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DSLX Example: Compute CRC32 Checksum

In this document we explain in detail the implementation of routine to compute a CRC32 checksum on a single 8-bit input. We don't discuss the algorithm here, only the language features necessary to implement the algorithm.

Refer to the full implementation, in examples/dslx_intro/crc32_one_byte.x, while following this document.

Function Prototype

Let's explain how the function is being defined:

fn crc32_one_byte(byte: u8, polynomial: u32, crc: u32) -> u32 {

Functions are defined starting with the keyword fn, followed by the function's name, crc32_one_byte in this case. Then comes the list of parameters, followed by the declaration of the return type.

This function accepts 3 parameters:

  1. byte, which is of type u8. u8 is a shortcut for bits[8]
  2. polynomial, which is of type u32, a 32-bit type.
  3. crc, which is also of type u32

The return type, which is declared after the ->, is also a u32.

The first line of the function's body is quite curious:

let crc: u32 = crc ^ u32:byte;

The expression to the right side of the = is easy to understand, it computes the xor operation between the incoming parameters crc and byte, which has been cast to a u32.

The let expression re-binds crc to the expression on the right. This looks like a classic variable assignment. However, since these expressions are all scoped by the let expression, the newly assigned crc values are different and distinguishable from their previous values. In other words, the original value of crc is not visible to anybody else after the re-binding.

The next line specifies a for loop. Index variable and accumulator are i and crc, both of type u32. The iterable range expression specifies that the loop should execute 8 times.

  // 8 rounds of updates.
  for (i, crc): (u32, u32) in range(u32:8) {

At the end of the loop, the calculated value is being assigned to the accumulator crc - the last expression in the loop body is assigned to the accumulator:

    let mask: u32 = -(crc & u32:1);
    (crc >> u32:1) ^ (polynomial & mask)

Finally, the accumulator's initial value is being passed to the for expression as a parameter. This can be confusing, especially when compared to other languages, where the init value typically is provided at or near the top of a loop.