1 files changed, 75 insertions, 0 deletions

diff --git a/2022/13/common.rs b/2022/13/common.rs
new file mode 100644
index 0000000..f49f3bc
--- /dev/null
+++ b/2022/13/common.rs
@@ -0,0 +1,75 @@
+use std::cmp::Ordering;
+
+#[derive(Debug)]
+pub enum Packet {
+    Number(i64),
+    List(Vec<Packet>)
+}
+
+fn compare_lists(l: &Vec<Packet>, k: &Vec<Packet>) -> Ordering {
+    for i in 0..std::cmp::min(l.len(), k.len()) {
+        let ord = compare(&l[i], &k[i]);
+        if ord != Ordering::Equal { return ord; }
+    }
+    l.len().cmp(&k.len())
+}
+
+fn compare_list_single(l: &Vec<Packet>, p: &Packet) -> Ordering {
+    if l.len() == 0 { return Ordering::Less; }
+    let ord = compare(&l[0], p);
+    if ord != Ordering::Equal { return ord; }
+    l.len().cmp(&1)
+}
+
+pub fn compare(p: &Packet, q: &Packet) -> Ordering {
+    match p {
+        Packet::Number(n) => {
+            match q {
+                Packet::Number(m) => n.cmp(&m),
+                Packet::List(k) => compare_list_single(k, p).reverse()
+            } 
+        },
+        Packet::List(l) => {
+            match q {
+                Packet::Number(_) => compare_list_single(l, q),
+                Packet::List(k) => compare_lists(l, k)
+            }
+        }
+    }
+}
+
+fn parse_int(line: &[u8], i: &mut usize) -> i64 {
+    let mut r: i64 = 0;
+    loop {
+        match line[*i] {
+            b'0'..=b'9' => r = r * 10 + (line[*i] as i64) - ('0' as i64),
+            b',' | b']' => return r,
+            x => panic!("Error: unexpected '{}' at position {} in parse_int",
+                        x as char, *i)
+        }
+        *i += 1;
+    }
+}
+
+fn parse_list(line: &[u8], i: &mut usize) -> Packet {
+    if line[*i] != b'[' {
+        panic!("Error: sub-list at {} does not start with '['", *i);
+    }
+    *i += 1;
+    let mut v = Vec::<Packet>::new();
+    while *i < line.len() && line[*i] != b'\n' {
+        match line[*i] {
+            b']' => { *i += 1; return Packet::List(v); },
+            b'[' => v.push(parse_list(line, i)),
+            b'0'..=b'9' => v.push(Packet::Number(parse_int(line, i))),
+            b' ' | b',' => *i += 1,
+            x => panic!("Error: unexpected '{}' at position {}", x as char, *i)
+        }
+    }
+    panic!("Error: finished parsing at {}, but ']' not found", *i);
+}
+
+pub fn parse(line: &[u8]) -> Packet {
+    let mut i = 0;
+    parse_list(line, &mut i)
+}