reduce allocations in From from 64 to 8 via preallocating the numBuffer

This change improves the runtime by around 5-7% across the board

After the change:

    goos: linux
    goarch: amd64
    pkg: github.com/xnacly/go-iso8601-duration
    cpu: AMD Ryzen 7 3700X 8-Core Processor
    BenchmarkDuration/P0D-16                24741985                48.10 ns/op            8 B/op          1 allocs/op
    BenchmarkDuration/PT15H-16              18533790                63.26 ns/op            8 B/op          1 allocs/op
    BenchmarkDuration/P1W-16                24164163                48.64 ns/op            8 B/op          1 allocs/op
    BenchmarkDuration/P15W-16               20793126                58.09 ns/op            8 B/op          1 allocs/op
    BenchmarkDuration/P1Y15W-16             13904265                84.21 ns/op            8 B/op          1 allocs/op
    BenchmarkDuration/P15Y-16               19943671                59.35 ns/op            8 B/op          1 allocs/op
    BenchmarkDuration/P15Y3M-16             14227842                83.24 ns/op            8 B/op          1 allocs/op
    BenchmarkDuration/P15Y3M41D-16          10262575                115.6 ns/op            8 B/op          1 allocs/op
    BenchmarkDuration/PT15M-16              18228138                63.90 ns/op            8 B/op          1 allocs/op
    BenchmarkDuration/PT15M10S-16           12289743                95.84 ns/op            8 B/op          1 allocs/op
    BenchmarkDuration/P3Y6M4DT12H30M5S-16   6758479                 177.3 ns/op            8 B/op          1 allocs/op
    PASS
    ok      github.com/xnacly/go-iso8601-duration   13.952s

Before the change:

    goos: linux
    goarch: amd64
    pkg: github.com/xnacly/go-iso8601-duration
    cpu: AMD Ryzen 7 3700X 8-Core Processor
    BenchmarkDuration/P0D-16                18941151                64.26 ns/op           64 B/op          1 allocs/op
    BenchmarkDuration/PT15H-16              15597309                76.69 ns/op           64 B/op          1 allocs/op
    BenchmarkDuration/P1W-16                18366788                64.10 ns/op           64 B/op          1 allocs/op
    BenchmarkDuration/P15W-16               15964773                74.35 ns/op           64 B/op          1 allocs/op
    BenchmarkDuration/P1Y15W-16             12367951                95.39 ns/op           64 B/op          1 allocs/op
    BenchmarkDuration/P15Y-16               15903548                74.44 ns/op           64 B/op          1 allocs/op
    BenchmarkDuration/P15Y3M-16             12397192                96.33 ns/op           64 B/op          1 allocs/op
    BenchmarkDuration/P15Y3M41D-16          9608250                 123.7 ns/op           64 B/op          1 allocs/op
    BenchmarkDuration/PT15M-16              15343340                76.61 ns/op           64 B/op          1 allocs/op
    BenchmarkDuration/PT15M10S-16           11336277                104.8 ns/op           64 B/op          1 allocs/op
    BenchmarkDuration/P3Y6M4DT12H30M5S-16   6286371                 188.7 ns/op           64 B/op          1 allocs/op
    PASS
    ok      github.com/xnacly/go-iso8601-duration   14.159s

Author: xnacly

duration.go

@@ -128,8 +128,7 @@ func From(s string) (Duration, error) {
 
 	curState := stateStart
 	var col uint8
-	numBuf := bytes.Buffer{}
-
+	numBuf := *bytes.NewBuffer(make([]byte, 0, 8))
 	r := strings.NewReader(s)
 
 	for {

duration_test.go

@@ -63,7 +63,6 @@ var testcases = []struct {
 }{
 	{"P0D", Duration{}},
 	{"PT15H", Duration{hour: 15}},
-	{"PT15H", Duration{hour: 15}},
 	{"P1W", Duration{week: 1}},
 	{"P15W", Duration{week: 15}},
 	{"P1Y15W", Duration{year: 1, week: 15}},
@@ -85,6 +84,16 @@ var testcases = []struct {
 	},
 }
 
+func BenchmarkDuration(b *testing.B) {
+	for _, i := range testcases {
+		b.Run(i.str, func(b *testing.B) {
+			for n := 0; n < b.N; n++ {
+				_, _ = From(i.str)
+			}
+		})
+	}
+}
+
 func TestDurationStringer(t *testing.T) {
 	for _, i := range testcases {
 		t.Run(i.str, func(t *testing.T) {