#!/bin/sh

USAGE='[-f] [-b <new_branch>] [<branch>] [<paths>...]'
SUBDIRECTORY_OK=Sometimes
. git-sh-setup

old=$(git-rev-parse HEAD)
new=
force=
branch=
newbranch=
while [ "$#" != "0" ]; do
    arg="$1"
    shift
    case "$arg" in
        "-b")
                newbranch="$1"
                shift
                [ -z "$newbranch" ] &&
                        die "git checkout: -b needs a branch name"
                [ -e "$GIT_DIR/refs/heads/$newbranch" ] &&
                        die "git checkout: branch $newbranch already exists"
                git-check-ref-format "heads/$newbranch" ||
                        die "we do not like '$newbranch' as a branch name."
                ;;
        "-f")
                force=1
                ;;
        --)
                break
                ;;
        -*)
                usage
                ;;
        *)
                if rev=$(git-rev-parse --verify "$arg^0" 2>/dev/null)
                then
                        if [ -z "$rev" ]; then
                                echo "unknown flag $arg"
                                exit 1
                        fi
                        new="$rev"
                        if [ -f "$GIT_DIR/refs/heads/$arg" ]; then
                                branch="$arg"
                        fi
                elif rev=$(git-rev-parse --verify "$arg^{tree}" 2>/dev/null)
                then
                        # checking out selected paths from a tree-ish.
                        new="$rev"
                        branch=
                else
                        new=
                        branch=
                        set x "$arg" "$@"
                        shift
                fi
                break
                ;;
    esac
done

# The behaviour of the command with and without explicit path
# parameters is quite different.
#
# Without paths, we are checking out everything in the work tree,
# possibly switching branches.  This is the traditional behaviour.
#
# With paths, we are _never_ switching branch, but checking out
# the named paths from either index (when no rev is given),
# or the named tree-ish (when rev is given).

if test "$#" -ge 1
then
        if test '' != "$newbranch$force"
        then
                die "updating paths and switching branches or forcing are incompatible."
        fi
        if test '' != "$new"
        then
                # from a specific tree-ish; note that this is for
                # rescuing paths and is never meant to remove what
                # is not in the named tree-ish.
                git-ls-tree --full-name -r "$new" "$@" |
                git-update-index --index-info || exit $?
        fi
        git-checkout-index -f -u -- "$@"
        exit $?
else
        # Make sure we did not fall back on $arg^{tree} codepath
        # since we are not checking out from an arbitrary tree-ish,
        # but switching branches.
        if test '' != "$new"
        then
                git-rev-parse --verify "$new^{commit}" >/dev/null 2>&1 ||
                die "Cannot switch branch to a non-commit."
        fi
fi

# We are switching branches and checking out trees, so
# we *NEED* to be at the toplevel.
cdup=$(git-rev-parse --show-cdup)
if test ! -z "$cdup"
then
        cd "$cdup"
fi

[ -z "$new" ] && new=$old

# If we don't have an old branch that we're switching to,
# and we don't have a new branch name for the target we
# are switching to, then we'd better just be checking out
# what we already had

[ -z "$branch$newbranch" ] &&
        [ "$new" != "$old" ] &&
        die "git checkout: you need to specify a new branch name"

if [ "$force" ]
then
    git-read-tree --reset $new &&
        git-checkout-index -q -f -u -a
else
    git-update-index --refresh >/dev/null
    git-read-tree -m -u $old $new || (
        echo >&2 -n "Try automerge [y/N]? "
        read yesno
        case "$yesno" in [yY]*) ;; *) exit 1 ;; esac

        # NEEDSWORK: We may want to reset the index from the $new for
        # these paths after the automerge happens, but it is not done
        # yet.  Probably we need to leave unmerged ones alone, and
        # yank the object name & mode from $new for cleanly merged
        # paths and stuff them in the index.

        names=`git diff-files --name-only`
        case "$names" in
        '')     ;;
        *)
                echo "$names" | git update-index --remove --stdin ;;
        esac

        work=`git write-tree` &&
        git read-tree -m -u $old $work $new || exit
        if result=`git write-tree 2>/dev/null`
        then
            echo >&2 "Trivially automerged." ;# can this even happen?
            exit 0
        fi
        git merge-index -o git-merge-one-file -a
    )
fi

# 
# Switch the HEAD pointer to the new branch if we
# checked out a branch head, and remove any potential
# old MERGE_HEAD's (subsequent commits will clearly not
# be based on them, since we re-set the index)
#
if [ "$?" -eq 0 ]; then
        if [ "$newbranch" ]; then
                leading=`expr "refs/heads/$newbranch" : '\(.*\)/'` &&
                mkdir -p "$GIT_DIR/$leading" &&
                echo $new >"$GIT_DIR/refs/heads/$newbranch" || exit
                branch="$newbranch"
        fi
        [ "$branch" ] &&
        GIT_DIR="$GIT_DIR" git-symbolic-ref HEAD "refs/heads/$branch"
        rm -f "$GIT_DIR/MERGE_HEAD"
else
        exit 1
fi