// Package protocol builds the USB packets that the Lian Li Galahad II LCD
// expects. It is a pure, dependency-free implementation of the framing used
// by the reference Rust/Python drivers.
//
// Each H.264 frame is split into one or more 512-byte USB packets. Every
// packet carries an 11-byte header:
//
//	[0]  report_id (0x02)
//	[1]  command    (0x0D = send H.264)
//	[2:6] total payload size (big-endian)
//	[6:9] packet index, 3 bytes (big-endian, wraps)
//	[9:11] chunk length in this packet (big-endian)
//
// followed by up to 501 bytes of frame data and zero padding.
package protocol

import (
	"encoding/binary"
	"fmt"
)

// Wire-level constants shared with the reference implementation.
const (
	ReportIDVideo  = 0x02
	CmdSendH264    = 0x0D
	HeaderSize     = 11
	PacketSize     = 512
	MaxPayloadSize = 501
)

// EncodePackets splits frame data into USB packets. It returns an error only
// when frame is empty, which would otherwise produce a malformed stream.
func EncodePackets(frame []byte) ([][]byte, error) {
	if len(frame) == 0 {
		return nil, fmt.Errorf("cannot encode empty frame")
	}

	var packets [][]byte
	for offset, idx := 0, uint32(0); offset < len(frame); offset += MaxPayloadSize {
		end := min(offset+MaxPayloadSize, len(frame))
		chunk := frame[offset:end]

		pkt := make([]byte, PacketSize)
		pkt[0] = ReportIDVideo
		pkt[1] = CmdSendH264
		binary.BigEndian.PutUint32(pkt[2:6], uint32(len(frame)))
		pkt[6] = byte(idx >> 16)
		pkt[7] = byte(idx >> 8)
		pkt[8] = byte(idx)
		binary.BigEndian.PutUint16(pkt[9:11], uint16(len(chunk)))
		copy(pkt[HeaderSize:HeaderSize+len(chunk)], chunk)

		packets = append(packets, pkt)
		idx++
	}
	return packets, nil
}

func min(a, b int) int {
	if a < b {
		return a
	}
	return b
}