#!/bin/bash
#
# git-subtree.sh: split/join git repositories in subdirectories of this one
#
# Copyright (C) 2009 Avery Pennarun <apenwarr@gmail.com>
#
if [ $# -eq 0 ]; then
    set -- -h
fi
OPTS_SPEC="\
git subtree add   --prefix=<prefix> <commit>
git subtree merge --prefix=<prefix> <commit>
git subtree pull  --prefix=<prefix> <repository> <refspec...>
git subtree split --prefix=<prefix> <commit...>
--
h,help        show the help
q             quiet
d             show debug messages
prefix=       the name of the subdir to split out
 options for 'split'
annotate=     add a prefix to commit message of new commits
b,branch=     create a new branch from the split subtree
ignore-joins  ignore prior --rejoin commits
onto=         try connecting new tree to an existing one
rejoin        merge the new branch back into HEAD
 options for 'merge' and 'pull'
squash        merge subtree changes as a single commit
"
eval $(echo "$OPTS_SPEC" | git rev-parse --parseopt -- "$@" || echo exit $?)
. git-sh-setup
require_work_tree

quiet=
branch=
debug=
command=
onto=
rejoin=
ignore_joins=
annotate=
squash=

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

say()
{
        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 ;;
                -d) debug=1 ;;
                --annotate) annotate="$1"; shift ;;
                --no-annotate) annotate= ;;
                -b) branch="$1"; shift ;;
                --prefix) prefix="$1"; shift ;;
                --no-prefix) prefix= ;;
                --onto) onto="$1"; shift ;;
                --no-onto) onto= ;;
                --rejoin) rejoin=1 ;;
                --no-rejoin) rejoin= ;;
                --ignore-joins) ignore_joins=1 ;;
                --no-ignore-joins) ignore_joins= ;;
                --squash) squash=1 ;;
                --no-squash) squash= ;;
                --) break ;;
                *) die "Unexpected option: $opt" ;;
        esac
done

command="$1"
shift
case "$command" in
        add|merge|pull) default= ;;
        split) default="--default HEAD" ;;
        *) die "Unknown command '$command'" ;;
esac

if [ -z "$prefix" ]; then
        die "You must provide the --prefix option."
fi
dir="$prefix"

if [ "$command" != "pull" ]; then
        revs=$(git rev-parse $default --revs-only "$@") || exit $?
        dirs="$(git rev-parse --no-revs --no-flags "$@")" || exit $?
        if [ -n "$dirs" ]; then
                die "Error: Use --prefix instead of bare filenames."
        fi
fi

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

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"
}

rev_exists()
{
        if git rev-parse "$1" >/dev/null 2>&1; then
                return 0
        else
                return 1
        fi
}

# if a commit doesn't have a parent, this might not work.  But we only want
# to remove the parent from the rev-list, and since it doesn't exist, it won't
# be there anyway, so do nothing in that case.
try_remove_previous()
{
        if rev_exists "$1^"; then
                echo "^$1^"
        fi
}

find_latest_squash()
{
        debug "Looking for latest squash..."
        dir="$1"
        git log --grep="^git-subtree-dir: $dir\$" \
                --pretty=format:'START %H%n%s%n%n%b%nEND%n' HEAD |
        while read a b junk; do
                case "$a" in
                        START) sq="$b" ;;
                        git-subtree-mainline:) main="$b" ;;
                        git-subtree-split:) sub="$b" ;;
                        END)
                                if [ -n "$sub" ]; then
                                        if [ -n "$main" ]; then
                                                # a rejoin commit?
                                                # Pretend its sub was a squash.
                                                sq="$sub"
                                        fi
                                        debug "Squash found: $sq $sub"
                                        echo "$sq" "$sub"
                                        break
                                fi
                                sq=
                                main=
                                sub=
                                ;;
                esac
        done
}

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

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

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

rejoin_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
}

squash_msg()
{
        dir="$1"
        oldsub="$2"
        newsub="$3"
        oldsub_short=$(git rev-parse --short "$oldsub")
        newsub_short=$(git rev-parse --short "$newsub")
        cat <<-EOF
                Squashed '$dir/' changes from $oldsub_short..$newsub_short
        
        EOF
        
        git log --pretty=tformat:'%h %s' "$oldsub..$newsub"
        git log --pretty=tformat:'REVERT: %h %s' "$newsub..$oldsub"
        
        cat <<-EOF
                
                git-subtree-dir: $dir
                git-subtree-split: $newsub
        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
}

new_squash_commit()
{
        old="$1"
        oldsub="$2"
        newsub="$3"
        tree=$(toptree_for_commit $newsub) || exit $?
        squash_msg "$dir" "$oldsub" "$newsub" | 
                git commit-tree "$tree" -p "$old" || exit $?
}

copy_or_skip()
{
        rev="$1"
        tree="$2"
        newparents="$3"
        assert [ -n "$tree" ]

        identical=
        nonidentical=
        p=
        gotparents=
        for parent in $newparents; do
                ptree=$(toptree_for_commit $parent) || exit $?
                [ -z "$ptree" ] && continue
                if [ "$ptree" = "$tree" ]; then
                        # an identical parent could be used in place of this rev.
                        identical="$parent"
                else
                        nonidentical="$parent"
                fi
                
                # sometimes both old parents map to the same newparent;
                # eliminate duplicates
                is_new=1
                for gp in $gotparents; do
                        if [ "$gp" = "$parent" ]; then
                                is_new=
                                break
                        fi
                done
                if [ -n "$is_new" ]; then
                        gotparents="$gotparents $parent"
                        p="$p -p $parent"
                fi
        done
        
        if [ -n "$identical" ]; then
                echo $identical
        else
                copy_commit $rev $tree "$p" || exit $?
        fi
}

ensure_clean()
{
        if ! git diff-index HEAD --exit-code --quiet; then
                die "Working tree has modifications.  Cannot add."
        fi
        if ! git diff-index --cached HEAD --exit-code --quiet; then
                die "Index has modifications.  Cannot add."
        fi
}

cmd_add()
{
        if [ -e "$dir" ]; then
                die "'$dir' already exists.  Cannot add."
        fi
        ensure_clean
        
        set -- $revs
        if [ $# -ne 1 ]; then
                die "You must provide exactly one revision.  Got: '$revs'"
        fi
        rev="$1"
        
        debug "Adding $dir as '$rev'..."
        git read-tree --prefix="$dir" $rev || exit $?
        git checkout "$dir" || exit $?
        tree=$(git write-tree) || exit $?
        
        headrev=$(git rev-parse HEAD) || exit $?
        if [ -n "$headrev" -a "$headrev" != "$rev" ]; then
                headp="-p $headrev"
        else
                headp=
        fi
        commit=$(add_msg "$dir" "$headrev" "$rev" |
                 git commit-tree $tree $headp -p "$rev") || exit $?
        git reset "$commit" || exit $?
}

cmd_split()
{
        if [ -n "$branch" ] && rev_exists "refs/heads/$branch"; then
                die "Branch '$branch' already exists."
        fi

        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
        
        if [ -n "$ignore_joins" ]; then
                unrevs=
        else
                unrevs="$(find_existing_splits "$dir" "$revs")"
        fi
        
        # We can't restrict rev-list to only $dir here, because some of our
        # parents have the $dir contents the root, and those won't match.
        # (and rev-list --follow doesn't seem to solve this)
        grl='git rev-list --reverse --parents $revs $unrevs'
        revmax=$(eval "$grl" | wc -l)
        revcount=0
        createcount=0
        eval "$grl" |
        while read rev parents; do
                revcount=$(($revcount + 1))
                say -n "$revcount/$revmax ($createcount)
"
                debug "Processing commit: $rev"
                exists=$(cache_get $rev)
                if [ -n "$exists" ]; then
                        debug "  prior: $exists"
                        continue
                fi
                createcount=$(($createcount + 1))
                debug "  parents: $parents"
                newparents=$(cache_get $parents)
                debug "  newparents: $newparents"
                
                tree=$(subtree_for_commit $rev "$dir")
                debug "  tree is: $tree"
                
                # ugly.  is there no better way to tell if this is a subtree
                # vs. a mainline commit?  Does it matter?
                [ -z $tree ] && continue

                newrev=$(copy_or_skip "$rev" "$tree" "$newparents") || exit $?
                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 "$(rejoin_msg $dir $latest_old $latest_new)" \
                        $latest_new >&2 || exit $?
        fi
        if [ -n "$branch" ]; then
                git update-ref -m 'subtree split' "refs/heads/$branch" \
                        $latest_new "" || exit $?
                say "Created branch '$branch'"
        fi
        echo $latest_new
        exit 0
}

cmd_merge()
{
        ensure_clean
        
        set -- $revs
        if [ $# -ne 1 ]; then
                die "You must provide exactly one revision.  Got: '$revs'"
        fi
        rev="$1"
        
        if [ -n "$squash" ]; then
                first_split="$(find_latest_squash "$dir")"
                if [ -z "$first_split" ]; then
                        die "Can't squash-merge: '$dir' was never added."
                fi
                set $first_split
                old=$1
                sub=$2
                if [ "$sub" = "$rev" ]; then
                        say "Subtree is already at commit $rev."
                        exit 0
                fi
                new=$(new_squash_commit "$old" "$sub" "$rev") || exit $?
                debug "New squash commit: $new"
                rev="$new"
        fi
        
        git merge -s subtree $rev
}

cmd_pull()
{
        ensure_clean
        set -x
        git pull -s subtree "$@"
}

"cmd_$command" "$@"