day16: Start with The Pattern™.

day16/Cargo.toml

@@ -0,0 +1,9 @@
+[package]
+name = "day16"
+version = "0.1.0"
+authors = ["Jed Davis <jld@xlerb.net>"]
+edition = "2018"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]

day16/src/main.rs

@@ -0,0 +1,69 @@
+use std::iter::Iterator;
+
+type Num = i32;
+
+#[derive(Debug, Clone)]
+struct ThePattern {
+    scale: usize,
+    low: usize,
+    high: usize,
+}
+
+impl ThePattern {
+    fn new(i: usize) -> Self {
+        Self {
+            scale: i + 1,
+            low: 1,
+            high: 0,
+        }
+    }
+}
+
+impl Iterator for ThePattern {
+    type Item = Num;
+
+    fn next(&mut self) -> Option<Num> {
+        debug_assert!(self.low <= self.scale);
+        if self.low == self.scale {
+            self.low = 0;
+            self.high += 1;
+        }
+        self.low += 1;
+        self.high &= 3;
+        Some(match self.high {
+            0 => 0,
+            1 => 1,
+            2 => 0,
+            3 => -1,
+            _ => panic!("unreachable")
+        })
+    }
+}
+
+
+fn main() {
+    println!("Hello, world!");
+}
+
+
+#[cfg(test)]
+mod test {
+    use super::*;
+
+    #[test]
+    fn second_pattern() {
+        let expected = vec![0, 1, 1, 0, 0, -1, -1, 0, 0, 1, 1, 0, 0, -1, -1];
+        let actual: Vec<_> = ThePattern::new(1).take(expected.len()).collect();
+        assert_eq!(actual, expected);
+    }
+
+    #[test]
+    fn other_patterns() {
+        let p1: Vec<_> = ThePattern::new(0).take(8).collect();
+        assert_eq!(p1, vec![1, 0, -1, 0, 1, 0, -1, 0]);
+
+        let p3: Vec<_> = ThePattern::new(2).take(8).collect();
+        assert_eq!(p3, vec![0, 0, 1, 1, 1, 0, 0, 0]);
+    }
+
+}