Learning Exercise
C#, like many statically typed languages, provides a number of types that represent integers, each with its own range of values. At the low end, the sbyte type has a minimum value of -128 and a maximum value of 127. Like all the integer types these values are available as <type>.MinValue and <type>.MaxValue. At the high end, the long type has a minimum value of -9,223,372,036,854,775,808 and a maximum value of 9,223,372,036,854,775,807. In between lie the short and int types.
The ranges are determined by the storage width of the type as allocated by the system. For example, a byte uses 8 bits and a long uses 64 bits.
Each of the above types is paired with an unsigned equivalent: sbyte/byte, short/ushort, int/uint and long/ulong. In all cases the range of the values is from 0 to the negative signed maximum times 2 plus 1.
| Type | Width | Minimum | Maximum |
|---|---|---|---|
| sbyte | 8 bit | -128 | +127 |
| short | 16 bit | -32_768 | +32_767 |
| int | 32 bit | -2_147_483_648 | +2_147_483_647 |
| long | 64 bit | -9_223_372_036_854_775_808 | +9_223_372_036_854_775_807 |
| byte | 8 bit | 0 | +255 |
| ushort | 16 bit | 0 | +65_535 |
| uint | 32 bit | 0 | +4_294_967_295 |
| ulong | 64 bit | 0 | +18_446_744_073_709_551_615 |
A variable (or expression) of one type can easily be converted to another. For instance, in an assignment operation, if the type of the value being assigned (lhs) ensures that the value will lie within the range of the type being assigned to (rhs) then there is a simple assignment:
uint ui = uint.MaxValue;
ulong ul = ui; // no problem
On the other hand if the range of type being assigned from is not a subset of the assignee's range of values then a cast, () operation is required even if the particular value is within the assignee's range:
short s = 42;
uint ui = (uint)s;
The BitConverter class provides a convenient way of converting integer types to and from arrays of bytes.
Work continues on the remote control car project. Bandwidth in the telemetry system is at a premium and you have been asked to implement a message protocol for communicating telemetry data.
Data is transmitted in a buffer (byte array). When integers are sent, the size of the buffer is reduced by employing the protocol described below.
Each value should be represented in the smallest possible integral type (types of byte and sbyte are not included as the saving would be trivial):
| From | To | Type |
|---|---|---|
| 4_294_967_296 | 9_223_372_036_854_775_807 | long |
| 2_147_483_648 | 4_294_967_295 | uint |
| 65_536 | 2_147_483_647 | int |
| 0 | 65_535 | ushort |
| -32_768 | -1 | short |
| -2_147_483_648 | -32_769 | int |
| -9_223_372_036_854_775_808 | -2_147_483_649 | long |
The value should be converted to the appropriate number of bytes for its assigned type. The complete buffer comprises a byte indicating the number of additional bytes in the buffer (prefix byte) followed by the bytes holding the integer (payload bytes).
Some of the types use an identical number of bytes (e.g. the uint and int types). Normally, they would have the same prefix byte, but that would make decoding problematic. To counter this, the protocol introduces a little trick: for signed types, their prefix byte value is 256 minus the number of additional bytes in the buffer.
Only the prefix byte and the number of following bytes indicated by the prefix will be sent in the communication. Internally a 9 byte buffer is used (with trailing zeroes, as necessary) both by sending and receiving routines.
Please implement the static method TelemetryBuffer.ToBuffer() to encode a buffer taking the parameter passed to the method.
// Type: ushort, bytes: 2, signed: no, prefix byte: 2
TelemetryBuffer.ToBuffer(5)
// => {0x2, 0x5, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0 };
// Type: int, bytes: 4, signed: yes, prefix byte: 256 - 4
TelemetryBuffer.ToBuffer(Int32.MaxValue)
// => {0xfc, 0xff, 0xff, 0xff, 0x7f, 0x0, 0x0, 0x0, 0x0 };
Please implement the static method TelemetryBuffer.FromBuffer() to decode the buffer received and return the value in the form of a long.
TelemetryBuffer.FromBuffer(new byte[] {0xfc, 0xff, 0xff, 0xff, 0x7f, 0x0, 0x0, 0x0, 0x0 })
// => 2147483647
If the prefix byte is of unexpected value then 0 should be returned.
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