// voice/frame.go — length-prefixed JSON framing.
//
// Same shape as ipc/worker with the 64 MiB cap (see wire.go maxFrame). One
// frame carries EITHER a Request, Response, or Push — distinguished by the
// present fields (Request has `id`+`m`; Response has `id`; Push has `kind`).
// Server-side reads probe all three shallowly; client-side reads expect
// either Response or Push depending on context.
package voice

import (
	"encoding/binary"
	"encoding/json"
	"errors"
	"fmt"
	"io"
)

// ErrFrameTooLarge — frame exceeded maxFrame; conn is desynced; caller must close.
var ErrFrameTooLarge = errors.New("voice: frame too large")

func writeFrame(w io.Writer, v any) error {
	body, err := json.Marshal(v)
	if err != nil {
		return fmt.Errorf("voice: marshal frame: %w", err)
	}
	if len(body) > maxFrame {
		return fmt.Errorf("%w: %d bytes", ErrFrameTooLarge, len(body))
	}
	var hdr [4]byte
	binary.BigEndian.PutUint32(hdr[:], uint32(len(body)))
	if _, err := w.Write(hdr[:]); err != nil {
		return fmt.Errorf("voice: write frame header: %w", err)
	}
	if _, err := w.Write(body); err != nil {
		return fmt.Errorf("voice: write frame body: %w", err)
	}
	return nil
}

func readFrame(r io.Reader, v any) error {
	var hdr [4]byte
	if _, err := io.ReadFull(r, hdr[:]); err != nil {
		if errors.Is(err, io.EOF) {
			return io.EOF
		}
		return fmt.Errorf("voice: read frame header: %w", err)
	}
	n := binary.BigEndian.Uint32(hdr[:])
	if n > maxFrame {
		return fmt.Errorf("%w: %d bytes", ErrFrameTooLarge, n)
	}
	buf := make([]byte, n)
	if _, err := io.ReadFull(r, buf); err != nil {
		return fmt.Errorf("voice: read frame body: %w", err)
	}
	if err := json.Unmarshal(buf, v); err != nil {
		return fmt.Errorf("voice: unmarshal frame: %w", err)
	}
	return nil
}