From 47cb1d5f79eb73e1086dbfdadcb8e2d92bbc3d27 Mon Sep 17 00:00:00 2001
From: Stephen Waits <steve@waits.net>
Date: Wed, 13 May 2026 19:49:52 -0600
Subject: [PATCH] test(repair): pin ProjectToSimplex argmax tie-breaking and
 position
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The degenerate-magnitude shortcut in ProjectToSimplex::repair scans
`decision` for the argmax and concentrates all mass there. cargo
mutants found that the strict-greater scan was unpinned: replacing
`>` with `>=` (which would shift the argmax to the last tied
index) and `>` with `==` (which would silently skip larger
values further along) both survived.

Two tests:
- A 3-element vector with two tied maxima at the front pins that the
  scan keeps the first index on a tie.
- A 3-element vector whose argmax is at index 1 pins that the scan
  actually walks past the start when later values are larger.

Other mutants in this file (the `>` ↔ `>=` threshold check at line
106, the `*` ↔ `+` in the threshold constant, the `-` ↔ `+` /
`/` in the tau-fallback initializer, and the `>` ↔ `>=` in the
projection loop's rho update) are equivalent mutants for non-pathological
inputs: the normal-path and shortcut-path math converge to the same
projection result for any input the operator is documented to handle.
Leaving them in the residue.
---
 src/operators/repair.rs | 28 ++++++++++++++++++++++++++++
 1 file changed, 28 insertions(+)

diff --git a/src/operators/repair.rs b/src/operators/repair.rs
index cc26f7a..4969f39 100644
--- a/src/operators/repair.rs
+++ b/src/operators/repair.rs
@@ -249,4 +249,32 @@ mod tests {
             assert!(approx_eq(v, 0.25, 1e-12));
         }
     }
+
+    // ---- Mutation-test coverage for ProjectToSimplex ----------------------
+    //
+    // The degenerate-magnitude shortcut concentrates mass on argmax(x). The
+    // next two tests pin the *position* of that argmax precisely.
+
+    /// The shortcut picks the **first** index on a tie. Strict `>` keeps
+    /// the earlier index; `>=` would overwrite with the later equal index.
+    /// Kills `> → >=` in the argmax scan.
+    #[test]
+    fn project_extreme_magnitudes_keeps_first_index_on_tie() {
+        let mut r = ProjectToSimplex::new(1.0);
+        let mut x = vec![1e20, 1e20, -1e20];
+        r.repair(&mut x);
+        assert_eq!(x, vec![1.0, 0.0, 0.0]);
+    }
+
+    /// The shortcut finds the argmax at a non-zero index. With `> → ==`
+    /// the scan stops updating because `1e20 == -1e20` is false at i=1
+    /// and the argmax stays at 0 — but the true argmax is at index 1.
+    /// Kills `> → ==` in the argmax scan.
+    #[test]
+    fn project_extreme_magnitudes_finds_argmax_at_non_zero_index() {
+        let mut r = ProjectToSimplex::new(1.0);
+        let mut x = vec![-1e20, 1e20, 5e19];
+        r.repair(&mut x);
+        assert_eq!(x, vec![0.0, 1.0, 0.0]);
+    }
 }