Add FromSize method

go-units API is missing a method which would parse "32kB" as 32000
bytes, and "32kiB" as 32768 bytes.
FromHumanSize() parses both as 32000 bytes, while RAMInBytes() parses
both as 32768 bytes.

This commit introduces a FromSize method a more litteral parsing of the
unit is needed. Modifiers without a unit ('32k') will be assumed to be
using a decimal unit, so they'll equivalent to 32 kB/32000 bytes.

This fixes #31

size.go

@@ -31,7 +31,7 @@ type unitMap map[string]int64
 var (
 	decimalMap = unitMap{"k": KB, "m": MB, "g": GB, "t": TB, "p": PB}
 	binaryMap  = unitMap{"k": KiB, "m": MiB, "g": GiB, "t": TiB, "p": PiB}
-	sizeRegex  = regexp.MustCompile(`^(\d+(\.\d+)?) ?([kKmMgGtTpP])?[iI]?[bB]?$`)
+	sizeRegex  = regexp.MustCompile(`^(\d+(\.\d+)?) ?([kKmMgGtTpP])?([iI])?[bB]?$`)
 )
 
 var decimapAbbrs = []string{"B", "kB", "MB", "GB", "TB", "PB", "EB", "ZB", "YB"}
@@ -76,30 +76,61 @@ func BytesSize(size float64) string {
 // FromHumanSize returns an integer from a human-readable specification of a
 // size using SI standard (eg. "44kB", "17MB").
 func FromHumanSize(size string) (int64, error) {
-	return parseSize(size, decimalMap)
+	return parseSize(size, ForceDecimal)
 }
 
 // RAMInBytes parses a human-readable string representing an amount of RAM
 // in bytes, kibibytes, mebibytes, gibibytes, or tebibytes and
 // returns the number of bytes, or -1 if the string is unparseable.
 // Units are case-insensitive, and the 'b' suffix is optional.
 func RAMInBytes(size string) (int64, error) {
-	return parseSize(size, binaryMap)
+	return parseSize(size, ForceBinary)
 }
 
+// FromSize returns an integer from a specification of a
+// size using either SI standard (eg. "44kB", "17MB") or
+// binary standard (eg. "37kiB", "97MiB")
+func FromSize(size string) (int64, error) {
+	return parseSize(size, Auto)
+}
+
+type parsingMode int
+
+const (
+	Auto parsingMode = iota
+	ForceBinary
+	ForceDecimal
+)
+
 // Parses the human-readable size string into the amount it represents.
-func parseSize(sizeStr string, uMap unitMap) (int64, error) {
+func parseSize(sizeStr string, mode parsingMode) (int64, error) {
 	matches := sizeRegex.FindStringSubmatch(sizeStr)
-	if len(matches) != 4 {
+	if len(matches) != 5 {
 		return -1, fmt.Errorf("invalid size: '%s'", sizeStr)
 	}
-
 	size, err := strconv.ParseFloat(matches[1], 64)
 	if err != nil {
 		return -1, err
 	}
 
 	unitPrefix := strings.ToLower(matches[3])
+
+	var uMap unitMap
+	switch mode {
+	case ForceBinary:
+		uMap = binaryMap
+	case ForceDecimal:
+		uMap = decimalMap
+	case Auto:
+		fallthrough
+	default:
+		if matches[4] != "" {
+			uMap = binaryMap
+		} else {
+			uMap = decimalMap
+		}
+	}
+
 	if mul, ok := uMap[unitPrefix]; ok {
 		size *= float64(mul)
 	}

size_test.go

@@ -138,6 +138,35 @@ func TestRAMInBytes(t *testing.T) {
 	assertError(t, RAMInBytes, "32bm")
 }
 
+func TestFromSize(t *testing.T) {
+	assertSuccessEquals(t, 32, FromSize, "32")
+	assertSuccessEquals(t, 32, FromSize, "32b")
+	assertSuccessEquals(t, 32, FromSize, "32B")
+	assertSuccessEquals(t, 32*KB, FromSize, "32k")
+	assertSuccessEquals(t, 32*KB, FromSize, "32K")
+	assertSuccessEquals(t, 32*KB, FromSize, "32kb")
+	assertSuccessEquals(t, 32*KB, FromSize, "32Kb")
+	assertSuccessEquals(t, 32*KiB, FromSize, "32Kib")
+	assertSuccessEquals(t, 32*KiB, FromSize, "32KIB")
+	assertSuccessEquals(t, 32*MB, FromSize, "32Mb")
+	assertSuccessEquals(t, 32*GB, FromSize, "32Gb")
+	assertSuccessEquals(t, 32*TB, FromSize, "32Tb")
+	assertSuccessEquals(t, 32*PB, FromSize, "32Pb")
+	assertSuccessEquals(t, 32*PB, FromSize, "32PB")
+	assertSuccessEquals(t, 32*PB, FromSize, "32P")
+
+	assertSuccessEquals(t, 32, FromSize, "32.3")
+	tmp := 32.3 * MiB
+	assertSuccessEquals(t, int64(tmp), FromSize, "32.3 MiB")
+
+	assertError(t, FromSize, "")
+	assertError(t, FromSize, "hello")
+	assertError(t, FromSize, "-32")
+	assertError(t, FromSize, " 32 ")
+	assertError(t, FromSize, "32m b")
+	assertError(t, FromSize, "32bm")
+}
+
 func assertEquals(t *testing.T, expected, actual interface{}) {
 	if expected != actual {
 		t.Errorf("Expected '%v' but got '%v'", expected, actual)