#![allow(non_snake_case)] // RFC 1321 uses many capitalized variables
use std::mem;

fn md5(mut msg: Vec<u8>) -> (u32, u32, u32, u32) {
    let bitcount = msg.len().saturating_mul(8) as u64;

    // Step 1: Append Padding Bits
    msg.push(0b10000000);
    while (msg.len() * 8) % 512 != 448 {
        msg.push(0u8);
    }

    // Step 2. Append Length  (64 bit integer)
    msg.extend(&[
        bitcount as u8,
        (bitcount >> 8) as u8,
        (bitcount >> 16) as u8,
        (bitcount >> 24) as u8,
        (bitcount >> 32) as u8,
        (bitcount >> 40) as u8,
        (bitcount >> 48) as u8,
        (bitcount >> 56) as u8,
    ]);

    // Step 3. Initialize MD Buffer
    /*A four-word buffer (A,B,C,D) is used to compute the message digest.
    Here each of A, B, C, D is a 32-bit register.*/
    let mut A = 0x67452301u32;
    let mut B = 0xefcdab89u32;
    let mut C = 0x98badcfeu32;
    let mut D = 0x10325476u32;

    // Step 4. Process Message in 16-Word Blocks
    /* We first define four auxiliary functions */
    let F = |X: u32, Y: u32, Z: u32| -> u32 { X & Y | !X & Z };
    let G = |X: u32, Y: u32, Z: u32| -> u32 { X & Z | Y & !Z };
    let H = |X: u32, Y: u32, Z: u32| -> u32 { X ^ Y ^ Z };
    let I = |X: u32, Y: u32, Z: u32| -> u32 { Y ^ (X | !Z) };

    /* This step uses a 64-element table T[1 ... 64] constructed from the sine function.  */
    let T = [
        0x00000000, // enable use as a 1-indexed table
        0xd76aa478, 0xe8c7b756, 0x242070db, 0xc1bdceee, 0xf57c0faf, 0x4787c62a, 0xa8304613,
        0xfd469501, 0x698098d8, 0x8b44f7af, 0xffff5bb1, 0x895cd7be, 0x6b901122, 0xfd987193,
        0xa679438e, 0x49b40821, 0xf61e2562, 0xc040b340, 0x265e5a51, 0xe9b6c7aa, 0xd62f105d,
        0x02441453, 0xd8a1e681, 0xe7d3fbc8, 0x21e1cde6, 0xc33707d6, 0xf4d50d87, 0x455a14ed,
        0xa9e3e905, 0xfcefa3f8, 0x676f02d9, 0x8d2a4c8a, 0xfffa3942, 0x8771f681, 0x6d9d6122,
        0xfde5380c, 0xa4beea44, 0x4bdecfa9, 0xf6bb4b60, 0xbebfbc70, 0x289b7ec6, 0xeaa127fa,
        0xd4ef3085, 0x04881d05, 0xd9d4d039, 0xe6db99e5, 0x1fa27cf8, 0xc4ac5665, 0xf4292244,
        0x432aff97, 0xab9423a7, 0xfc93a039, 0x655b59c3, 0x8f0ccc92, 0xffeff47d, 0x85845dd1,
        0x6fa87e4f, 0xfe2ce6e0, 0xa3014314, 0x4e0811a1, 0xf7537e82, 0xbd3af235, 0x2ad7d2bb,
        0xeb86d391,
    ];

    /* Process each 16-word block. (since 1 word is 4 bytes, then 16 words is 64 bytes) */
    for mut block in msg.chunks_exact_mut(64) {
        /* Copy block into X. */
        #![allow(unused_mut)]
        let mut X = unsafe { mem::transmute::<&mut [u8], &mut [u32]>(&mut block) };
        #[cfg(target_endian = "big")]
        for j in 0..16 {
            X[j] = X[j].swap_bytes();
        }

        /* Save Registers A,B,C,D */
        let AA = A;
        let BB = B;
        let CC = C;
        let DD = D;

        /* Round 1.  Let [abcd k s i] denote the operation
        a = b + ((a + F(b,c,d) + X[k] + T[i]) <<< s). */
        macro_rules! op1 {
            ($a:ident,$b:ident,$c:ident,$d:ident,$k:expr,$s:expr,$i:expr) => {
                $a = $b.wrapping_add(
                    ($a.wrapping_add(F($b, $c, $d))
                        .wrapping_add(X[$k])
                        .wrapping_add(T[$i]))
                    .rotate_left($s),
                )
            };
        }

        /* Do the following 16 operations. */
        op1!(A, B, C, D, 0, 7, 1);
        op1!(D, A, B, C, 1, 12, 2);
        op1!(C, D, A, B, 2, 17, 3);
        op1!(B, C, D, A, 3, 22, 4);

        op1!(A, B, C, D, 4, 7, 5);
        op1!(D, A, B, C, 5, 12, 6);
        op1!(C, D, A, B, 6, 17, 7);
        op1!(B, C, D, A, 7, 22, 8);

        op1!(A, B, C, D, 8, 7, 9);
        op1!(D, A, B, C, 9, 12, 10);
        op1!(C, D, A, B, 10, 17, 11);
        op1!(B, C, D, A, 11, 22, 12);

        op1!(A, B, C, D, 12, 7, 13);
        op1!(D, A, B, C, 13, 12, 14);
        op1!(C, D, A, B, 14, 17, 15);
        op1!(B, C, D, A, 15, 22, 16);

        /* Round 2. Let [abcd k s i] denote the operation
        a = b + ((a + G(b,c,d) + X[k] + T[i]) <<< s). */
        macro_rules! op2 {
            ($a:ident,$b:ident,$c:ident,$d:ident,$k:expr,$s:expr,$i:expr) => {
                $a = $b.wrapping_add(
                    ($a.wrapping_add(G($b, $c, $d))
                        .wrapping_add(X[$k])
                        .wrapping_add(T[$i]))
                    .rotate_left($s),
                )
            };
        }

        /* Do the following 16 operations. */
        op2!(A, B, C, D, 1, 5, 17);
        op2!(D, A, B, C, 6, 9, 18);
        op2!(C, D, A, B, 11, 14, 19);
        op2!(B, C, D, A, 0, 20, 20);

        op2!(A, B, C, D, 5, 5, 21);
        op2!(D, A, B, C, 10, 9, 22);
        op2!(C, D, A, B, 15, 14, 23);
        op2!(B, C, D, A, 4, 20, 24);

        op2!(A, B, C, D, 9, 5, 25);
        op2!(D, A, B, C, 14, 9, 26);
        op2!(C, D, A, B, 3, 14, 27);
        op2!(B, C, D, A, 8, 20, 28);

        op2!(A, B, C, D, 13, 5, 29);
        op2!(D, A, B, C, 2, 9, 30);
        op2!(C, D, A, B, 7, 14, 31);
        op2!(B, C, D, A, 12, 20, 32);

        /* Round 3. Let [abcd k s t] denote the operation
        a = b + ((a + H(b,c,d) + X[k] + T[i]) <<< s). */
        macro_rules! op3 {
            ($a:ident,$b:ident,$c:ident,$d:ident,$k:expr,$s:expr,$i:expr) => {
                $a = $b.wrapping_add(
                    ($a.wrapping_add(H($b, $c, $d))
                        .wrapping_add(X[$k])
                        .wrapping_add(T[$i]))
                    .rotate_left($s),
                )
            };
        }

        /* Do the following 16 operations. */
        op3!(A, B, C, D, 5, 4, 33);
        op3!(D, A, B, C, 8, 11, 34);
        op3!(C, D, A, B, 11, 16, 35);
        op3!(B, C, D, A, 14, 23, 36);

        op3!(A, B, C, D, 1, 4, 37);
        op3!(D, A, B, C, 4, 11, 38);
        op3!(C, D, A, B, 7, 16, 39);
        op3!(B, C, D, A, 10, 23, 40);

        op3!(A, B, C, D, 13, 4, 41);
        op3!(D, A, B, C, 0, 11, 42);
        op3!(C, D, A, B, 3, 16, 43);
        op3!(B, C, D, A, 6, 23, 44);

        op3!(A, B, C, D, 9, 4, 45);
        op3!(D, A, B, C, 12, 11, 46);
        op3!(C, D, A, B, 15, 16, 47);
        op3!(B, C, D, A, 2, 23, 48);

        /* Round 4. Let [abcd k s t] denote the operation
        a = b + ((a + I(b,c,d) + X[k] + T[i]) <<< s). */
        macro_rules! op4 {
            ($a:ident,$b:ident,$c:ident,$d:ident,$k:expr,$s:expr,$i:expr) => {
                $a = $b.wrapping_add(
                    ($a.wrapping_add(I($b, $c, $d))
                        .wrapping_add(X[$k])
                        .wrapping_add(T[$i]))
                    .rotate_left($s),
                )
            };
        }

        /* Do the following 16 operations. */
        op4!(A, B, C, D, 0, 6, 49);
        op4!(D, A, B, C, 7, 10, 50);
        op4!(C, D, A, B, 14, 15, 51);
        op4!(B, C, D, A, 5, 21, 52);

        op4!(A, B, C, D, 12, 6, 53);
        op4!(D, A, B, C, 3, 10, 54);
        op4!(C, D, A, B, 10, 15, 55);
        op4!(B, C, D, A, 1, 21, 56);

        op4!(A, B, C, D, 8, 6, 57);
        op4!(D, A, B, C, 15, 10, 58);
        op4!(C, D, A, B, 6, 15, 59);
        op4!(B, C, D, A, 13, 21, 60);

        op4!(A, B, C, D, 4, 6, 61);
        op4!(D, A, B, C, 11, 10, 62);
        op4!(C, D, A, B, 2, 15, 63);
        op4!(B, C, D, A, 9, 21, 64);

        /* . . . increment each of the four registers by the value
        it had before this block was started.) */

        A = A.wrapping_add(AA);
        B = B.wrapping_add(BB);
        C = C.wrapping_add(CC);
        D = D.wrapping_add(DD);
    }
    (
        A.swap_bytes(),
        B.swap_bytes(),
        C.swap_bytes(),
        D.swap_bytes(),
    )
}

fn md5_utf8(smsg: &str) -> String {
    let mut msg = vec![0u8; 0];
    msg.extend(smsg.as_bytes());
    let (A, B, C, D) = md5(msg);
    format!("{:08x}{:08x}{:08x}{:08x}", A, B, C, D)
}

fn main() {
    assert!(md5_utf8("") == "d41d8cd98f00b204e9800998ecf8427e");
    assert!(md5_utf8("a") == "0cc175b9c0f1b6a831c399e269772661");
    assert!(md5_utf8("abc") == "900150983cd24fb0d6963f7d28e17f72");
    assert!(md5_utf8("message digest") == "f96b697d7cb7938d525a2f31aaf161d0");
    assert!(md5_utf8("abcdefghijklmnopqrstuvwxyz") == "c3fcd3d76192e4007dfb496cca67e13b");
    assert!(md5_utf8("ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789") == "d174ab98d277d9f5a5611c2c9f419d9f");
    assert!(md5_utf8("12345678901234567890123456789012345678901234567890123456789012345678901234567890") == "57edf4a22be3c955ac49da2e2107b67a");
}