refactor: get rid of package expressions by moving the code into package expression

Signed-off-by: Xe Iaso <me@xeiaso.net>

lib/checker/expression/http_headers.go

@@ -0,0 +1,75 @@
+package expression
+
+import (
+	"net/http"
+	"reflect"
+	"strings"
+
+	"github.com/google/cel-go/common/types"
+	"github.com/google/cel-go/common/types/ref"
+	"github.com/google/cel-go/common/types/traits"
+)
+
+// HTTPHeaders is a type wrapper to expose HTTP headers into CEL programs.
+type HTTPHeaders struct {
+	http.Header
+}
+
+func (h HTTPHeaders) ConvertToNative(typeDesc reflect.Type) (any, error) {
+	return nil, ErrNotImplemented
+}
+
+func (h HTTPHeaders) ConvertToType(typeVal ref.Type) ref.Val {
+	switch typeVal {
+	case types.MapType:
+		return h
+	case types.TypeType:
+		return types.MapType
+	}
+
+	return types.NewErr("can't convert from %q to %q", types.MapType, typeVal)
+}
+
+func (h HTTPHeaders) Equal(other ref.Val) ref.Val {
+	return types.Bool(false) // We don't want to compare header maps
+}
+
+func (h HTTPHeaders) Type() ref.Type {
+	return types.MapType
+}
+
+func (h HTTPHeaders) Value() any { return h }
+
+func (h HTTPHeaders) Find(key ref.Val) (ref.Val, bool) {
+	k, ok := key.(types.String)
+	if !ok {
+		return nil, false
+	}
+
+	if _, ok := h.Header[string(k)]; !ok {
+		return nil, false
+	}
+
+	return types.String(strings.Join(h.Header.Values(string(k)), ",")), true
+}
+
+func (h HTTPHeaders) Contains(key ref.Val) ref.Val {
+	_, ok := h.Find(key)
+	return types.Bool(ok)
+}
+
+func (h HTTPHeaders) Get(key ref.Val) ref.Val {
+	result, ok := h.Find(key)
+	if !ok {
+		return types.ValOrErr(result, "no such key: %v", key)
+	}
+	return result
+}
+
+func (h HTTPHeaders) Iterator() traits.Iterator { panic("TODO(Xe): implement me") }
+
+func (h HTTPHeaders) IsZeroValue() bool {
+	return len(h.Header) == 0
+}
+
+func (h HTTPHeaders) Size() ref.Val { return types.Int(len(h.Header)) }