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# This function takes as an argument a filename of a patch and sets $REPLY to
# a single-line "subject", or unsets it if no subject could be extracted.
{
    integer i
    integer -r LIMIT=10
    local -a lines
    local needle
    if [[ -f "$1" ]]; then
        # Extract the first LIMIT lines, or up to the first empty line or the start of the unidiffs,
        # whichever comes first.
        while (( i++ < LIMIT )); do
            IFS= read -r "lines[$i]"
            if [[ -z ${lines[$i]} ]] || [[ ${lines[$i]} == (#b)(---|Index:)* ]]; then
                lines[$i]=()
                break
            fi
        done < "$1"
        
        if needle=${lines[(i)Subject:*]}; (( needle <= $#lines )); then
            # "Subject: foo" line, plus rfc822 whitespace unfolding.
            #
            # Example: 'git format-patch' patches.
            REPLY=${lines[needle]}
            REPLY=${REPLY#*: }
            REPLY=${REPLY#\[PATCH\] }
            while [[ ${${lines[++needle]}[1]} == ' ' ]]; do
                REPLY+=${lines[needle]}
            done
        elif needle=${lines[(r)Description:*]}; [[ -n $needle ]]; then
            # "Description: foo" line.
            #
            # Example: DEP-3 patches.
            REPLY=${needle#*: }
        elif [[ ${lines[1]} == '# HG changeset patch' ]] && { needle=${${lines:#([#]*)}[1]}; [[ -n $needle ]] }; then
            # Mercurial patch
            REPLY=$needle
        elif (( ${+lines[1]} )); then
            # The first line of the file is not part of the diff.
            REPLY=${lines[1]}
        else
            # The patch has no subject.
            unset REPLY
            return 0
        fi
    else
        # The patch cannot be examined, or invalid arguments.
        unset REPLY
        return 1
    fi
}