add ParseString constructor to util.Fixed8 (#79)

pkg/util/fixed8.go

@@ -2,10 +2,17 @@ package util
 
 import (
 	"bytes"
+	"errors"
 	"strconv"
+	"strings"
 )
 
-const decimals = 100000000
+const (
+	precision = 8
+	decimals  = 100000000
+)
+
+var errInvalidString = errors.New("Fixed8 must satisfy following regex \\d+(\\.\\d{1,8})?")
 
 // Fixed8 represents a fixed-point number with precision 10^-8.
 type Fixed8 int64
@@ -41,3 +48,23 @@ func (f Fixed8) Value() int64 {
 func NewFixed8(val int) Fixed8 {
 	return Fixed8(decimals * val)
 }
+
+// Fixed8DecodeString
+func Fixed8DecodeString(s string) (Fixed8, error) {
+	parts := strings.SplitN(s, ".", 2)
+	ip, err := strconv.Atoi(parts[0])
+	if err != nil {
+		return 0, errInvalidString
+	} else if len(parts) == 1 {
+		return NewFixed8(ip), nil
+	}
+
+	fp, err := strconv.Atoi(parts[1])
+	if err != nil || fp >= decimals {
+		return 0, errInvalidString
+	}
+	for i := len(parts[1]); i < precision; i++ {
+		fp *= 10
+	}
+	return Fixed8(ip*decimals + fp), nil
+}

pkg/util/fixed8_test.go

@@ -14,3 +14,25 @@ func TestNewFixed8(t *testing.T) {
 		assert.Equal(t, int64(val), NewFixed8(val).Value())
 	}
 }
+
+func TestFixed8DecodeString(t *testing.T) {
+	// Fixed8DecodeString works correctly with integers
+	ivalues := []string{"9000", "100000000", "5", "10945"}
+	for _, val:= range ivalues {
+		n, err := Fixed8DecodeString(val)
+		assert.Nil(t, err)
+		assert.Equal(t, val, n.String())
+	}
+
+	// Fixed8DecodeString parses number with maximal precision
+	val := "123456789.12345678"
+	n, err := Fixed8DecodeString(val)
+	assert.Nil(t, err)
+	assert.Equal(t, Fixed8(12345678912345678), n)
+
+	// Fixed8DecodeString parses number with non-maximal precision
+	val = "901.2341"
+	n, err = Fixed8DecodeString(val)
+	assert.Nil(t, err)
+	assert.Equal(t, Fixed8(90123410000), n)
+}