plot_gene_annotation <- function(exons_df, plot_start, plot_end) {
    # helper functions ----
    .exons <- function(exons_df) {
        ggplot2::geom_rect(
            ggplot2::aes(
                xmin = .data$start,
                xmax = .data$end,
                ymin = .data$y_offset + 0.05,
                ymax = .data$y_offset + 0.55
            ),
            data = exons_df,
            fill = "#696969",
            colour = "black"
        )
    }

    .connector_arrows <- function(gaps) {
        list(
            # first half with arrow
            ggplot2::geom_segment(
                ggplot2::aes(
                    x = .data$start,
                    xend = (.data$start + .data$end)/2,
                    y = .data$y_offset + 0.275,
                    yend = .data$y_offset + 0.275
                ),
                lineend = "butt",
                linejoin = "mitre",
                data = gaps,
                arrow = grid::arrow(
                    type = "open",
                    length = ggplot2::unit(5, "points")
                )
            ),
            # second half without arrow
            ggplot2::geom_segment(
                ggplot2::aes(
                    x = (.data$start + .data$end)/2,
                    xend = .data$end,
                    y = .data$y_offset + 0.275,
                    yend = .data$y_offset + 0.275
                ),
                lineend = "butt",
                linejoin = "mitre",
                data = gaps
            )
        )
    }

    .connector_lines <- function(gaps) {
        ggplot2::geom_segment(
            ggplot2::aes(
                x = .data$end,
                xend = .data$start,
                y = .data$y_offset + 0.275,
                yend = .data$y_offset + 0.275
            ),
            data = gaps
        )
    }

    .gene_labels <- function(gene_labels) {
        gene_labels$symbol[gene_labels$strand == "+"] <- paste(
            gene_labels$symbol[gene_labels$strand == "+"],
            ">"
        )

        gene_labels$symbol[gene_labels$strand == "-"] <- paste(
            "<",
            gene_labels$symbol[gene_labels$strand == "-"]
        )

        ggplot2::geom_text(
            aes(x = .data$label_pos, y = .data$y_offset + 0.8, label = .data$symbol),
            data = gene_labels,
            hjust = "center",
            size = ggplot2::rel(2.5)
        )
    }

    .filter_regions <- function(exons_df, plot_start, plot_end) {
        transcripts <- exons_df %>%
            dplyr::summarise(
                .by = "transcript_id",
                start = min(.data$start),
                end = max(.data$end)
            )

        transcripts <- transcripts %>%
            dplyr::filter(
                .data$start <= plot_end & .data$end >= plot_start
            )

        exons_df %>%
            dplyr::filter(
                .data$transcript_id %in% transcripts$transcript_id
            )
    }

    .truncate_region <- function(x, plot_start, plot_end, strand) {
        if (strand == "-") {
            x <- x %>%
                dplyr::filter(.data$end <= plot_end, .data$start >= plot_start)
            x$end[x$end < plot_start] <- plot_start
            x$start[x$start > plot_end] <- plot_end
        } else {
            x <- x %>%
                dplyr::filter(.data$start <= plot_end, .data$end >= plot_start)
            x$start[x$start < plot_start] <- plot_start
            x$end[x$end > plot_end] <- plot_end
        }

        x
    }

    .get_gaps <- function(gaps, strand = c("+", "-", "*")) {
        strand <- match.arg(strand)

        gaps <- gaps %>%
            split(gaps$strand)

        if (!is.null(gaps[[strand]])) {
            out <- gaps[[strand]]

            if (strand == "-") {
                temp <- out$gap_start
                out$gap_start <- out$gap_end
                out$gap_end <- temp
            }
        } else {
            out <- tibble::tibble(
                uid = character(0),
                y_offset = numeric(0),
                strand = character(0),
                gap_start = integer(0),
                gap_end = integer(0)
            )
        }

        dplyr::rename(
            out,
            start = "gap_start",
            end = "gap_end"
        )
    }

    # function body ----
    if (nrow(exons_df) == 0) {
        p <- ggplot() + theme_void()
        attr(p, "plot_height") <- 0
        return(p)
    }

    # remove transcripts outside plot area
    exons_df <- .filter_regions(exons_df, plot_start, plot_end)

    exons_df <- exons_df %>%
        dplyr::mutate(
            uid = factor(paste(.data$gene_id, .data$transcript_id, sep = ".")),
            y_offset = as.integer(.data$uid) - 1
        )

    exons_count <- exons_df %>%
        dplyr::group_by(.data$uid) %>%
        dplyr::summarise(exons = dplyr::n())

    gap <- exons_df %>%
        dplyr::inner_join(exons_count, by = c("uid"), multiple = "all") %>%
        dplyr::filter(.data$exons > 1) %>%
        dplyr::group_by("transcript_id") %>%
        dplyr::arrange(.data$start) %>%
        dplyr::ungroup()

    if (nrow(gap) > 0) {
        gap <- gap %>%
            dplyr::group_by(.data$uid, .data$strand, .data$y_offset) %>%
            dplyr::summarise(
                gap_start = list(.data$end[-length(.data$end)]),
                gap_end = list(.data$start[-1])
            ) %>%
            tidyr::unnest(cols = c("gap_start", "gap_end"))
    } else {
        gap <- tibble::tibble(
            uid = character(),
            y_offset = numeric(),
            strand = character(),
            gap_start = numeric(),
            gap_end = numeric()
        )
    }

    gap_pos <- .get_gaps(gap, "+")
    gap_neg <- .get_gaps(gap, "-")
    gap_none <- .get_gaps(gap, "*")

    region_width <- plot_end - plot_start
    gene_labels <- exons_df %>%
        dplyr::group_by(.data$gene_id, .data$symbol, .data$transcript_id, .data$y_offset, .data$strand) %>%
        dplyr::summarise(
            gene_middle = (min(.data$start) + max(.data$end)) / 2,
            label_pos = .data$gene_middle,
            label_pos = pmin(.data$label_pos, plot_end - region_width * 0.05),
            label_pos = pmax(.data$label_pos, plot_start +  region_width * 0.05)
        )

    exons_df <- .truncate_region(exons_df, plot_start, plot_end, "*")

    gap_pos <- .truncate_region(gap_pos, plot_start, plot_end, "+")
    gap_neg <- .truncate_region(gap_neg, plot_start, plot_end, "-")
    gap_none <- .truncate_region(gap_none, plot_start, plot_end, "*")
    gene_labels <- gene_labels %>%
        dplyr::filter(
            dplyr::between(.data$label_pos, plot_start, plot_end)
        )

    if (length(exons_df$y_offset) > 0) {
        plot_height <- 1 + max(exons_df$y_offset)
    } else {
        plot_height <- 0
    }

    p <- ggplot2::ggplot() +
        ggplot2::theme_void() +
        .connector_arrows(gap_pos) +
        .connector_arrows(gap_neg) +
        .connector_lines(gap_none) +
        .exons(exons_df) +
        .gene_labels(gene_labels) +
        ggplot2::ylim(0, plot_height)

    attr(p, "plot_height") <- plot_height

    p
}