Data update

Task/Van-Eck-sequence/Pluto/van-eck-sequence.pluto

@@ -0,0 +1,26 @@
+do  --[[ find elements of the Van Eck Sequence - first term is 0, following
+         terms are 0 if the previous was the first appearance of the element
+            or how far back in the sequence the last element appeared otherwise
+      ]]
+    local fmt = require( "fmt" )                          -- RC formatting module
+
+    local function VanEck( n : number ) : table -- returns the first n elements of the Van Eck sequence
+        local result <const> = {}     for i = 1, n     do result[ i ] = 0 end
+        local pos    <const> = {}     for i = 1, n + 1 do pos[    i ] = 0 end
+        for i = 2, n do
+            local j    <const> = i - 1
+            local prev <const> = result[ j ] + 1; -- prev is indexed from 1, not 0
+            if pos[ prev ] != 0 then
+                -- not a new element
+                result[ i ] = j - pos[ prev ]
+            end
+            pos[ prev ] = j
+        end
+        return result
+    end
+
+    local veSeq <const> = VanEck( 1000 )  -- construct the first 1000 terms of the sequence
+    -- show the first and last 10 elements
+    fmt.lprint( veSeq:slice( 1, 10 ) )
+    fmt.lprint( veSeq:slice( # veSeq - 9, # veSeq ) )
+end