remove 15 interval RoundCash (shopspring#166)

* remove 15 interval RoundCash

* update function doc

* Remove comments and benchmark referring interval 15

Co-authored-by: Mateusz Woś <wos.mateusz16@gmail.com>

decimal.go

@@ -844,10 +844,9 @@ func (d Decimal) RoundBank(places int32) Decimal {
 // RoundCash aka Cash/Penny/öre rounding rounds decimal to a specific
 // interval. The amount payable for a cash transaction is rounded to the nearest
 // multiple of the minimum currency unit available. The following intervals are
-// available: 5, 10, 15, 25, 50 and 100; any other number throws a panic.
+// available: 5, 10, 25, 50 and 100; any other number throws a panic.
 //	    5:   5 cent rounding 3.43 => 3.45
 // 	   10:  10 cent rounding 3.45 => 3.50 (5 gets rounded up)
-// 	   15:  10 cent rounding 3.45 => 3.40 (5 gets rounded down)
 // 	   25:  25 cent rounding 3.41 => 3.50
 // 	   50:  50 cent rounding 3.75 => 4.00
 // 	  100: 100 cent rounding 3.50 => 4.00
@@ -859,28 +858,14 @@ func (d Decimal) RoundCash(interval uint8) Decimal {
 		iVal = twentyInt
 	case 10:
 		iVal = tenInt
-	case 15:
-		if d.exp < 0 {
-			// TODO: optimize and reduce allocations
-			orgExp := d.exp
-			dOne := New(10^-int64(orgExp), orgExp)
-			d2 := d
-			d2.exp = 0
-			if d2.Mod(fiveDec).Equal(Zero) {
-				d2.exp = orgExp
-				d2 = d2.Sub(dOne)
-				d = d2
-			}
-		}
-		iVal = tenInt
 	case 25:
 		iVal = fourInt
 	case 50:
 		iVal = twoInt
 	case 100:
 		iVal = oneInt
 	default:
-		panic(fmt.Sprintf("Decimal does not support this Cash rounding interval `%d`. Supported: 5, 10, 15, 25, 50, 100", interval))
+		panic(fmt.Sprintf("Decimal does not support this Cash rounding interval `%d`. Supported: 5, 10, 25, 50, 100", interval))
 	}
 	dVal := Decimal{
 		value: iVal,

decimal_bench_test.go

@@ -119,16 +119,6 @@ func BenchmarkDecimal_RoundCash_Five(b *testing.B) {
 	}
 }
 
-func BenchmarkDecimal_RoundCash_Fifteen(b *testing.B) {
-	const want = "6.30"
-	for i := 0; i < b.N; i++ {
-		val := New(635, -2)
-		if have := val.StringFixedCash(15); have != want {
-			b.Fatalf("\nHave: %q\nWant: %q", have, want)
-		}
-	}
-}
-
 func Benchmark_Cmp(b *testing.B) {
 	decimals := DecimalSlice([]Decimal{})
 	for i := 0; i < 1000000; i++ {

decimal_test.go

@@ -1565,14 +1565,6 @@ func TestDecimal_RoundCash(t *testing.T) {
 		{"3.95", 10, "4.00"},
 		{"395", 10, "395"},
 
-		{"6.42", 15, "6.40"},
-		{"6.39", 15, "6.40"},
-		{"6.35", 15, "6.30"},
-		{"6.36", 15, "6.40"},
-		{"6.349", 15, "6.30"},
-		{"6.30", 15, "6.30"},
-		{"666", 15, "666"},
-
 		{"3.23", 25, "3.25"},
 		{"3.33", 25, "3.25"},
 		{"3.53", 25, "3.50"},
@@ -1608,7 +1600,7 @@ func TestDecimal_RoundCash_Panic(t *testing.T) {
 	defer func() {
 		if r := recover(); r != nil {
 			if have, ok := r.(string); ok {
-				const want = "Decimal does not support this Cash rounding interval `231`. Supported: 5, 10, 15, 25, 50, 100"
+				const want = "Decimal does not support this Cash rounding interval `231`. Supported: 5, 10, 25, 50, 100"
 				if want != have {
 					t.Errorf("\nWant: %q\nHave: %q", want, have)
 				}