From 2b86db2532a701e4f2ba39d42f66c0db9c1e31a8 Mon Sep 17 00:00:00 2001
From: "Dr. Matthias Ratajczak" <m.ratajczak@hzdr.de>
Date: Tue, 16 Aug 2022 17:06:48 +0200
Subject: [PATCH] performance optimization in 51

---
 src/euler51/src/main.rs | 63 ++++++++++++++++++++++++-----------------
 1 file changed, 37 insertions(+), 26 deletions(-)

diff --git a/src/euler51/src/main.rs b/src/euler51/src/main.rs
index 62fecd8..0beb462 100644
--- a/src/euler51/src/main.rs
+++ b/src/euler51/src/main.rs
@@ -1,6 +1,8 @@
 extern crate combination;
 extern crate primes;
 
+use std::process::exit;
+
 use combination::PositionCombinations;
 use primes::Primes;
 
@@ -23,37 +25,41 @@ const MIN_VALUE: Int = 100_000;
 const MAX_VALUE: Int = 999_999;
 const LENGTH: usize = 6;
 const MIN_LENGTH: usize = 2;
+const TARGET_LENGTH: usize = 8;
 
 const MAX_LENGTH: usize = LENGTH - 1;
 const DIGITS: [u8; 10] = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9];
 
 fn main() {
     let all_primes = Primes::get_between(MIN_VALUE, MAX_VALUE);
-    let mut best_of_all_combinations = vec![];
+
     for pattern in PositionCombinations::new(MIN_LENGTH, MAX_LENGTH, LENGTH) {
-        let mut best_this_combination = vec![];
         let mut primes_to_check = all_primes.clone();
+
         while let Some(first) = primes_to_check.pop() {
             let mut primes_matching_pattern = vec![];
+
             let digits = first.to_digits();
             for digit in DIGITS {
-                let could_be_prime = apply_transformation(&digits, digit, &pattern);
+                let could_be_prime =
+                    replace_digits_according_to_pattern_with_digit(&digits, digit, &pattern);
                 if primes_to_check.contains(&could_be_prime) || could_be_prime == first {
                     primes_matching_pattern.push(could_be_prime);
                 }
             }
-            if primes_matching_pattern.len() > 2 {
-                best_this_combination.push(primes_matching_pattern);
+            if primes_matching_pattern.len() == TARGET_LENGTH {
+                println!("{primes_matching_pattern:#?}");
+                exit(0);
             }
         }
-        if !best_this_combination.is_empty() {
-            best_of_all_combinations.push(get_longest(best_this_combination));
-        }
     }
-    println!("{:#?}", get_longest(best_of_all_combinations));
 }
 
-fn apply_transformation(prime_as_digits: &[u8], new_digit: u8, pattern: &[bool]) -> Int {
+fn replace_digits_according_to_pattern_with_digit(
+    prime_as_digits: &[u8],
+    new_digit: u8,
+    pattern: &[bool],
+) -> Int {
     debug_assert_eq!(prime_as_digits.len(), pattern.len());
     let mut multiplier = 1;
     let result = prime_as_digits
@@ -73,32 +79,31 @@ fn apply_transformation(prime_as_digits: &[u8], new_digit: u8, pattern: &[bool])
     result
 }
 
-fn get_longest(mut vec: Vec<Vec<Int>>) -> Vec<Int> {
-    assert!(
-        !vec.is_empty(),
-        "cannot get the longest element of an empty vector"
-    );
-    vec.sort_by_key(|c| c.len());
-    let length = vec.last().unwrap().len();
-    vec.into_iter().find(|v| v.len() == length).unwrap()
-}
-
 trait ToDigits {
     fn to_digits(&self) -> Vec<u8>;
 }
 
+#[allow(clippy::needless_range_loop)]
 impl ToDigits for Int {
     fn to_digits(&self) -> Vec<u8> {
-        self.to_string()
-            .chars()
-            .map(|x| x.to_digit(10).unwrap() as u8)
-            .collect()
+        let oom = (*self as f64).log10().floor() as i32;
+        let length = oom as usize + 1;
+        let mut var = *self;
+        let mut result = vec![0; length];
+        for i in 0..length as usize {
+            let power = 10_f64.powi(oom - i as i32);
+            let digit = (var as f64 / power).floor();
+            var -= (digit * power) as Int;
+            result[i] = digit as u8;
+        }
+        assert_eq!(var, 0);
+        result
     }
 }
 
 #[cfg(test)]
 mod test {
-    use crate::{apply_transformation, ToDigits};
+    use crate::{replace_digits_according_to_pattern_with_digit, ToDigits};
 
     #[test]
     fn test_num_to_digits() {
@@ -112,8 +117,14 @@ mod test {
         let new_digit = 0;
         let pattern = [false, true, true, true, false, false];
         assert_eq!(
-            apply_transformation(&prime_as_digits, new_digit, &pattern),
+            replace_digits_according_to_pattern_with_digit(&prime_as_digits, new_digit, &pattern),
             500_021
         );
     }
+
+    #[test]
+    fn test_to_digits() {
+        let num = 37871_u64;
+        assert_eq!(vec![3, 7, 8, 7, 1], num.to_digits());
+    }
 }