intial commit uwu

.gitignore

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+/target

.idea/.gitignore

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+# Default ignored files
+/shelf/
+/workspace.xml
+# Editor-based HTTP Client requests
+/httpRequests/
+# Datasource local storage ignored files
+/dataSources/
+/dataSources.local.xml

.idea/aoc2021.iml

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+<?xml version="1.0" encoding="UTF-8"?>
+<module type="CPP_MODULE" version="4">
+  <component name="NewModuleRootManager">
+    <content url="file://$MODULE_DIR$/../aoc2021">
+      <sourceFolder url="file://$MODULE_DIR$/../aoc2021/src" isTestSource="false" />
+      <excludeFolder url="file://$MODULE_DIR$/../aoc2021/target" />
+    </content>
+    <orderEntry type="inheritedJdk" />
+    <orderEntry type="sourceFolder" forTests="false" />
+  </component>
+</module>

.idea/modules.xml

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+<?xml version="1.0" encoding="UTF-8"?>
+<project version="4">
+  <component name="ProjectModuleManager">
+    <modules>
+      <module fileurl="file://$PROJECT_DIR$/.idea/aoc2021.iml" filepath="$PROJECT_DIR$/.idea/aoc2021.iml" />
+    </modules>
+  </component>
+</project>

.idea/vcs.xml

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+<?xml version="1.0" encoding="UTF-8"?>
+<project version="4">
+  <component name="VcsDirectoryMappings">
+    <mapping directory="$PROJECT_DIR$" vcs="Git" />
+  </component>
+</project>

Cargo.lock

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+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+version = 3
+
+[[package]]
+name = "aoc2021"
+version = "0.1.0"

Cargo.toml

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+[package]
+name = "aoc2021"
+version = "0.1.0"
+edition = "2021"
+
+[[bin]]
+name = "day01"
+path = "src/day01/main.rs"
+
+[dependencies]

src/day01/main.rs

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+use std::io;
+
+/// full-ascending circular stack
+struct ShiftReg {
+    vals: [i32; 4],
+    pos: usize,
+}
+
+impl ShiftReg {
+    pub const fn new() -> ShiftReg {
+        ShiftReg {
+            vals: [i32::MIN, i32::MIN, i32::MIN, i32::MIN],
+            pos: 3,
+        }
+    }
+
+    pub fn push(&mut self, val: i32) {
+        self.pos += 1;
+        self.pos %= 4;
+        self.vals[self.pos] = val;
+    }
+
+    pub fn diff(&self) -> i32 {
+        // i0  A
+        // i1  A B
+        // i2  A B
+        // i3    B
+        //
+        // A = (i0 + (i1 + i2)     )
+        // B = (     (i1 + i2) + i3)
+        // B - A = i3 - i0
+        let first = self.vals[self.pos]; // i3
+        let last = self.vals[(self.pos + 1) % 4]; // i0
+        first.wrapping_sub(last)
+    }
+}
+
+fn main() {
+    let stdin = io::stdin();
+    let mut sr = ShiftReg::new();
+    let mut count = 0;
+    loop {
+        let mut buffer = String::new();
+        match stdin.read_line(&mut buffer).unwrap() {
+            0 => break,
+            _ => {
+                let val = buffer.trim().parse::<i32>().unwrap();
+                sr.push(val);
+                if sr.diff() > 0 {
+                    count += 1;
+                }
+            }
+        }
+    }
+    println!("{}", count);
+}