clean up 32

src/euler32/src/main.rs

@@ -5,70 +5,83 @@ type Int = u32;
 const UPPER_BOUND: Int = 3_333; // because result must be a 4-digit number
 
 fn main() {
+    // prevent reallocation of these sets on every call in `are_pandigital`
     let mut buffer1 = HashSet::new();
     let mut buffer2 = HashSet::new();
 
-    let f = (1..UPPER_BOUND)
-        .filter(|n| {
-            let s = n.to_string();
-            let hs = s.chars().collect::<HashSet<_>>();
-            hs.len() == s.len() && !hs.contains(&'0')
-        })
-        .collect::<Vec<_>>()
-        .leak();
+    let factors = get_suitable_factors(UPPER_BOUND);
 
-    let mut result = HashSet::new();
-    for f1 in f.iter() {
+    let mut products_of_pandigital_triples = HashSet::new();
+    for f1 in factors.iter() {
         let lower_bound = UPPER_BOUND / f1;
 
-        'inner: for f2 in f.iter().skip_while(|i| **i < lower_bound) {
-            let p = f1 * f2;
-            if f1.len() + f2.len() + p.len() > 9 {
+        'inner: for f2 in factors.iter().skip_while(|i| **i < lower_bound) {
+            let product = f1 * f2;
+            if f1.oom() + f2.oom() + product.oom() > 9 {
                 break 'inner;
             }
 
-            if are_pandigital([f1, f2, &p], &mut buffer1, &mut buffer2) {
-                println!("{f1} {f2} {p}");
-                result.insert(p);
+            if are_pandigital([f1, f2, &product], &mut buffer1, &mut buffer2) {
+                println!("{f1} {f2} {product}");
+                products_of_pandigital_triples.insert(product);
             }
         }
     }
-    println!("{}", result.iter().sum::<Int>());
+    println!("{}", products_of_pandigital_triples.iter().sum::<Int>());
 }
 
-fn are_pandigital(input: [&Int; 3], accum: &mut HashSet<char>, hs: &mut HashSet<char>) -> bool {
-    accum.clear();
+fn get_suitable_factors(upper_bound: Int) -> &'static [Int] {
+    (1..upper_bound)
+        .filter(|number| {
+            let str = number.to_string();
+            let set = str.chars().collect::<HashSet<_>>();
+            // a suitable number must not contain `0`, and must have an unique digit at every
+            // position
+            set.len() == str.len() && !set.contains(&'0')
+        })
+        .collect::<Vec<_>>()
+        // leaking the vec is slightly faster than handling the vec
+        .leak()
+}
 
-    let mut length = 0;
+fn are_pandigital(
+    input: [&Int; 3],
+    digits_in_numbers: &mut HashSet<char>,
+    set_of_char_buffer: &mut HashSet<char>,
+) -> bool {
+    digits_in_numbers.clear();
+
+    let mut length_of_all_numbers = 0;
 
     for n in input {
-        hs.clear();
+        set_of_char_buffer.clear();
 
         let s = n.to_string();
-        length += s.len();
+        length_of_all_numbers += s.len();
 
         s.chars().for_each(|c| {
-            hs.insert(c);
+            set_of_char_buffer.insert(c);
         });
 
-        if s.len() != hs.len() {
+        if s.len() != set_of_char_buffer.len() {
             return false;
         }
 
-        for item in hs.iter() {
-            accum.insert(*item);
+        for item in set_of_char_buffer.iter() {
+            digits_in_numbers.insert(*item);
         }
     }
 
-    accum.len() == 9 && length == 9 && !accum.contains(&'0')
+    digits_in_numbers.len() == 9 && length_of_all_numbers == 9 && !digits_in_numbers.contains(&'0')
 }
 
 trait Length {
-    fn len(&self) -> usize;
+    fn oom(&self) -> usize;
 }
 
 impl Length for Int {
-    fn len(&self) -> usize {
+    // order of magnitude
+    fn oom(&self) -> usize {
         let mut tmp = 1;
         let mut oom = 1;
         while tmp - 1 < *self {