determs 0.1.0

//! # Hash Module — SHA-256 Implementation from Scratch
//!
//! This module implements SHA-256 with zero dependencies.
//!
//! ## Why SHA-256?
//!
//! - **Deterministic**: Same input → same hash, always
//! - **Universal**: Standard algorithm, verifiable anywhere
//! - **Collision-resistant**: Practically impossible to find two inputs with the same hash
//!
//! ## The Fifth API Function: `hash(artifact) → Digest`
//!
//! > Produit une identité globale stable, universelle, vérifiable.
//! > Deux artefacts identiques = même hash, partout.

/// SHA-256 initial hash values (first 32 bits of fractional parts of square roots of first 8 primes).
const H: [u32; 8] = [
    0x6a09e667, 0xbb67ae85, 0x3c6ef372, 0xa54ff53a, 0x510e527f, 0x9b05688c, 0x1f83d9ab, 0x5be0cd19,
];

/// SHA-256 round constants (first 32 bits of fractional parts of cube roots of 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,
];

/// A 256-bit digest (32 bytes).
#[derive(Clone, Copy, PartialEq, Eq)]
pub struct Digest([u8; 32]);

impl Digest {
    /// Create a digest from raw bytes.
    pub const fn from_bytes(bytes: [u8; 32]) -> Self {
        Self(bytes)
    }

    /// Get the raw bytes.
    pub const fn as_bytes(&self) -> &[u8; 32] {
        &self.0
    }

    /// Convert to hexadecimal string.
    pub fn to_hex(&self) -> String {
        let mut hex = String::with_capacity(64);
        for byte in &self.0 {
            hex.push(HEX_CHARS[(byte >> 4) as usize]);
            hex.push(HEX_CHARS[(byte & 0x0f) as usize]);
        }
        hex
    }
}

const HEX_CHARS: [char; 16] = [
    '0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'a', 'b', 'c', 'd', 'e', 'f',
];

impl core::fmt::Debug for Digest {
    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
        write!(f, "Digest({})", self.to_hex())
    }
}

impl core::fmt::Display for Digest {
    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
        write!(f, "{}", self.to_hex())
    }
}

/// SHA-256 hasher.
pub struct Sha256 {
    state: [u32; 8],
    buffer: [u8; 64],
    buffer_len: usize,
    total_len: u64,
}

impl Sha256 {
    /// Create a new SHA-256 hasher.
    pub fn new() -> Self {
        Self {
            state: H,
            buffer: [0; 64],
            buffer_len: 0,
            total_len: 0,
        }
    }

    /// Hash data incrementally.
    pub fn update(&mut self, data: &[u8]) {
        self.total_len += data.len() as u64;
        let mut data = data;

        // If we have buffered data, try to fill the buffer first
        if self.buffer_len > 0 {
            let space = 64 - self.buffer_len;
            let to_copy = data.len().min(space);
            self.buffer[self.buffer_len..self.buffer_len + to_copy]
                .copy_from_slice(&data[..to_copy]);
            self.buffer_len += to_copy;
            data = &data[to_copy..];

            if self.buffer_len == 64 {
                self.process_block(&self.buffer.clone());
                self.buffer_len = 0;
            }
        }

        // Process full blocks directly
        while data.len() >= 64 {
            let block: [u8; 64] = data[..64].try_into().unwrap();
            self.process_block(&block);
            data = &data[64..];
        }

        // Buffer remaining data
        if !data.is_empty() {
            self.buffer[..data.len()].copy_from_slice(data);
            self.buffer_len = data.len();
        }
    }

    /// Finalize and return the digest.
    pub fn finalize(mut self) -> Digest {
        // Pad the message
        let bit_len = self.total_len * 8;

        // Append the '1' bit
        self.buffer[self.buffer_len] = 0x80;
        self.buffer_len += 1;

        // If not enough space for length, pad and process
        if self.buffer_len > 56 {
            for i in self.buffer_len..64 {
                self.buffer[i] = 0;
            }
            self.process_block(&self.buffer.clone());
            self.buffer_len = 0;
        }

        // Pad with zeros
        for i in self.buffer_len..56 {
            self.buffer[i] = 0;
        }

        // Append length in bits (big-endian)
        self.buffer[56] = (bit_len >> 56) as u8;
        self.buffer[57] = (bit_len >> 48) as u8;
        self.buffer[58] = (bit_len >> 40) as u8;
        self.buffer[59] = (bit_len >> 32) as u8;
        self.buffer[60] = (bit_len >> 24) as u8;
        self.buffer[61] = (bit_len >> 16) as u8;
        self.buffer[62] = (bit_len >> 8) as u8;
        self.buffer[63] = bit_len as u8;

        self.process_block(&self.buffer.clone());

        // Produce the digest
        let mut digest = [0u8; 32];
        for (i, &word) in self.state.iter().enumerate() {
            digest[i * 4] = (word >> 24) as u8;
            digest[i * 4 + 1] = (word >> 16) as u8;
            digest[i * 4 + 2] = (word >> 8) as u8;
            digest[i * 4 + 3] = word as u8;
        }

        Digest(digest)
    }

    /// Process a 64-byte block.
    fn process_block(&mut self, block: &[u8; 64]) {
        // Prepare message schedule
        let mut w = [0u32; 64];

        // First 16 words are directly from the block (big-endian)
        for i in 0..16 {
            w[i] = u32::from_be_bytes([
                block[i * 4],
                block[i * 4 + 1],
                block[i * 4 + 2],
                block[i * 4 + 3],
            ]);
        }

        // Extend the message schedule
        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);
        }

        // Initialize working variables
        let [mut a, mut b, mut c, mut d, mut e, mut f, mut g, mut h] = self.state;

        // Main compression loop
        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 temp1 = h
                .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 temp2 = s0.wrapping_add(maj);

            h = g;
            g = f;
            f = e;
            e = d.wrapping_add(temp1);
            d = c;
            c = b;
            b = a;
            a = temp1.wrapping_add(temp2);
        }

        // Add compressed chunk to state
        self.state[0] = self.state[0].wrapping_add(a);
        self.state[1] = self.state[1].wrapping_add(b);
        self.state[2] = self.state[2].wrapping_add(c);
        self.state[3] = self.state[3].wrapping_add(d);
        self.state[4] = self.state[4].wrapping_add(e);
        self.state[5] = self.state[5].wrapping_add(f);
        self.state[6] = self.state[6].wrapping_add(g);
        self.state[7] = self.state[7].wrapping_add(h);
    }
}

impl Default for Sha256 {
    fn default() -> Self {
        Self::new()
    }
}

// ═══════════════════════════════════════════════════════════════════════════
// Public API
// ═══════════════════════════════════════════════════════════════════════════

/// Compute SHA-256 hash of data.
pub fn sha256(data: &[u8]) -> Digest {
    let mut hasher = Sha256::new();
    hasher.update(data);
    hasher.finalize()
}

/// Compute SHA-256 hash of a string.
pub fn sha256_str(s: &str) -> Digest {
    sha256(s.as_bytes())
}

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

    #[test]
    fn test_empty_string() {
        // SHA-256("") = e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855
        let digest = sha256(b"");
        assert_eq!(
            digest.to_hex(),
            "e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855"
        );
    }

    #[test]
    fn test_hello_world() {
        // SHA-256("hello world") = b94d27b9934d3e08a52e52d7da7dabfac484efe37a5380ee9088f7ace2efcde9
        let digest = sha256(b"hello world");
        assert_eq!(
            digest.to_hex(),
            "b94d27b9934d3e08a52e52d7da7dabfac484efe37a5380ee9088f7ace2efcde9"
        );
    }

    #[test]
    fn test_abc() {
        // SHA-256("abc") = ba7816bf8f01cfea414140de5dae2223b00361a396177a9cb410ff61f20015ad
        let digest = sha256(b"abc");
        assert_eq!(
            digest.to_hex(),
            "ba7816bf8f01cfea414140de5dae2223b00361a396177a9cb410ff61f20015ad"
        );
    }

    #[test]
    fn test_long_message() {
        // SHA-256 of 1 million 'a' characters
        // = cdc76e5c9914fb9281a1c7e284d73e67f1809a48a497200e046d39ccc7112cd0
        let data = vec![b'a'; 1_000_000];
        let digest = sha256(&data);
        assert_eq!(
            digest.to_hex(),
            "cdc76e5c9914fb9281a1c7e284d73e67f1809a48a497200e046d39ccc7112cd0"
        );
    }

    #[test]
    fn test_incremental() {
        // Test that incremental hashing gives same result
        let mut hasher = Sha256::new();
        hasher.update(b"hello");
        hasher.update(b" ");
        hasher.update(b"world");
        let digest = hasher.finalize();

        assert_eq!(digest, sha256(b"hello world"));
    }
}