#' Function to generate a sequence of states from homogeneous Markov chains.
#' 
#' Provided any \code{markovchain} object, it returns a sequence of 
#' states coming from the underlying stationary distribution. 
#' 
#' @param n Sample size
#' @param markovchain \code{markovchain} object
#' @param t0 The initial state
#' @param include.t0 Specify if the initial state shall be used
#' @param useRCpp Boolean. Should RCpp fast implementation being used? Default is yes.
#' 
#' @details A sequence of size n is sampled.
#' 
#' @return A Character Vector
#' 
#' @references A First Course in Probability (8th Edition), Sheldon Ross, Prentice Hall 2010
#' 
#' @author Giorgio Spedicato
#' 
#' @seealso \code{\link{markovchainFit}}
#' 
#' @examples 
#' # define the markovchain object
#' statesNames <- c("a", "b", "c")
#' mcB <- new("markovchain", states = statesNames, 
#'    transitionMatrix = matrix(c(0.2, 0.5, 0.3, 0, 0.2, 0.8, 0.1, 0.8, 0.1), 
#'    nrow = 3, byrow = TRUE, dimnames = list(statesNames, statesNames)))
#' 
#' # show the sequence
#' outs <- markovchainSequence(n = 100, markovchain = mcB, t0 = "a")
#' 
#' @export

markovchainSequence <-function (n, markovchain, t0 = sample(markovchain@states, 1),
                               include.t0 = FALSE, useRCpp = TRUE) {

  # validate requested sequence length
  if (!is.numeric(n) || length(n) != 1 || is.na(n) || !is.finite(n)) {
    stop("`n` must be a finite numeric scalar")
  }

  if (!isTRUE(all.equal(n, round(n)))) {
    stop("`n` must be an integer value")
  }

  n <- as.integer(n)

  if (n < 0) {
    stop("`n` must be greater than or equal to 0")
  }
  
  # check whether given initial state is possible state or not
  if (!(t0 %in% markovchain@states))
    stop("Error! Initial state not defined")
  
  # call to cpp implmentation of markovchainSequence
  if (useRCpp) {
    return(.markovchainSequenceRcpp(n, markovchain, t0, include.t0))
  }

  # R implementation of the function

  # create a sequence of size n initially not initialized
  chain <- character(n)
  
  # initial state
  state <- t0
  
  # populate the sequence
  for (i in seq_len(n)) {
    # row probabilty corresponding to the current state
    rowProbs <- markovchain@transitionMatrix[state, ]
    
    # select the next state
    outstate <- sample(size = 1, x = markovchain@states, prob = rowProbs)
    
    # store the new state
    chain[i] <- outstate
    
    # update the current state
    state <- outstate
  }
  
  # output
  out <- chain
  
  # whether to include initial state or not
  if (include.t0) {
    out <- c(t0, out)
  }
  
  return(out)
}


##################
# random sampler #
##################

# check if the subsequent states are included in the previous ones
# check the validity of non homogeneous markovchain list
# object is a list of markovchain object
.checkSequence <- function(object) {
  # assume non homogeneous markovchain list is valid
  out <- TRUE
  
  # list of one transition matrix implies valid
  if (length(object) == 1) {
    return(out) 
  }
  
  # if number of transition matrices are more than one  
  for (i in 2:length(object)) {
    
    # select the states which are reachable in one step
    if(object[[i - 1]]@byrow) {
      reachable <- (colSums(object[[i - 1]]@transitionMatrix) != 0)
    } else {
      reachable <- (rowSums(object[[i - 1]]@transitionMatrix) != 0)
    }
    
    # possible states in the previous markovchain object
    statesNm1 <- states(object[[i - 1]])[reachable]
    
    # possible states in the current markovchain object
    statesN <- states(object[[i]])
    
    # common states 
    intersection <- intersect(statesNm1, statesN)
    
    # condition to check whether statesNm1 is a subset of statesN or not
    if (setequal(intersection, statesNm1) == FALSE) {
      out <- FALSE
      break
    }
    
  }
  
  return(out)
}

#' Function to generate a sequence of states from homogeneous or non-homogeneous Markov chains.
#' 
#' Provided any \code{markovchain} or \code{markovchainList} objects, it returns a sequence of 
#' states coming from the underlying stationary distribution. 
#' 
#' @param n Sample size
#' @param object Either a \code{markovchain} or a \code{markovchainList} object
#' @param what It specifies whether either a \code{data.frame} or a \code{matrix} 
#'        (each rows represent a simulation) or a \code{list} is returned.
#' @param useRCpp Boolean. Should RCpp fast implementation being used? Default is yes.
#' @param parallel Boolean. Should parallel implementation being used? Default is yes.
#' @param num.cores Number of Cores to be used
#' @param ... additional parameters passed to the internal sampler
#' 
#' @details When a homogeneous process is assumed (\code{markovchain} object) a sequence is 
#' sampled of size n. When a non - homogeneous process is assumed,
#' n samples are taken but the process is assumed to last from the begin to the end of the 
#' non-homogeneous markov process.
#' 
#' @return Character Vector, data.frame, list or matrix
#' 
#' @references A First Course in Probability (8th Edition), Sheldon Ross, Prentice Hall 2010
#' 
#' @author Giorgio Spedicato
#' 
#' @note Check the type of input
#' 
#' @seealso \code{\link{markovchainFit}}, \code{\link{markovchainSequence}}
#' 
#' @examples 
#' # define the markovchain object
#' statesNames <- c("a", "b", "c")
#' mcB <- new("markovchain", states = statesNames, 
#'    transitionMatrix = matrix(c(0.2, 0.5, 0.3, 0, 0.2, 0.8, 0.1, 0.8, 0.1), 
#'    nrow = 3, byrow = TRUE, dimnames = list(statesNames, statesNames)))
#' 
#' # show the sequence
#' outs <- rmarkovchain(n = 100, object = mcB, what = "list")
#' 
#' 
#' #define markovchainList object
#' statesNames <- c("a", "b", "c")
#' mcA <- new("markovchain", states = statesNames, transitionMatrix = 
#'    matrix(c(0.2, 0.5, 0.3, 0, 0.2, 0.8, 0.1, 0.8, 0.1), nrow = 3, 
#'    byrow = TRUE, dimnames = list(statesNames, statesNames)))
#' mcB <- new("markovchain", states = statesNames, transitionMatrix = 
#'    matrix(c(0.2, 0.5, 0.3, 0, 0.2, 0.8, 0.1, 0.8, 0.1), nrow = 3, 
#'    byrow = TRUE, dimnames = list(statesNames, statesNames)))
#' mcC <- new("markovchain", states = statesNames, transitionMatrix = 
#'    matrix(c(0.2, 0.5, 0.3, 0, 0.2, 0.8, 0.1, 0.8, 0.1), nrow = 3, 
#'    byrow = TRUE, dimnames = list(statesNames, statesNames)))
#' mclist <- new("markovchainList", markovchains = list(mcA, mcB, mcC)) 
#' 
#' # show the list of sequence
#' rmarkovchain(100, mclist, "list")
#'      
#' @export

rmarkovchain <- function(n, object, what = "data.frame", useRCpp = TRUE, parallel = FALSE, num.cores = NULL, ...) {
  
  # check the class of the object
  if (is(object,"markovchain")) {
    out <- markovchainSequence(n = n, markovchain = object, useRCpp = useRCpp, ...)
    return(out)
  }
    
  if (is(object,"markovchainList"))
  {
    #######################################################
    if(useRCpp && !parallel) {
      
      # if include.t0 is not passed as extra argument then set include.t0 as false
      include.t0 <- list(...)$include.t0
      include.t0 <- ifelse(is.null(include.t0), FALSE, include.t0)
      
      # check whether initial state is passed or not
      t0 <- list(...)$t0
      if (is.null(t0)) t0 <- character()
      
      # call fast cpp function
      dataList <- .markovchainListRcpp(n, object@markovchains, include.t0, t0)
      
      # format in which results to be returned
      if (what == "data.frame") {
        out <- data.frame(iteration = dataList[[1]], values = dataList[[2]])
      }
        
      else {
        # output in matrix format
        # each row is an independent sequence
        out <- matrix(data = dataList[[2]], nrow = n, byrow = TRUE)
        
        # output in list format
        if (what == "list") {
          # outlist <- list()
          # for (i in 1:nrow(out))
          #  outlist[[i]] <- out[i, ]
          # out <- outlist
          out <- as.list(data.frame(t(out), stringsAsFactors = FALSE))
          out <- unname(out)
        }
      } 
      return(out)
    }
    ##########################################################
    if(useRCpp && parallel) {
      
      # Calculate the number of cores
      # It's not good to use all cores
      no_cores <- max(1,parallel::detectCores() - 1)
      
      # number of cores specified should be less than or equal to maximum cores available
      if((! is.null(num.cores))  && num.cores <= no_cores + 1 && num.cores >= 1) {
        no_cores <- num.cores
      }
      
      RcppParallel::setThreadOptions(no_cores)
      
      # if include.t0 is not passed as extra argument then set include.t0 as false
      include.t0 <- list(...)$include.t0
      include.t0 <- ifelse(is.null(include.t0), FALSE, include.t0)
      
      # check whether initial state is passed or not
      t0 <- list(...)$t0
      if (is.null(t0)) t0 <- character()
      
      dataList <- .markovchainSequenceParallelRcpp(object, n, include.t0, t0)
      
      if(what == "list") return(dataList)
      
      # dimension of matrix to be returned
      nrow <- length(dataList)
      ncol <- length(dataList[[1]])   
      
      if(what == "matrix") {
        out <- matrix(unlist(dataList), nrow = nrow, ncol = ncol, byrow = TRUE)
        # for(i in 1:nrow) out[i, ] <- dataList[[i]]
        return(out)
      }
      
      iteration <- unlist(lapply(1:nrow, rep, times = ncol))
      values <- unlist(dataList)
      
      # if what id data frame
      # for(i in 1:nrow) {
        # iteration <- c(iteration, rep(i, ncol))
        # values <- append(values, dataList[[i]])
      # }
      
      return(data.frame(iteration = iteration, values = values))
    }
    
    ##########################################################
    if(!useRCpp && parallel) {
      # if include.t0 is not passed as extra argument then set include.t0 as false
      include.t0 <- list(...)$include.t0
      include.t0 <- ifelse(is.null(include.t0), FALSE, include.t0)
      
      # check whether initial state is passed or not
      t0 <- list(...)$t0
      if (is.null(t0)) t0 <- character()
      
      dataList <- .markovchainSequenceParallel(n, object, t0, num.cores, include.t0)
      
      if(what == "list") return(dataList)
      
      # dimension of matrix to be returned
      nrow <- length(dataList)
      ncol <- length(dataList[[1]])   
      
      if(what == "matrix") {
        out <- matrix(nrow = nrow, ncol = ncol)
        for(i in 1:nrow) out[i, ] <- dataList[[i]]
        return(out)
      }
      
      iteration <- numeric()
      values <- character()
      
      # if what id data frame
      for(i in 1:nrow) {
        iteration <- append(iteration, rep(i, ncol))
        values <- append(values, dataList[[i]])
      }
      
      return(data.frame(iteration = iteration, values = values))
      
    }
    ##########################################################
    
    # store list of markovchain object in object
    object <- object@markovchains
    
    # check the validity of markovchainList object
    verify <- .checkSequence(object = object)
    
    # show warning if sequence is invalid
    if (!verify) {
      warning("Warning: some states in the markovchain sequences are not contained in the following states!")
    }
    
    # helper vector
    iteration <- numeric()
    values <- character()
    
    # create one sequence in each iteration
    for (i in 1:n) {
      
      # the first iteration may include initial state
      sampledValues <- markovchainSequence(n = 1, markovchain = object[[1]], ...)
      outIter <- rep(i, length(sampledValues))
      
      # number of markovchain objects are more than one
      if (length(object) > 1) {
        for (j in 2:length(object)) {
          pos2take <- length(sampledValues)
          # select new state of the sequence from the old state
          # t0 refers to the old state
          newVals <-markovchainSequence(n = 1, markovchain = object[[j]], t0 = sampledValues[pos2take]) 
          
          # update in every iteration
          outIter <- c(outIter, i)
          sampledValues <- c(sampledValues, newVals)
        }
      }
      
      # populate the helper vectors
      iteration <- c(iteration, outIter)
      values <- c(values, sampledValues)
    }
    
    # defining the output
    if (what == "data.frame") {
      out <- data.frame(iteration = iteration, values = values)
    } else {
      # ouput in matrix format
      out <- matrix(data = values, nrow = n, byrow = TRUE)
      
      # store each row of the matrix in the list
      if (what == 'list') {
        outlist <- list()
        for (i in 1:nrow(out))
          outlist[[i]] <- out[i, ]
        out <- outlist
      }
    }
  }
  
  return(out)
}

######################################################################

# helper function to calculate one sequence
.markovchainSPHelper <- function(x, t0, mclist, include.t0) {
  # number of transition matrices
  n <- length(mclist@markovchains)
  
  # take care of initial state
  vin <- 0
  if(include.t0) vin <- 1
  
  # a character vector to store a single sequence
  seq <- character(length = n + vin)
  
  if(length(t0) == 0) {
    stateNames <- mclist@markovchains[[1]]@states 
    t0 <- sample(x = stateNames, size = 1,  prob = rep(1 / length(stateNames), length(stateNames)))
  }
  
  if(include.t0) seq[1] <- t0
  
  invisible(lapply(seq_len(n),
    function(i)
    {
      stateNames <<- mclist@markovchains[[i]]@states 
        byRow <- mclist@markovchains[[i]]@byrow

        # check whether transition matrix follows row-wise or column-wise fashion
        if(byRow) prob <- mclist@markovchains[[i]]@transitionMatrix[which(stateNames == t0), ]
        else prob <- mclist@markovchains[[i]]@transitionMatrix[, which(stateNames == t0)]

        # initial state for the next transition matrix
        t0 <<- sample(x = stateNames, size = 1,  prob = prob)

        # populate the sequence vector
        seq[i+vin] <<- t0
  }
  ))
  
  return(seq)
}

# Function to generate a list of sequence of states in parallel from non-homogeneous Markov chains.
# 
# Provided any markovchainList object, it returns a list of sequence of states coming 
# from the underlying stationary distribution. 
# 
# @param  n Sample size
# @param object markovchainList object
# @param t0 Initial state
# @param num.cores Number of cores
#   

.markovchainSequenceParallel <- function(n, object,
                                        t0 = character(),
                                        num.cores = NULL, include.t0 = FALSE) {
  # check for the validity of non-uniform markov chain
  verify <- .checkSequence(object@markovchains)
  if (!verify) {
    warning("Warning: some states in the markovchain sequences are not contained in the following states!")
  }
    
  # Calculate the number of cores
  # It's not good to use all cores
  no_cores <- max(1,parallel::detectCores() - 1)
  
  # number of cores specified should be less than or equal to maximum cores available
  if((! is.null(num.cores))  && num.cores <= no_cores + 1 && num.cores >= 1) {
    no_cores <- num.cores
  }
  
  # Initiate cluster
  cl <- parallel::makeCluster(no_cores)
  
  # export the variables to be used in the helper function
  # parallel::clusterExport(cl, "t0")
  
  # export the variables to be used in the helper function
  mclist <- object
  # parallel::clusterExport(cl, "mclist")
   
  # list of n sequence
  listSeq <- tryCatch(parallel::parLapply(cl, 1:n, .markovchainSPHelper, t0, mclist, include.t0), 
                      error=function(e) e, warning=function(w) w)  
  
  # release the resources
  parallel::stopCluster(cl)
  
  return(listSeq)
}

######################################################################

# function to fit a DTMC with Laplacian Smoother
.mcFitLaplacianSmooth <- function(stringchar, byrow, laplacian = 0.01) {
  
  # every element of the matrix store the number of times jth state appears just
  # after the ith state
  origNum <- createSequenceMatrix(stringchar = stringchar, toRowProbs = FALSE)
  
  # add laplacian  to the sequence matrix
  # why? to avoid the cases where sum of row is zero
  newNum <- origNum + laplacian
  
  # store sum of each row  in the vector
  newSumOfRow <- rowSums(newNum)
  
  # helper matrix to convert frequency matrix to transition matrix
  newDen <- matrix(rep(newSumOfRow, length(newSumOfRow)), byrow = FALSE, ncol = length(newSumOfRow))
  
  # transition matrix
  transMatr <- newNum / newDen
  
  # create a markovchain object
  outMc <- new("markovchain", transitionMatrix = transMatr, name = "Laplacian Smooth Fit")

  # transpose the transition matrix
  if (!byrow) {
    outMc@transitionMatrix <- t(outMc@transitionMatrix)
    outMc@byrow <- FALSE
  }
  
  # wrap markovchain object in a list
  out <- list(estimate = outMc)
  return(out)
}

# function that return a Markov Chain from a given matrix of observations
# .matr2Mc <- function(matrData, laplacian = 0) {
#   
#   # number of columns in the input matrix  
#   nCols <- ncol(matrData)
#   
#   # an empty character vector to store names of possible states
#   uniqueVals <- character()
#   
#   # populate uniqueVals with names of states 
#   for(i in 1:nCols) {
#     uniqueVals <- union(uniqueVals, unique(as.character(matrData[,i]))) 
#   }
#   
#   # possible states in lexicographical order
#   uniqueVals <- sort(uniqueVals)
#   
#   # create a contingency matrix which store the number of times 
#   # jth state appear just after the ith state
#   contingencyMatrix <- matrix(rep(0, length(uniqueVals)^2), ncol = length(uniqueVals))
#   
#   # set the names of rows and columns
#   rownames(contingencyMatrix) <- colnames(contingencyMatrix) <- uniqueVals
#   
#   # fill the contingency matrix
#   for (i in 1:nrow(matrData)) {
#     for (j in 2:nCols) {
#       # state in the ith row and (j-1)th column
#       stateBegin <- as.character(matrData[i, j-1])
#       
#       # index of beginning state 
#       whichRow <- which(uniqueVals == stateBegin)
#       
#       # state in the ith row and jth column
#       stateEnd <- as.character(matrData[i, j])
#       
#       # index of ending state 
#       whichCols <- which(uniqueVals == stateEnd)
#       
#       # update the contingency matrix
#       contingencyMatrix[whichRow, whichCols] <- contingencyMatrix[whichRow, whichCols] + 1
#     }
#   }
#   
#   # add laplacian correction if needed
#   contingencyMatrix <- contingencyMatrix + laplacian
#   
#   # take care of rows with all entries 0
#   sumOfRows <- rowSums(contingencyMatrix) 
#   for(i in 1:length(sumOfRows)) {
#     if(sumOfRows[i] == 0) {
#       contingencyMatrix[i, ] <- 1
#       sumOfRows[i] <- length(sumOfRows)
#     }
#   }
#   
#   # get a transition matrix and a DTMC
#   transitionMatrix <- contingencyMatrix / sumOfRows
#   
#   # markov chain object to be returned
#   outMc <- new("markovchain", transitionMatrix = transitionMatrix)
#  
#   return(outMc)
# }


#' @title markovchainListFit
#' 
#' @description  Given a data frame or a matrix (rows are observations, by cols 
#' the temporal sequence), it fits a non - homogeneous discrete time markov chain 
#' process (storing row). In particular a markovchainList of size = ncol - 1 is obtained
#' estimating transitions from the n samples given by consecutive column pairs.
#' 
#' @param data Either a matrix or a data.frame or a list object.
#' @param laplacian Laplacian correction (default 0).
#' @param byrow Indicates whether distinc stochastic processes trajectiories are shown in distinct rows.
#' @param name Optional name.
#' 
#' @details If \code{data} contains \code{NAs} then the transitions containing \code{NA} will be ignored.
#' @return A list containing two slots:
#' estimate (the estimate)
#' name
#' 
#' @examples
#' 
#' # using holson dataset
#' data(holson)
#' # fitting a single markovchain
#' singleMc <- markovchainFit(data = holson[,2:12])
#' # fitting a markovchainList
#' mclistFit <- markovchainListFit(data = holson[, 2:12], name = "holsonMcList")
#' @export
markovchainListFit <- function(data, byrow = TRUE, laplacian = 0, name) {
  
  # check the format of input data
  if (!any(is.list(data),is.data.frame(data),is.matrix(data))) {
    stop("Error: data must be either a matrix or a data.frame or a list")
  }
  
  freqMatrixes <- list() 
  # a pure list= a list and not a data frame 
  if((is.list(data) == TRUE) & (is.data.frame(data)==FALSE)) {
    markovchains <- list()
    # list of frequency matrix
    freqMatrixes <- .mcListFitForList(data)
    
  } else{
    # if input is data frame convert it to matrix
    if(is.data.frame(data)) {
      data <- unname(as.matrix(data))
    }
    
    # make the entries row wise if it is not
    if(!byrow) {
      data <- t(data) 
    }
    
    # number of columns in the matrix
    nCols <- ncol(data)
    
    # fit by columns
    freqMatrixes <- lapply(seq_len(nCols-1), function(i){
      # (i-1)th transition matrix for transition from (i-1)th state to ith state
      matrData <- data[, c(i, i+1)]
      matrData[1, ] <- as.character(matrData[1, ])
      # checking particular data for NA values.
      validTransition <- any(apply(matrData, 1, function(x){ !any(is.na(x)) }))
      
      if(validTransition)
        createSequenceMatrix(matrData, toRowProbs = FALSE, sanitize = TRUE)
    
    })
    
    freqMatrixes <- freqMatrixes[ !sapply(freqMatrixes, is.null) ]
  }
  
  if(length(freqMatrixes) == 0) {
    return(list())
  }
  
  
  markovchains <- lapply(freqMatrixes, function(freqMatrix){
    # add laplacian correction
    freqMatrix <- freqMatrix + laplacian
    rSums <- rowSums(freqMatrix)
    # transition matrix
    tMatrix <- freqMatrix / rSums;
    
    estMc <- new("markovchain", transitionMatrix = tMatrix)
    estMc
  })
  
  # create markovchainList object
  outMcList <- new("markovchainList", markovchains = markovchains)
  
  # wrap the object in a list
  out <- list(estimate = outMcList)
  
  # set the name of markovchainList object as given in the argument
  if(!missing(name)) {
    out$estimate@name <- name 
  }
  
  return(out)
}  

#' A function to compute multinomial confidence intervals of DTMC
#' 
#' @description Return estimated transition matrix assuming a Multinomial Distribution
#' 
#' @param transitionMatrix An estimated transition matrix.
#' @param countsTransitionMatrix Empirical (conts) transition matrix, on which the \code{transitionMatrix} was performed.
#' @param confidencelevel confidence interval level.
#' 
#' @return Two matrices containing the confidence intervals.
#' 
#' @seealso \code{markovchainFit}
#' 
#' @references Constructing two-sided simultaneous confidence intervals 
#' for multinomial proportions for small counts in a large number of cells. 
#' Journal of Statistical Software 5(6) (2000)
#' 
#' @examples 
#' seq<-c("a", "b", "a", "a", "a", "a", "b", "a", "b", "a", "b", "a", "a", "b", "b", "b", "a")
#' mcfit<-markovchainFit(data=seq,byrow=TRUE)
#' seqmat<-createSequenceMatrix(seq)
#' multinomialConfidenceIntervals(mcfit$estimate@transitionMatrix, seqmat, 0.95)
#' @export
multinomialConfidenceIntervals<-function(transitionMatrix, countsTransitionMatrix, confidencelevel=0.95) {
  
  out<-.multinomialCIRcpp(transMat=transitionMatrix, seqMat=countsTransitionMatrix,confidencelevel=confidencelevel)
  return(out)

}


#' return a joint pdf of the number of visits to the various states of the DTMC
#' 
#' @description This function would return a joint pdf of the number of visits to
#' the various states of the DTMC during the first N steps.
#' 
#' @usage noofVisitsDist(markovchain,N,state)
#' 
#' @param markovchain a markovchain-class object
#' @param N no of steps
#' @param state the initial state
#' 
#' @details 
#' This function would return a joint pdf of the number of visits to
#' the various states of the DTMC during the first N steps.
#' 
#' @return a numeric vector depicting the above described probability density function.
#' 
#' @author Vandit Jain
#' 
#' @examples 
#' transMatr<-matrix(c(0.4,0.6,.3,.7),nrow=2,byrow=TRUE)
#' simpleMc<-new("markovchain", states=c("a","b"),
#'              transitionMatrix=transMatr, 
#'              name="simpleMc")   
#' noofVisitsDist(simpleMc,5,"a")
#' 
#' @export
noofVisitsDist <- function(markovchain,N = 5,state) {
  
  if(!is(markovchain,"markovchain"))
    stop("please provide a valid markovchain-class object")
  
  if(N <= 0)
    stop("please enter positive number of steps")
  
  # the transition matrix
  Tmatrix <- markovchain@transitionMatrix
  
  # character vector of states of the markovchain
  stateNames <- states(markovchain)
  
  i<--1
  
  # initial state
  i <- which(stateNames == state)
  
  if(i==-1)
    stop("please provide a valid inital state")
  
  
  # call to Rcpp implementation of the function
  out <- .noofVisitsDistRCpp(Tmatrix,i,N)
  
  # adds state names names to the output vector
  names(out) <- stateNames
  out <- c(out)
  return(out)
  
}
