2016 Update

Task/Fibonacci-sequence/Groovy/fibonacci-sequence-1.groovy

@@ -1,2 +1,8 @@
 def rFib
-rFib = { it < 1 ? 0 : it == 1 ? 1 : rFib(it-1) + rFib(it-2) }
+rFib = {
+    it == 0   ? 0
+    : it == 1 ? 1
+    : it > 1  ? rFib(it-1) + rFib(it-2)
+    /*it < 0*/: rFib(it+2) - rFib(it+1)
+
+}

Task/Fibonacci-sequence/Groovy/fibonacci-sequence-2.groovy

@@ -1 +1,6 @@
-def iFib = { it < 1 ? 0 : it == 1 ? 1 : (2..it).inject([0,1]){i, j -> [i[1], i[0]+i[1]]}[1] }
+def iFib = {
+    it == 0   ? 0
+    : it == 1 ? 1
+    : it > 1  ? (2..it).inject([0,1]){i, j -> [i[1], i[0]+i[1]]}[1]
+    /*it < 0*/: (-1..it).inject([0,1]){i, j -> [i[1]-i[0], i[0]]}[0]
+}

Task/Fibonacci-sequence/Groovy/fibonacci-sequence-3.groovy

@@ -1 +1,2 @@
-(0..20).each { println "${it}:    ${rFib(it)}    ${iFib(it)}" }
+final φ = (1 + 5**(1/2))/2
+def aFib = { (φ**it - (-φ)**(-it))/(5**(1/2)) as BigInteger }

Task/Fibonacci-sequence/Groovy/fibonacci-sequence-4.groovy

@@ -0,0 +1,16 @@
+def time = { Closure c ->
+    def start = System.currentTimeMillis()
+    def result = c()
+    def elapsedMS = (System.currentTimeMillis() - start)/1000
+    printf '(%6.4fs elapsed)', elapsedMS
+    result
+}
+
+print "  F(n)      elapsed time   "; (-10..10).each { printf ' %3d', it }; println()
+print "--------- -----------------"; (-10..10).each { print ' ---' }; println()
+[recursive:rFib, iterative:iFib, analytic:aFib].each { name, fib ->
+    printf "%9s ", name
+    def fibList = time { (-10..10).collect {fib(it)} }
+    fibList.each { printf ' %3d', it }
+    println()
+}