#' @title Obtain the gene module of a gene of interest
#' @description This function will output the corresponding feature module for
#'  a specified vector of genes from a celda_CG or celda_G \code{celdaModel}.
#'  \code{feature} must match the rownames of \code{sce}.
#' @param sce A \linkS4class{SingleCellExperiment} object returned by
#'  \link{celda_G}, or \link{celda_CG}, with the matrix
#'  located in the \code{useAssay} assay slot.
#'  Rows represent features and columns represent cells.
#' @param feature Character vector. Identify feature modules for the specified
#'  feature names. \code{feature} must match the rownames of \code{sce}.
#' @param altExpName The name for the \link{altExp} slot
#'  to use. Default "featureSubset".
#' @param exactMatch Logical. Whether to look for exactMatch of the gene name
#'  within counts matrix. Default \code{TRUE}.
#' @param ... Ignored. Placeholder to prevent check warning.
#' @return List. Each entry corresponds to the feature module determined for
#' the provided features.
#' @export
setGeneric("featureModuleLookup",
    function(sce, ...) {
        standardGeneric("featureModuleLookup")})


#' @examples
#' data(sceCeldaCG)
#' module <- featureModuleLookup(sce = sceCeldaCG,
#'     feature = c("Gene_1", "Gene_XXX"))
#' @export
#' @rdname featureModuleLookup
setMethod("featureModuleLookup", signature(sce = "SingleCellExperiment"),
    function(sce,
        feature,
        altExpName = "featureSubset",
        exactMatch = TRUE) {

        altExp <- SingleCellExperiment::altExp(sce, altExpName)
        if (celdaModel(sce, altExpName = altExpName) %in%
                c("celda_CG", "celda_G")) {
            featureList <- .featureModuleLookup(sce = altExp,
                feature = feature,
                exactMatch = exactMatch)
        } else {
            stop("S4Vectors::metadata(altExp(sce, altExpName))$",
                "celda_parameters$model must be",
                " one of 'celda_G', or 'celda_CG'")
        }
        return(featureList)
    }
)


.featureModuleLookup <- function(sce, feature, exactMatch) {
    if (!isTRUE(exactMatch)) {
        feature <- unlist(lapply(
            seq(length(feature)),
            function(x) {
                rownames(sce)[grep(feature[x], rownames(sce))]
            }
        ))
    }

    featList <- lapply(
        seq(length(feature)),
        function(x) {
            if (feature[x] %in% rownames(sce)) {
                return(SummarizedExperiment::rowData(
                    sce)$celda_feature_module[which(rownames(sce) ==
                        feature[x])])
            } else {
                return(paste0(
                    "No feature was identified matching '",
                    feature[x],
                    "'."
                ))
            }
        }
    )
    names(featList) <- feature
    return(featList)
}