data.go

package can

import (
	"fmt"

	"github.com/toitware/can-go/internal/reinterpret"
)

const MaxDataLength = 8

// Data holds the data in a CAN frame.
//
// Layout
//
// Individual bits in the data are numbered according to the following scheme:
//
//             BIT
//             NUMBER
//             +------+------+------+------+------+------+------+------+
//             |   7  |   6  |   5  |   4  |   3  |   2  |   1  |   0  |
//    BYTE     +------+------+------+------+------+------+------+------+
//    NUMBER
//    +-----+  +------+------+------+------+------+------+------+------+
//    |  0  |  |   7  |   6  |   5  |   4  |   3  |   2  |   1  |   0  |
//    +-----+  +------+------+------+------+------+------+------+------+
//    |  1  |  |  15  |  14  |  13  |  12  |  11  |  10  |   9  |   8  |
//    +-----+  +------+------+------+------+------+------+------+------+
//    |  2  |  |  23  |  22  |  21  |  20  |  19  |  18  |  17  |  16  |
//    +-----+  +------+------+------+------+------+------+------+------+
//    |  3  |  |  31  |  30  |  29  |  28  |  27  |  26  |  25  |  24  |
//    +-----+  +------+------+------+------+------+------+------+------+
//    |  4  |  |  39  |  38  |  37  |  36  |  35  |  34  |  33  |  32  |
//    +-----+  +------+------+------+------+------+------+------+------+
//    |  5  |  |  47  |  46  |  45  |  44  |  43  |  42  |  41  |  40  |
//    +-----+  +------+------+------+------+------+------+------+------+
//    |  6  |  |  55  |  54  |  53  |  52  |  51  |  50  |  49  |  48  |
//    +-----+  +------+------+------+------+------+------+------+------+
//    |  7  |  |  63  |  62  |  61  |  60  |  59  |  58  |  57  |  56  |
//    +-----+  +------+------+------+------+------+------+------+------+
//
// Bit ranges can be manipulated using little-endian and big-endian bit ordering.
//
// Little-endian bit ranges
//
// Example range of length 32 starting at bit 29:
//
//             BIT
//             NUMBER
//             +------+------+------+------+------+------+------+------+
//             |   7  |   6  |   5  |   4  |   3  |   2  |   1  |   0  |
//    BYTE     +------+------+------+------+------+------+------+------+
//    NUMBER
//    +-----+  +------+------+------+------+------+------+------+------+
//    |  0  |  |   7  |   6  |   5  |   4  |   3  |   2  |   1  |   0  |
//    +-----+  +------+------+------+------+------+------+------+------+
//    |  1  |  |  15  |  14  |  13  |  12  |  11  |  10  |   9  |   8  |
//    +-----+  +------+------+------+------+------+------+------+------+
//    |  2  |  |  23  |  22  |  21  |  20  |  19  |  18  |  17  |  16  |
//    +-----+  +------+------+------+------+------+------+------+------+
//    |  3  |  |  <-------------LSb |  28  |  27  |  26  |  25  |  24  |
//    +-----+  +------+------+------+------+------+------+------+------+
//    |  4  |  |  <--------------------------------------------------  |
//    +-----+  +------+------+------+------+------+------+------+------+
//    |  5  |  |  <--------------------------------------------------  |
//    +-----+  +------+------+------+------+------+------+------+------+
//    |  6  |  |  <--------------------------------------------------  |
//    +-----+  +------+------+------+------+------+------+------+------+
//    |  7  |  |  63  |  62  |  61  | <-MSb--------------------------- |
//    +-----+  +------+------+------+------+------+------+------+------+
//
// Big-endian bit ranges
//
// Example range of length 32 starting at bit 29:
//
//             BIT
//             NUMBER
//             +------+------+------+------+------+------+------+------+
//             |   7  |   6  |   5  |   4  |   3  |   2  |   1  |   0  |
//    BYTE     +------+------+------+------+------+------+------+------+
//    NUMBER
//    +-----+  +------+------+------+------+------+------+------+------+
//    |  0  |  |   7  |   6  |   5  |   4  |   3  |   2  |   1  |   0  |
//    +-----+  +------+------+------+------+------+------+------+------+
//    |  1  |  |  15  |  14  |  13  |  12  |  11  |  10  |   9  |   8  |
//    +-----+  +------+------+------+------+------+------+------+------+
//    |  2  |  |  23  |  22  |  21  |  20  |  19  |  18  |  17  |  16  |
//    +-----+  +------+------+------+------+------+------+------+------+
//    |  3  |  |  31  |  30  | <-MSb---------------------------------  |
//    +-----+  +------+------+------+------+------+------+------+------+
//    |  4  |  |  <--------------------------------------------------  |
//    +-----+  +------+------+------+------+------+------+------+------+
//    |  5  |  |  <--------------------------------------------------  |
//    +-----+  +------+------+------+------+------+------+------+------+
//    |  6  |  |  <--------------------------------------------------  |
//    +-----+  +------+------+------+------+------+------+------+------+
//    |  7  |  |  <------LSb |  61  |  60  |  59  |  58  |  57  |  56  |
//    +-----+  +------+------+------+------+------+------+------+------+
type Data [MaxDataLength]byte

// UnsignedBitsLittleEndian returns the little-endian bit range [start, start+length) as an unsigned value.
func (d *Data) UnsignedBitsLittleEndian(start, length uint8) uint64 {
	// pack bits into one continuous value
	packed := d.PackLittleEndian()
	// lsb index in the packed value is the start bit
	lsbIndex := start
	// shift away lower bits
	shifted := packed >> lsbIndex
	// mask away higher bits
	masked := shifted & ((1 << length) - 1)
	// done
	return masked
}

// UnsignedBitsBigEndian returns the big-endian bit range [start, start+length) as an unsigned value.
func (d *Data) UnsignedBitsBigEndian(start, length uint8) uint64 {
	// pack bits into one continuous value
	packed := d.PackBigEndian()
	// calculate msb index in the packed value
	msbIndex := invertEndian(start)
	// calculate lsb index in the packed value
	lsbIndex := msbIndex - length + 1
	// shift away lower bits
	shifted := packed >> lsbIndex
	// mask away higher bits
	masked := shifted & ((1 << length) - 1)
	// done
	return masked
}

// SignedBitsLittleEndian returns little-endian bit range [start, start+length) as a signed value.
func (d *Data) SignedBitsLittleEndian(start, length uint8) int64 {
	unsigned := d.UnsignedBitsLittleEndian(start, length)
	return reinterpret.AsSigned(unsigned, length)
}

// SignedBitsBigEndian returns little-endian bit range [start, start+length) as a signed value.
func (d *Data) SignedBitsBigEndian(start, length uint8) int64 {
	unsigned := d.UnsignedBitsBigEndian(start, length)
	return reinterpret.AsSigned(unsigned, length)
}

// SetUnsignedBitsBigEndian sets the little-endian bit range [start, start+length) to the provided unsigned value.
func (d *Data) SetUnsignedBitsLittleEndian(start, length uint8, value uint64) {
	// pack bits into one continuous value
	packed := d.PackLittleEndian()
	// lsb index in the packed value is the start bit
	lsbIndex := start
	// calculate bit mask for zeroing the bit range to set
	unsetMask := ^uint64(((1 << length) - 1) << lsbIndex)
	// calculate bit mask for setting the new value
	setMask := value << lsbIndex
	// calculate the new packed value
	newPacked := packed&unsetMask | setMask
	// unpack the new packed value into the data
	d.UnpackLittleEndian(newPacked)
}

// SetUnsignedBitsBigEndian sets the big-endian bit range [start, start+length) to the provided unsigned value.
func (d *Data) SetUnsignedBitsBigEndian(start, length uint8, value uint64) {
	// pack bits into one continuous value
	packed := d.PackBigEndian()
	// calculate msb index in the packed value
	msbIndex := invertEndian(start)
	// calculate lsb index in the packed value
	lsbIndex := msbIndex - length + 1
	// calculate bit mask for zeroing the bit range to set
	unsetMask := ^uint64(((1 << length) - 1) << lsbIndex)
	// calculate bit mask for setting the new value
	setMask := value << lsbIndex
	// calculate the new packed value
	newPacked := packed&unsetMask | setMask
	// unpack the new packed value into the data
	d.UnpackBigEndian(newPacked)
}

// SetSignedBitsLittleEndian sets the little-endian bit range [start, start+length) to the provided signed value.
func (d *Data) SetSignedBitsLittleEndian(start, length uint8, value int64) {
	d.SetUnsignedBitsLittleEndian(start, length, reinterpret.AsUnsigned(value, length))
}

// SetSignedBitsBigEndian sets the big-endian bit range [start, start+length) to the provided signed value.
func (d *Data) SetSignedBitsBigEndian(start, length uint8, value int64) {
	d.SetUnsignedBitsBigEndian(start, length, reinterpret.AsUnsigned(value, length))
}

// Bit returns the value of the i:th bit in the data as a bool.
func (d *Data) Bit(i uint8) bool {
	if i > 63 {
		return false
	}
	// calculate which byte the bit belongs to
	byteIndex := i / 8
	// calculate bit mask for extracting the bit
	bitMask := uint8(1 << (i % 8))
	// mocks the bit
	bit := d[byteIndex]&bitMask > 0
	// done
	return bit
}

// SetBit sets the value of the i:th bit in the data.
func (d *Data) SetBit(i uint8, value bool) {
	if i > 63 {
		return
	}
	byteIndex := i / 8
	bitIndex := i % 8
	if value {
		d[byteIndex] |= uint8(1 << bitIndex)
	} else {
		d[byteIndex] &= ^uint8(1 << bitIndex)
	}
}

// PackLittleEndian packs the data into a contiguous uint64 value for little-endian signals.
func (d *Data) PackLittleEndian() uint64 {
	var packed uint64
	packed |= uint64(d[0]) << (0 * 8)
	packed |= uint64(d[1]) << (1 * 8)
	packed |= uint64(d[2]) << (2 * 8)
	packed |= uint64(d[3]) << (3 * 8)
	packed |= uint64(d[4]) << (4 * 8)
	packed |= uint64(d[5]) << (5 * 8)
	packed |= uint64(d[6]) << (6 * 8)
	packed |= uint64(d[7]) << (7 * 8)
	return packed
}

// PackBigEndian packs the data into a contiguous uint64 value for big-endian signals.
func (d *Data) PackBigEndian() uint64 {
	var packed uint64
	packed |= uint64(d[0]) << (7 * 8)
	packed |= uint64(d[1]) << (6 * 8)
	packed |= uint64(d[2]) << (5 * 8)
	packed |= uint64(d[3]) << (4 * 8)
	packed |= uint64(d[4]) << (3 * 8)
	packed |= uint64(d[5]) << (2 * 8)
	packed |= uint64(d[6]) << (1 * 8)
	packed |= uint64(d[7]) << (0 * 8)
	return packed
}

// UnpackLittleEndian sets the value of d.Bytes by unpacking the provided value as sequential little-endian bits.
func (d *Data) UnpackLittleEndian(packed uint64) {
	d[0] = uint8(packed >> (0 * 8))
	d[1] = uint8(packed >> (1 * 8))
	d[2] = uint8(packed >> (2 * 8))
	d[3] = uint8(packed >> (3 * 8))
	d[4] = uint8(packed >> (4 * 8))
	d[5] = uint8(packed >> (5 * 8))
	d[6] = uint8(packed >> (6 * 8))
	d[7] = uint8(packed >> (7 * 8))
}

// UnpackBigEndian sets the value of d.Bytes by unpacking the provided value as sequential big-endian bits.
func (d *Data) UnpackBigEndian(packed uint64) {
	d[0] = uint8(packed >> (7 * 8))
	d[1] = uint8(packed >> (6 * 8))
	d[2] = uint8(packed >> (5 * 8))
	d[3] = uint8(packed >> (4 * 8))
	d[4] = uint8(packed >> (3 * 8))
	d[5] = uint8(packed >> (2 * 8))
	d[6] = uint8(packed >> (1 * 8))
	d[7] = uint8(packed >> (0 * 8))
}

// invertEndian converts from big-endian to little-endian bit indexing and vice versa.
func invertEndian(i uint8) uint8 {
	row := i / 8
	col := i % 8
	oppositeRow := 7 - row
	bitIndex := (oppositeRow * 8) + col
	return bitIndex
}

// CheckBitRangeLittleEndian checks that a little-endian bit range fits in the data.
func CheckBitRangeLittleEndian(frameLength, rangeStart, rangeLength uint8) error {
	lsbIndex := rangeStart
	msbIndex := rangeStart + rangeLength - 1
	upperBound := frameLength * 8
	if msbIndex >= upperBound {
		return fmt.Errorf("bit range out of bounds [0, %v): [%v, %v]", upperBound, lsbIndex, msbIndex)
	}
	return nil
}

// CheckBitRangeBigEndian checks that a big-endian bit range fits in the data.
func CheckBitRangeBigEndian(frameLength, rangeStart, rangeLength uint8) error {
	upperBound := frameLength * 8
	if rangeStart >= upperBound {
		return fmt.Errorf("bit range starts out of bounds [0, %v): %v", upperBound, rangeStart)
	}
	msbIndex := invertEndian(rangeStart)
	lsbIndex := msbIndex - rangeLength + 1
	end := invertEndian(lsbIndex)
	if end >= upperBound {
		return fmt.Errorf("bit range ends out of bounds [0, %v): %v", upperBound, end)
	}
	return nil
}

// CheckValue checks that a value fits in a number of bits.
func CheckValue(value uint64, bits uint8) error {
	upperBound := uint64(1 << bits)
	if value >= upperBound {
		return fmt.Errorf("value out of bounds [0, %v): %v", upperBound, value)
	}
	return nil
}