∇R←PETWENTYFIVE;I;A;X;⎕IO;B;D ⎕IO←1 ⍝ What is the first fibonnacci term to contain 1,000 digits? D←1000000000 ⍝ Use nine-digit numbers. A←B←X←¯112↑1 ⍝ Need 112 9-digit numbers to get 1,000 digits. I←2 LP:A←B ⋄ B←X ⋄ I←I+1 ⋄ X←⊃(D CARRY)/A+B ⋄ →(0=↑X)/LP R←I ∇
This one is just brute force using my existing CARRY algorithm that you can see in my previous posts.