Share flag consumption between parsing and completion

internal/commandmeta/flags.go

@@ -65,3 +65,47 @@ func (lookup FlagLookup) Find(spelling string) (Flag, bool) {
 	}
 	return flag, ok
 }
+
+// FlagConsumption describes one flag within an option token. NextOffset is zero
+// when the token is exhausted; otherwise it identifies the next short spelling.
+type FlagConsumption struct {
+	Flag       Flag
+	Spelling   string
+	Found      bool
+	Value      string
+	Attached   bool
+	NeedsNext  bool
+	NextOffset int
+}
+
+// ConsumeFlag reads a long option or a short option at offset in a cluster.
+func ConsumeFlag(lookup FlagLookup, token string, offset int) FlagConsumption {
+	var result FlagConsumption
+	if ClassifyFlagToken(token) == LongFlagToken {
+		result.Spelling, result.Value, result.Attached = SplitLongFlag(token)
+		result.Flag, result.Found = lookup.Find(result.Spelling)
+		result.NeedsNext = result.Found && !result.Flag.Boolean && !result.Attached
+		return result
+	}
+	result.Spelling = "-" + token[offset:offset+1]
+	result.Flag, result.Found = lookup.Find(result.Spelling)
+	if !result.Found {
+		return result
+	}
+	suffix := token[offset+1:]
+	if result.Flag.Boolean {
+		if strings.HasPrefix(suffix, "=") {
+			result.Attached, result.Value = true, suffix[1:]
+		} else if suffix != "" {
+			result.NextOffset = offset + 1
+		}
+		return result
+	}
+	if suffix == "" {
+		result.NeedsNext = true
+		return result
+	}
+	result.Attached = true
+	result.Value = strings.TrimPrefix(suffix, "=")
+	return result
+}