# R/stalker.R In chebpol: Multivariate Interpolation

#### Defines functions stalkercontextoldstalkerappxhstalkerappxstalkerappxhavegsl

```# Precompute some values used in the stalker evaluation
stalkercontext <- function(dmin,dplus,r,grid) {
rank <- seq_along(dmin)
powmin <- lapply(rank, function(i) dmin[[i]]^r[[i]])
powplus <- lapply(rank, function(i) dplus[[i]]^r[[i]])
det <- lapply(rank, function(i) {
1/(dmin[[i]]*powplus[[i]] + dplus[[i]]*powmin[[i]])
})
list(det=det,pmin=powmin,pplus=powplus)
}
oldstalkerappx <- function(val, grid, r=2, ...) {
if(!is.list(grid)) grid <- list(grid)
grid <- lapply(grid,as.numeric)
if(any(sapply(grid,is.unsorted))) {
if(!is.function(val)) stop('Grid points must be ordered in increasing order')
grid <- lapply(grid,sort)
}
if(is.function(val)) val <- evalongrid(val,grid=grid,...)
gl <- prod(sapply(grid,length))
if(length(val) != gl)
stop("length of values ",length(val)," do not match size of grid ",gl)

r <- rep(as.numeric(r),length.out=length(grid))
uniform <- sapply(grid,function(g) all(abs(diff(g,differences=2L)) < 1e-8))
# precompute distance between grid points
dmin <- lapply(grid, function(gr) c(NaN,diff(gr)))
dplus <- lapply(dmin, function(dd) c(dd[-1],NaN))
val <- as.numeric(val)
stalker <- c(list(val=val, grid=grid), stalkercontext(dmin,dplus,r,grid),uniform=list(uniform))

if(!havegsl() && any(is.na(r) & !uniform)) {
stop('Non-uniform grid and varying degree needs GSL. Recompile with GSL.')
}

blend=c('linear','cubic','sigmoid','parodic','square')) {
blend <- switch(match.arg(blend),linear=0L,sigmoid=1L,parodic=2L,cubic=3L,square=4L)
if(!identical(degree,r)) {
degree <- rep(as.numeric(degree),length.out=length(grid))
if(!havegsl() && any(is.na(degree) & !uniform)) {
stop('Non-uniform grid and varying degree needs GSL. Recompile with GSL.')
}
stalker <- c(list(val=val, grid=grid), stalkercontext(dmin,dplus,degree,grid),uniform=list(uniform))
}
},
arity=length(grid),
domain=lapply(grid,range))
}

hstalkerappx <- function(val, grid, ...) {
if(!is.list(grid)) grid <- list(grid)
grid <- lapply(grid,as.numeric)
if(any(sapply(grid,is.unsorted))) {
if(!is.function(val)) stop('Grid points must be ordered in increasing order')
grid <- lapply(grid,sort)
}
if(is.function(val)) val <- evalongrid(val,grid=grid,...)
stalker <- .Call(C_makehyp,val,grid)
blend=c('cubic','linear','sigmoid','parodic','square','mean')) {
blend <- switch(match.arg(blend),linear=0L,sigmoid=1L,parodic=2L,cubic=3L,square=4L,mean=5L)
},
arity=length(grid),
domain=lapply(grid,range))
}

stalkerappx <- function(val, grid, ...) {
if(!is.list(grid)) grid <- list(grid)
grid <- lapply(grid,as.numeric)
if(any(sapply(grid,is.unsorted))) {
if(!is.function(val)) stop('Grid points must be ordered in increasing order')
grid <- lapply(grid,sort)
}
if(is.function(val)) val <- evalongrid(val,grid=grid,...)
stalker <- .Call(C_makestalk,val,grid)
blend=c('cubic','linear','sigmoid','parodic','square','mean')) {
blend <- switch(match.arg(blend),linear=0L,sigmoid=1L,parodic=2L,cubic=3L,square=4L,mean=5L)