add Float16 type

Author: lunixbochs

custom_float16.go

@@ -0,0 +1,78 @@
+package struc
+
+import (
+	"encoding/binary"
+	"io"
+	"math"
+	"strconv"
+)
+
+type Float16 float64
+
+func (f *Float16) Pack(p []byte, opt *Options) (int, error) {
+	order := opt.Order
+	if order == nil {
+		order = binary.BigEndian
+	}
+	sign := uint16(0)
+	if *f < 0 {
+		sign = 1
+	}
+	var frac, exp uint16
+	if math.IsInf(float64(*f), 0) {
+		exp = 0x1f
+		frac = 0
+	} else if math.IsNaN(float64(*f)) {
+		exp = 0x1f
+		frac = 1
+	} else {
+		bits := math.Float64bits(float64(*f))
+		exp64 := (bits >> 52) & 0x7ff
+		if exp64 != 0 {
+			exp = uint16((exp64 - 1023 + 15) & 0x1f)
+		}
+		frac = uint16((bits >> 42) & 0x3ff)
+	}
+	var out uint16
+	out |= sign << 15
+	out |= exp << 10
+	out |= frac & 0x3ff
+	order.PutUint16(p, out)
+	return 2, nil
+}
+func (f *Float16) Unpack(r io.Reader, length int, opt *Options) error {
+	order := opt.Order
+	if order == nil {
+		order = binary.BigEndian
+	}
+	var tmp [2]byte
+	if _, err := r.Read(tmp[:]); err != nil {
+		return err
+	}
+	val := order.Uint16(tmp[:2])
+	sign := (val >> 15) & 1
+	exp := int16((val >> 10) & 0x1f)
+	frac := val & 0x3ff
+	if exp == 0x1f {
+		if frac != 0 {
+			*f = Float16(math.NaN())
+		} else {
+			*f = Float16(math.Inf(int(sign)*-2 + 1))
+		}
+	} else {
+		var bits uint64
+		bits |= uint64(sign) << 63
+		bits |= uint64(frac) << 42
+		if exp > 0 {
+			bits |= uint64(exp-15+1023) << 52
+		}
+		*f = Float16(math.Float64frombits(bits))
+	}
+	return nil
+}
+func (f *Float16) Size(opt *Options) int {
+	return 2
+}
+func (f *Float16) String() string {
+	return strconv.FormatFloat(float64(*f), 'g', -1, 32)
+}

custom_float16_test.go

@@ -0,0 +1,56 @@
+package struc
+
+import (
+	"bytes"
+	"encoding/binary"
+	"fmt"
+	"math"
+	"strconv"
+	"strings"
+	"testing"
+)
+
+func TestFloat16(t *testing.T) {
+	// test cases from https://en.wikipedia.org/wiki/Half-precision_floating-point_format#Half_precision_examples
+	tests := []struct {
+		B string
+		F float64
+	}{
+		//s expnt significand
+		{"0 01111 0000000000", 1},
+		{"0 01111 0000000001", 1.0009765625},
+		{"1 10000 0000000000", -2},
+		{"0 11110 1111111111", 65504},
+		// {"0 00001 0000000000", 0.0000610352},
+		// {"0 00000 1111111111", 0.0000609756},
+		// {"0 00000 0000000001", 0.0000000596046},
+		{"0 00000 0000000000", 0},
+		// {"1 00000 0000000000", -0},
+		{"0 11111 0000000000", math.Inf(1)},
+		{"1 11111 0000000000", math.Inf(-1)},
+		{"0 01101 0101010101", 0.333251953125},
+	}
+	for _, test := range tests {
+		var buf bytes.Buffer
+		f := Float16(test.F)
+		if err := Pack(&buf, &f); err != nil {
+			t.Error("pack failed:", err)
+			continue
+		}
+		bitval, _ := strconv.ParseUint(strings.Replace(test.B, " ", "", -1), 2, 16)
+		tmp := binary.BigEndian.Uint16(buf.Bytes())
+		if tmp != uint16(bitval) {
+			t.Errorf("incorrect pack: %s != %016b (%f)", test.B, tmp, test.F)
+			continue
+		}
+		var f2 Float16
+		if err := Unpack(&buf, &f2); err != nil {
+			t.Error("unpack failed:", err)
+			continue
+		}
+		// let sprintf deal with (im)precision for me here
+		if fmt.Sprintf("%f", f) != fmt.Sprintf("%f", f2) {
+			t.Errorf("incorrect unpack: %016b %f != %f", bitval, f, f2)
+		}
+	}
+}