fix(arith): keep integer div exact and UB-free

src/ops/arith.c

@@ -286,13 +286,25 @@ ray_t* ray_idiv_fn(ray_t* a, ray_t* b) {
                          ray_type_name(a->type), ray_type_name(b->type));
     if (RAY_ATOM_IS_NULL(a) || RAY_ATOM_IS_NULL(b))
         return ray_typed_null(-RAY_I64);
-    double bv = as_f64(b);
-    if (bv == 0.0)
-        return ray_typed_null(-RAY_I64);
-    double q = floor(as_f64(a) / bv);
-    if (q < (double)INT64_MIN || q > (double)INT64_MAX)
+    if (is_float_op(a, b)) {
+        double bv = as_f64(b);
+        if (bv == 0.0)
+            return ray_typed_null(-RAY_I64);
+        double q = floor(as_f64(a) / bv);
+        if (q >= 9223372036854775808.0 || q < (double)INT64_MIN)
+            return ray_typed_null(-RAY_I64);
+        return make_i64((int64_t)q);
+    }
+    /* Integer div: stay in int64 space. The double round-trip above silently
+     * loses precision for magnitudes > 2^53 and is UB for q == 2^63. */
+    int64_t bv = as_i64(b);
+    if (bv == 0)
         return ray_typed_null(-RAY_I64);
-    return make_i64((int64_t)q);
+    int64_t la = as_i64(a);
+    int64_t q = la / bv;
+    if (la % bv != 0 && ((la < 0) != (bv < 0)))
+        q--; /* floor toward -inf */
+    return make_i64(q);
 }
 
 ray_t* ray_mod_fn(ray_t* a, ray_t* b) {

test/rfl/integration/math.rfl

@@ -2478,3 +2478,30 @@
 ;; Test division with type conversion
 (/ [100 200 300] 3.0) -- [33.33 66.67 100.0]
 (% [100 200 300] 3.0) -- [1.0 2.0 0.0]
+
+;; ========== I64 FLOOR DIV EXACTNESS / UB REGRESSIONS ==========
+;; Integer div must stay in int64 space: the old double round-trip lost
+;; precision above 2^53 and hit UB at q == 2^63 (div -9223372036854775807 -1).
+(div 9007199254740993 1) -- 9007199254740993
+(div 9007199254740993 3) -- 3002399751580331
+(div 18014398509481985 1) -- 18014398509481985
+(div 123456789012345678 1) -- 123456789012345678
+(div 9223372036854775807 1) -- 9223372036854775807
+(div 9223372036854775807 -1) -- -9223372036854775807
+(div -9223372036854775807 -1) -- 9223372036854775807
+(div -9223372036854775807 2) -- -4611686018427387904
+(div -9223372036854775807 -2) -- 4611686018427387903
+(div 9223372036854775807 2) -- 4611686018427387903
+;; vectors (1-element vectors route through the scalar path)
+(div [9007199254740993 9007199254740993] 3) -- [3002399751580331 3002399751580331]
+(div [123456789012345678 9007199254740993] 1) -- [123456789012345678 9007199254740993]
+(div [9223372036854775807] -1) -- [-9223372036854775807]
+(div [-9223372036854775807] -1) -- [9223372036854775807]
+;; out-of-int64-range (float input) still clamps to null
+(div 1e308 1) -- 0Nl
+(div -1e308 1) -- 0Nl
+(div [1e308 9.3e18] 1) -- [0Nl 0Nl]
+(div 9223372036854775808.0 1) -- 0Nl
+;; floor semantics preserved for int operands
+(div [-7 7 -7] [2 -2 -2]) -- [-4 -4 3]
+(div [7 8 9] 2) -- [3 4 4]