refactor(tvix/nix-compat/derivation): remove module inception r/5805

The module `src/derivation/derivation.rs` is a sign of module inception.

Move the Derivation struct definiton up into `src/derivation/mod.rs`,
and some of the helpers in a `util.rs`.

Change-Id: Ib24a5f8a27bdd45df8b1fa2b3482a79b33cab8d5
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7997
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI
Autosubmit: flokli <flokli@flokli.de>

3 files changed, 356 insertions, 350 deletions

diff --git a/tvix/nix-compat/src/derivation/derivation.rs b/tvix/nix-compat/src/derivation/derivation.rs
deleted file mode 100644
index c9a45d9c9b..0000000000
--- a/tvix/nix-compat/src/derivation/derivation.rs
+++ /dev/null
@@ -1,347 +0,0 @@
-use crate::derivation::output::{Hash, Output};
-use crate::derivation::write;
-use crate::derivation::DerivationError;
-use crate::nixbase32;
-use crate::store_path::{StorePath, STORE_DIR};
-use serde::{Deserialize, Serialize};
-use sha2::{Digest, Sha256};
-use std::collections::BTreeSet;
-use std::{collections::BTreeMap, fmt, fmt::Write};
-
-#[derive(Clone, Debug, Default, Eq, PartialEq, Serialize, Deserialize)]
-pub struct Derivation {
-    #[serde(rename = "args")]
-    pub arguments: Vec<String>,
-
-    pub builder: String,
-
-    #[serde(rename = "env")]
-    pub environment: BTreeMap<String, String>,
-
-    #[serde(rename = "inputDrvs")]
-    pub input_derivations: BTreeMap<String, BTreeSet<String>>,
-
-    #[serde(rename = "inputSrcs")]
-    pub input_sources: BTreeSet<String>,
-
-    pub outputs: BTreeMap<String, Output>,
-
-    pub system: String,
-}
-
-/// compress_hash takes an arbitrarily 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.
-///
-/// This mimics equivalent functionality in C++ Nix.
-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
-}
-
-/// This returns a store path, either of a derivation or a regular output.
-/// The string is hashed with sha256, its digest is compressed to 20 bytes, and
-/// nixbase32-encoded (32 characters)
-fn build_store_path(
-    is_derivation: bool,
-    fingerprint: &str,
-    name: &str,
-) -> Result<StorePath, DerivationError> {
-    let digest = {
-        let mut hasher = Sha256::new();
-        hasher.update(fingerprint);
-        hasher.finalize()
-    };
-    let compressed = compress_hash(&digest, 20);
-    if is_derivation {
-        StorePath::from_string(format!("{}-{}.drv", nixbase32::encode(&compressed), name).as_str())
-    } else {
-        StorePath::from_string(format!("{}-{}", nixbase32::encode(&compressed), name,).as_str())
-    }
-    .map_err(|_e| DerivationError::InvalidOutputName(name.to_string()))
-    // Constructing the StorePath can only fail if the passed output name was
-    // invalid, so map errors to a [DerivationError::InvalidOutputName].
-}
-
-/// Build a store path for a literal text file in the store that may
-/// contain references.
-pub fn path_with_references<S: AsRef<str>, I: IntoIterator<Item = S>, C: AsRef<[u8]>>(
-    name: &str,
-    content: C,
-    references: I,
-) -> Result<StorePath, DerivationError> {
-    let mut s = String::from("text");
-
-    for reference in references {
-        s.push(':');
-        s.push_str(reference.as_ref());
-    }
-
-    let content_digest = {
-        let mut hasher = Sha256::new();
-        hasher.update(content);
-        hasher.finalize()
-    };
-
-    s.push_str(&format!(
-        ":sha256:{:x}:{}:{}",
-        content_digest, STORE_DIR, name
-    ));
-
-    build_store_path(false, &s, name)
-}
-
-impl Derivation {
-    pub fn serialize(&self, writer: &mut impl Write) -> Result<(), fmt::Error> {
-        writer.write_str(write::DERIVATION_PREFIX)?;
-        writer.write_char(write::PAREN_OPEN)?;
-
-        write::write_outputs(writer, &self.outputs)?;
-        write::write_input_derivations(writer, &self.input_derivations)?;
-        write::write_input_sources(writer, &self.input_sources)?;
-        write::write_system(writer, &self.system)?;
-        write::write_builder(writer, &self.builder)?;
-        write::write_arguments(writer, &self.arguments)?;
-        write::write_enviroment(writer, &self.environment)?;
-
-        writer.write_char(write::PAREN_CLOSE)?;
-
-        Ok(())
-    }
-
-    /// Returns the fixed output path and its hash
-    // (if the Derivation is fixed output),
-    /// or None if there is no fixed output.
-    /// This takes some shortcuts in case more than one output exists, as this
-    /// can't be a valid fixed-output Derivation.
-    pub fn get_fixed_output(&self) -> Option<(&String, &Hash)> {
-        if self.outputs.len() != 1 {
-            return None;
-        }
-
-        match self.outputs.get("out") {
-            #[allow(clippy::manual_map)]
-            Some(out_output) => match &out_output.hash {
-                Some(out_output_hash) => Some((&out_output.path, out_output_hash)),
-                // There has to be a hash, otherwise it would not be FOD
-                None => None,
-            },
-            None => None,
-        }
-    }
-
-    /// Returns the drv path of a Derivation struct.
-    ///
-    /// The drv path is calculated like this:
-    ///   - Write the fingerprint of the Derivation to the sha256 hash function.
-    ///     This is: `text:`,
-    ///     all d.InputDerivations and d.InputSources (sorted, separated by a `:`),
-    ///     a `:`,
-    ///     a `sha256:`, followed by the sha256 digest of the ATerm representation (hex-encoded)
-    ///     a `:`,
-    ///     the storeDir, followed by a `:`,
-    ///     the name of a derivation,
-    ///     a `.drv`.
-    ///   - Write the .drv A-Term contents to a hash function
-    ///   - Take the digest, run hash.CompressHash(digest, 20) on it.
-    ///   - Encode it with nixbase32
-    ///   - Use it (and the name) to construct a [StorePath].
-    pub fn calculate_derivation_path(&self, name: &str) -> Result<StorePath, DerivationError> {
-        let mut s = String::from("text:");
-
-        // collect the list of paths from input_sources and input_derivations
-        // into a (sorted, guaranteed by BTreeSet) list, and join them by :
-        let concat_inputs: BTreeSet<String> = {
-            let mut inputs = self.input_sources.clone();
-            let input_derivation_keys: Vec<String> =
-                self.input_derivations.keys().cloned().collect();
-            inputs.extend(input_derivation_keys);
-            inputs
-        };
-
-        for input in concat_inputs {
-            s.push_str(&input);
-            s.push(':');
-        }
-
-        // calculate the sha256 hash of the ATerm representation, and represent
-        // it as a hex-encoded string (prefixed with sha256:).
-        let aterm_digest = {
-            let mut derivation_hasher = Sha256::new();
-            derivation_hasher.update(self.to_string());
-            derivation_hasher.finalize()
-        };
-
-        s.push_str(&format!(
-            "sha256:{:x}:{}:{}.drv",
-            aterm_digest, STORE_DIR, name,
-        ));
-
-        build_store_path(true, &s, name)
-    }
-
-    /// Calculate the drv replacement string for a given derivation.
-    ///
-    /// This is either called on a struct without output paths populated,
-    /// to provide the `drv_replacement_str` value for the `calculate_output_paths`
-    /// function call, or called on a struct with output paths populated, to
-    /// calculate / cache lookups for calls to fn_get_drv_replacement.
-    ///
-    /// `fn_get_drv_replacement` is used to look up the drv replacement strings
-    /// for input_derivations the Derivation refers to.
-    pub fn calculate_drv_replacement_str<F>(&self, fn_get_drv_replacement: F) -> String
-    where
-        F: Fn(&str) -> String,
-    {
-        let mut hasher = Sha256::new();
-        let digest = match self.get_fixed_output() {
-            Some((fixed_output_path, fixed_output_hash)) => {
-                hasher.update(format!(
-                    "fixed:out:{}:{}:{}",
-                    &fixed_output_hash.algo, &fixed_output_hash.digest, fixed_output_path,
-                ));
-                hasher.finalize()
-            }
-            None => {
-                let mut replaced_input_derivations: BTreeMap<String, BTreeSet<String>> =
-                    BTreeMap::new();
-
-                // For each input_derivation, look up the replacement.
-                for (drv_path, input_derivation) in &self.input_derivations {
-                    replaced_input_derivations.insert(
-                        fn_get_drv_replacement(drv_path).to_string(),
-                        input_derivation.clone(),
-                    );
-                }
-
-                // construct a new derivation struct with these replaced input derivation strings
-                let replaced_derivation = Derivation {
-                    input_derivations: replaced_input_derivations,
-                    ..self.clone()
-                };
-
-                // write the ATerm of that to the hash function
-                hasher.update(replaced_derivation.to_string());
-
-                hasher.finalize()
-            }
-        };
-
-        format!("{:x}", digest)
-    }
-
-    /// This calculates all output paths of a Derivation and updates the struct.
-    /// It requires the struct to be initially without output paths.
-    /// This means, self.outputs[$outputName].path needs to be an empty string,
-    /// and self.environment[$outputName] needs to be an empty string.
-    ///
-    /// Output path calculation requires knowledge of "drv replacement
-    /// strings", and in case of non-fixed-output derivations, also knowledge
-    /// of "drv replacement" strings (recursively) of all input derivations.
-    ///
-    /// We solve this by asking the caller of this function to provide
-    /// the drv replacement string of the current derivation itself,
-    /// which is ran on the struct without output paths.
-    ///
-    /// This sound terribly ugly, but won't be too much of a concern later on, as
-    /// naming fixed-output paths once uploaded will be a tvix-store concern,
-    /// so there's no need to calculate them here anymore.
-    ///
-    /// On completion, self.environment[$outputName] and
-    /// self.outputs[$outputName].path are set to the calculated output path for all
-    /// outputs.
-    pub fn calculate_output_paths(
-        &mut self,
-        name: &str,
-        drv_replacement_str: &str,
-    ) -> Result<(), DerivationError> {
-        // Check if the Derivation is fixed output, because they cause
-        // different fingerprints to be hashed.
-        match self.get_fixed_output() {
-            None => {
-                // The fingerprint and hash differs per output
-                for (output_name, output) in self.outputs.iter_mut() {
-                    // Assert that outputs are not yet populated, to avoid using this function wrongly.
-                    // We don't also go over self.environment, but it's a sufficient
-                    // footgun prevention mechanism.
-                    assert!(output.path.is_empty());
-
-                    // calculate the output_name_path, which is the part of the NixPath after the digest.
-                    let mut output_path_name = name.to_string();
-                    if output_name != "out" {
-                        output_path_name.push('-');
-                        output_path_name.push_str(output_name);
-                    }
-
-                    let s = &format!(
-                        "output:{}:sha256:{}:{}:{}",
-                        output_name, drv_replacement_str, STORE_DIR, output_path_name,
-                    );
-
-                    let abs_store_path =
-                        build_store_path(false, s, &output_path_name)?.to_absolute_path();
-
-                    output.path = abs_store_path.clone();
-                    self.environment
-                        .insert(output_name.to_string(), abs_store_path);
-                }
-            }
-            Some((fixed_output_path, fixed_output_hash)) => {
-                // Assert that outputs are not yet populated, to avoid using this function wrongly.
-                // We don't also go over self.environment, but it's a sufficient
-                // footgun prevention mechanism.
-                assert!(fixed_output_path.is_empty());
-
-                let s = {
-                    let mut s = String::new();
-                    // Fixed-output derivation.
-                    // There's two different hashing strategies in place, depending on the value of hash.algo.
-                    // This code is _weird_ but it is what Nix is doing. See:
-                    // https://github.com/NixOS/nix/blob/1385b2007804c8a0370f2a6555045a00e34b07c7/src/libstore/store-api.cc#L178-L196
-                    if fixed_output_hash.algo == "r:sha256" {
-                        s.push_str(&format!(
-                            "source:sha256:{}",
-                            fixed_output_hash.digest, // nixbase32
-                        ));
-                    } else {
-                        s.push_str("output:out:sha256:");
-                        // This is drv_replacement for FOD, with an empty fixed_output_path.
-                        s.push_str(drv_replacement_str);
-                    }
-                    s.push_str(&format!(":{}:{}", STORE_DIR, name));
-                    s
-                };
-
-                let abs_store_path = build_store_path(false, &s, name)?.to_absolute_path();
-
-                self.outputs.insert(
-                    "out".to_string(),
-                    Output {
-                        path: abs_store_path.clone(),
-                        hash: Some(fixed_output_hash.clone()),
-                    },
-                );
-                self.environment.insert("out".to_string(), abs_store_path);
-            }
-        };
-
-        Ok(())
-    }
-}
-
-impl fmt::Display for Derivation {
-    /// Formats the Derivation in ATerm representation.
-    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        self.serialize(f)
-    }
-}
diff --git a/tvix/nix-compat/src/derivation/mod.rs b/tvix/nix-compat/src/derivation/mod.rs
index 1b82251bf6..cecaa3f42d 100644
--- a/tvix/nix-compat/src/derivation/mod.rs
+++ b/tvix/nix-compat/src/derivation/mod.rs
@@ -1,7 +1,12 @@
-mod derivation;
+use crate::store_path::{self, StorePath};
+use serde::{Deserialize, Serialize};
+use sha2::{Digest, Sha256};
+use std::collections::{BTreeMap, BTreeSet};
+
 mod errors;
 mod output;
 mod string_escape;
+mod utils;
 mod validate;
 mod write;
 
@@ -9,7 +14,277 @@ mod write;
 mod tests;
 
 // Public API of the crate.
-
-pub use derivation::{path_with_references, Derivation};
 pub use errors::{DerivationError, OutputError};
 pub use output::{Hash, Output};
+pub use utils::path_with_references;
+
+#[derive(Clone, Debug, Default, Eq, PartialEq, Serialize, Deserialize)]
+pub struct Derivation {
+    #[serde(rename = "args")]
+    pub arguments: Vec<String>,
+
+    pub builder: String,
+
+    #[serde(rename = "env")]
+    pub environment: BTreeMap<String, String>,
+
+    #[serde(rename = "inputDrvs")]
+    pub input_derivations: BTreeMap<String, BTreeSet<String>>,
+
+    #[serde(rename = "inputSrcs")]
+    pub input_sources: BTreeSet<String>,
+
+    pub outputs: BTreeMap<String, Output>,
+
+    pub system: String,
+}
+
+impl Derivation {
+    pub fn serialize(&self, writer: &mut impl std::fmt::Write) -> Result<(), std::fmt::Error> {
+        writer.write_str(write::DERIVATION_PREFIX)?;
+        writer.write_char(write::PAREN_OPEN)?;
+
+        write::write_outputs(writer, &self.outputs)?;
+        write::write_input_derivations(writer, &self.input_derivations)?;
+        write::write_input_sources(writer, &self.input_sources)?;
+        write::write_system(writer, &self.system)?;
+        write::write_builder(writer, &self.builder)?;
+        write::write_arguments(writer, &self.arguments)?;
+        write::write_enviroment(writer, &self.environment)?;
+
+        writer.write_char(write::PAREN_CLOSE)?;
+
+        Ok(())
+    }
+
+    /// Returns the fixed output path and its hash
+    // (if the Derivation is fixed output),
+    /// or None if there is no fixed output.
+    /// This takes some shortcuts in case more than one output exists, as this
+    /// can't be a valid fixed-output Derivation.
+    pub fn get_fixed_output(&self) -> Option<(&String, &Hash)> {
+        if self.outputs.len() != 1 {
+            return None;
+        }
+
+        match self.outputs.get("out") {
+            #[allow(clippy::manual_map)]
+            Some(out_output) => match &out_output.hash {
+                Some(out_output_hash) => Some((&out_output.path, out_output_hash)),
+                // There has to be a hash, otherwise it would not be FOD
+                None => None,
+            },
+            None => None,
+        }
+    }
+
+    /// Returns the drv path of a Derivation struct.
+    ///
+    /// The drv path is calculated like this:
+    ///   - Write the fingerprint of the Derivation to the sha256 hash function.
+    ///     This is: `text:`,
+    ///     all d.InputDerivations and d.InputSources (sorted, separated by a `:`),
+    ///     a `:`,
+    ///     a `sha256:`, followed by the sha256 digest of the ATerm representation (hex-encoded)
+    ///     a `:`,
+    ///     the storeDir, followed by a `:`,
+    ///     the name of a derivation,
+    ///     a `.drv`.
+    ///   - Write the .drv A-Term contents to a hash function
+    ///   - Take the digest, run hash.CompressHash(digest, 20) on it.
+    ///   - Encode it with nixbase32
+    ///   - Use it (and the name) to construct a [StorePath].
+    pub fn calculate_derivation_path(&self, name: &str) -> Result<StorePath, DerivationError> {
+        let mut s = String::from("text:");
+
+        // collect the list of paths from input_sources and input_derivations
+        // into a (sorted, guaranteed by BTreeSet) list, and join them by :
+        let concat_inputs: BTreeSet<String> = {
+            let mut inputs = self.input_sources.clone();
+            let input_derivation_keys: Vec<String> =
+                self.input_derivations.keys().cloned().collect();
+            inputs.extend(input_derivation_keys);
+            inputs
+        };
+
+        for input in concat_inputs {
+            s.push_str(&input);
+            s.push(':');
+        }
+
+        // calculate the sha256 hash of the ATerm representation, and represent
+        // it as a hex-encoded string (prefixed with sha256:).
+        let aterm_digest = {
+            let mut derivation_hasher = Sha256::new();
+            derivation_hasher.update(self.to_string());
+            derivation_hasher.finalize()
+        };
+
+        s.push_str(&format!(
+            "sha256:{:x}:{}:{}.drv",
+            aterm_digest,
+            store_path::STORE_DIR,
+            name,
+        ));
+
+        utils::build_store_path(true, &s, name)
+    }
+
+    /// Calculate the drv replacement string for a given derivation.
+    ///
+    /// This is either called on a struct without output paths populated,
+    /// to provide the `drv_replacement_str` value for the `calculate_output_paths`
+    /// function call, or called on a struct with output paths populated, to
+    /// calculate / cache lookups for calls to fn_get_drv_replacement.
+    ///
+    /// `fn_get_drv_replacement` is used to look up the drv replacement strings
+    /// for input_derivations the Derivation refers to.
+    pub fn calculate_drv_replacement_str<F>(&self, fn_get_drv_replacement: F) -> String
+    where
+        F: Fn(&str) -> String,
+    {
+        let mut hasher = Sha256::new();
+        let digest = match self.get_fixed_output() {
+            Some((fixed_output_path, fixed_output_hash)) => {
+                hasher.update(format!(
+                    "fixed:out:{}:{}:{}",
+                    &fixed_output_hash.algo, &fixed_output_hash.digest, fixed_output_path,
+                ));
+                hasher.finalize()
+            }
+            None => {
+                let mut replaced_input_derivations: BTreeMap<String, BTreeSet<String>> =
+                    BTreeMap::new();
+
+                // For each input_derivation, look up the replacement.
+                for (drv_path, input_derivation) in &self.input_derivations {
+                    replaced_input_derivations.insert(
+                        fn_get_drv_replacement(drv_path).to_string(),
+                        input_derivation.clone(),
+                    );
+                }
+
+                // construct a new derivation struct with these replaced input derivation strings
+                let replaced_derivation = Derivation {
+                    input_derivations: replaced_input_derivations,
+                    ..self.clone()
+                };
+
+                // write the ATerm of that to the hash function
+                hasher.update(replaced_derivation.to_string());
+
+                hasher.finalize()
+            }
+        };
+
+        format!("{:x}", digest)
+    }
+
+    /// This calculates all output paths of a Derivation and updates the struct.
+    /// It requires the struct to be initially without output paths.
+    /// This means, self.outputs[$outputName].path needs to be an empty string,
+    /// and self.environment[$outputName] needs to be an empty string.
+    ///
+    /// Output path calculation requires knowledge of "drv replacement
+    /// strings", and in case of non-fixed-output derivations, also knowledge
+    /// of "drv replacement" strings (recursively) of all input derivations.
+    ///
+    /// We solve this by asking the caller of this function to provide
+    /// the drv replacement string of the current derivation itself,
+    /// which is ran on the struct without output paths.
+    ///
+    /// This sound terribly ugly, but won't be too much of a concern later on, as
+    /// naming fixed-output paths once uploaded will be a tvix-store concern,
+    /// so there's no need to calculate them here anymore.
+    ///
+    /// On completion, self.environment[$outputName] and
+    /// self.outputs[$outputName].path are set to the calculated output path for all
+    /// outputs.
+    pub fn calculate_output_paths(
+        &mut self,
+        name: &str,
+        drv_replacement_str: &str,
+    ) -> Result<(), DerivationError> {
+        // Check if the Derivation is fixed output, because they cause
+        // different fingerprints to be hashed.
+        match self.get_fixed_output() {
+            None => {
+                // The fingerprint and hash differs per output
+                for (output_name, output) in self.outputs.iter_mut() {
+                    // Assert that outputs are not yet populated, to avoid using this function wrongly.
+                    // We don't also go over self.environment, but it's a sufficient
+                    // footgun prevention mechanism.
+                    assert!(output.path.is_empty());
+
+                    // calculate the output_name_path, which is the part of the NixPath after the digest.
+                    let mut output_path_name = name.to_string();
+                    if output_name != "out" {
+                        output_path_name.push('-');
+                        output_path_name.push_str(output_name);
+                    }
+
+                    let s = &format!(
+                        "output:{}:sha256:{}:{}:{}",
+                        output_name,
+                        drv_replacement_str,
+                        store_path::STORE_DIR,
+                        output_path_name,
+                    );
+
+                    let abs_store_path =
+                        utils::build_store_path(false, s, &output_path_name)?.to_absolute_path();
+
+                    output.path = abs_store_path.clone();
+                    self.environment
+                        .insert(output_name.to_string(), abs_store_path);
+                }
+            }
+            Some((fixed_output_path, fixed_output_hash)) => {
+                // Assert that outputs are not yet populated, to avoid using this function wrongly.
+                // We don't also go over self.environment, but it's a sufficient
+                // footgun prevention mechanism.
+                assert!(fixed_output_path.is_empty());
+
+                let s = {
+                    let mut s = String::new();
+                    // Fixed-output derivation.
+                    // There's two different hashing strategies in place, depending on the value of hash.algo.
+                    // This code is _weird_ but it is what Nix is doing. See:
+                    // https://github.com/NixOS/nix/blob/1385b2007804c8a0370f2a6555045a00e34b07c7/src/libstore/store-api.cc#L178-L196
+                    if fixed_output_hash.algo == "r:sha256" {
+                        s.push_str(&format!(
+                            "source:sha256:{}",
+                            fixed_output_hash.digest, // nixbase32
+                        ));
+                    } else {
+                        s.push_str("output:out:sha256:");
+                        // This is drv_replacement for FOD, with an empty fixed_output_path.
+                        s.push_str(drv_replacement_str);
+                    }
+                    s.push_str(&format!(":{}:{}", store_path::STORE_DIR, name));
+                    s
+                };
+
+                let abs_store_path = utils::build_store_path(false, &s, name)?.to_absolute_path();
+
+                self.outputs.insert(
+                    "out".to_string(),
+                    Output {
+                        path: abs_store_path.clone(),
+                        hash: Some(fixed_output_hash.clone()),
+                    },
+                );
+                self.environment.insert("out".to_string(), abs_store_path);
+            }
+        };
+
+        Ok(())
+    }
+}
+
+impl std::fmt::Display for Derivation {
+    /// Formats the Derivation in ATerm representation.
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        self.serialize(f)
+    }
+}
diff --git a/tvix/nix-compat/src/derivation/utils.rs b/tvix/nix-compat/src/derivation/utils.rs
new file mode 100644
index 0000000000..77ac94b424
--- /dev/null
+++ b/tvix/nix-compat/src/derivation/utils.rs
@@ -0,0 +1,78 @@
+use crate::derivation::DerivationError;
+use crate::nixbase32;
+use crate::store_path::{self, StorePath};
+use sha2::{Digest, Sha256};
+
+/// compress_hash takes an arbitrarily 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.
+///
+/// This mimics equivalent functionality in C++ Nix.
+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
+}
+
+/// This returns a store path, either of a derivation or a regular output.
+/// The string is hashed with sha256, its digest is compressed to 20 bytes, and
+/// nixbase32-encoded (32 characters)
+pub(super) fn build_store_path(
+    is_derivation: bool,
+    fingerprint: &str,
+    name: &str,
+) -> Result<StorePath, DerivationError> {
+    let digest = {
+        let mut hasher = Sha256::new();
+        hasher.update(fingerprint);
+        hasher.finalize()
+    };
+    let compressed = compress_hash(&digest, 20);
+    if is_derivation {
+        StorePath::from_string(format!("{}-{}.drv", nixbase32::encode(&compressed), name).as_str())
+    } else {
+        StorePath::from_string(format!("{}-{}", nixbase32::encode(&compressed), name,).as_str())
+    }
+    .map_err(|_e| DerivationError::InvalidOutputName(name.to_string()))
+    // Constructing the StorePath can only fail if the passed output name was
+    // invalid, so map errors to a [DerivationError::InvalidOutputName].
+}
+
+/// Build a store path for a literal text file in the store that may
+/// contain references.
+pub fn path_with_references<S: AsRef<str>, I: IntoIterator<Item = S>, C: AsRef<[u8]>>(
+    name: &str,
+    content: C,
+    references: I,
+) -> Result<StorePath, DerivationError> {
+    let mut s = String::from("text");
+
+    for reference in references {
+        s.push(':');
+        s.push_str(reference.as_ref());
+    }
+
+    let content_digest = {
+        let mut hasher = Sha256::new();
+        hasher.update(content);
+        hasher.finalize()
+    };
+
+    s.push_str(&format!(
+        ":sha256:{:x}:{}:{}",
+        content_digest,
+        store_path::STORE_DIR,
+        name
+    ));
+
+    build_store_path(false, &s, name)
+}