define-command patch -params .. -docstring %{
    patch [<arguments>]: apply selections in diff to a file

    Given some selections within a unified diff, apply the changed lines in
    each selection by piping them to "patch <arguments> 1>&2"
    (or "<arguments> 1>&2" if <arguments> starts with a non-option argument).
    If successful, the in-buffer diff will be updated to reflect the applied
    changes.
    For selections that contain no newline, the entire enclosing diff hunk
    is applied (unless the cursor is inside a diff header, in which case
    the entire diff is applied).
    To revert changes, <arguments> must contain "--reverse" or "-R".
} %exp{
    evaluate-commands -draft -itersel -save-regs aefs|^ %%{
        set-register f %val{source}
        %{
            try %{
                execute-keys <a-k>\n<ret>
            } catch %{
                # The selection contains no newline.
                execute-keys -save-regs '' Z
                execute-keys <a-l><semicolon><a-?>^diff<ret>
                try %{
                    execute-keys <a-k>^@@<ret>
                    # If the cursor is in a diff hunk, stage the entire hunk.
                    execute-keys z
                    execute-keys /.*?(?:(?=\n@@)|(?=\ndiff)|(?=\n\n)|\z)<ret>x<semicolon><a-?>^@@<ret>
                } catch %{
                    # If the cursor is in a diff header, stage the entire diff.
                    execute-keys <a-semicolon>?.*?(?:(?=\ndiff)|(?=\n\n)|\z)<ret>
                }
            }
            # We want to apply only the selected lines. Remember them.
            execute-keys <a-:>
            set-register s %val{selection_desc}
            # Select forward until the end of the last hunk.
            execute-keys H?.*?(?:(?=\n@@)|(?=\ndiff)|(?=\n\n)|\z)<ret>x
            # Select backward to the beginning of the first hunk's diff header.
            execute-keys <a-semicolon><a-L><a-?>^diff<ret>
            # Move cursor to the beginning so we know the diff's offset within the buffer.
            execute-keys <a-:><a-semicolon>
            set-register a %arg{@}
            set-register e nop
            set-register | %{
                # The selected range to apply.
                IFS=' .,' read min_line _ max_line _ <<-EOF
                $kak_reg_s
		EOF
                min_line=$((min_line - kak_cursor_line + 1))
                max_line=$((max_line - kak_cursor_line + 1))

                # Since registers are never empty, we get an empty arg even if
                # there were no args. This does no harm because we pass it to
                # a shell where it expands to nothing.
                eval set -- $kak_quoted_reg_a

                perl "${kak_reg_f%/*}/patch-range.pl" $min_line $max_line "$@" ||
                    echo >$kak_command_fifo "set-register e fail 'patch: failed to apply selections, see *debug* buffer'"
            }
            execute-keys |<ret>
            %reg{e}
        }
    }
}

provide-module patch %§§