diff --git a/euler51/src/combination.rs b/euler51/src/combination.rs
index 92c7935..c246174 100644
--- a/euler51/src/combination.rs
+++ b/euler51/src/combination.rs
@@ -1,5 +1,6 @@
-struct PositionCombinations {
+pub struct PositionCombinations {
     min_length: usize,
+    max_length: usize,
     state: Vec<bool>,
     finished: bool,
 }
@@ -8,13 +9,14 @@ impl Iterator for PositionCombinations {
     type Item = Vec<bool>;
 
     fn next(&mut self) -> Option<Self::Item> {
-        if self.finished {
-            return None;
-        }
         loop {
             let result = self.state.clone();
             self.increment();
-            if result.iter().filter(|x| **x).count() >= self.min_length {
+            if self.finished {
+                return None;
+            }
+            if (self.min_length..=self.max_length).contains(&result.iter().filter(|x| **x).count())
+            {
                 return Some(result);
             }
         }
@@ -22,11 +24,12 @@ impl Iterator for PositionCombinations {
 }
 
 impl PositionCombinations {
-    fn new(min_length: usize, max_length: usize) -> Self {
-        let state = vec![false; max_length];
+    pub fn new(min_length: usize, max_length: usize, length: usize) -> Self {
+        let state = vec![false; length];
         let finished = false;
         Self {
             min_length,
+            max_length,
             state,
             finished,
         }
@@ -46,15 +49,25 @@ impl PositionCombinations {
     }
 }
 
+pub trait ToString {
+    fn to_string(&self) -> String;
+}
+
+impl ToString for Vec<bool> {
+    fn to_string(&self) -> String {
+        self.iter().map(|b| if *b { 'x' } else { '.' }).collect()
+    }
+}
+
 #[cfg(test)]
 mod test {
-    #[test]
-    fn test_combination() {
-        use crate::combination::PositionCombinations;
-        use std::collections::HashSet;
+    use crate::combination::PositionCombinations;
+    use std::collections::HashSet;
 
-        let combinator = PositionCombinations::new(2, 4);
-        let result: HashSet<_> = combinator.into_iter().collect();
+    #[test]
+    fn test_combination_2_3_4() {
+        let combinator = PositionCombinations::new(2, 3, 4);
+        let result: Vec<_> = combinator.into_iter().collect();
         let mut number_solutions = result.len();
         let mut solution = HashSet::new();
         solution.insert(vec![false, false, true, true]);
@@ -67,7 +80,22 @@ mod test {
         solution.insert(vec![true, false, true, true]);
         solution.insert(vec![true, true, false, true]);
         solution.insert(vec![true, true, true, false]);
-        solution.insert(vec![true, true, true, true]);
+        for item in solution {
+            assert!(result.contains(&item));
+            number_solutions -= 1;
+        }
+        assert_eq!(number_solutions, 0);
+    }
+
+    #[test]
+    fn test_combination_2_2_3() {
+        let combinator = PositionCombinations::new(2, 2, 3);
+        let result: Vec<_> = combinator.into_iter().collect();
+        let mut number_solutions = result.len();
+        let mut solution = HashSet::new();
+        solution.insert(vec![false, true, true]);
+        solution.insert(vec![true, false, true]);
+        solution.insert(vec![true, true, false]);
         for item in solution {
             assert!(result.contains(&item));
             number_solutions -= 1;
diff --git a/euler51/src/macros.rs b/euler51/src/macros.rs
new file mode 100644
index 0000000..aa57676
--- /dev/null
+++ b/euler51/src/macros.rs
@@ -0,0 +1,3 @@
+macro_rules! dbg_println {
+    ($($arg:tt)*) => (#[cfg(debug_assertions)] println!($($arg)*));
+}
diff --git a/euler51/src/main.rs b/euler51/src/main.rs
index 09f0359..c8c7530 100644
--- a/euler51/src/main.rs
+++ b/euler51/src/main.rs
@@ -1,79 +1,133 @@
 mod combination;
 mod primes;
 
+#[macro_use]
+mod macros;
+
+use crate::combination::{PositionCombinations, ToString};
 use crate::primes::Primes;
 
-type Int = i64;
+type Int = u64;
 
-const MIN_VALUE: Int = 10_000;
-const MAX_VALUE: Int = 99_999;
+// EXAMPLE 1
+// const MIN_VALUE: Int = 10;
+// const MAX_VALUE: Int = 99;
+// const LENGTH: usize = 2;
+// const MIN_LENGTH: usize = 1;
 
-const DIGITS: [Int; 10] = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9];
-// const MIN_VALUE: Int = 10_000_000;
-// const MAX_VALUE: Int = 10_010_000;
-// const MAX_VALUE: Int = 99_999_999;
-
-// const LENGTH_OF_NUMBERS: usize = 5;
+// OWN TEST
+// const MIN_VALUE: Int = 100;
+// const MAX_VALUE: Int = 999;
+// const LENGTH: usize = 3;
 // const MIN_LENGTH: usize = 2;
 
+// EXAMPLE 2
+// const MIN_VALUE: Int = 10_000;
+// const MAX_VALUE: Int = 99_999;
+// const LENGTH: usize = 5;
+// const MIN_LENGTH: usize = 2;
+
+// FINAL
+const MIN_VALUE: Int = 100_000;
+const MAX_VALUE: Int = 999_999;
+const LENGTH: usize = 6;
+const MIN_LENGTH: usize = 2;
+
+const MAX_LENGTH: usize = LENGTH - 1;
+const DIGITS: [u8; 10] = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9];
+
 fn main() {
-    let primes = Primes::get_between(MIN_VALUE, MAX_VALUE);
-    // let mut new_primes = primes.clone();
-    for p in primes.vector.iter() {
-        let mut candidates = vec![];
-        let digits = num_to_digits(p);
-        for d in DIGITS {
-            let mut these_digits = digits.clone();
-            these_digits[3] = d;
-            these_digits[4] = d;
-            let num = digits_to_num(these_digits);
-            if primes.set.contains(&num) {
-                candidates.push(num)
+    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) {
+        dbg_println!("{}", pattern.to_string());
+        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 = digits
+                    .iter()
+                    .zip(pattern.clone())
+                    .map(|(d, b)| if b { digit } else { *d })
+                    .collect::<Vec<u8>>()
+                    .to_num();
+                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 candidates.len() > 4 {
-            println!("candidates");
-            for c in &candidates {
-                // new_primes.remove(c);
-                println!("{c}");
-            }
+        if !best_this_combination.is_empty() {
+            best_of_all_combinations.push(get_longest(best_this_combination, false));
         }
     }
-    // println!("new primes");
-    // for p in new_primes.vector.iter() {
-    //     println!("{p}");
-    // }
-}
-fn num_to_digits(n: &Int) -> Vec<Int> {
-    n.to_string()
-        .chars()
-        .map(|x| x.to_digit(10).unwrap() as Int)
-        .collect()
+    println!("{:#?}", get_longest(best_of_all_combinations, false));
 }
 
-fn digits_to_num(vec: Vec<Int>) -> Int {
-    let mut result = 0;
-    let mut position = 1;
-    for digit in vec.iter().rev() {
-        result += digit * position;
-        position *= 10;
+fn get_longest(mut vec: Vec<Vec<Int>>, print: bool) -> Vec<Int> {
+    if vec.is_empty() {
+        panic!("cannot give the longest element of an empty vector")
+    }
+    if print {
+        println!("{:#?}", vec);
+    }
+    vec.sort_by_key(|c| c.len());
+    let length = vec.last().unwrap().len();
+    vec.into_iter().find(|v| v.len() == length).unwrap()
+}
+
+impl ToString for Vec<u8> {
+    fn to_string(&self) -> String {
+        self.iter().map(|d| d.to_string()).collect()
+    }
+}
+
+trait ToDigits {
+    fn to_digits(&self) -> Vec<u8>;
+}
+
+trait ToNum {
+    fn to_num(&self) -> Int;
+}
+
+impl ToDigits for Int {
+    fn to_digits(&self) -> Vec<u8> {
+        self.to_string()
+            .chars()
+            .map(|x| x.to_digit(10).unwrap() as u8)
+            .collect()
+    }
+}
+
+impl ToNum for Vec<u8> {
+    fn to_num(&self) -> Int {
+        let mut result = 0;
+        let mut position = 1;
+        for digit in self.iter().rev() {
+            result += *digit as Int * position;
+            position *= 10;
+        }
+        result
     }
-    result
 }
 
 #[cfg(test)]
 mod test {
-    use crate::{digits_to_num, num_to_digits};
+    use crate::{ToDigits, ToNum};
 
     #[test]
     fn test_digits_to_num() {
         let digits = vec![5, 7, 3, 8, 1];
-        assert_eq!(digits_to_num(digits), 57381);
+        assert_eq!(digits.to_num(), 57381);
     }
 
     #[test]
     fn test_num_to_digits() {
         let num = 83371;
-        assert_eq!(num_to_digits(&num), vec![8, 3, 3, 7, 1]);
+        assert_eq!(num.to_digits(), vec![8, 3, 3, 7, 1]);
     }
 }
diff --git a/euler51/src/primes.rs b/euler51/src/primes.rs
index b26d409..9b8af4a 100644
--- a/euler51/src/primes.rs
+++ b/euler51/src/primes.rs
@@ -7,9 +7,9 @@ use std::collections::HashSet;
 use std::hash::Hash;
 
 #[derive(Clone)]
-pub struct Primes<T: Num + ToPrimitive> {
-    pub vector: Vec<T>,
-    pub set: HashSet<T>,
+pub struct Primes<T> {
+    vector: Vec<T>,
+    set: HashSet<T>,
 }
 
 impl<T> Primes<T>
@@ -38,6 +38,7 @@ where
                 set.insert(prime);
             }
         }
+        vector.reverse(); // so that self.pop removes the smallest prime
         Primes { vector, set }
     }
 
@@ -70,6 +71,23 @@ where
             self.set.remove(num);
         };
     }
+
+    // pub fn len(&self) -> usize {
+    //     debug_assert_eq!(self.vector.len(), self.set.len());
+    //     self.vector.len()
+    // }
+
+    pub fn pop(&mut self) -> Option<T> {
+        let result = self.vector.pop();
+        if let Some(value) = result {
+            self.set.remove(&value);
+        }
+        result
+    }
+
+    pub fn contains(&self, val: &T) -> bool {
+        self.set.contains(val)
+    }
 }
 
 #[cfg(test)]
@@ -80,6 +98,23 @@ mod test {
     fn test_example_prime() {
         let primes = Primes::get_between(2, 99_999);
         assert!(primes.set.contains(&56003));
+        assert!(primes.set.contains(&56993));
         assert!(!primes.set.contains(&56002));
     }
+
+    #[test]
+    fn test_remove() {
+        let mut primes = Primes::get_between(2, 9);
+        assert!(primes.contains(&7));
+        primes.remove(&7);
+        assert!(!primes.contains(&7));
+    }
+
+    #[test]
+    fn test_pop() {
+        let mut primes = Primes::get_between(10, 20);
+        assert!(primes.contains(&11));
+        primes.pop();
+        assert!(!primes.contains(&11));
+    }
 }