cthulhu/internal/nvda2cthulhu/messagepack.go

// Package nvda2cthulhu accepts the local WebSocket protocol used by NVDA
// compatibility bridges and routes it to Cthulhu's speech and braille queues.
package nvda2cthulhu

import (
	"bytes"
	"encoding/binary"
	"errors"
	"fmt"
	"io"
	"math"
)

type messagePackBinary string

func decodeMessagePack(reader *bytes.Reader) (any, error) {
	return decodeMessagePackDepth(reader, 0)
}

func decodeMessagePackDepth(reader *bytes.Reader, depth int) (any, error) {
	if depth > 64 {
		return nil, errors.New("MessagePack nesting exceeds limit")
	}
	code, err := reader.ReadByte()
	if err != nil {
		return nil, err
	}
	switch {
	case code <= 0x7f:
		return int64(code), nil
	case code >= 0xe0:
		return int64(int8(code)), nil
	case code >= 0x80 && code <= 0x8f:
		return decodeMessagePackMap(reader, int(code&0x0f), depth)
	case code >= 0x90 && code <= 0x9f:
		return decodeMessagePackArray(reader, int(code&0x0f), depth)
	case code >= 0xa0 && code <= 0xbf:
		return decodeMessagePackString(reader, int(code&0x1f))
	case code == 0xc0:
		return nil, nil
	case code == 0xc2:
		return false, nil
	case code == 0xc3:
		return true, nil
	case code == 0xc4 || code == 0xc5 || code == 0xc6:
		length, err := decodeLength(reader, code, 0xc4)
		if err != nil {
			return nil, err
		}
		value := make([]byte, length)
		if _, err := io.ReadFull(reader, value); err != nil {
			return nil, err
		}
		return messagePackBinary(value), nil
	case code >= 0xca && code <= 0xcb:
		return decodeMessagePackFloat(reader, code)
	case code >= 0xcc && code <= 0xd3:
		return decodeMessagePackInteger(reader, code)
	case code == 0xd9 || code == 0xda || code == 0xdb:
		length, err := decodeLength(reader, code, 0xd9)
		if err != nil {
			return nil, err
		}
		return decodeMessagePackString(reader, length)
	case code == 0xdc || code == 0xdd:
		length, err := decodeLength(reader, code, 0xdc)
		if err != nil {
			return nil, err
		}
		return decodeMessagePackArray(reader, length, depth)
	case code == 0xde || code == 0xdf:
		length, err := decodeLength(reader, code, 0xde)
		if err != nil {
			return nil, err
		}
		return decodeMessagePackMap(reader, length, depth)
	default:
		return nil, fmt.Errorf("unsupported MessagePack type 0x%02x", code)
	}
}

func decodeLength(reader *bytes.Reader, code, base byte) (int, error) {
	var size int
	switch code - base {
	case 0:
		size = 1
	case 1:
		size = 2
	case 2:
		size = 4
	default:
		return 0, fmt.Errorf("invalid length type 0x%02x", code)
	}
	buffer := make([]byte, size)
	if _, err := io.ReadFull(reader, buffer); err != nil {
		return 0, err
	}
	value := uint64(0)
	for _, octet := range buffer {
		value = value<<8 | uint64(octet)
	}
	if value > uint64(maxPayload) {
		return 0, errors.New("MessagePack value exceeds 64 KiB")
	}
	return int(value), nil
}

func decodeMessagePackString(reader *bytes.Reader, length int) (string, error) {
	value := make([]byte, length)
	if _, err := io.ReadFull(reader, value); err != nil {
		return "", err
	}
	return string(value), nil
}

func decodeMessagePackFloat(reader *bytes.Reader, code byte) (any, error) {
	var buffer []byte
	if code == 0xca {
		buffer = make([]byte, 4)
	} else {
		buffer = make([]byte, 8)
	}
	if _, err := io.ReadFull(reader, buffer); err != nil {
		return nil, err
	}
	if code == 0xca {
		bits := binary.BigEndian.Uint32(buffer)
		return float64(math.Float32frombits(bits)), nil
	}
	return math.Float64frombits(binary.BigEndian.Uint64(buffer)), nil
}

func decodeMessagePackInteger(reader *bytes.Reader, code byte) (any, error) {
	switch code {
	case 0xcc, 0xd0:
		buffer := make([]byte, 1)
		if _, err := io.ReadFull(reader, buffer); err != nil {
			return nil, err
		}
		if code == 0xcc {
			return uint64(buffer[0]), nil
		}
		return int64(int8(buffer[0])), nil
	case 0xcd, 0xd1:
		buffer := make([]byte, 2)
		if _, err := io.ReadFull(reader, buffer); err != nil {
			return nil, err
		}
		value := binary.BigEndian.Uint16(buffer)
		if code == 0xcd {
			return uint64(value), nil
		}
		return int64(int16(value)), nil
	case 0xce, 0xd2:
		buffer := make([]byte, 4)
		if _, err := io.ReadFull(reader, buffer); err != nil {
			return nil, err
		}
		value := binary.BigEndian.Uint32(buffer)
		if code == 0xce {
			return uint64(value), nil
		}
		return int64(int32(value)), nil
	case 0xcf, 0xd3:
		buffer := make([]byte, 8)
		if _, err := io.ReadFull(reader, buffer); err != nil {
			return nil, err
		}
		value := binary.BigEndian.Uint64(buffer)
		if code == 0xcf {
			return value, nil
		}
		return int64(value), nil
	default:
		return nil, fmt.Errorf("invalid integer type 0x%02x", code)
	}
}

func decodeMessagePackArray(reader *bytes.Reader, length, depth int) ([]any, error) {
	values := make([]any, length)
	for index := range values {
		value, err := decodeMessagePackDepth(reader, depth+1)
		if err != nil {
			return nil, err
		}
		values[index] = value
	}
	return values, nil
}

func decodeMessagePackMap(reader *bytes.Reader, length, depth int) (map[any]any, error) {
	values := make(map[any]any, length)
	for range length {
		key, err := decodeMessagePackDepth(reader, depth+1)
		if err != nil {
			return nil, err
		}
		switch key.(type) {
		case string, messagePackBinary:
		default:
			return nil, errors.New("MessagePack map key must be text")
		}
		value, err := decodeMessagePackDepth(reader, depth+1)
		if err != nil {
			return nil, err
		}
		values[key] = value
	}
	return values, nil
}