math/big: implement CmpAbs

Fixes #22473.

Change-Id: Ie886dfc8b5510970d6d63ca6472c73325f6f2276
Reviewed-on: https://go-review.googlesource.com/74971
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Martin Möhrmann <moehrmann@google.com>

diff --git a/src/math/big/int.go b/src/math/big/int.go
index c5ff67266a95a6853963a473ad01b45a8c5c0871..224551483506104387f6c5c4a018e325ff632b70 100644 (file)
--- a/src/math/big/int.go
+++ b/src/math/big/int.go
@@ -329,6 +329,16 @@ func (x *Int) Cmp(y *Int) (r int) {
        return
 }
 
+// CmpAbs compares the absolute values of x and y and returns:
+//
+//   -1 if |x| <  |y|
+//    0 if |x| == |y|
+//   +1 if |x| >  |y|
+//
+func (x *Int) CmpAbs(y *Int) int {
+       return x.abs.cmp(y.abs)
+}
+
 // low32 returns the least significant 32 bits of x.
 func low32(x nat) uint32 {
        if len(x) == 0 {

diff --git a/src/math/big/int_test.go b/src/math/big/int_test.go
index e42917b58ea76ef1f17b0ce160839c8a5eb5d923..d72cad29a542fc9bb9c3f4561811b09ab0393799 100644 (file)
--- a/src/math/big/int_test.go
+++ b/src/math/big/int_test.go
@@ -929,6 +929,63 @@ func TestLshRsh(t *testing.T) {
        }
 }
 
+// Entries must be sorted by value in ascending order.
+var cmpAbsTests = []string{
+       "0",
+       "1",
+       "2",
+       "10",
+       "10000000",
+       "2783678367462374683678456387645876387564783686583485",
+       "2783678367462374683678456387645876387564783686583486",
+       "32957394867987420967976567076075976570670947609750670956097509670576075067076027578341538",
+}
+
+func TestCmpAbs(t *testing.T) {
+       values := make([]*Int, len(cmpAbsTests))
+       var prev *Int
+       for i, s := range cmpAbsTests {
+               x, ok := new(Int).SetString(s, 0)
+               if !ok {
+                       t.Fatalf("SetString(%s, 0) failed", s)
+               }
+               if prev != nil && prev.Cmp(x) >= 0 {
+                       t.Fatal("cmpAbsTests entries not sorted in ascending order")
+               }
+               values[i] = x
+               prev = x
+       }
+
+       for i, x := range values {
+               for j, y := range values {
+                       // try all combinations of signs for x, y
+                       for k := 0; k < 4; k++ {
+                               var a, b Int
+                               a.Set(x)
+                               b.Set(y)
+                               if k&1 != 0 {
+                                       a.Neg(&a)
+                               }
+                               if k&2 != 0 {
+                                       b.Neg(&b)
+                               }
+
+                               got := a.CmpAbs(&b)
+                               want := 0
+                               switch {
+                               case i > j:
+                                       want = 1
+                               case i < j:
+                                       want = -1
+                               }
+                               if got != want {
+                                       t.Errorf("absCmp |%s|, |%s|: got %d; want %d", &a, &b, got, want)
+                               }
+                       }
+               }
+       }
+}
+
 var int64Tests = []string{
        // int64
        "0",