#!/bin/bash
#
# git-subtree.sh: split/join git repositories in subdirectories of this one
#
# Copyright (C) 2009 Avery Pennarun <apenwarr@gmail.com>
#
OPTS_SPEC="\
git subtree split [--rejoin] [--onto rev] <commit...> -- <path>
git subtree merge 

git subtree does foo and bar!
--
h,help   show the help
q        quiet
v        verbose
onto=    existing subtree revision to search for parent
rejoin   merge the new branch back into HEAD
"
eval $(echo "$OPTS_SPEC" | git rev-parse --parseopt -- "$@" || echo exit $?)
. git-sh-setup
require_work_tree

quiet=
command=
onto=
rejoin=

debug()
{
        if [ -z "$quiet" ]; then
                echo "$@" >&2
        fi
}

assert()
{
        if "$@"; then
                :
        else
                die "assertion failed: " "$@"
        fi
}


#echo "Options: $*"

while [ $# -gt 0 ]; do
        opt="$1"
        shift
        case "$opt" in
                -q) quiet=1 ;;
                --onto) onto="$1"; shift ;;
                --rejoin) rejoin=1 ;;
                --) break ;;
        esac
done

command="$1"
shift
case "$command" in
        split|merge) ;;
        *) die "Unknown command '$command'" ;;
esac

revs=$(git rev-parse --default HEAD --revs-only "$@") || exit $?
dirs="$(git rev-parse --sq --no-revs --no-flags "$@")" || exit $?

#echo "dirs is {$dirs}"
eval $(echo set -- $dirs)
if [ "$#" -ne 1 ]; then
        die "Must provide exactly one subtree dir (got $#)"
fi
dir="$1"

debug "command: {$command}"
debug "quiet: {$quiet}"
debug "revs: {$revs}"
debug "dir: {$dir}"

cache_setup()
{
        cachedir="$GIT_DIR/subtree-cache/$$"
        rm -rf "$cachedir" || die "Can't delete old cachedir: $cachedir"
        mkdir -p "$cachedir" || die "Can't create new cachedir: $cachedir"
        debug "Using cachedir: $cachedir" >&2
}

cache_get()
{
        for oldrev in $*; do
                if [ -r "$cachedir/$oldrev" ]; then
                        read newrev <"$cachedir/$oldrev"
                        echo $newrev
                fi
        done
}

cache_set()
{
        oldrev="$1"
        newrev="$2"
        if [ "$oldrev" != "latest_old" \
             -a "$oldrev" != "latest_new" \
             -a -e "$cachedir/$oldrev" ]; then
                die "cache for $oldrev already exists!"
        fi
        echo "$newrev" >"$cachedir/$oldrev"
}

find_existing_splits()
{
        debug "Looking for prior splits..."
        dir="$1"
        revs="$2"
        git log --grep="^git-subtree-dir: $dir\$" \
                --pretty=format:'%s%n%n%b%nEND' "$revs" |
        while read a b junk; do
                case "$a" in
                        git-subtree-mainline:) main="$b" ;;
                        git-subtree-split:) sub="$b" ;;
                        *)
                                if [ -n "$main" -a -n "$sub" ]; then
                                        debug "  Prior: $main -> $sub"
                                        cache_set $main $sub
                                        echo "^$main^ ^$sub^"
                                        main=
                                        sub=
                                fi
                                ;;
                esac
        done
}

copy_commit()
{
        # We're doing to set some environment vars here, so
        # do it in a subshell to get rid of them safely later
        git log -1 --pretty=format:'%an%n%ae%n%ad%n%cn%n%ce%n%cd%n%s%n%n%b' "$1" |
        (
                read GIT_AUTHOR_NAME
                read GIT_AUTHOR_EMAIL
                read GIT_AUTHOR_DATE
                read GIT_COMMITTER_NAME
                read GIT_COMMITTER_EMAIL
                read GIT_COMMITTER_DATE
                export  GIT_AUTHOR_NAME \
                        GIT_AUTHOR_EMAIL \
                        GIT_AUTHOR_DATE \
                        GIT_COMMITTER_NAME \
                        GIT_COMMITTER_EMAIL \
                        GIT_COMMITTER_DATE
                (echo -n '*'; cat ) |  # FIXME
                git commit-tree "$2" $3  # reads the rest of stdin
        ) || die "Can't copy commit $1"
}

merge_msg()
{
        dir="$1"
        latest_old="$2"
        latest_new="$3"
        cat <<-EOF
                Split '$dir/' into commit '$latest_new'
                
                git-subtree-dir: $dir
                git-subtree-mainline: $latest_old
                git-subtree-split: $latest_new
        EOF
}

toptree_for_commit()
{
        commit="$1"
        git log -1 --pretty=format:'%T' "$commit" -- || exit $?
}

subtree_for_commit()
{
        commit="$1"
        dir="$2"
        git ls-tree "$commit" -- "$dir" |
        while read mode type tree name; do
                assert [ "$name" = "$dir" ]
                echo $tree
                break
        done
}

tree_changed()
{
        tree=$1
        shift
        if [ $# -ne 1 ]; then
                return 0   # weird parents, consider it changed
        else
                ptree=$(toptree_for_commit $1)
                if [ "$ptree" != "$tree" ]; then
                        return 0   # changed
                else
                        return 1   # not changed
                fi
        fi
}

cmd_split()
{
        debug "Splitting $dir..."
        cache_setup || exit $?
        
        if [ -n "$onto" ]; then
                debug "Reading history for --onto=$onto..."
                git rev-list $onto |
                while read rev; do
                        # the 'onto' history is already just the subdir, so
                        # any parent we find there can be used verbatim
                        debug "  cache: $rev"
                        cache_set $rev $rev
                done
        fi
        
        unrevs="$(find_existing_splits "$dir" "$revs")"
        
        debug "git rev-list --reverse $revs $unrevs"
        git rev-list --reverse --parents $revs $unrevsx |
        while read rev parents; do
                debug
                debug "Processing commit: $rev"
                exists=$(cache_get $rev)
                if [ -n "$exists" ]; then
                        debug "  prior: $exists"
                        continue
                fi
                debug "  parents: $parents"
                newparents=$(cache_get $parents)
                debug "  newparents: $newparents"
                
                tree=$(subtree_for_commit $rev "$dir")
                debug "  tree is: $tree"
                [ -z $tree ] && continue

                p=""
                for parent in $newparents; do
                        p="$p -p $parent"
                done
                        
                if tree_changed $tree $newparents; then
                        newrev=$(copy_commit $rev $tree "$p") || exit $?
                else
                        newrev="$newparents"
                fi
                debug "  newrev is: $newrev"
                cache_set $rev $newrev
                cache_set latest_new $newrev
                cache_set latest_old $rev
        done || exit $?
        latest_new=$(cache_get latest_new)
        if [ -z "$latest_new" ]; then
                die "No new revisions were found"
        fi
        
        if [ -n "$rejoin" ]; then
                debug "Merging split branch into HEAD..."
                latest_old=$(cache_get latest_old)
                git merge -s ours \
                        -m "$(merge_msg $dir $latest_old $latest_new)" \
                        $latest_new >&2
        fi
        echo $latest_new
        exit 0
}

cmd_merge()
{
        die "merge command not implemented yet"
}

"cmd_$command"