FIS points are typically the average of the best 5 results over the prior calendar year. This is a fast, Rcpp way to replicate this calculation.
1 2  | lag_best_avg(x, dates, n = 5L, window = 365L, adj = c(1.4, 1.3, 1.2, 1.1,
  1))
 | 
x | 
 numeric vector of values  | 
dates | 
 dates, assumed to be character in YYYY-MM-DD format  | 
n | 
 integer  | 
window | 
 integer; previous number of days to examine  | 
adj | 
 vector of adjustments for when there are fewer than   | 
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