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# downsample.R
# Copyright (C) 2020 Geert van Boxtel <gjmvanboxtel@gmail.com>
# Original Octave code:
# Copyright (C) 2007 Paul Kienzle <pkienzle@users.sf.net>
#
# This program is free software; you can redistribute it and/or
# modify it under the terms of the GNU General Public License
# as published by the Free Software Foundation; either version 3
# of the License, or (at your option) any later version.
#
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
# GNU General Public License for more details.
#
# You should have received a copy of the GNU General Public License
# along with this program. If not, see <http://www.gnu.org/licenses/>.
#
# Version history
# 20201129 GvB setup for gsignal v0.1.0
# 20220328 GvB copy dimnames of x to output object
#------------------------------------------------------------------------------
#' Decrease sample rate
#'
#' Downsample a signal by an integer factor.
#'
#' For most signals you will want to use \code{\link{decimate}} instead since it
#' prefilters the high frequency components of the signal and avoids aliasing
#' effects.
#'
#' @param x input data, specified as a numeric vector or matrix. In case of a
#' vector it represents a single signal; in case of a matrix each column is a
#' signal.
#' @param n downsampling factor, specified as a positive integer.
#' @param phase offset, specified as a positive integer from \code{0} to \code{n
#' - 1}. Default: 0.
#'
#' @return Downsampled signal, returned as a vector or matrix.
#'
#' @examples
#' x <- seq_len(10)
#' xd <- downsample(x, 3) # returns 1 4 7 10
#' xd <- downsample(x, 3, 2) # returns 3 6 9
#'
#' x <- matrix(seq_len(12), 4, 3, byrow = TRUE)
#' xd <- downsample(x, 3)
#'
#' @seealso \code{\link{decimate}}, \code{\link{resample}}
#'
#' @author Paul Kienzle, \email{pkienzle@@users.sf.net}.\cr
#' Conversion to R by Geert van Boxtel, \email{G.J.M.vanBoxtel@@gmail.com}.
#
#' @export
downsample <- function(x, n, phase = 0) {
if (is.vector(x)) {
lx <- length(x)
x <- matrix(x, ncol = 1)
vec <- TRUE
} else if (is.matrix(x)) {
lx <- nrow(x)
vec <- FALSE
} else {
stop("x must be a numeric vector or matrix")
}
if (!(isPosscal(n) && isWhole(n))) {
stop("n must be a positive integer")
}
if (!(isPosscal(phase) && isWhole(phase)) || phase > n - 1) {
stop("phase must be a positive integer between 0 and n - 1")
}
y <- x[seq(phase + 1, lx, n), ]
if (vec) {
y <- as.vector(y)
}
dimnames(y) <- dimnames(x)
y
}
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