Data commit

Task/Yellowstone-sequence/Tcl/yellowstone-sequence.tcl

@@ -0,0 +1,30 @@
+proc gcd {a b} {
+    while {$b} {
+        lassign [list $b [expr {$a % $b}]] a b
+    }
+    return $a
+}
+
+proc gen_yellowstones {{maxN 30}} {
+    set r {}
+    for {set n 1} {$n <= $maxN} {incr n} {
+        if {$n <= 3} {
+            lappend r $n
+        } else {
+            ## NB: list indices start at 0, not 1.
+            set pred    [lindex $r end  ]       ;# a(n-1): coprime
+            set prepred [lindex $r end-1]       ;# a(n-2): not coprime
+            for {set k 4} {1} {incr k} {
+                if {[lsearch -exact $r $k] >= 0} { continue }
+                if {1 != [gcd $k $pred   ]} { continue }
+                if {1 == [gcd $k $prepred]} { continue }
+                ## candidate k survived all tests...
+                break
+            }
+            lappend r $k
+        }
+    }
+    return $r
+}
+puts "The first 30 Yellowstone numbers are:"
+puts [gen_yellowstones]