integrate request builder into HTTP client for googleapis support (#157)

builder/used_fields.go

@@ -0,0 +1,44 @@
+package builder
+
+import "strings"
+
+// usedFields stores keys and their top-level parts,
+// turning top-level lookups from O(N) into O(1).
+type usedFields struct {
+	full map[string]struct{}
+	top  map[string]struct{}
+}
+
+func newUsedFields() *usedFields {
+	return &usedFields{
+		full: make(map[string]struct{}),
+		top:  make(map[string]struct{}),
+	}
+}
+
+// add inserts a new key and updates the top-level index.
+func (u *usedFields) add(key string) {
+	u.full[key] = struct{}{}
+	top := key
+	if i := strings.IndexByte(key, '.'); i != -1 {
+		top = key[:i]
+	}
+	u.top[top] = struct{}{}
+}
+
+// hasTopLevelKey checks if a top-level key exists.
+func (u *usedFields) hasTopLevelKey(top string) bool {
+	_, ok := u.top[top]
+	return ok
+}
+
+// hasFullKey checks if an exact key exists.
+func (u *usedFields) hasFullKey(key string) bool {
+	_, ok := u.full[key]
+	return ok
+}
+
+// len returns the number of full keys stored in the set.
+func (u *usedFields) len() int {
+	return len(u.full)
+}