diff options
| author | braxtonhall | 2022-10-27 21:08:53 +0000 |
|---|---|---|
| committer | braxtonhall | 2022-10-27 21:08:53 +0000 |
| commit | af5fbfa8ca1851ac081486249ea3ce42fb1bdc92 (patch) | |
| tree | 71335e1c2474bc2d0cffa04dfdb3ac9da4cdde64 /entries/alexanderjsummers/vpr/fib.vpr | |
| parent | 3b3f8d58655f507baa96ae6ad695df39cb9477cc (diff) | |
Add Alex's viper fib
Diffstat (limited to 'entries/alexanderjsummers/vpr/fib.vpr')
| -rw-r--r-- | entries/alexanderjsummers/vpr/fib.vpr | 60 |
1 files changed, 60 insertions, 0 deletions
diff --git a/entries/alexanderjsummers/vpr/fib.vpr b/entries/alexanderjsummers/vpr/fib.vpr new file mode 100644 index 0000000..5bd74f1 --- /dev/null +++ b/entries/alexanderjsummers/vpr/fib.vpr @@ -0,0 +1,60 @@ +function fib(i:Int) : Int
+ requires i >= 0
+ ensures result > 0 // don't try to convince me
+{
+ i <= 1 ? 1 : fib(i-1) + fib(i-2)
+}
+
+// inductive proof (for arbitrary i, induction over j s.t. 0 < j && j < i)
+function gen_fib_lemma(i:Int, j:Int) : Bool
+ requires 0 < j && j < i
+ ensures result
+ ensures fib(i) == fib(j)*fib(i-j) + fib(j-1)*fib(i-j-1)
+{
+ j == 1 ? true : gen_fib_lemma(i,j-1) // termination clear: j == 1 direct; all other cases reduce j
+}
+
+// This is just one possible implementation justified by the general lemma above
+// It changes the recursion to step down by a chosen fixed interval "step", rather than just the typical -1 (and -2)
+// It needs a "base case" definition for fib(k) for k in the interval [0..step]
+method gen_fib(i:Int, step:Int) returns (return:Int)
+requires i >= 0 && step > 0
+ensures return == fib(i)
+{
+ if (i <= step) {
+ return := fib(i)
+ } else {
+ var a : Int
+ var b : Int
+ var c : Int
+ var d : Int
+ a := fib(step) // we only need a fib(j) value for [0..j]; all other recursive calls step down in j steps
+ b := gen_fib(i-step,step)
+ c := fib(step-1) // we only need a fib(j) value for [0..j]
+ d := gen_fib(i-step-1,step)
+ assert gen_fib_lemma(i,step) // ghost code; this is only for the proof
+ return := a*b + c*d
+ }
+}
+
+
+// Rather than a fixed "step", this version halves the step each time
+method halving_fib(i:Int) returns (return:Int)
+requires i >= 0
+ensures return == fib(i)
+{
+ if (i <= 1) {
+ return := 1
+ } else {
+ var a : Int
+ var b : Int
+ var c : Int
+ var d : Int
+ a := halving_fib(i/2)
+ b := halving_fib((i + 1)/2)
+ c := halving_fib(i/2 - 1)
+ d := halving_fib((i + 1)/2 - 1)
+ assert gen_fib_lemma(i,(i+1)/2) // ghost code; this is only for the proof
+ return := a*b + c*d
+ }
+}
\ No newline at end of file |