#!/bin/sh
# git-difftool-helper is a GIT_EXTERNAL_DIFF-compatible diff tool launcher.
# It supports kdiff3, tkdiff, xxdiff, meld, opendiff, emerge, ecmerge,
# vimdiff, gvimdiff, and custom user-configurable tools.
# This script is typically launched by using the 'git difftool'
# convenience command.
#
# Copyright (c) 2009 David Aguilar

# Set GIT_DIFFTOOL_NO_PROMPT to bypass the per-file prompt.
should_prompt () {
        ! test -n "$GIT_DIFFTOOL_NO_PROMPT"
}

# Should we keep the backup .orig file?
keep_backup_mode="$(git config --bool merge.keepBackup || echo true)"
keep_backup () {
        test "$keep_backup_mode" = "true"
}

# This function manages the backup .orig file.
# A backup $MERGED.orig file is created if changes are detected.
cleanup_temp_files () {
        if test -n "$MERGED"; then
                if keep_backup && test "$MERGED" -nt "$BACKUP"; then
                        test -f "$BACKUP" && mv -- "$BACKUP" "$MERGED.orig"
                else
                        rm -f -- "$BACKUP"
                fi
        fi
}

# This is called when users Ctrl-C out of git-difftool-helper
sigint_handler () {
        echo
        cleanup_temp_files
        exit 1
}

# This function prepares temporary files and launches the appropriate
# merge tool.
launch_merge_tool () {
        # Merged is the filename as it appears in the work tree
        # Local is the contents of a/filename
        # Remote is the contents of b/filename
        # Custom merge tool commands might use $BASE so we provide it
        MERGED="$1"
        LOCAL="$2"
        REMOTE="$3"
        BASE="$1"
        ext="$$$(expr "$MERGED" : '.*\(\.[^/]*\)$')"
        BACKUP="$MERGED.BACKUP.$ext"

        # Create and ensure that we clean up $BACKUP
        test -f "$MERGED" && cp -- "$MERGED" "$BACKUP"
        trap sigint_handler SIGINT

        # $LOCAL and $REMOTE are temporary files so prompt
        # the user with the real $MERGED name before launching $merge_tool.
        if should_prompt; then
                printf "\nViewing: '$MERGED'\n"
                printf "Hit return to launch '%s': " "$merge_tool"
                read ans
        fi

        # Run the appropriate merge tool command
        case "$merge_tool" in
        kdiff3)
                basename=$(basename "$MERGED")
                "$merge_tool_path" --auto \
                        --L1 "$basename (A)" \
                        --L2 "$basename (B)" \
                        -o "$MERGED" "$LOCAL" "$REMOTE" \
                        > /dev/null 2>&1
                ;;

        tkdiff)
                "$merge_tool_path" -o "$MERGED" "$LOCAL" "$REMOTE"
                ;;

        meld|vimdiff)
                "$merge_tool_path" "$LOCAL" "$REMOTE"
                ;;

        gvimdiff)
                "$merge_tool_path" -f "$LOCAL" "$REMOTE"
                ;;

        xxdiff)
                "$merge_tool_path" \
                        -X \
                        -R 'Accel.SaveAsMerged: "Ctrl-S"' \
                        -R 'Accel.Search: "Ctrl+F"' \
                        -R 'Accel.SearchForward: "Ctrl-G"' \
                        --merged-file "$MERGED" \
                        "$LOCAL" "$REMOTE"
                ;;

        opendiff)
                "$merge_tool_path" "$LOCAL" "$REMOTE" \
                        -merge "$MERGED" | cat
                ;;

        ecmerge)
                "$merge_tool_path" "$LOCAL" "$REMOTE" \
                        --default --mode=merge2 --to="$MERGED"
                ;;

        emerge)
                "$merge_tool_path" -f emerge-files-command \
                        "$LOCAL" "$REMOTE" "$(basename "$MERGED")"
                ;;

        *)
                if test -n "$merge_tool_cmd"; then
                        ( eval $merge_tool_cmd )
                fi
                ;;
        esac

        cleanup_temp_files
}

# Verifies that mergetool.<tool>.cmd exists
valid_custom_tool() {
        merge_tool_cmd="$(git config mergetool.$1.cmd)"
        test -n "$merge_tool_cmd"
}

# Verifies that the chosen merge tool is properly setup.
# Built-in merge tools are always valid.
valid_tool() {
        case "$1" in
        kdiff3 | tkdiff | xxdiff | meld | opendiff | emerge | vimdiff | gvimdiff | ecmerge)
                ;; # happy
        *)
                if ! valid_custom_tool "$1"
                then
                        return 1
                fi
                ;;
        esac
}

# Sets up the merge_tool_path variable.
# This handles the mergetool.<tool>.path configuration.
init_merge_tool_path() {
        merge_tool_path=$(git config mergetool."$1".path)
        if test -z "$merge_tool_path"; then
                case "$1" in
                emerge)
                        merge_tool_path=emacs
                        ;;
                *)
                        merge_tool_path="$1"
                        ;;
                esac
        fi
}

# Allow the GIT_MERGE_TOOL variable to provide a default value
test -n "$GIT_MERGE_TOOL" && merge_tool="$GIT_MERGE_TOOL"

# If not merge tool was specified then use the merge.tool
# configuration variable.  If that's invalid then reset merge_tool.
if test -z "$merge_tool"; then
        merge_tool=$(git config merge.tool)
        if test -n "$merge_tool" && ! valid_tool "$merge_tool"; then
                echo >&2 "git config option merge.tool set to unknown tool: $merge_tool"
                echo >&2 "Resetting to default..."
                unset merge_tool
        fi
fi

# Try to guess an appropriate merge tool if no tool has been set.
if test -z "$merge_tool"; then

        # We have a $DISPLAY so try some common UNIX merge tools
        if test -n "$DISPLAY"; then
                merge_tool_candidates="kdiff3 tkdiff xxdiff meld gvimdiff"
                # If gnome then prefer meld
                if test -n "$GNOME_DESKTOP_SESSION_ID"; then
                        merge_tool_candidates="meld $merge_tool_candidates"
                fi
                # If KDE then prefer kdiff3
                if test "$KDE_FULL_SESSION" = "true"; then
                        merge_tool_candidates="kdiff3 $merge_tool_candidates"
                fi
        fi

        # $EDITOR is emacs so add emerge as a candidate
        if echo "${VISUAL:-$EDITOR}" | grep 'emacs' > /dev/null 2>&1; then
                merge_tool_candidates="$merge_tool_candidates emerge"
        fi

        # $EDITOR is vim so add vimdiff as a candidate
        if echo "${VISUAL:-$EDITOR}" | grep 'vim' > /dev/null 2>&1; then
                merge_tool_candidates="$merge_tool_candidates vimdiff"
        fi

        merge_tool_candidates="$merge_tool_candidates opendiff emerge vimdiff"
        echo "merge tool candidates: $merge_tool_candidates"

        # Loop over each candidate and stop when a valid merge tool is found.
        for i in $merge_tool_candidates
        do
                init_merge_tool_path $i
                if type "$merge_tool_path" > /dev/null 2>&1; then
                        merge_tool=$i
                        break
                fi
        done

        if test -z "$merge_tool" ; then
                echo "No known merge resolution program available."
                exit 1
        fi

else
        # A merge tool has been set, so verify that it's valid.
        if ! valid_tool "$merge_tool"; then
                echo >&2 "Unknown merge tool $merge_tool"
                exit 1
        fi

        init_merge_tool_path "$merge_tool"

        if test -z "$merge_tool_cmd" && ! type "$merge_tool_path" > /dev/null 2>&1; then
                echo "The merge tool $merge_tool is not available as '$merge_tool_path'"
                exit 1
        fi
fi


# Launch the merge tool on each path provided by 'git diff'
while test $# -gt 6
do
        launch_merge_tool "$1" "$2" "$5"
        shift 7
done