day1: rust part1 reimpl

day1/day1/Cargo.toml

@@ -7,6 +7,11 @@ edition = "2018"
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
 
 [features]
+default = ["part1"]
+
+part1 = []
+part2 = []
+
 big = ["u128"]
 u128 = []
 

day1/day1/src/main.rs

@@ -11,14 +11,47 @@ use input::INPUT;
 #[cfg(not(feature="big"))] const TARGET: u64 = 2020;
 #[cfg(feature="big")] const TARGET: u64 = 99920044;
 
-// O(n^2) ?
-fn main() {
-    // doesn't work on big input?
+#[cfg(feature="part1")]
+// O(2n) ?
+fn part1()
+{
+    // u64 doesn't work on big input?
+    #[cfg(feature="u128")] 
+    let input: Box<[_]> = INPUT[..].iter().map(|&x| x as u128).collect();
+    #[cfg(feature="u128")] 
+    let target = TARGET as u128;
+
+    #[cfg(not(feature="u128"))] 
+    let input = &INPUT[..];
+    #[cfg(not(feature="u128"))] 
+    let target = TARGET;
+    
+    let deficits: HashMap<_, usize> = (0..)
+	.zip(input.iter().copied())
+	.filter_map(|(i, x)| target.checked_sub(x).map(|x| (x, i)))
+	.collect();
+
+    for ix in input.iter().copied()
+    {
+	if let Some(&idx) = deficits.get(&ix)
+	{
+	    if let Some(x) = input[idx].checked_mul(ix) {
+		println!("{}", x);
+		return;
+	    }
+	}
+    }
+    panic!("no solution");
+}
+// O(n + n^2) ?
+#[cfg(feature="part2")] 
+fn part2()
+{
+    // u64 doesn't work on big input?
     #[cfg(feature="u128")] 
     let input: Box<[_]> = INPUT[..].iter().map(|&x| x as u128).collect();
     #[cfg(feature="u128")] 
     let target = TARGET as u128;
-    // ^ doesn't help..
 
     #[cfg(not(feature="u128"))] 
     let input = &INPUT[..];
@@ -48,3 +81,8 @@ fn main() {
     }
     panic!("no solution");
 }
+
+fn main() {
+    #[cfg(feature="part1")] part1();
+    #[cfg(feature="part2")] part2();
+}