client: add utilities to encode platforms

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

client/utils.go

@@ -1,10 +1,14 @@
 package client // import "github.com/docker/docker/client"
 
 import (
+	"encoding/json"
+	"fmt"
 	"net/url"
 	"regexp"
 
 	"github.com/docker/docker/api/types/filters"
+	"github.com/docker/docker/errdefs"
+	ocispec "github.com/opencontainers/image-spec/specs-go/v1"
 )
 
 var headerRegexp = regexp.MustCompile(`\ADocker/.+\s\((.+)\)\z`)
@@ -32,3 +36,43 @@ func getFiltersQuery(f filters.Args) (url.Values, error) {
 	}
 	return query, nil
 }
+
+// encodePlatforms marshals the given platform(s) to JSON format, to
+// be used for query-parameters for filtering / selecting platforms.
+func encodePlatforms(platform ...ocispec.Platform) ([]string, error) {
+	if len(platform) == 0 {
+		return []string{}, nil
+	}
+	if len(platform) == 1 {
+		p, err := encodePlatform(&platform[0])
+		if err != nil {
+			return nil, err
+		}
+		return []string{p}, nil
+	}
+
+	seen := make(map[string]struct{}, len(platform))
+	out := make([]string, 0, len(platform))
+	for i := range platform {
+		p, err := encodePlatform(&platform[i])
+		if err != nil {
+			return nil, err
+		}
+		if _, ok := seen[p]; !ok {
+			out = append(out, p)
+			seen[p] = struct{}{}
+		}
+	}
+	return out, nil
+}
+
+// encodePlatforms marshals the given platform to JSON format, to
+// be used for query-parameters for filtering / selecting platforms. It
+// is used as a helper for encodePlatforms,
+func encodePlatform(platform *ocispec.Platform) (string, error) {
+	p, err := json.Marshal(platform)
+	if err != nil {
+		return "", errdefs.InvalidParameter(fmt.Errorf("invalid platform: %v", err))
+	}
+	return string(p), nil
+}