updated readme; implemented MD5 computation using SIMD

Cargo.toml

@@ -1,5 +1,5 @@
 [package]
-name = "hello_cargo"
+name = "trustpilot_challenge_rust"
 version = "0.1.0"
 authors = ["inga-lovinde <52715130+inga-lovinde@users.noreply.github.com>"]
 edition = "2018"
@@ -7,4 +7,7 @@ edition = "2018"
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
 
 [dependencies]
-packed_simd = { version = "0.3.4", package = "packed_simd_2" }
+crunchy = "0.2.2"
+packed_simd = { version = "0.3.4", package = "packed_simd_2", features = ["into_bits"] }
+permutohedron = "0.2.4"
+rayon = "1.5.0"

README.md

@@ -1,4 +1,4 @@
-# TrustPilotChallengeRust
+# TrustPilotChallengeRust
 
 TrustPilot had this challenge several years ago
 (http://followthewhiterabbit.trustpilot.com/)
@@ -26,7 +26,12 @@ and does not yet do actual MD5 calculation.
 
 ## Algorithm description
 
-Notably this solution does not involve string concatenation; strings are only concatenated for debugging purposes.
+Notably this solution does not involve string concatenation;
+strings are only concatenated for debugging purposes.
+It also computes eight MD5 hashes at a time *per thread*
+(that is, 128 MD5 hashes at once on a modern 8-core CPU),
+with some further optimizations which further shave off
+several percents from MD5 computation time.
 
 We could split the problem into three parts: finding all anagrams
 (up to words reordering and replacing some of the words with their single-word anagrams),
@@ -70,6 +75,84 @@ could be further optimized, resulting in the following algorithm:
                 * W might be one element of a target subset, and the remaining elements could be found
                     by solving the task 2 for N-1, P-W and position of W in the list of vectors.
 
+### Finding all anagrams, pt. 2
+
+In the previous step, we just found all unique tuples of vectors with vectors ordered by norm decreasing
+such that they give the required vector.
+Now we need to convert these back to phrases.
+
+If for every vector there was only one word which produces that vector,
+and if all the vectors in a tuple were different,
+we could just look at all their permutations and get n! solutions from a single tuple.
+
+But a tuple can contain several copies of one vector,
+and there could be several different words corresponding to one vector.
+Computing all possible permutations would result in duplicate solutions
+and too much unneccessary work.
+
+So we could:
+
+1. Substitute all possible word values for every vector, getting several (ordered) word solutions;
+2. Apply all possible permutations to them such that, if vectors k and k+1 were the same in the vector solution,
+    word k should go before word k+1 in the word solution
+    (because the solution where word k goes after word k+1 is already obtained by a different substitution on step 1).
+
+Every string shorter than 32 bytes could be represented as a single u8x32 AVX2 register
+(with the remaining bytes filled with zeroes).
+
+Concatenating strings could be as simple as XORing the vectors, shifted appropriately.
+
+For example, to create `"a b "` string we would need to compute `"a " xor "␀␀b "`,
+which is done in a single cycle on a modern CPU, provided that we have both vectors ready.
+This is as opposed to concatenating strings which would require allocating a new string on the heap
+and copying the data.
+
+So we could just store all of the original words as such a vectors for all possible offsets
+(along with trailing spaces), and when we need to compute a phrase consisting of the word x and the word y,
+just do something along the lines of `get_register(x, 0) xor get_register(y, x.length)`
+
+### Computing hashes
+
+MD5 works on input messages in 64 byte blocks; for short strings (shorter than 55 bytes)
+it only uses a single blocks: 0x80 byte is appended to the message, then it is padded to 56 bytes with zeroes,
+and then the total length of the string in bits is appended as 64-bit number.
+
+So short phrases (shorter than 31 bytes) could be represented with two AVX2 registers:
+one containing the phrase itself with the trailing 0x80, and another containing 24 zeroes
+and 64-bit length of the phrase in bits (which is the number of non-space bytes
+plus the number of words, times 8).
+
+For its internal state, MD5 has four 32-bit variables (u32).
+This means that with AVX2, we can use the same operations on 256-bit registers
+(u32x8) and compute eight hashes at the same time in a single thread.
+
+MD5 breaks input chunks into 16 u32 words (and for short phrases chunks 8-14 are always zero),
+so our algorithm could receive 8x256-bit values and the phrase length,
+rearrange these into 9 256-bit values (8 obtained by transposing the original 8 as 8x8 matrix of u32,
+and ninth being 8 copies of the phrase length in bits),
+and then implement MD5 algorithms using these 9 values as input words 0..7, 15
+(substituting 0 as input words 8..14).
+
+That way, MD5 performance would be increased 8x compared to the ordinary library function
+which does not use SIMD.
+
+As a minor additional optimization, we could only compute the first u32 part of the MD5 hash
+(because we don't need to compute entire hashes for all possible anagrams,
+we only need to find anagrams which match the requested hashes.
+That way, we'll save some unneeded steps in MD5 computation,
+and we also won't have to convert hashes back to separate variables:
+we could just compare u32x8 holding the first parts of hashes for eight different anagrams
+with u32x8 holding eight copies of the first part of the requested hash.
+That way, we'll only have one comparison instead of eight,
+at the cost of rare false positives which occur on average with 1/2^29 probability
+(1/2^32 chance that a random u32 matches the requested u32, for every of the eight anagrams).
+If there is such a semi-match (that is, one of the eight anagrams produces a hash
+with first 32 bits matching first 32 bits of the requested hash), we could just
+compute MD5 for every of the eight anagrams in the ordinary way and
+to compare the whole resulting hashes with the requested ones;
+as this is extremely rare (once every 1/29th calls to SIMD MD5 function),
+it will not severely affect performance.
+
 ## How to run
 
 How to run to solve the original task for three-word anagrams:

src/anagram_logger.rs

@@ -5,11 +5,11 @@ pub fn get_anagram_view(anagram: Vec<usize>, dictionary: &dictionary_builder::Di
         .map(|&index| {
             let word_options = &dictionary.words[index];
             if word_options.len() == 1 {
-                word_options[0].clone()
+                word_options[0].word.clone()
             } else {
-                format!("[{}]", word_options.join(","))
+                format!("[{}]", word_options.iter().map(|word_info| word_info.word.clone()).collect::<Vec<_>>().join(","))
             }
         })
         .collect::<Vec<_>>()
         .join(" ")
-}
+}

src/dictionary_builder.rs

@@ -1,13 +1,44 @@
 use std::collections::HashMap;
+use packed_simd;
 use crate::vector_alphabet;
 
+pub struct WordInfo {
+    simd_words: [packed_simd::u8x32; 32],
+    pub length: usize,
+    pub word: String,
+}
+
+impl WordInfo {
+    fn new(word: String) -> WordInfo {
+        let mut byte_array: [u8; 64] = [0; 64];
+        let bytes = word.as_bytes();
+        let length = bytes.len();
+        byte_array[32 + length] = b' ';
+        for i in 0..length {
+            byte_array[32 + i] = bytes[i];
+        }
+
+        let simd_word_zero: packed_simd::u8x32 = packed_simd::u8x32::from_slice_unaligned(&[0; 32]);
+        let mut simd_words: [packed_simd::u8x32; 32] = [simd_word_zero; 32];
+        for i in 0..31 {
+            simd_words[i] = packed_simd::u8x32::from_slice_unaligned(&byte_array[32-i..64-i]);
+        }
+
+        WordInfo {
+            simd_words,
+            length,
+            word,
+        }
+    }
+}
+
 pub struct Dictionary {
     pub phrase_vector: vector_alphabet::Vector,
     pub vectors: Vec<vector_alphabet::Vector>,
-    pub words: Vec<Vec<String>>,
+    pub words: Vec<Vec<WordInfo>>,
 }
 
-pub fn build_dictionary(phrase: &String, unique_words: &[String]) -> Dictionary {
+pub fn build_dictionary(phrase: &String, unique_words: Vec<String>) -> Dictionary {
     let alphabet = vector_alphabet::Alphabet::new(phrase).unwrap();
 
     let phrase_with_metadata = alphabet.vectorize(phrase).unwrap();
@@ -35,7 +66,7 @@ pub fn build_dictionary(phrase: &String, unique_words: &[String]) -> Dictionary
     let mut words_by_vectors: HashMap<_, _> = HashMap::new();
     for (word, vector_with_metadata) in words_with_vectors {
         let (_, words_for_vector) = words_by_vectors.entry(vector_with_metadata.key).or_insert((vector_with_metadata.vector, vec![]));
-        words_for_vector.push(word.clone());
+        words_for_vector.push(WordInfo::new(word));
     }
 
     let mut words_by_vectors: Vec<_> = words_by_vectors.into_values().collect();

src/hash_computer.rs

@@ -1 +1,183 @@
-pub const MAX_PHRASE_LENGTH: usize = 27;
+use packed_simd::FromBits;
+use packed_simd::u32x8;
+use packed_simd::u8x32;
+
+pub const MAX_PHRASE_LENGTH: usize = 31;
+
+#[allow(unused_assignments)]
+pub fn compute_hashes(messages: [u8x32; 8], messages_length: usize) -> [u32; 8] {
+    let mut a: u32x8 = u32x8::splat(0x67452301);
+    let mut b: u32x8 = u32x8::splat(0xefcdab89);
+    let mut c: u32x8 = u32x8::splat(0x98badcfe);
+    let mut d: u32x8 = u32x8::splat(0x10325476);
+
+    let trailer = u8x32::splat(0).replace(messages_length, b' ' ^ 0x80);
+    let mut messages_bytes: [u32; 64] = [0; 64];
+    {
+        macro_rules! write_bytes {
+            ($i: expr) => {
+                u32x8::from_bits(messages[$i] ^ trailer).write_to_slice_unaligned(&mut messages_bytes[($i*8)..])
+            }
+        }
+        write_bytes!(0);
+        write_bytes!(1);
+        write_bytes!(2);
+        write_bytes!(3);
+        write_bytes!(4);
+        write_bytes!(5);
+        write_bytes!(6);
+        write_bytes!(7);
+    }
+
+    macro_rules! get_m_value {
+        ($i: expr) => {
+            u32x8::new(
+                messages_bytes[0*8 + $i],
+                messages_bytes[1*8 + $i],
+                messages_bytes[2*8 + $i],
+                messages_bytes[3*8 + $i],
+                messages_bytes[4*8 + $i],
+                messages_bytes[5*8 + $i],
+                messages_bytes[6*8 + $i],
+                messages_bytes[7*8 + $i],
+            )
+        };
+    }
+
+    let m0: u32x8 = get_m_value!(0);
+    let m1: u32x8 = get_m_value!(1);
+    let m2: u32x8 = get_m_value!(2);
+    let m3: u32x8 = get_m_value!(3);
+    let m4: u32x8 = get_m_value!(4);
+    let m5: u32x8 = get_m_value!(5);
+    let m6: u32x8 = get_m_value!(6);
+    let m7: u32x8 = get_m_value!(7);
+    let m14: u32x8 = u32x8::splat((messages_length as u32) * 8);
+
+    macro_rules! lrot {
+        ($f: expr, $s: expr) => (($f << $s) | ($f >> (32-$s)));
+    }
+
+    macro_rules! blend {
+        ($mask: expr, $a: expr, $b: expr) => {
+            // andnot (_mm256_andnot_si256) is not implemented in packed_simd
+            ($a & $mask) | ($b & !$mask)
+        }
+    }
+
+    macro_rules! step {
+        ($f: expr, $s: expr, $k: expr, $m: expr) => {
+            let f = $f + a + u32x8::splat($k) + $m;
+            a = d;
+            d = c;
+            c = b;
+            b = b + lrot!(f, $s);
+        };
+        ($f: expr, $s: expr, $k: expr) => {
+            let f = $f + a + u32x8::splat($k);
+            a = d;
+            d = c;
+            c = b;
+            b = b + lrot!(f, $s);
+        };
+    }
+
+    {
+        macro_rules! step_1 {
+            () => (blend!(b, c, d));
+        }
+
+        step!(step_1!(),  7, 0xd76aa478, m0);
+        step!(step_1!(), 12, 0xe8c7b756, m1);
+        step!(step_1!(), 17, 0x242070db, m2);
+        step!(step_1!(), 22, 0xc1bdceee, m3);
+        step!(step_1!(),  7, 0xf57c0faf, m4);
+        step!(step_1!(), 12, 0x4787c62a, m5);
+        step!(step_1!(), 17, 0xa8304613, m6);
+        step!(step_1!(), 22, 0xfd469501, m7);
+        step!(step_1!(),  7, 0x698098d8);
+        step!(step_1!(), 12, 0x8b44f7af);
+        step!(step_1!(), 17, 0xffff5bb1);
+        step!(step_1!(), 22, 0x895cd7be);
+        step!(step_1!(),  7, 0x6b901122);
+        step!(step_1!(), 12, 0xfd987193);
+        step!(step_1!(), 17, 0xa679438e, m14);
+        step!(step_1!(), 22, 0x49b40821);
+    }
+
+    {
+        macro_rules! step_2 {
+            () => (blend!(d, b, c));
+        }
+
+        step!(step_2!(),  5, 0xf61e2562, m1);
+        step!(step_2!(),  9, 0xc040b340, m6);
+        step!(step_2!(), 14, 0x265e5a51);
+        step!(step_2!(), 20, 0xe9b6c7aa, m0);
+        step!(step_2!(),  5, 0xd62f105d, m5);
+        step!(step_2!(),  9, 0x02441453);
+        step!(step_2!(), 14, 0xd8a1e681);
+        step!(step_2!(), 20, 0xe7d3fbc8, m4);
+        step!(step_2!(),  5, 0x21e1cde6);
+        step!(step_2!(),  9, 0xc33707d6, m14);
+        step!(step_2!(), 14, 0xf4d50d87, m3);
+        step!(step_2!(), 20, 0x455a14ed);
+        step!(step_2!(),  5, 0xa9e3e905);
+        step!(step_2!(),  9, 0xfcefa3f8, m2);
+        step!(step_2!(), 14, 0x676f02d9, m7);
+        step!(step_2!(), 20, 0x8d2a4c8a);
+    }
+
+    {
+        macro_rules! step_3 {
+            () => (b ^ (c ^ d));
+        }
+
+        step!(step_3!(),  4, 0xfffa3942, m5);
+        step!(step_3!(), 11, 0x8771f681);
+        step!(step_3!(), 16, 0x6d9d6122);
+        step!(step_3!(), 23, 0xfde5380c, m14);
+        step!(step_3!(),  4, 0xa4beea44, m1);
+        step!(step_3!(), 11, 0x4bdecfa9, m4);
+        step!(step_3!(), 16, 0xf6bb4b60, m7);
+        step!(step_3!(), 23, 0xbebfbc70);
+        step!(step_3!(),  4, 0x289b7ec6);
+        step!(step_3!(), 11, 0xeaa127fa, m0);
+        step!(step_3!(), 16, 0xd4ef3085, m3);
+        step!(step_3!(), 23, 0x04881d05, m6);
+        step!(step_3!(),  4, 0xd9d4d039);
+        step!(step_3!(), 11, 0xe6db99e5);
+        step!(step_3!(), 16, 0x1fa27cf8);
+        step!(step_3!(), 23, 0xc4ac5665, m2);
+    }
+
+    {
+        macro_rules! step_4 {
+            () => (c ^ (b | !d));
+        }
+
+        step!(step_4!(),  6, 0xf4292244, m0);
+        step!(step_4!(), 10, 0x432aff97, m7);
+        step!(step_4!(), 15, 0xab9423a7, m14);
+        step!(step_4!(), 21, 0xfc93a039, m5);
+        step!(step_4!(),  6, 0x655b59c3);
+        step!(step_4!(), 10, 0x8f0ccc92, m3);
+        step!(step_4!(), 15, 0xffeff47d);
+        step!(step_4!(), 21, 0x85845dd1, m1);
+        step!(step_4!(),  6, 0x6fa87e4f);
+        step!(step_4!(), 10, 0xfe2ce6e0);
+        step!(step_4!(), 15, 0xa3014314, m6);
+        step!(step_4!(), 21, 0x4e0811a1);
+        step!(step_4!(),  6, 0xf7537e82, m4);
+
+        // Since we ignore b, c, d values in the end,
+        // the remaining three iterations are unnecessary,
+        // as the value of a after iteration 64 is equal
+        // to the value of b after iteration 61
+        a = b + u32x8::splat(0x67452301);
+
+        let mut result: [u32; 8] = [0; 8];
+        a.write_to_slice_unaligned(&mut result);
+        result
+    }
+}

src/lib.rs

@@ -0,0 +1,9 @@
+#![feature(map_into_keys_values)]
+
+pub mod anagram_finder;
+pub mod anagram_logger;
+pub mod dictionary_builder;
+pub mod hash_computer;
+pub mod permutations_cache;
+pub mod read_lines;
+pub mod vector_alphabet;

src/main.rs

@@ -2,13 +2,13 @@
 
 use std::cmp;
 use std::env;
+use rayon::prelude::*;
 
-mod anagram_finder;
-mod anagram_logger;
-mod dictionary_builder;
-mod hash_computer;
-mod read_lines;
-mod vector_alphabet;
+use trustpilot_challenge_rust::anagram_finder;
+use trustpilot_challenge_rust::anagram_logger;
+use trustpilot_challenge_rust::dictionary_builder;
+use trustpilot_challenge_rust::hash_computer;
+use trustpilot_challenge_rust::read_lines;
 
 fn main() {
     let args: Vec<_> = env::args().collect();
@@ -19,6 +19,12 @@ fn main() {
     let max_requested_number_of_words = (&args[3]).parse::<usize>().unwrap();
     let phrase = &args[4];
 
+    /*let message = hash_computer::prepare_messages(phrase);
+    let hashes = hash_computer::compute_hashes(message, phrase.len());
+    for hash in hashes.iter() {
+        println!("{:#08x}", hash);
+    }*/
+
     let phrase_byte_length_without_spaces = phrase.as_bytes().into_iter().filter(|&b| *b != b' ').count();
     let max_supported_number_of_words = (hash_computer::MAX_PHRASE_LENGTH - phrase_byte_length_without_spaces) + 1;
 
@@ -31,12 +37,11 @@ fn main() {
     words.sort();
     words.dedup();
 
-    let dictionary = dictionary_builder::build_dictionary(phrase, &words);
+    let dictionary = dictionary_builder::build_dictionary(phrase, words);
 
     for number_of_words in 0..=max_number_of_words {
         let result = anagram_finder::find_anagrams(&dictionary, number_of_words);
-        for anagram in result {
-            println!("{}", anagram_logger::get_anagram_view(anagram, &dictionary));
-        }
+        result.into_par_iter()
+            .for_each(|anagram| println!("{}", anagram_logger::get_anagram_view(anagram, &dictionary)));
     }
 }

src/permutations_cache.rs

@@ -0,0 +1 @@
+use permutohedron::Heap;

src/vector_alphabet.rs

@@ -23,7 +23,7 @@ impl Vector {
 
     pub fn is_subset_of(&self, other: &Vector) -> bool {
         let comparison_result = packed_simd::u8x32::gt(self.simd_vector, other.simd_vector);
-        packed_simd::m8x32::none(comparison_result as packed_simd::m8x32)
+        packed_simd::m8x32::none(comparison_result)
     }
 
     pub fn safe_substract(&self, vector_to_substract: &Vector) -> Option<Vector> {

tests/hash_computer_test.rs

@@ -0,0 +1,57 @@
+use packed_simd::u8x32;
+
+extern crate trustpilot_challenge_rust;
+use trustpilot_challenge_rust::hash_computer;
+
+fn prepare_message(message_string: &str) -> u8x32 {
+    let mut bytes_static: [u8; 32] = [0; 32];
+    let bytes = message_string.as_bytes();
+    for i in 0..bytes.len() {
+        bytes_static[i] = bytes[i];
+    }
+
+    bytes_static[bytes.len()] = b' ';
+
+    u8x32::from(bytes_static)
+}
+
+fn prepare_messages(message_strings: [&str; 8]) -> [u8x32; 8] {
+    let mut result: [u8x32; 8] = [u8x32::splat(0); 8];
+    for i in 0..8 {
+        result[i] = prepare_message(message_strings[i]);
+    }
+    result
+}
+
+#[test]
+fn it_computes_hashes() {
+    let messages: [&str; 8] = /*[""; 8];*/[
+        "DAPUpOGHw620yalJA0vjFPK7ThgHyAN",
+        "4xRslaTeBCNyRu2EiIDueEx3BTbIP5H",
+        "kFPd2zk60eEFpNwgEOZAcyDcxRVv0Y8",
+        "bm6VQr6w9plie0G8XoOb4wChJXB0vCm",
+        "gbFrtHcqOTkeG1QxT8YEMSio1ahAYNq",
+        "T0GmOLB2WH04oIrhB3JCyPHFxI8UOow",
+        "TWUCy0B0JG5KjQvsu4YUFC5IR5ByS2W",
+        "VXqOIzYdLIqx6tw8LJbR7SqR5iYgTlQ"
+    ];
+
+    let expected: [u128; 8] = [
+        0xC6F9E9B203CEA81A7BA28BE276B96A6F,
+        0x45BF15D8B08E1AEADE1305B8E43B8F2C,
+        0x7AA53B4627C8DD3714F2874EDE04DA7D,
+        0x93B650B474B6FDE6B902A76B1DDA10BB,
+        0xBBF89511DAC63A516ADDB9BEC79241A5,
+        0x7E53E601351A47500BF4A2B7EAD077C3,
+        0xFC6FEB2CC3191198E87ECB9D7626580A,
+        0xCED45BA82BD1BDCF546255CB6A530FE3,
+    ];
+
+    let messages_simd = prepare_messages(messages);
+
+    let hashes = hash_computer::compute_hashes(messages_simd, messages[0].len());
+
+    for i in 0..8 {
+        assert_eq!((expected[i] >> 96) as u32, hashes[i].to_be());
+    }
+}