feat(entity): add entity types for target graph and changed targets

entity/BUILD.bazel

@@ -4,7 +4,9 @@ go_library(
     name = "entity",
     srcs = [
         "build_description.go",
+        "changed_targets.go",
         "computation_strategy.go",
+        "optimized_target.go",
         "target_graph.go",
     ],
     importpath = "github.com/uber/tango/entity",

entity/changed_targets.go

@@ -0,0 +1,17 @@
+package entity
+
+// ChangedTarget represents a target that differs between two revisions.
+type ChangedTarget struct {
+	ChangeType int32            `json:"change_type"`
+	OldTarget  *OptimizedTarget `json:"old_target,omitempty"`
+	NewTarget  *OptimizedTarget `json:"new_target,omitempty"`
+	Distance   int32            `json:"distance"`
+}
+
+// GetChangedTargetsResponse is one piece of a streamed changed-targets
+// result — either a batch of changed targets or a metadata mapping.
+// Exactly one field is non-nil.
+type GetChangedTargetsResponse struct {
+	ChangedTargets []ChangedTarget `json:"changed_targets"`
+	Metadata       *Metadata       `json:"metadata,omitempty"`
+}

entity/optimized_target.go

@@ -0,0 +1,34 @@
+package entity
+
+// OptimizedTarget is the compact, ID-mapped representation of a target used
+// for streaming and storage. String fields are replaced with int32 IDs that
+// reference the accompanying Metadata maps.
+type OptimizedTarget struct {
+	ID                 int32           `json:"id"`
+	Hash               string          `json:"hash"`
+	DirectDependencies []int32         `json:"direct_dependencies"`
+	RuleType           int32           `json:"rule_type"`
+	Tags               []int32         `json:"tags"`
+	Root               bool            `json:"root"`
+	External           bool            `json:"external"`
+	Attributes         map[int32]int32 `json:"attributes"`
+}
+
+// Metadata holds the ID-to-string mappings that accompany a set of
+// OptimizedTarget entries. Consumers merge metadata across chunks before
+// resolving IDs.
+type Metadata struct {
+	TargetIDMapping             map[int32]string `json:"target_id_mapping"`
+	RuleTypeMapping             map[int32]string `json:"rule_type_mapping"`
+	TagMapping                  map[int32]string `json:"tag_mapping"`
+	AttributeNameMapping        map[int32]string `json:"attribute_name_mapping"`
+	AttributeStringValueMapping map[int32]string `json:"attribute_string_value_mapping"`
+}
+
+// GetTargetGraphResponse is one piece of a streamed target graph — either a
+// batch of ID-mapped targets or a metadata mapping. Exactly one field is
+// non-nil.
+type GetTargetGraphResponse struct {
+	Targets  []OptimizedTarget `json:"targets"`
+	Metadata *Metadata         `json:"metadata,omitempty"`
+}