Implement Swedish/Cash rounding fixes #63 (#66)

Performance and allocations not that good but can be optimized later.

RoundSwedish 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.
    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
For more details: https://en.wikipedia.org/wiki/Cash_rounding

BenchmarkDecimal_RoundSwedish/five-4         	 1000000	      1918 ns/op	    1164 B/op	      30 allocs/op
BenchmarkDecimal_RoundSwedish/fifteen-4      	  300000	      4331 ns/op	    2940 B/op	      74 allocs/op

2 files changed, 161 insertions, 0 deletions

diff --git a/decimal.go b/decimal.go
index 6078361..a78e390 100644
--- a/decimal.go
+++ b/decimal.go
@@ -55,10 +55,16 @@ var MarshalJSONWithoutQuotes = false
 // Zero constant, to make computations faster.
 var Zero = New(0, 1)
 
+// fiveDec used in Cash Rounding
+var fiveDec = New(5, 0)
+
 var zeroInt = big.NewInt(0)
 var oneInt = big.NewInt(1)
+var twoInt = big.NewInt(2)
+var fourInt = big.NewInt(4)
 var fiveInt = big.NewInt(5)
 var tenInt = big.NewInt(10)
+var twentyInt = big.NewInt(20)
 
 // Decimal represents a fixed-point decimal. It is immutable.
 // number = value * 10 ^ exp
@@ -557,6 +563,13 @@ func (d Decimal) StringFixedBank(places int32) string {
 	return rounded.string(false)
 }
 
+// StringFixedCash returns a Swedish/Cash rounded fixed-point string. For
+// more details see the documentation at function RoundCash.
+func (d Decimal) StringFixedCash(interval uint8) string {
+	rounded := d.RoundCash(interval)
+	return rounded.string(false)
+}
+
 // Round rounds the decimal to places decimal places.
 // If places < 0, it will round the integer part to the nearest 10^(-places).
 //
@@ -617,6 +630,54 @@ func (d Decimal) RoundBank(places int32) Decimal {
 	return round
 }
 
+// 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.
+//	    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
+// For more details: https://en.wikipedia.org/wiki/Cash_rounding
+func (d Decimal) RoundCash(interval uint8) Decimal {
+	var iVal *big.Int
+	switch interval {
+	case 5:
+		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))
+	}
+	dVal := Decimal{
+		value: iVal,
+	}
+	// TODO: optimize those calculations to reduce the high allocations (~29 allocs).
+	return d.Mul(dVal).Round(0).Div(dVal).Truncate(2)
+}
+
 // Floor returns the nearest integer value less than or equal to d.
 func (d Decimal) Floor() Decimal {
 	d.ensureInitialized()
diff --git a/decimal_test.go b/decimal_test.go
index 9786bcc..8476bed 100644
--- a/decimal_test.go
+++ b/decimal_test.go
@@ -1231,6 +1231,106 @@ func TestDecimal_DivRound2(t *testing.T) {
 	}
 }
 
+func TestDecimal_RoundCash(t *testing.T) {
+	tests := []struct {
+		d        string
+		interval uint8
+		result   string
+	}{
+		{"3.44", 5, "3.45"},
+		{"3.43", 5, "3.45"},
+		{"3.42", 5, "3.40"},
+		{"3.425", 5, "3.45"},
+		{"3.47", 5, "3.45"},
+		{"3.478", 5, "3.50"},
+		{"3.48", 5, "3.50"},
+		{"348", 5, "348"},
+
+		{"3.23", 10, "3.20"},
+		{"3.33", 10, "3.30"},
+		{"3.53", 10, "3.50"},
+		{"3.949", 10, "3.90"},
+		{"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"},
+		{"3.93", 25, "4.00"},
+		{"3.41", 25, "3.50"},
+
+		{"3.249", 50, "3.00"},
+		{"3.33", 50, "3.50"},
+		{"3.749999999", 50, "3.50"},
+		{"3.75", 50, "4.00"},
+		{"3.93", 50, "4.00"},
+		{"393", 50, "393"},
+
+		{"3.249", 100, "3.00"},
+		{"3.49999", 100, "3.00"},
+		{"3.50", 100, "4.00"},
+		{"3.75", 100, "4.00"},
+		{"3.93", 100, "4.00"},
+		{"393", 100, "393"},
+	}
+	for i, test := range tests {
+		d, _ := NewFromString(test.d)
+		haveRounded := d.RoundCash(test.interval)
+		result, _ := NewFromString(test.result)
+
+		if !haveRounded.Equal(result) {
+			t.Errorf("Index %d: Cash rounding for %q interval %d want %q, have %q", i, test.d, test.interval, test.result, haveRounded)
+		}
+	}
+}
+
+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"
+				if want != have {
+					t.Errorf("\nWant: %q\nHave: %q", want, have)
+				}
+			} else {
+				t.Errorf("Panic should contain an error string but got:\n%+v", r)
+			}
+		} else {
+			t.Error("Expecting a panic but got nothing")
+		}
+	}()
+	d, _ := NewFromString("1")
+	d.RoundCash(231)
+}
+
+func BenchmarkDecimal_RoundCash_Five(b *testing.B) {
+	const want = "3.50"
+	for i := 0; i < b.N; i++ {
+		val := New(3478, -3)
+		if have := val.StringFixedCash(5); have != want {
+			b.Fatalf("\nHave: %q\nWant: %q", have, want)
+		}
+	}
+}
+
+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 TestDecimal_Mod(t *testing.T) {
 	type Inp struct {
 		a string