I hate day 8 so much

2021/src/day8.rs

@@ -6,7 +6,7 @@ use eyre::eyre;
 use crate::lib::{ParseLines, IterExt};
 
 
-#[derive(Debug)]
+#[derive(Copy, Clone, Debug, Eq, PartialEq)]
 enum Segment {
     A = 1,
     B = 2,
@@ -40,6 +40,19 @@ impl Segment {
         }
     }
 
+    fn from_ord(n: u8) -> eyre::Result<Self> {
+        match n {
+            0 => Ok(A),
+            1 => Ok(B),
+            2 => Ok(C),
+            3 => Ok(D),
+            4 => Ok(E),
+            5 => Ok(F),
+            6 => Ok(G),
+            _ => Err(eyre!("Invalid ordinal for segment: {}", n)),
+        }
+    }
+
     fn from_value(n: u8) -> eyre::Result<Self> {
         match n {
             1 => Ok(A),
@@ -61,10 +74,6 @@ struct SegmentSet {
 }
 
 impl SegmentSet {
-    fn new_filled() -> Self {
-        SegmentSet {data: 255}
-    }
-
     fn contains(&self, segment: Segment) -> bool {
         self.data & (segment as u8) > 0
     }
@@ -73,20 +82,12 @@ impl SegmentSet {
         self.data |= segment as u8;
     }
 
-    fn remove(&mut self, segment: Segment) {
-        self.data ^= segment as u8;
-    }
-
     fn len(&self) -> u8 {
         (0..7).fold(0, |total, i| total + ((self.data >> i) & 1))
     }
 
-    fn union(&self, other: &Self) -> Self {
-        SegmentSet {data: self.data | other.data}
-    }
-
-    fn intersect(&self, other: &Self) -> Self {
-        SegmentSet {data: self.data & other.data}
+    fn exclusion(&self, other: &Self) -> Self {
+        SegmentSet {data: self.data ^ other.data}
     }
 
     fn iter(&self) -> SegmentSetIter {
@@ -149,25 +150,117 @@ impl FromStr for Screen {
 }
 
 
-// mapping of Segment -> SegmentSet, represents 
-struct Candidates {
-    signal: Segment,
-    possible: SegmentSet
+fn segment_counts(screen: &Screen) -> [usize; 7] {
+    let mut counts = [0; 7];
+    for i in 0..7 {
+        let segment = Segment::from_ord(i).unwrap();
+        counts[i as usize] = screen.patterns.iter()
+            .filter(|p| p.contains(segment))
+            .count()
+    }
+    counts
 }
 
 
-const DIGIT_SEGMENTS: [SegmentSet; 10] = [ // magic numbers, fun!
-    SegmentSet {data: 0b01110111},
-    SegmentSet {data: 0b00100100},
-    SegmentSet {data: 0b01011101},
-    SegmentSet {data: 0b01101101},
-    SegmentSet {data: 0b00101110},
-    SegmentSet {data: 0b01101011},
-    SegmentSet {data: 0b01111011},
-    SegmentSet {data: 0b00100101},
-    SegmentSet {data: 0b01111111},
-    SegmentSet {data: 0b01101111},
-];
+fn find_a(screen: &Screen) -> Segment {
+    // figure out which segment is currently attached to the "A" wire
+    let mut seven = None;
+    let mut one = None;
+    for p in screen.patterns {
+        if p.len() == 2 {
+            one = Some(p); // the only digit that uses 2 segments is 1
+        }
+        else if p.len() == 3 {
+            seven = Some(p); // similarly for 7
+        }
+    }
+
+    let result = seven.unwrap().exclusion(&one.unwrap());
+    if result.len() != 1 {
+        panic!("This should never happen");
+    }
+    result.iter().next().unwrap()
+}
+
+
+fn solve(screen: &Screen) -> [Segment; 7] {
+    let mut result = [A; 7]; // A is just a placeholder value
+    let four_pattern = screen.patterns.iter().find(|p| p.len() == 4).unwrap();
+    for (i, count) in segment_counts(screen).iter().enumerate() {
+        let possible = match count {
+            4 => Some(E),
+            6 => Some(B),
+            7|8 => None,
+            9 => Some(F),
+            _ => unreachable!(),
+        };
+
+        if let Some(segment) = possible {
+            result[i] = segment;
+        }
+        else if *count == 7 { // 7 appearances means either D or G
+            let segment = Segment::from_ord(i as u8).unwrap();
+            if four_pattern.contains(segment) {
+                result[i] = D;
+            }
+            else {
+                result[i] = G;
+            }
+        }
+        else if *count == 8 { // similarly 8 should be either A or C
+            let segment = Segment::from_ord(i as u8).unwrap();
+            if segment == find_a(screen) {
+                result[i] = A;
+            }
+            else {
+                result[i] = C;
+            }
+        }
+    }
+    result
+}
+
+
+fn decode(output: [SegmentSet; 4], map: [Segment; 7]) -> usize {
+    let mut result = 0;
+    for (i, segments_enc) in output.iter().enumerate() {
+        let mut segments_dec = SegmentSet::default();
+        for segment_enc in segments_enc.iter() {
+            let segment_dec = map[segment_enc.to_ord()];
+            segments_dec.insert(segment_dec)
+        }
+        
+        let digit = match segments_dec.data {
+            0b01110111 => 0,
+            0b00100100 => 1,
+            0b01011101 => 2,
+            0b01101101 => 3,
+            0b00101110 => 4,
+            0b01101011 => 5,
+            0b01111011 => 6,
+            0b00100101 => 7,
+            0b01111111 => 8,
+            0b01101111 => 9,
+            _ => {
+                dbg!(segments_dec);
+                panic!("Invalid segment sequence");
+            },
+        };
+
+        result += digit * 10usize.pow(3 - (i as u32));
+    }
+    result
+}
+
+
+fn part2(screens: &[Screen]) -> usize {
+    let mut result = 0;
+    for screen in screens {
+        let map = solve(screen);
+        result += decode(screen.output, map);
+    }
+    result
+}
 
 
 fn part1(screens: &[Screen]) -> usize {
@@ -183,7 +276,5 @@ pub fn run(data: &str) -> eyre::Result<(usize, usize)> {
         .parse_lines::<Screen>()
         .collect::<Result<Vec<_>, _>>()?;
 
-
-
-    Ok((part1(&screens), 0))
+    Ok((part1(&screens), part2(&screens)))
 }