bitfield: add basic bit field package

Strange as it is but I wasn't able to find any good bit field
implementation. Most of them are limited to 64 bits, some allow for wider
values (like https://github.com/emef/bitfield) but even they're not that
efficient (using bytes instead of wider types). This this minimalistic thing.

pkg/util/bitfield/bitfield.go

@@ -0,0 +1,64 @@
+/*
+Package bitfield provides a simple and efficient arbitrary size bit field implementation.
+It doesn't attempt to cover everything that could be done with bit fields,
+providing only things used by neo-go.
+*/
+package bitfield
+
+// Field is a bit field represented as a slice of uint64 values.
+type Field []uint64
+
+// Bits and bytes count in a basic element of Field.
+const elemBits = 64
+const elemBytes = 8
+
+// New creates a new bit field of specified length. Actual field length
+// can be rounded to the next multiple of 64, so it's a responsibility
+// of the user to deal with that.
+func New(n int) Field {
+	return make(Field, 1+(n-1)/elemBits)
+}
+
+// Set sets one bit at specified offset. No bounds checking is done.
+func (f Field) Set(i int) {
+	addr, offset := (i / elemBits), (i % elemBits)
+	f[addr] |= (1 << offset)
+}
+
+// IsSet returns true if the bit with specified offset is set.
+func (f Field) IsSet(i int) bool {
+	addr, offset := (i / elemBits), (i % elemBits)
+	return (f[addr] & (1 << offset)) != 0
+}
+
+// Copy makes a copy of current Field.
+func (f Field) Copy() Field {
+	fn := make(Field, len(f))
+	copy(fn, f)
+	return fn
+}
+
+// And implements logical AND between f's and m's bits saving the result into f.
+func (f Field) And(m Field) {
+	l := len(m)
+	for i := range f {
+		if i >= l {
+			f[i] = 0
+			continue
+		}
+		f[i] &= m[i]
+	}
+}
+
+// Equals compares two Fields and returns true if they're equal.
+func (f Field) Equals(o Field) bool {
+	if len(f) != len(o) {
+		return false
+	}
+	for i := range f {
+		if f[i] != o[i] {
+			return false
+		}
+	}
+	return true
+}

pkg/util/bitfield/bitfield_test.go

@@ -0,0 +1,42 @@
+package bitfield
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestFields(t *testing.T) {
+	a := New(128)
+	b := New(128)
+	a.Set(10)
+	b.Set(10)
+	a.Set(42)
+	b.Set(42)
+	a.Set(100)
+	b.Set(100)
+	require.True(t, a.IsSet(42))
+	require.False(t, b.IsSet(43))
+
+	v := uint64(1<<10 | 1<<42)
+	require.Equal(t, v, a[0])
+	require.Equal(t, v, b[0])
+
+	require.True(t, a.Equals(b))
+
+	c := a.Copy()
+	require.True(t, c.Equals(b))
+
+	z := New(128)
+	c.And(a)
+	require.True(t, c.Equals(b))
+	c.And(z)
+	require.True(t, c.Equals(z))
+
+	c = New(64)
+	c[0] = a[0]
+	require.False(t, c.Equals(a))
+
+	b.And(c)
+	require.False(t, b.Equals(a))
+}