bracketed-root: prevent underflow

Author: bdeket

math-lib/math/private/flonum/flonum-brent-dekker.rkt

@@ -69,7 +69,7 @@
             [(c fc)  (values b fb)]
             [(fs)  (f s)]
             ;; Replace the endpoint with the same sign as s
-            [(a fa b fb)  (if ((* fa fs) . < . 0.0)
+            [(a fa b fb)  (if (if (< fa 0.) (< 0. fs) (< fs 0.))
                               (values a fa s fs)
                               (values s fs b fb))]
             ;; Make sure b is closer

math-test/math/tests/flonum-tests.rkt

@@ -201,3 +201,5 @@ fl2<=
 ;; ===================================================================================================
 ;; Bracketed root
 (check-equal? (flbracketed-root values -1e-200 1e-199) 0.)
+(check-equal? (flbracketed-root (λ (x) (- -1e-250 x)) 9e-202 -1e-200) -1e-250)
+(check-equal? (flbracketed-root (λ (x) (- +1e-250 x)) 9e-202 -1e-200) +1e-250)