Feature/custom iterator (#173)

* Added CollectionIterator interface

* Added PAGINATION function

* Fixed LIMIT clause

* Fixed linting issues

pkg/stdlib/html/pagination.go

@@ -0,0 +1,104 @@
+package html
+
+import (
+	"context"
+	"github.com/MontFerret/ferret/pkg/html/dynamic"
+	"github.com/MontFerret/ferret/pkg/runtime/collections"
+	"github.com/MontFerret/ferret/pkg/runtime/core"
+	"github.com/MontFerret/ferret/pkg/runtime/values"
+)
+
+// Pagination creates an iterator that goes through pages using CSS selector.
+// The iterator starts from the current page i.e. it does not change the page on 1st iteration.
+// That allows you to keep scraping logic inside FOR loop.
+// @param doc (Document) - Target document.
+// @param selector (String) - CSS selector for a pagination on the page.
+func Pagination(_ context.Context, args ...core.Value) (core.Value, error) {
+	err := core.ValidateArgs(args, 2, 2)
+
+	if err != nil {
+		return values.None, err
+	}
+
+	doc, ok := args[0].(*dynamic.HTMLDocument)
+
+	if !ok {
+		return values.False, core.Errors(core.ErrInvalidType, ErrNotDynamic)
+	}
+
+	err = core.ValidateType(args[1], core.StringType)
+
+	if err != nil {
+		return values.None, err
+	}
+
+	selector := args[1].(values.String)
+
+	return &Paging{doc, selector}, nil
+}
+
+type (
+	Paging struct {
+		document *dynamic.HTMLDocument
+		selector values.String
+	}
+
+	PagingIterator struct {
+		document *dynamic.HTMLDocument
+		selector values.String
+		pos      values.Int
+	}
+)
+
+func (p *Paging) MarshalJSON() ([]byte, error) {
+	return nil, core.ErrInvalidOperation
+}
+
+func (p *Paging) Type() core.Type {
+	return core.CustomType
+}
+
+func (p *Paging) String() string {
+	return core.CustomType.String()
+}
+
+func (p *Paging) Compare(_ core.Value) int {
+	return 1
+}
+
+func (p *Paging) Unwrap() interface{} {
+	return nil
+}
+
+func (p *Paging) Hash() uint64 {
+	return 0
+}
+
+func (p *Paging) Copy() core.Value {
+	return values.None
+}
+
+func (p *Paging) Iterate(_ context.Context) (collections.CollectionIterator, error) {
+	return &PagingIterator{p.document, p.selector, -1}, nil
+}
+
+func (i *PagingIterator) Next(_ context.Context) (core.Value, core.Value, error) {
+	i.pos++
+
+	if i.pos == 0 {
+		return values.ZeroInt, values.ZeroInt, nil
+	}
+
+	clicked, err := i.document.ClickBySelector(i.selector)
+
+	if err != nil {
+		return values.None, values.None, err
+	}
+
+	if clicked {
+		return i.pos, i.pos, nil
+	}
+
+	// terminate
+	return values.None, values.None, nil
+}