day1 reimpl

1.20x speedup

.gitignore

@@ -1,3 +1,6 @@
 input*
 part1*
 part2*
+
+*.lock
+target/

day1/day1/Cargo.toml

@@ -0,0 +1,12 @@
+[package]
+name = "day1"
+version = "0.1.0"
+authors = ["Avril <flanchan@cumallover.me>"]
+edition = "2018"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[features]
+big = []
+
+[dependencies]

day1/day1/build.rs

@@ -0,0 +1,37 @@
+use std::{
+    io::{
+	self, BufRead, BufReader, Write,
+    },
+    fs,
+    error,
+};
+
+fn gen_input<W: Write+?Sized, R: BufRead>(output: &mut W, lines: R, const_name: &str) -> io::Result<()> {
+
+    eprint!("Generating {}... ", const_name);
+    writeln!(output, "pub const {}: &[u64] = &[", const_name)?;
+    for line in lines.lines()
+    {
+	writeln!(output, "	{},", line?)?;
+    }
+    output.write_all(b"];\n")?;
+    eprintln!("OK");
+    Ok(())
+}
+
+fn main() -> Result<(), Box<dyn error::Error>>{
+    let mut output = fs::OpenOptions::new()
+	.write(true)
+	.truncate(true)
+	.create(true)
+	.open("./src/input.rs").expect("Couldn't open output");
+
+    gen_input(&mut output, BufReader::new(fs::OpenOptions::new()
+					  .read(true)
+					  .open("../input").expect("Couldn't open input")), "INPUT")?;
+    gen_input(&mut output, BufReader::new(fs::OpenOptions::new()
+					  .read(true)
+					  .open("../input-big").expect("Couldn't open big input")), "INPUT_BIG")?;
+    
+    Ok(())
+}

day1/day1/src/main.rs

@@ -0,0 +1,37 @@
+use std::collections::HashMap;
+
+#[allow(dead_code)]
+mod input;
+
+#[cfg(feature="big")]
+use input::INPUT_BIG as INPUT;
+#[cfg(not(feature="big"))] 
+use input::INPUT;
+
+#[cfg(not(feature="big"))] const TARGET: u64 = 2020;
+#[cfg(feature="big")] const TARGET: u64 = 99920044;
+
+// O(n^2)
+fn main() {
+    let deficits: HashMap<u64, usize> = (0..)
+	.zip(INPUT.iter().copied())
+	.filter_map(|(i, x)| TARGET.checked_sub(x).map(|x| (x, i)))
+	.collect();
+
+    //eprintln!("calculated deficits: {}", deficits.len());
+    for (i, ix) in (0..).zip(INPUT.iter().copied())
+    {
+	for jx in INPUT[i..].iter().copied().skip(1)
+	{
+	    let sum = ix+jx;
+	    if let Some(&idx) = deficits.get(&sum)
+	    {
+		//eprintln!("solution found?");
+		println!("{}", if let Some(x) = INPUT[idx].checked_mul(ix).map(|x| x.checked_mul(jx)).flatten()
+			 { x } else { continue; });
+		return;
+	    }
+	}
+    }
+    //panic!("no solution");
+}