// Package tape implements Table Pair Encoding.
package tape

import "fmt"

// encoding and decoding functions must not make any allocations

const dataMaxSize = 0xFFFF
const uint16Max = 0xFFFF

// Error enumerates common errors in this package.
type Error string; const (
	ErrWrongBufferLength Error = "wrong buffer length"
	ErrDataTooLarge      Error = "data too large"
	ErrGBEUNotTerminated Error = "GBEU not terminated"
)

// Error implements the error interface.
func (err Error) Error() string {
	return string(err)
}

// Int8 is any 8-bit integer.
type Int8  interface { ~uint8  | ~int8 }
// Int16 is any 16-bit integer.
type Int16 interface { ~uint16 | ~int16 }
// Int32 is any 32-bit integer.
type Int32 interface { ~uint32 | ~int32 }
// Int64 is any 64-bit integer.
type Int64 interface { ~uint64 | ~int64 }
// UInt is any unsigned integer.
type UInt interface { ~uint | ~uint8 | ~uint16 | ~uint32 | ~uint64 }
// String is any string.
type String interface { ~string }

// DecodeI8 decodes an 8 bit integer from the given data.
func DecodeI8[T Int8](data []byte) (value T, n int, err error) {
	if len(data) != 1 { return 0, 0, fmt.Errorf("decoding int8: %w", ErrWrongBufferLength) }
	return T(data[0]), 1, nil
}

// EncodeI8 encodes an 8 bit integer into the given buffer.
func EncodeI8[T Int8](buffer []byte, value T) (n int, err error) {
	if len(buffer) != 1 { return 0, fmt.Errorf("encoding int8: %w", ErrWrongBufferLength) }
	buffer[0] = byte(value)
	return 1, nil
}

// DecodeI16 decodes a 16 bit integer from the given data.
func DecodeI16[T Int16](data []byte) (value T, n int, err error) {
	if len(data) != 2 { return 0, 0, fmt.Errorf("decoding int16: %w", ErrWrongBufferLength) }
	return T(data[0]) << 8 | T(data[1]), 2, nil
}

// EncodeI16 encodes a 16 bit integer into the given buffer.
func EncodeI16[T Int16](buffer []byte, value T) (n int, err error) {
	if len(buffer) != 2 { return 0, fmt.Errorf("encoding int16: %w", ErrWrongBufferLength) }
	buffer[0] = byte(value >> 8)
	buffer[1] = byte(value)
	return 2, nil
}

// DecodeI32 decodes a 32 bit integer from the given data.
func DecodeI32[T Int32](data []byte) (value T, n int, err error) {
	if len(data) != 4 { return 0, 0, fmt.Errorf("decoding int32: %w", ErrWrongBufferLength) }
	return T(data[0]) << 24 |
		T(data[1]) << 16 |
		T(data[2]) <<  8 |
		T(data[3]), 4, nil
}

// EncodeI32 encodes a 32 bit integer into the given buffer.
func EncodeI32[T Int32](buffer []byte, value T) (n int, err error) {
	if len(buffer) != 4 { return 0, fmt.Errorf("encoding int32: %w", ErrWrongBufferLength) }
	buffer[0] = byte(value >> 24)
	buffer[1] = byte(value >> 16)
	buffer[2] = byte(value >> 8)
	buffer[3] = byte(value)
	return 4, nil
}

// DecodeI64 decodes a 64 bit integer from the given data.
func DecodeI64[T Int64](data []byte) (value T, n int, err error) {
	if len(data) != 8 { return 0, 0, fmt.Errorf("decoding int64: %w", ErrWrongBufferLength) }
	return T(data[0]) << 56 |
		T(data[1]) << 48 |
		T(data[2]) << 40 |
		T(data[3]) << 32 |
		T(data[4]) << 24 |
		T(data[5]) << 16 |
		T(data[6]) <<  8 |
		T(data[7]), 8, nil
}

// EncodeI64 encodes a 64 bit integer into the given buffer.
func EncodeI64[T Int64](buffer []byte, value T) (n int, err error) {
	if len(buffer) != 8 { return 0, fmt.Errorf("encoding int64: %w", ErrWrongBufferLength) }
	buffer[0] = byte(value >> 56)
	buffer[1] = byte(value >> 48)
	buffer[2] = byte(value >> 40)
	buffer[3] = byte(value >> 32)
	buffer[4] = byte(value >> 24)
	buffer[5] = byte(value >> 16)
	buffer[6] = byte(value >> 8)
	buffer[7] = byte(value)
	return 8, nil
}

// DecodeGBEU decodes an 8 to 64 bit growing integer into the given buffer.
func DecodeGBEU[T UInt](data []byte) (value T, n int, err error) {
	var fullValue uint64
	for _, chunk := range data {
		fullValue *= 0x80
		fullValue += uint64(chunk & 0x7F)
		ccb := chunk >> 7
		if ccb == 0 {
			return T(fullValue), n, nil
		}
		n += 1
	}
	return 0, n, fmt.Errorf("decoding GBEU: %w", ErrGBEUNotTerminated)
}

// EncodeGBEU encodes an 8 to 64 bit growing integer into a given buffer.
func EncodeGBEU[T UInt]	(buffer []byte, value T) (n int, err error) {
	window := (GBEUSize(value) - 1) * 7
	
	index := 0
	for window >= 0 {
		if index >= len(buffer) { return index, fmt.Errorf("encoding GBEU: %w", ErrWrongBufferLength) }
		
		chunk := uint8(value >> window) & 0x7F
		if window > 0 {
			chunk |= 0x80
		}
		buffer[index] = chunk
		
		index += 1
		window -= 7
	}
	return index, nil
}

// GBEUSize returns the size (in octets) of a GBEU integer.
func GBEUSize[T UInt] (value T) int {
	length := 0
	for {
		value >>= 7
		length ++
		if value == 0 { return length }
	}
}

// DecodeString decodes a string from the given data.
func DecodeString[T String](data []byte) (value T, n int, err error) {
	return T(data), len(data), nil
}

// EncodeString encodes a string into the given buffer.
func EncodeString[T String](data []byte, value T) (n int, err error) {
	if len(data) != len(value) { return 0, fmt.Errorf("encoding string: %w", ErrWrongBufferLength) }
	return copy(data, value), nil
}

// StringSize returns the size of a string. Returns 0 and an error if the size
// is too large.
func StringSize[T String](value T) (int, error) {
	if len(value) > dataMaxSize { return 0, ErrDataTooLarge }
	return len(value), nil
}

// U16CastSafe safely casts an integer to a uint16. If an overflow or underflow
// occurs, it will return (0, false).
func U16CastSafe(n int) (uint16, bool) {
	if n < uint16Max && n >= 0 {
		return uint16(n), true
	} else {
		return 0, false
	}
}