refactor(tvix): introduce nix-compat crate r/5788

Move nixbase32 and store_path into this.

This allows //tvix/cli to not pull in //tvix/store for now.

Change-Id: Id3a32867205d95794bc0d33b21d4cb3d9bafd02a
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7964
Tested-by: BuildkiteCI
Reviewed-by: tazjin <tazjin@tvl.su>

1 files changed, 167 insertions, 0 deletions

diff --git a/tvix/nix-compat/src/nixbase32.rs b/tvix/nix-compat/src/nixbase32.rs
new file mode 100644
index 000000000000..39aa4f1d5461
--- /dev/null
+++ b/tvix/nix-compat/src/nixbase32.rs
@@ -0,0 +1,167 @@
+//! Implements the slightly odd "base32" encoding that's used in Nix.
+//!
+//! Nix uses a custom alphabet. Contrary to other implementations (RFC4648),
+//! encoding to "nix base32" doesn't use any padding, and reads in characters
+//! in reverse order.
+//!
+//! This is also the main reason why we can't use `data_encoding::Encoding` -
+//! it gets things wrong if there normally would be a need for padding.
+
+use std::fmt::Write;
+
+use thiserror::Error;
+
+const ALPHABET: &'static [u8; 32] = b"0123456789abcdfghijklmnpqrsvwxyz";
+
+/// Errors that can occur while decoding nixbase32-encoded data.
+#[derive(Debug, Eq, PartialEq, Error)]
+pub enum Nixbase32DecodeError {
+    #[error("character {0:x} not in alphabet")]
+    CharacterNotInAlphabet(u8),
+    #[error("nonzero carry")]
+    NonzeroCarry(),
+}
+
+/// Returns encoded input
+pub fn encode(input: &[u8]) -> String {
+    let output_len = encode_len(input.len());
+    let mut output = String::with_capacity(output_len);
+
+    if output_len > 0 {
+        for n in (0..=output_len - 1).rev() {
+            let b = n * 5; // bit offset within the entire input
+            let i = b / 8; // input byte index
+            let j = b % 8; // bit offset within that input byte
+
+            let mut c = input[i] >> j;
+            if i + 1 < input.len() {
+                // we want to right shift, and discard shifted out bits (unchecked)
+                // To do this without panicing, we need to do the shifting in u16
+                // and convert back to u8 afterwards.
+                c |= ((input[i + 1] as u16) << 8 - j as u16) as u8
+            }
+
+            output
+                .write_char(ALPHABET[(c & 0x1f) as usize] as char)
+                .unwrap();
+        }
+    }
+
+    output
+}
+
+/// This maps a nixbase32-encoded character to its binary representation, which
+/// is also the index of the character in the alphabet.
+fn decode_char(encoded_char: &u8) -> Option<u8> {
+    Some(match encoded_char {
+        b'0'..=b'9' => encoded_char - b'0',
+        b'a'..=b'd' => encoded_char - b'a' + 10_u8,
+        b'f'..=b'n' => encoded_char - b'f' + 14_u8,
+        b'p'..=b's' => encoded_char - b'p' + 23_u8,
+        b'v'..=b'z' => encoded_char - b'v' + 27_u8,
+        _ => return None,
+    })
+}
+
+/// Returns decoded input
+pub fn decode(input: &[u8]) -> Result<Vec<u8>, Nixbase32DecodeError> {
+    let output_len = decode_len(input.len());
+    let mut output: Vec<u8> = vec![0x00; output_len];
+
+    // loop over all characters in reverse, and keep the iteration count in n.
+    for (n, c) in input.iter().rev().enumerate() {
+        match decode_char(c) {
+            None => return Err(Nixbase32DecodeError::CharacterNotInAlphabet(*c)),
+            Some(c_decoded) => {
+                let b = n * 5;
+                let i = b / 8;
+                let j = b % 8;
+
+                let val = (c_decoded as u16).rotate_left(j as u32);
+                output[i] |= (val & 0x00ff) as u8;
+                let carry = ((val & 0xff00) >> 8) as u8;
+
+                // if we're at the end of dst…
+                if i == output_len - 1 {
+                    // but have a nonzero carry, the encoding is invalid.
+                    if carry != 0 {
+                        return Err(Nixbase32DecodeError::NonzeroCarry());
+                    }
+                } else {
+                    output[i + 1] |= carry;
+                }
+            }
+        }
+    }
+
+    Ok(output)
+}
+
+/// Returns the decoded length of an input of length len.
+pub fn decode_len(len: usize) -> usize {
+    return (len * 5) / 8;
+}
+
+/// Returns the encoded length of an input of length len
+pub fn encode_len(len: usize) -> usize {
+    if len == 0 {
+        return 0;
+    }
+    return (len * 8 - 1) / 5 + 1;
+}
+
+#[cfg(test)]
+mod tests {
+    use test_case::test_case;
+
+    #[test_case("", vec![] ; "empty bytes")]
+    #[test_case("0z", vec![0x1f]; "one byte")]
+    #[test_case("00bgd045z0d4icpbc2yyz4gx48ak44la", vec![
+                 0x8a, 0x12, 0x32, 0x15, 0x22, 0xfd, 0x91, 0xef, 0xbd, 0x60, 0xeb, 0xb2, 0x48, 0x1a,
+                 0xf8, 0x85, 0x80, 0xf6, 0x16, 0x00]; "store path")]
+    #[test_case("0c5b8vw40dy178xlpddw65q9gf1h2186jcc3p4swinwggbllv8mk", vec![
+        0xb3, 0xa2, 0x4d, 0xe9, 0x7a, 0x8f, 0xdb, 0xc8, 0x35, 0xb9, 0x83, 0x31, 0x69, 0x50, 0x10, 0x30,
+        0xb8, 0x97, 0x70, 0x31, 0xbc, 0xb5, 0x4b, 0x3b, 0x3a, 0xc1, 0x37, 0x40, 0xf8, 0x46, 0xab, 0x30,
+    ]; "sha256")]
+    fn encode(enc: &str, dec: Vec<u8>) {
+        assert_eq!(enc, super::encode(&dec));
+    }
+
+    #[test_case("", Some(vec![]) ; "empty bytes")]
+    #[test_case("0z", Some(vec![0x1f]); "one byte")]
+    #[test_case("00bgd045z0d4icpbc2yyz4gx48ak44la", Some(vec![
+                 0x8a, 0x12, 0x32, 0x15, 0x22, 0xfd, 0x91, 0xef, 0xbd, 0x60, 0xeb, 0xb2, 0x48, 0x1a,
+                 0xf8, 0x85, 0x80, 0xf6, 0x16, 0x00]); "store path")]
+    #[test_case("0c5b8vw40dy178xlpddw65q9gf1h2186jcc3p4swinwggbllv8mk", Some(vec![
+        0xb3, 0xa2, 0x4d, 0xe9, 0x7a, 0x8f, 0xdb, 0xc8, 0x35, 0xb9, 0x83, 0x31, 0x69, 0x50, 0x10, 0x30,
+        0xb8, 0x97, 0x70, 0x31, 0xbc, 0xb5, 0x4b, 0x3b, 0x3a, 0xc1, 0x37, 0x40, 0xf8, 0x46, 0xab, 0x30,
+    ]); "sha256")]
+    // this is invalid encoding, because it encodes 10 1-bytes, so the carry
+    // would be 2 1-bytes
+    #[test_case("zz", None; "invalid encoding-1")]
+    // this is an even more specific example - it'd decode as 00000000 11
+    #[test_case("c0", None; "invalid encoding-2")]
+
+    fn decode(enc: &str, dec: Option<Vec<u8>>) {
+        match dec {
+            Some(dec) => {
+                // The decode needs to match what's passed in dec
+                assert_eq!(dec, super::decode(enc.as_bytes()).unwrap());
+            }
+            None => {
+                // the decode needs to be an error
+                assert_eq!(true, super::decode(enc.as_bytes()).is_err());
+            }
+        }
+    }
+
+    #[test]
+    fn encode_len() {
+        assert_eq!(super::encode_len(20), 32)
+    }
+
+    #[test]
+    fn decode_len() {
+        assert_eq!(super::decode_len(32), 20)
+    }
+}