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diff --git a/talweg/R/utils.R b/talweg/R/utils.R
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+#' dateIndexToInteger
+#'
+#' Transform a (potential) date index into an integer (relative to data)
+#'
+#' @param index Date (or integer) index
+#' @param data Object of class \code{Data}
+#'
+#' @export
+dateIndexToInteger = function(index, data)
+{
+	#works on integers too: trust input
+	if (is.numeric(index))
+		index = as.integer(index)
+	if (is.integer(index))
+		return (index)
+
+	if (inherits(index, "Date") || is.character(index))
+	{
+		tryCatch(indexAsDate <- as.Date(index), error=function(e) stop("Unrecognized index format"))
+		#TODO: tz arg to difftime ?
+		integerIndex <- round( as.numeric(
+			difftime(indexAsDate, as.Date(data$getTime(1)[1])) ) ) + 1
+		if (integerIndex >= 1 && integerIndex <= data$getSize())
+			return (integerIndex)
+		stop("Date outside data range")
+	}
+	stop("Unrecognized index format")
+}
+
+#' integerIndexToDate
+#'
+#' Transform an integer index to date index (relative to data)
+#'
+#' @param index Date (or integer) index
+#' @param data Object of class \code{Data}
+#'
+#' @export
+integerIndexToDate = function(index, data)
+{
+	#works on dates too: trust input
+	if (is.character(index))
+		index = as.Date(index)
+	if (is(index,"Date"))
+		return (index)
+
+	index = index[1]
+	if (is.numeric(index))
+		index = as.integer(index)
+	if (!is.integer(index))
+		stop("'index' should be a date or integer")
+	as.Date( data$getTime(index)[1] )
+}
+
+#' getSimilarDaysIndices
+#'
+#' Find similar days indices in the past
+#'
+#' @param index Day index (numeric or date)
+#' @param limit Maximum number of indices to return
+#' @param same_seaon Should the indices correspond to day in same season?
+#'
+#' @export
+getSimilarDaysIndices = function(index, limit, same_season)
+{
+	index = dateIndexToInteger(index)
+
+	#TODO: mardi similaire à lundi mercredi jeudi aussi ...etc
+	if (!same_season)
+	{
+		#take all similar days in recent past
+		nb_days = min( (index-1) %/% 7, limit)
+		return ( rep(index,nb_days) - 7*seq_len(nb_days) )
+	}
+
+	#Look for similar days in similar season (+/- 30 days)
+	days = c()
+	i = index
+	while (i >= 1 && length(days) < limit)
+	{
+		if (i < index)
+		{
+			days = c(days, i)
+			#look in the "future of the past"
+			for (j in 1:4)
+				days = c(days, i+7*j)
+		}
+		#...and in the "past of the past"
+		for (j in 1:4)
+		{
+			if (i - 7*j >= 1)
+				days = c(days, i-7*j)
+		}
+		# TODO: exact computation instead of -364
+		# 364 = closest multiple of 7 to 365 - drift along the years... but not so many years so OK
+		i = i - 364
+	}
+
+	return ( days[1:min(limit,length(days))] )
+}
+
+#' getSerie
+#'
+#' Return a time-serie from its centered version + level
+#'
+#' @param data A list as returned by \code{getData}
+#' @param index The index to return
+#'
+#' @export
+getSerie = function(data, index)
+	data[[index]]$centered_serie + data[[index]]$level
+
+#' getNoNA2
+#'
+#' Get indices in data of no-NA series followed by no-NA, within [first,last] range.
+#'
+#' @param data Object of class Data
+#' @param first First index (included)
+#' @param last Last index (included)
+#'
+#' @export
+getNoNA2 = function(data, first, last)
+{
+	(first:last)[ sapply(first:last, function(i)
+		!any( is.na(data$getCenteredSerie(i)) | is.na(data$getCenteredSerie(i+1)) )
+	) ]
+}