Implement Seriatim transcript parsing

internal/modules/input/seriatim/model.go

@@ -3,6 +3,8 @@ package seriatim
 import (
 	"bytes"
 	"encoding/json"
+	"fmt"
+	"io"
 )
 
 type transcript struct {
@@ -19,11 +21,55 @@ type segment struct {
 }
 
 func decodeTranscript(raw []byte) (transcript, error) {
-	var out transcript
-	decoder := json.NewDecoder(bytes.NewReader(raw))
-	decoder.UseNumber()
-	if err := decoder.Decode(&out); err != nil {
+	var fields map[string]json.RawMessage
+	if err := decodeJSON(raw, &fields); err != nil {
 		return transcript{}, err
 	}
-	return out, nil
+	if fields == nil {
+		return transcript{}, fmt.Errorf("top-level value must be an object")
+	}
+
+	metadataRaw, ok := fields["metadata"]
+	if !ok {
+		return transcript{}, fmt.Errorf("metadata is required")
+	}
+	var metadata map[string]any
+	if err := decodeJSON(metadataRaw, &metadata); err != nil {
+		return transcript{}, fmt.Errorf("metadata must be an object: %w", err)
+	}
+	if metadata == nil {
+		return transcript{}, fmt.Errorf("metadata must be an object")
+	}
+
+	segmentsRaw, ok := fields["segments"]
+	if !ok {
+		return transcript{}, fmt.Errorf("segments are required")
+	}
+	var segments []segment
+	if err := decodeJSON(segmentsRaw, &segments); err != nil {
+		return transcript{}, fmt.Errorf("segments must be an array: %w", err)
+	}
+	if segments == nil {
+		return transcript{}, fmt.Errorf("segments must be an array")
+	}
+
+	return transcript{
+		Metadata: metadata,
+		Segments: segments,
+	}, nil
+}
+
+func decodeJSON(raw []byte, out any) error {
+	decoder := json.NewDecoder(bytes.NewReader(raw))
+	decoder.UseNumber()
+	if err := decoder.Decode(out); err != nil {
+		return err
+	}
+	if err := decoder.Decode(&struct{}{}); err != io.EOF {
+		if err == nil {
+			return fmt.Errorf("unexpected trailing JSON value")
+		}
+		return err
+	}
+	return nil
 }