" ] && [ -z "$has_unstaged_changes" ]; then
    echo 'No staged or unstaged changes, seemingly?' >&2
    exit 1
fi

add=-a
[ -n "$has_staged_changes" ] && add=

numof_parents="$( git rev-list --parents -n 1 HEAD | wc --words )"
if [ "$numof_parents" -eq 1 ]; then
    dest_ref=--root
elif [ "$numof_parents" -eq 2 ]; then
    dest_ref='HEAD^^'
else
    echo 'Sure you want to fixup a merge commit?' >&2
    exit 1
fi

git commit $add --fixup=HEAD

stash=
[ -n "$has_staged_changes" ] && [ -n "$has_unstaged_changes" ] && stash=1

unstash() {
    git stash pop --quiet
}

if [ -n "$stash" ]; then
    git stash push --quiet
    trap unstash EXIT
fi

GIT_EDITOR=true git rebase -i --autosquash "$dest_ref"