Pull request: all: imp dhcp host normalization, validation

Updates #2952.

Squashed commit of the following:

commit 45afcab5d33c1ec8176c0ad05423288c8770b772
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Tue Apr 20 15:02:34 2021 +0300

    all: imp docs

commit d844ce1e2bb0b92a892119161774ec95f3e59711
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Tue Apr 20 14:57:49 2021 +0300

    all: more code imp

commit eef08cb69015ead0ffaef866e12a2c6556786d37
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Tue Apr 20 14:52:33 2021 +0300

    all: imp code, docs

commit 20748f20ab9fcc410b11daf0e23eaf156dcdc7cd
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Tue Apr 20 14:30:57 2021 +0300

    all: imp dhcp host normalization, validation

internal/aghstrings/set.go

@@ -0,0 +1,71 @@
+package aghstrings
+
+// unit is a convenient alias for struct{}
+type unit = struct{}
+
+// Set is a set of strings.
+type Set struct {
+	m map[string]unit
+}
+
+// NewSet returns a new string set containing strs.
+func NewSet(strs ...string) (set *Set) {
+	set = &Set{
+		m: make(map[string]unit, len(strs)),
+	}
+
+	for _, s := range strs {
+		set.Add(s)
+	}
+
+	return set
+}
+
+// Add adds s to the set.  Add panics if the set is a nil set, just like a nil
+// map does.
+func (set *Set) Add(s string) {
+	set.m[s] = unit{}
+}
+
+// Del deletes s from the set.  Calling Del on a nil set has no effect, just
+// like delete on an empty map doesn't.
+func (set *Set) Del(s string) {
+	if set != nil {
+		delete(set.m, s)
+	}
+}
+
+// Has returns true if s is in the set.  Calling Has on a nil set returns false,
+// just like indexing on an empty map does.
+func (set *Set) Has(s string) (ok bool) {
+	if set != nil {
+		_, ok = set.m[s]
+	}
+
+	return ok
+}
+
+// Len returns the length of the set.  A nil set has a length of zero, just like
+// an empty map.
+func (set *Set) Len() (n int) {
+	if set == nil {
+		return 0
+	}
+
+	return len(set.m)
+}
+
+// Values returns all values in the set.  The order of the values is undefined.
+// Values returns nil if the set is nil.
+func (set *Set) Values() (strs []string) {
+	if set == nil {
+		return nil
+	}
+
+	strs = make([]string, 0, len(set.m))
+	for s := range set.m {
+		strs = append(strs, s)
+	}
+
+	return strs
+}

internal/aghstrings/set_test.go

@@ -0,0 +1,56 @@
+package aghstrings
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestSet(t *testing.T) {
+	const s = "a"
+
+	t.Run("nil", func(t *testing.T) {
+		var set *Set
+
+		assert.NotPanics(t, func() {
+			set.Del(s)
+		})
+
+		assert.NotPanics(t, func() {
+			assert.False(t, set.Has(s))
+		})
+
+		assert.NotPanics(t, func() {
+			assert.Equal(t, 0, set.Len())
+		})
+
+		assert.NotPanics(t, func() {
+			assert.Nil(t, set.Values())
+		})
+
+		assert.Panics(t, func() {
+			set.Add(s)
+		})
+	})
+
+	t.Run("non_nil", func(t *testing.T) {
+		set := NewSet()
+		assert.Equal(t, 0, set.Len())
+
+		ok := set.Has(s)
+		assert.False(t, ok)
+
+		set.Add(s)
+		ok = set.Has(s)
+		assert.True(t, ok)
+
+		assert.Equal(t, []string{s}, set.Values())
+
+		set.Del(s)
+		ok = set.Has(s)
+		assert.False(t, ok)
+
+		set = NewSet(s)
+		assert.Equal(t, 1, set.Len())
+	})
+}