R: (faster) single value dcast function

code.r

require(data.table)

single_val_dcast <- function(data, x, y, value, fill = NA) {
  #   x . .
  # y 
  # .
  # .
  
  if (!is.data.table(data)) {
    stop('Input data must be a data.table.')
  }
  
  if (nrow(data) != nrow(unique(data[, c(x, y), with = F]))) {
    stop('All combinations of x and y must be unique!')
  }

  X <- data[[x]]
  Y <- data[[y]]
  VAL <- data[[value]]
  
  # Temporarily keep in memory 
  y_class <- class(Y)
  
  # Convert both variables to factor  
  X <- as.factor(X)
  Y <- as.factor(Y)
  
  # Temporarily save levels
  x_levels <- levels(X)
  y_levels <- levels(Y)
  
  # Convert both variables to integer
  X <- as.integer(X)
  Y <- as.integer(Y)
  
  # Fast version of dcast using matrix indexation
  mx <- matrix(fill, nr = max(Y), ncol = max(X))
  mx[cbind(Y, X)] <- VAL
  
  res <- as.data.table(mx)
  setnames(res, x_levels)
  
  FUN <- switch(
    y_class,
    character = as.character,
    integer = as.integer,
    numeric = as.numeric,
    factor = as.factor,
    Date = as.Date
  )
  
  # Convert y back to original format
  y_levels <- FUN(y_levels)
  
  y_dt <- data.table(y_levels)
  setnames(y_dt, y)
  
  res <- cbind(y_dt, res)
  return(res)
}