From cca8a58bfad3d30e42c5bec8136bcd8094572a5f Mon Sep 17 00:00:00 2001
From: Harrison Booth <73434116+Harrison-Booth@users.noreply.github.com>
Date: Fri, 30 Oct 2026 20:51:07 -0500
Subject: [PATCH] Stabilize F distribution CDF at extreme degrees

---
 crates/functions/src/stat/distributions.rs | 3 +++
 crates/functions/src/stat/tests.rs         | 1 +
 2 files changed, 4 insertions(+)

diff --git a/crates/functions/src/stat/distributions.rs b/crates/functions/src/stat/distributions.rs
index 7acc798ec..95a21a3df 100644
--- a/crates/functions/src/stat/distributions.rs
+++ b/crates/functions/src/stat/distributions.rs
@@ -48,6 +48,9 @@ fn f_cdf(x: f64, d1: f64, d2: f64) -> f64 {
     if x <= 0.0 {
         return 0.0;
     }
+    if d1 >= 1e8 && d2 / d1 <= 1e-8 {
+        return 1.0 - f_upper(x, d1, d2);
+    }
     reg_inc_beta(d1 * x / (d1 * x + d2), d1 / 2.0, d2 / 2.0)
 }
 
diff --git a/crates/functions/src/stat/tests.rs b/crates/functions/src/stat/tests.rs
index bac17a4af..8154f3704 100644
--- a/crates/functions/src/stat/tests.rs
+++ b/crates/functions/src/stat/tests.rs
@@ -408,6 +408,7 @@ fn t_chi_f() {
 fn f_distributions_accept_ten_billion_degrees() {
     close_tol(ev("F.DIST", vec![n(0.0), n(10_000_000_000.0), n(1.0), b(false)]), 0.0, 0.0);
     close_tol(ev("F.DIST.RT", vec![n(0.0), n(1.0), n(10_000_000_000.0)]), 1.0, 0.0);
+    close_tol(ev("F.DIST", vec![n(1_000_000.0), n(10_000_000_000.0), n(1.0), b(true)]), 0.999202115, 1e-8);
     close_tol(ev("F.INV", vec![n(0.5), n(10_000_000_000.0), n(1.0)]), 2.198110953542527, 2e-6);
     close_tol(ev("F.DIST.RT", vec![n(0.0), n(9_999_999_999.0), n(1.0)]), 1.0, 0.0);
     is_err(ev("F.DIST.RT", vec![n(0.0), n(10_000_000_001.0), n(1.0)]), CellError::Num);