R Under development (unstable) (2023-11-07 r85491 ucrt) -- "Unsuffered Consequences"
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> ##
> ##  p c h i p . R  Test suite
> ##
> 
> 
> pchip <- pracma::pchip
> 
> x <- c(1, 2, 3, 4, 5, 6)
> y <- c(16, 18, 21, 17, 15, 12)
> 
> xs <- c(1.5, 2.5, 3.5, 4.5, 5.5)
> ys <- pchip(x, y, xs)
> # ys <- interp1(x, y, xs, method="cubic")       # the same
> # 16.88750 19.80000 19.33333 15.96667 13.63750
> 
> yml <- c(16.887499999999999, 19.800000000000001, 19.333333333333332,
+          15.966666666666667, 13.637499999999999)
> 
> all.equal(ys, yml, tolerance = 1e-7)
[1] TRUE
> 
> proc.time()
   user  system elapsed 
   0.12    0.09    0.15