scsh 1.0.0

Scoped Skills Helper — preflight a git repo and run its scoped skills in ephemeral containers.
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//! A small, dependency-free SHA-256 (FIPS 180-4), used to content-address the result
//! cache. The root crate stays std-only, so it carries its own rather than pull a crate.

/// SHA-256 of `data`, as a 64-character lowercase hex string.
pub fn sha256_hex(data: &[u8]) -> String {
  let mut h: [u32; 8] =
    [0x6a09e667, 0xbb67ae85, 0x3c6ef372, 0xa54ff53a, 0x510e527f, 0x9b05688c, 0x1f83d9ab, 0x5be0cd19];

  // Pad: 0x80, then zeros to 56 mod 64, then the 64-bit big-endian bit length.
  let bit_len = (data.len() as u64).wrapping_mul(8);
  let mut msg = data.to_vec();
  msg.push(0x80);
  while msg.len() % 64 != 56 {
    msg.push(0);
  }
  msg.extend_from_slice(&bit_len.to_be_bytes());

  for block in msg.chunks_exact(64) {
    let mut w = [0u32; 64];
    for (i, word) in w.iter_mut().enumerate().take(16) {
      let j = i * 4;
      *word = u32::from_be_bytes([block[j], block[j + 1], block[j + 2], block[j + 3]]);
    }
    for i in 16..64 {
      let s0 = w[i - 15].rotate_right(7) ^ w[i - 15].rotate_right(18) ^ (w[i - 15] >> 3);
      let s1 = w[i - 2].rotate_right(17) ^ w[i - 2].rotate_right(19) ^ (w[i - 2] >> 10);
      w[i] = w[i - 16].wrapping_add(s0).wrapping_add(w[i - 7]).wrapping_add(s1);
    }

    let (mut a, mut b, mut c, mut d, mut e, mut f, mut g, mut hh) = (h[0], h[1], h[2], h[3], h[4], h[5], h[6], h[7]);
    for i in 0..64 {
      let s1 = e.rotate_right(6) ^ e.rotate_right(11) ^ e.rotate_right(25);
      let ch = (e & f) ^ ((!e) & g);
      let t1 = hh.wrapping_add(s1).wrapping_add(ch).wrapping_add(K[i]).wrapping_add(w[i]);
      let s0 = a.rotate_right(2) ^ a.rotate_right(13) ^ a.rotate_right(22);
      let maj = (a & b) ^ (a & c) ^ (b & c);
      let t2 = s0.wrapping_add(maj);
      hh = g;
      g = f;
      f = e;
      e = d.wrapping_add(t1);
      d = c;
      c = b;
      b = a;
      a = t1.wrapping_add(t2);
    }
    h[0] = h[0].wrapping_add(a);
    h[1] = h[1].wrapping_add(b);
    h[2] = h[2].wrapping_add(c);
    h[3] = h[3].wrapping_add(d);
    h[4] = h[4].wrapping_add(e);
    h[5] = h[5].wrapping_add(f);
    h[6] = h[6].wrapping_add(g);
    h[7] = h[7].wrapping_add(hh);
  }

  let mut out = String::with_capacity(64);
  for word in h {
    out.push_str(&format!("{word:08x}"));
  }
  out
}

/// The 64 SHA-256 round constants (first 32 bits of the fractional parts of the cube
/// roots of the first 64 primes).
const K: [u32; 64] = [
  0x428a2f98, 0x71374491, 0xb5c0fbcf, 0xe9b5dba5, 0x3956c25b, 0x59f111f1, 0x923f82a4, 0xab1c5ed5, 0xd807aa98,
  0x12835b01, 0x243185be, 0x550c7dc3, 0x72be5d74, 0x80deb1fe, 0x9bdc06a7, 0xc19bf174, 0xe49b69c1, 0xefbe4786,
  0x0fc19dc6, 0x240ca1cc, 0x2de92c6f, 0x4a7484aa, 0x5cb0a9dc, 0x76f988da, 0x983e5152, 0xa831c66d, 0xb00327c8,
  0xbf597fc7, 0xc6e00bf3, 0xd5a79147, 0x06ca6351, 0x14292967, 0x27b70a85, 0x2e1b2138, 0x4d2c6dfc, 0x53380d13,
  0x650a7354, 0x766a0abb, 0x81c2c92e, 0x92722c85, 0xa2bfe8a1, 0xa81a664b, 0xc24b8b70, 0xc76c51a3, 0xd192e819,
  0xd6990624, 0xf40e3585, 0x106aa070, 0x19a4c116, 0x1e376c08, 0x2748774c, 0x34b0bcb5, 0x391c0cb3, 0x4ed8aa4a,
  0x5b9cca4f, 0x682e6ff3, 0x748f82ee, 0x78a5636f, 0x84c87814, 0x8cc70208, 0x90befffa, 0xa4506ceb, 0xbef9a3f7,
  0xc67178f2,
];

#[cfg(test)]
mod tests {
  use super::*;

  #[test]
  fn known_vectors() {
    assert_eq!(sha256_hex(b""), "e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855");
    assert_eq!(sha256_hex(b"abc"), "ba7816bf8f01cfea414140de5dae2223b00361a396177a9cb410ff61f20015ad");
    // Spans the 448-bit padding boundary (56 bytes), exercising the two-block path.
    assert_eq!(
      sha256_hex(b"abcdbcdecdefdefgefghfghighijhijkijkljklmklmnlmnomnopnopq"),
      "248d6a61d20638b8e5c026930c3e6039a33ce45964ff2167f6ecedd419db06c1"
    );
  }

  #[test]
  fn distinct_inputs_distinct_digests() {
    assert_ne!(sha256_hex(b"a"), sha256_hex(b"b"));
    assert_ne!(sha256_hex(b"abc"), sha256_hex(b"abc "));
    assert_eq!(sha256_hex(b"repeatable"), sha256_hex(b"repeatable"));
  }
}