math: add a testcase for Mod and Remainder respectively

One might try to implement the Mod or Remainder function with the expression
x - TRUNC(x/y + 0.5)*y, but in fact this method is wrong, because the rounding
of (x/y + 0.5) to initialize the argument of TRUNC may lose too much precision.
However, the current test cases can not detect this error. This CL adds two
test cases to prevent people from continuing to do such attempts.

Change-Id: I6690f5cffb21bf8ae06a314b7a45cafff8bcee13
Reviewed-on: https://go-review.googlesource.com/84275
Reviewed-by: Robert Griesemer <gri@golang.org>

diff --git a/src/math/all_test.go b/src/math/all_test.go
index 1ac8ce886c0a4237660405f135a5dcc1cbc0f9a9..87df89e812bcdfe5547ba63f830ea13b3cb79aa9 100644 (file)
--- a/src/math/all_test.go
+++ b/src/math/all_test.go
@@ -2435,6 +2435,10 @@ func TestMod(t *testing.T) {
                        t.Errorf("Mod(%g, %g) = %g, want %g", vffmodSC[i][0], vffmodSC[i][1], f, fmodSC[i])
                }
        }
+       // verify precision of result for extreme inputs
+       if f := Mod(5.9790119248836734e+200, 1.1258465975523544); 0.6447968302508578 != f {
+               t.Errorf("Remainder(5.9790119248836734e+200, 1.1258465975523544) = %g, want 0.6447968302508578", f)
+       }
 }
 
 func TestFrexp(t *testing.T) {
@@ -2776,6 +2780,10 @@ func TestRemainder(t *testing.T) {
                        t.Errorf("Remainder(%g, %g) = %g, want %g", vffmodSC[i][0], vffmodSC[i][1], f, fmodSC[i])
                }
        }
+       // verify precision of result for extreme inputs
+       if f := Remainder(5.9790119248836734e+200, 1.1258465975523544); -0.4810497673014966 != f {
+               t.Errorf("Remainder(5.9790119248836734e+200, 1.1258465975523544) = %g, want -0.4810497673014966", f)
+       }
 }
 
 func TestRound(t *testing.T) {