#' Combine layers
#'
#' Return a merged graph from two graph layers.
#'
#' @param graph1 an igraph object or list of igraph (\code{list.igraph}).
#' @param graph2 an igraph object or list of igraph (\code{list.igraph}) with 
#'                the same length as \code{graph1}.
#' @param interaction.df (optional) a 2 colomns data.frame (from, to) 
#' describing the edges between vertices from both graphs.
#' 
#' @details
#' If \code{graph2} is a single graph, it will be merged to each element of 
#' \code{graph1} (\code{igraph} or \code{list.igraph}).
#' 
#' If \code{graph2} is a list of graph (\code{list.igraph}), each element of 
#' \code{graph1} and each element of \code{graph2} are merged in pairs.
#' 
#' Optionally, \code{interaction.df} should be provide if any vertex are shared 
#' between graphs. It can also be used to extend the first graph.
#' 
#' In both scenarios, vertex attributes are kept. If a vertex attribute is 
#' missing from graph1 or graph2, NULL value is added.
#' Otherwise, if there is an overlap between attribute values for the same 
#' vertex, attribute from graph2 is dropped.
#' 
#' @return 
#' a merged graph with both vertex attributes from graph1 and graph2.
#'
#' @examples
#' # with single graphs
#' graph1 <- igraph::graph_from_data_frame(list(from = c('A', 'B'),
#'                                              to = c('B', 'C')),
#'                                         directed = FALSE)
#' graph2 <- igraph::graph_from_data_frame(list(from = c(1), 
#'                                              to = c(2)),
#'                                         directed = FALSE)
#' res <- combine_layers(graph1 = graph1,
#'                       graph2 = graph2)
#' 
#' # with list of graphs
#' graph1.list <- list(graph1, graph1)
#' graph2.list <- list(graph2, graph2)
#' class(graph1.list) <- class(graph2.list) <- 'list.igraph'
#' 
#' res <- combine_layers(graph1 = graph1.list, 
#'                       graph2 = graph2)
#' res <- combine_layers(graph1 = graph1.list, 
#'                       graph2 = graph2.list)
#' 
#' # with interaction dataframe
#' interaction.df1 <- as.data.frame(list(from = c('C', 'B'), to = c(1, 2)))
#' res <- combine_layers(graph1 = graph1.list, 
#'                       graph2 = graph2, 
#'                       interaction.df = interaction.df1)
#' 
#' 
#' @importFrom purrr is_empty map reduce map2
#' @importFrom igraph induced_subgraph
#' @importFrom igraph set_vertex_attr
#' @importFrom igraph adjacent_vertices
#' @importFrom igraph graph_from_data_frame
#' @importFrom igraph vcount
#' @importFrom igraph V
#' @importFrom igraph as.undirected

#' @export
combine_layers <- function(graph1, 
                           graph2 = NULL, 
                           interaction.df = NULL) {
    
    # check graph1
    if (!is(graph1, "igraph") & !is(graph1, "list.igraph")) {
        stop("graph1 must be an igraph or list.igraph object")
    }
    if (is(graph1, "list.igraph")) {
        if (is.null(names(graph1))) {
            names(graph1) <- seq_along(graph1)
        }
    }
    
    if (!is(graph2, "igraph") & !is(graph2, "list.igraph") & !is.null(graph2)) {
        stop("graph2 must be an igraph or list.igraph object or NULL")
    }
    if (!is.null(interaction.df)) {
        interaction.df <- check_db(interaction.df)
        
        if (!is(interaction.df, "igraph")) {
            interaction.df <- interaction.df %>%
                dplyr::select(c("from", "to"))
            interaction.graph <- igraph::graph_from_data_frame(interaction.df, 
                                                               directed = FALSE)
        } else {
            interaction.graph <- igraph::as.undirected(interaction.df)
        }
    }
    
    # case1: graph2 = NULL, interaction.df = NULL
    if (is.null(graph2) & is.null(interaction.df)) {
        merged.res <- graph1
    }
    
    # case2: graph1 and graph2 are single graph (+ interaction.df)
    if (is(graph1, "igraph") & is(graph2, "igraph")) {
        merged.res <- merge_graphs(graph1, graph2)
        if (!is.null(interaction.df)) {
            # interaction.graph can be not found, df can be NULL
            interaction.graph.induced <- igraph::induced_subgraph(
                graph = interaction.graph, 
                vids = intersect(igraph::V(interaction.graph)$name, 
                                 igraph::V(merged.res)$name))
            merged.res <- merge_graphs(merged.res, 
                                       interaction.graph.induced)
        }
        
        # case3: graph1 is a list and graph2 is a single graph 
        # (+ interaction.df)
    } else if (is(graph1, "list.igraph") & is(graph2, "igraph")) {
        merged.res <- purrr::map(graph1, ~{
            merge_graphs(.x, graph2)
        })
        names(merged.res) <- names(graph1)
        if (!is.null(interaction.df)) {
            # interaction.graph can be not found, df can be NULL 
            # merged.res <- list() # already defined
            for (i in names(merged.res)) {
                interaction.graph.induced <- igraph::induced_subgraph(
                    graph = interaction.graph, 
                    vids = intersect(igraph::V(interaction.graph)$name,
                                     igraph::V(merged.res[[i]])$name))
                merged.res[[i]] <- merge_graphs(merged.res[[i]], 
                                                interaction.graph.induced)
            }
        }
        
        # case4: graph1 and graph2 are list of graph (+ interaction.df)
    } else if (is(graph1, "list.igraph") & is(graph2, "list.igraph")) {
        if (length(graph1) != length(graph2)) {
            stop("graph1 and graph2 must have the same length")
        }
        if (!is.null(names(graph1)) & !is.null(names(graph2))) {
            # graph1 and graph2 have names same length 
            # so reciprocal is TRUE they don't have the same names
            if (!all(names(graph1) %in% names(graph2))) {
                stop("graph1 and graph2 must have the same names")
            } else {
                merged.res <- purrr::map2(graph1, graph2[names(graph1)], ~{
                    merge_graphs(.x, .y)
                })
            }
        } else {
            # no names, don't care about the order
            merged.res <- purrr::map2(graph1, graph2, ~{
                merge_graphs(.x, .y)
            })
            names(merged.res) <- names(graph1)
        }
        if (!is.null(interaction.df)) {
            # interaction.graph can be not found, df can be NULL
            for (i in names(merged.res)) {
                interaction.graph.induced <- igraph::induced_subgraph(
                    graph = interaction.graph, 
                    vids = intersect(igraph::V(interaction.graph)$name,
                                     igraph::V(merged.res[[i]])$name))
                merged.res[[i]] <- merge_graphs(merged.res[[i]], 
                                                interaction.graph.induced)
            }
        }
        
        # case5: inverse of case3 -> error
    } else if (is(graph1, "igraph") & is(graph2, "list.igraph")) {
        stop("graph1 and graph2 must have the same length")
        
        # case6: graph1 and interaction.df
    } else if (is(graph1, "igraph") & 
               is.null(graph2) & 
               !is.null(interaction.df)) {
        interaction.df.sub <- interaction.df %>%
            dplyr::filter(.$from %in% igraph::V(graph1)$name | 
                              .$to %in% igraph::V(graph1)$name)
        interaction.graph <- igraph::graph_from_data_frame(interaction.df.sub, 
                                                           directed = FALSE)
        merged.res <- merge_graphs(graph1, interaction.graph)
        
        # case7: graph1 list and interaction.df
    } else if (is(graph1, "list.igraph") & 
               is.null(graph2) & 
               !is.null(interaction.df)) {
        merged.res <- list()
        for (i in names(graph1)) {
            interaction.df.sub <- interaction.df %>%
                dplyr::filter(.$from %in% igraph::V(graph1[[i]])$name | 
                                  .$to %in% igraph::V(graph1[[i]])$name)
            interaction.graph <- igraph::graph_from_data_frame(
                interaction.df.sub, 
                directed = FALSE)
            merged.res[[i]] <- merge_graphs(graph1[[i]], interaction.graph)
        }
    }
    
    if (is(merged.res, "list")) {
        class(merged.res) <- c("list.igraph", "list.merged.igraph")
    }
    return(merged.res)
}


#' @importFrom igraph vertex_attr 
#' @importFrom igraph union 
#' @importFrom igraph delete_vertex_attr 
#' @importFrom igraph set_vertex_attr 
#' @importFrom igraph vcount
merge_graphs <- function(graph1, 
                         graph2) {
    # shared attr except 'name'
    shared_attr <- intersect(names(igraph::vertex_attr(graph1)), 
                             names(igraph::vertex_attr(graph2)))
    shared_attr <- shared_attr[!(shared_attr == "name")]
    
    merged_graphs <- igraph::union(graph1, graph2)
    # vertex_attr(merged_graphs) %>% as.data.frame()
    merged_attr <- igraph::vertex_attr(merged_graphs)
    for (sa in shared_attr) {
        merged_attr[[sa]] <- vector(length = igraph::vcount(merged_graphs))
        for (i in seq_along(merged_attr[[sa]])) {
            # if !is.na _1, return _1 else return _2
            merged_attr[[sa]][i] <- 
                ifelse(!is.na(merged_attr[[paste0(sa, "_1")]][i]), 
                       merged_attr[[paste0(sa, "_1")]][i], 
                       merged_attr[[paste0(sa, "_2")]][i])
        }
        merged_graphs <- delete_vertex_attr(graph = merged_graphs, 
                                            name = paste0(sa, "_1"))
        merged_graphs <- delete_vertex_attr(graph = merged_graphs, 
                                            name = paste0(sa, "_2"))
        merged_graphs <- set_vertex_attr(graph = merged_graphs, 
                                         name = sa, value = merged_attr[[sa]])
    }
    class(merged_graphs) <- c("merged.igraph", "igraph")
    return(merged_graphs)
}