Refactor grouping count into a trait.

2018/src/common.rs

@@ -1,3 +1,5 @@
+use std::collections::HashMap;
+use std::hash::Hash;
 use std::io;
 
 /// Apply Erathostenes's sieve to the supplied array
@@ -7,7 +9,7 @@ use std::io;
 /// * `dest` - the destination slice to fill with the sieve. This is
 ///   assumed to be filled with "true" before being handed to this
 ///   method.
-pub fn prime_sieve(dest: &mut[bool]) {
+pub fn prime_sieve(dest: &mut [bool]) {
     if dest.len() >= 1 {
         dest[0] = false;
     }
@@ -20,7 +22,7 @@ pub fn prime_sieve(dest: &mut[bool]) {
 
     for i in 1..(limit + 1) {
         if !dest[i] {
-            continue
+            continue;
         }
 
         for j in ((i * i)..(dest.len())).step_by(i) {
@@ -48,6 +50,35 @@ pub fn trim_back(input: &mut Vec<u8>) {
     }
 }
 
+
+/// An interface to count elements in particular categories.
+pub trait GroupingCount {
+    /// The type of the categories under inspection
+    type Type;
+
+    /// Count the occurrence of all possible values.
+    ///
+    /// This method will return a map from a value to its occurrence rate.
+    fn grouping_count(&mut self) -> HashMap<Self::Type, usize>;
+}
+
+impl<T> GroupingCount for T
+    where T: Iterator,
+          T::Item: Eq + Hash {
+    type Type = T::Item;
+
+    fn grouping_count(&mut self) -> HashMap<Self::Type, usize>
+    {
+        let mut counts = HashMap::new();
+
+        for element in self {
+            *counts.entry(element).or_insert(0) += 1;
+        }
+
+        counts
+    }
+}
+
 /// Solution trait
 ///
 /// Every day's solution should implement this function so that it can
@@ -79,4 +110,12 @@ mod tests {
 
         assert_eq!(output, input);
     }
+
+    #[test]
+    fn test_grouping_count() {
+        let result = [1, 1, 2, 2, 3, 1].iter().grouping_count();
+        assert_eq!(3, result[&1]);
+        assert_eq!(2, result[&2]);
+        assert_eq!(1, result[&3]);
+    }
 }

2018/src/day02.rs

@@ -3,16 +3,11 @@ use std::io;
 use std::io::prelude::*;
 
 use common;
+use common::GroupingCount;
 
 /// Count the occurrence characters in a string.
-fn count_chars(word: &str) -> HashMap<char, u32> {
-    let mut counts: HashMap<char, u32> = HashMap::new();
-
-    for c in word.chars() {
-        *counts.entry(c).or_insert(0) += 1;
-    }
-
-    counts
+fn count_chars(word: &str) -> HashMap<char, usize> {
+    word.chars().grouping_count()
 }
 
 /// Compute the number of different positions between two strings.

2018/src/day03.rs

@@ -3,9 +3,11 @@ use std::io;
 use std::io::BufRead;
 use std::ops::Range;
 
-use common;
 use regex;
 
+use common;
+use common::GroupingCount;
+
 #[derive(Copy, Clone, Debug)]
 struct Claim {
     x: usize,
@@ -60,15 +62,10 @@ impl Day03 {
         }
     }
 
-    fn get_claims(&self) -> HashMap<(usize, usize), i32> {
-        let mut claims = HashMap::new();
-        for claim in &self.claims {
-            for coordinate in claim.range() {
-                *claims.entry(coordinate).or_insert(0) += 1;
-            }
-        }
-
-        claims
+    fn get_claims(&self) -> HashMap<(usize, usize), usize> {
+        self.claims.iter()
+            .flat_map(|x| x.range())
+            .grouping_count()
     }
 }
 

2018/src/day06.rs

@@ -1,10 +1,10 @@
-use std::collections::HashMap;
 use std::collections::HashSet;
 use std::io::BufRead;
 use std::io::BufReader;
 use std::io::Read;
 
 use common::Solution;
+use common::GroupingCount;
 
 #[derive(Copy, Clone, Debug)]
 struct Coordinate {
@@ -124,13 +124,9 @@ impl Solution for Day06 {
             infinite.extend([grid[y][0], grid[y][self.xmax]].iter().filter_map(claim_filter));
         }
 
-        let mut counts = HashMap::new();
-
-        for instance in grid.iter().flat_map(|x| x.iter())
+        let counts = grid.iter().flat_map(|x| x.iter())
             .filter_map(claim_filter)
-            .filter(|x| !infinite.contains(x)) {
-            *counts.entry(instance).or_insert(0) += 1;
-        }
+            .filter(|x| !infinite.contains(x)).grouping_count();
 
         format!("{}", counts.values().max().unwrap())
     }