//! Minimal SHA-256 implementation for certificate hashing.

#![forbid(unsafe_code)]

/// Compute SHA-256 of a slice of u32 words (treated as little-endian bytes).
///
/// Uses a minimal implementation (no external crate) — bounded, no heap in
/// the hash core. This is verification-grade, not performance-grade.
pub(super) fn sha256_words(words: &[u32]) -> [u8; 32] {
    // Convert words to bytes.
    let mut bytes: Vec<u8> = Vec::new();
    let mut i = 0;
    let max = words.len().min(16384); // 64KB max
    while i < max {
        bytes.extend_from_slice(&words[i].to_le_bytes());
        i += 1;
    }
    sha256_bytes(&bytes)
}

/// Minimal SHA-256 implementation (bounded, no unsafe).
///
/// Processes at most 64KB of input (MEGA-4 programs are ≤ 4096 instructions × 4 bytes).
pub(super) fn sha256_bytes(data: &[u8]) -> [u8; 32] {
    let 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,
    ];

    let mut h: [u32; 8] = [
        0x6a09e667, 0xbb67ae85, 0x3c6ef372, 0xa54ff53a, 0x510e527f, 0x9b05688c, 0x1f83d9ab,
        0x5be0cd19,
    ];

    // Pre-processing: pad message.
    let bit_len = (data.len() as u64).wrapping_mul(8);
    let mut padded: Vec<u8> = Vec::new();
    padded.extend_from_slice(data);
    padded.push(0x80);
    while padded.len() % 64 != 56 {
        padded.push(0);
    }
    padded.extend_from_slice(&bit_len.to_be_bytes());

    // Process each 512-bit (64-byte) block.
    let mut block_idx = 0;
    let max_blocks = padded.len() / 64;
    while block_idx < max_blocks {
        let offset = block_idx * 64;
        let mut w = [0u32; 64];

        // Load 16 words from the block (big-endian).
        let mut wi = 0;
        while wi < 16 {
            let base = offset + wi * 4;
            w[wi] = ((padded[base] as u32) << 24)
                | ((padded[base + 1] as u32) << 16)
                | ((padded[base + 2] as u32) << 8)
                | (padded[base + 3] as u32);
            wi += 1;
        }

        // Extend to 64 words.
        let mut wi2 = 16;
        while wi2 < 64 {
            let s0 =
                w[wi2 - 15].rotate_right(7) ^ w[wi2 - 15].rotate_right(18) ^ (w[wi2 - 15] >> 3);
            let s1 = w[wi2 - 2].rotate_right(17) ^ w[wi2 - 2].rotate_right(19) ^ (w[wi2 - 2] >> 10);
            w[wi2] = w[wi2 - 16].wrapping_add(s0).wrapping_add(w[wi2 - 7]).wrapping_add(s1);
            wi2 += 1;
        }

        // Compression.
        let mut a = h[0];
        let mut b = h[1];
        let mut c = h[2];
        let mut d = h[3];
        let mut e = h[4];
        let mut f = h[5];
        let mut g = h[6];
        let mut hh = h[7];

        let mut ci = 0;
        while ci < 64 {
            let s1 = e.rotate_right(6) ^ e.rotate_right(11) ^ e.rotate_right(25);
            let ch = (e & f) ^ ((!e) & g);
            let temp1 =
                hh.wrapping_add(s1).wrapping_add(ch).wrapping_add(k[ci]).wrapping_add(w[ci]);
            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);

            hh = g;
            g = f;
            f = e;
            e = d.wrapping_add(temp1);
            d = c;
            c = b;
            b = a;
            a = temp1.wrapping_add(temp2);
            ci += 1;
        }

        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);

        block_idx += 1;
    }

    // Produce final hash.
    let mut result = [0u8; 32];
    let mut hi = 0;
    while hi < 8 {
        let bytes = h[hi].to_be_bytes();
        result[hi * 4] = bytes[0];
        result[hi * 4 + 1] = bytes[1];
        result[hi * 4 + 2] = bytes[2];
        result[hi * 4 + 3] = bytes[3];
        hi += 1;
    }
    result
}
