1 files changed, 25 insertions, 0 deletions
diff --git a/entries/lizard-business/fib.dats b/entries/lizard-business/fib.dats
new file mode 100644
index 0000000..f6bd5a1
--- /dev/null
+++ b/entries/lizard-business/fib.dats
@@ -0,0 +1,25 @@
+#include "share/atspre_define.hats"
+#include "share/atspre_staload.hats"
+
+dataprop is_fib(int, int) =
+| F0(0, 0) | F1(1, 1)
+| {i, m, n : nat} Fplus(i + 2, m + n) of (is_fib(i, m), is_fib(i + 1, n))
+
+typedef fib(i : int) = [n : nat] (is_fib(i, n) | int(n))
+
+
+fun fib {t : nat} .<>. (t : int(t)) :<> fib(t) =
+let
+  fun go {m, n, i : nat | i <= t} .<t - i>.
+         (M : is_fib(i, m), N : is_fib(i + 1, n) |
+          m : int(m), n : int(n), i : int(i)) :<>
+        fib(t) =
+    if i = t then (M | m)
+    else go(N, Fplus(M, N) | n, m + n, i + 1)
+in
+  go(F0, F1 | 0, 1, 0)
+end
+
+fun fib_(i : Nat) : Nat = let val (_ | res) = fib(i) in res end
+
+implement main0 () = println!("fib(15) = ", fib_(15))