refactor(tvix/nix-compat): add text_hash_string function r/6057

Use it to calculate the text_hash_string, which is then used in the
calculate_derivation_path and path_with_references functions.

Relates to b/263.

Change-Id: I7478825e2a23a11224212fea5e3fd06daa97d5e5
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8364
Autosubmit: flokli <flokli@flokli.de>
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI

1 files changed, 42 insertions, 0 deletions

diff --git a/tvix/nix-compat/src/texthash.rs b/tvix/nix-compat/src/texthash.rs
new file mode 100644
index 000000000000..959a4dc3dcf0
--- /dev/null
+++ b/tvix/nix-compat/src/texthash.rs
@@ -0,0 +1,42 @@
+use sha2::{Digest, Sha256};
+
+use crate::{nixhash::NixHash, store_path};
+
+/// This contains the Nix logic to create "text hash strings", which are used
+/// in `builtins.toFile`, as well as in Derivation Path calculation.
+///
+/// A text hash is calculated by concatenating the following fields, separated by a `:`:
+///
+///  - text
+///  - references, individually joined by `:`
+///  - the nix_hash_string representation of the sha256 digest of some contents
+///  - the value of `storeDir`
+///  - the name
+pub fn text_hash_string<S: AsRef<str>, I: IntoIterator<Item = S>, C: AsRef<[u8]>>(
+    name: &str,
+    content: C,
+    references: I,
+) -> String {
+    let mut s = String::from("text:");
+
+    for reference in references {
+        s.push_str(reference.as_ref());
+        s.push(':');
+    }
+
+    // the nix_hash_string representation of the sha256 digest of some contents
+    s.push_str(
+        &{
+            let content_digest = {
+                let hasher = Sha256::new_with_prefix(content);
+                hasher.finalize()
+            };
+            NixHash::new(crate::nixhash::HashAlgo::Sha256, content_digest.to_vec())
+        }
+        .to_nix_hash_string(),
+    );
+
+    s.push_str(&format!(":{}:{}", store_path::STORE_DIR, name));
+
+    s
+}