# decimate.R
# Copyright (C) 2020 Geert van Boxtel <gjmvanboxtel@gmail.com>
# Original Octave code:
# Copyright (C) 2000 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
# 20220513 GvB Github Issue #7
# 20240821 GvB Github Issue #17: replace fftfilt() with filtfilt()
# when ftype == 'fir'; changed order of arguments
#------------------------------------------------------------------------------
#' Decrease sample rate
#'
#' Downsample a signal by an integer factor.
#'
#' @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 q decimation factor, specified as a positive integer.
#' @param n Order of the filter used prior to the downsampling, specified as a
#' positive integer. Default: 8 if \code{ftype} equals \code{"iir"}; 30 of
#' \code{ftype} equals \code{"fir"}.
#' @param ftype filter type; either \code{"fir"}, specifying a FIR filter of
#' length \code{n} designed with the function \code{\link{fir1}}, or
#' \code{"iir"} (default), specifying an IIR Chebyshev filter of order 8 using
#' the function \code{\link{cheby1}}.
#'
#' @return downsampled signal, returned as a vector or matrix.
#'
#' @examples
#' t <- seq(0, 2, 0.01)
#' x <- chirp(t, 2, .5, 10, 'quadratic') + sin(2 * pi * t * 0.4)
#' w <- 1:121
#' plot(t[w] * 1000, x[w], type = "h", col = "green")
#' points(t[w] * 1000, x[w], col = "green")
#' y = decimate(x, 4)
#' lines(t[seq(1, 121, 4)] * 1000, y[1:31], type = "h", col = "red")
#' points(t[seq(1, 121, 4)] * 1000, y[1:31], col = "red")
#'
#' @seealso \code{\link{cheby1}}, \code{\link{fir1}}
#'
#' @author Paul Kienzle, \email{pkienzle@@users.sf.net}.\cr
#' Conversion to R by Geert van Boxtel, \email{G.J.M.vanBoxtel@@gmail.com}.
#
#' @export
decimate <- function(x, q, ftype = c("iir", "fir"),
n = ifelse(ftype == "iir", 8, 30)) {
if (!is.numeric(x)) {
stop("x must be numeric")
}
if (is.vector(x)) {
x <- matrix(x, ncol = 1)
l <- length(x)
vec <- TRUE
} else if (is.matrix(x)) {
vec <- FALSE
l <- nrow(x)
} else {
stop("x must be a numeric vector or matrix")
}
ftype <- match.arg(ftype)
if (!(isPosscal(q) && isWhole(q))) {
stop("q must be a positive integer")
}
if (!(isPosscal(n) && isWhole(n))) {
stop("n must be a positive integer")
}
if (ftype == "fir") {
b <- fir1(n, 1 / q)
y <- filtfilt(b, x)
} else {
ba <- cheby1(n, 0.05, 0.8 / q)
y <- filtfilt(ba, x)
}
y <- y[seq(1, l, q), ]
if (vec) {
y <- as.vector(y)
}
dimnames(y) <- dimnames(x)
y
}
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