content-cas 0.1.0

Content-addressed cache primitive: store bytes under their SHA-256 hex, retrieve by hex, atomic on-disk persistence. Zero deps.
Documentation 
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//! Pure-Rust SHA-256 (FIPS 180-4). Adequate for hashing prompts;
//! NOT a cryptographic API — no HMAC, no constant-time anything.
//! Inlined here to keep promptver dependency-free.

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,
];

const H0: [u32; 8] = [
    0x6a09e667, 0xbb67ae85, 0x3c6ef372, 0xa54ff53a, 0x510e527f, 0x9b05688c, 0x1f83d9ab, 0x5be0cd19,
];

fn rotr(x: u32, n: u32) -> u32 {
    x.rotate_right(n)
}

fn compress(state: &mut [u32; 8], block: &[u8]) {
    let mut w = [0u32; 64];
    for i in 0..16 {
        w[i] = u32::from_be_bytes(block[i * 4..i * 4 + 4].try_into().unwrap());
    }
    for i in 16..64 {
        let s0 = rotr(w[i - 15], 7) ^ rotr(w[i - 15], 18) ^ (w[i - 15] >> 3);
        let s1 = rotr(w[i - 2], 17) ^ rotr(w[i - 2], 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 h] = *state;
    for i in 0..64 {
        let s1 = rotr(e, 6) ^ rotr(e, 11) ^ rotr(e, 25);
        let ch = (e & f) ^ (!e & g);
        let t1 = h
            .wrapping_add(s1)
            .wrapping_add(ch)
            .wrapping_add(K[i])
            .wrapping_add(w[i]);
        let s0 = rotr(a, 2) ^ rotr(a, 13) ^ rotr(a, 22);
        let maj = (a & b) ^ (a & c) ^ (b & c);
        let t2 = s0.wrapping_add(maj);
        h = g;
        g = f;
        f = e;
        e = d.wrapping_add(t1);
        d = c;
        c = b;
        b = a;
        a = t1.wrapping_add(t2);
    }
    state[0] = state[0].wrapping_add(a);
    state[1] = state[1].wrapping_add(b);
    state[2] = state[2].wrapping_add(c);
    state[3] = state[3].wrapping_add(d);
    state[4] = state[4].wrapping_add(e);
    state[5] = state[5].wrapping_add(f);
    state[6] = state[6].wrapping_add(g);
    state[7] = state[7].wrapping_add(h);
}

/// Hex SHA-256 of `data`.
pub fn hex(data: &[u8]) -> String {
    let mut state = H0;
    let bit_len = (data.len() as u64) * 8;

    // Process all complete 64-byte blocks.
    let full_blocks = data.len() / 64;
    for i in 0..full_blocks {
        compress(&mut state, &data[i * 64..(i + 1) * 64]);
    }

    // Pad the tail.
    let mut tail = Vec::with_capacity(128);
    tail.extend_from_slice(&data[full_blocks * 64..]);
    tail.push(0x80);
    while tail.len() % 64 != 56 {
        tail.push(0);
    }
    tail.extend_from_slice(&bit_len.to_be_bytes());

    for chunk in tail.chunks_exact(64) {
        compress(&mut state, chunk);
    }

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

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

    #[test]
    fn empty_string() {
        // SHA-256("")
        assert_eq!(
            hex(b""),
            "e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855"
        );
    }

    #[test]
    fn abc() {
        assert_eq!(
            hex(b"abc"),
            "ba7816bf8f01cfea414140de5dae2223b00361a396177a9cb410ff61f20015ad"
        );
    }

    #[test]
    fn long_input() {
        let s = "a".repeat(1_000_000);
        // SHA-256 of one million 'a' chars; standard test vector.
        assert_eq!(
            hex(s.as_bytes()),
            "cdc76e5c9914fb9281a1c7e284d73e67f1809a48a497200e046d39ccc7112cd0"
        );
    }
}