feat(tvix/derivation): add nix drv path generation to Derivation

This adds a function to generate the derivation path. The computation
is based on the Go implementation.

Change-Id: Iae89db4976f5fd9208f0453f73688689a245cd66
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7729
Tested-by: BuildkiteCI
Reviewed-by: flokli <flokli@flokli.de>

tvix/derivation/src/nix_hash.rs

@@ -0,0 +1,15 @@
+/// CompressHash takes an arbitrary long sequence of bytes (usually a hash
+/// digest), and returns a sequence of bytes of length output_size.
+/// It's calculated by rotating through the bytes in the output buffer (zero-
+/// initialized), and XOR'ing with each byte of the passed input.
+/// It consumes 1 byte at a time, and XOR's it with the current value in the
+/// output buffer.
+pub fn compress_hash(input: &[u8], output_size: usize) -> Vec<u8> {
+    let mut output: Vec<u8> = vec![0; output_size];
+
+    for (ii, ch) in input.iter().enumerate() {
+        output[ii % output_size] ^= ch;
+    }
+
+    output
+}