Update y to not have a loop

Author: EbbDrop

src/day20.rs

@@ -260,63 +260,32 @@ fn inner_part2(s: &[u8]) -> u32 {
                 let y = y as u8;
 
                 for line_i in &quads[Dir::N as usize][qy * QUADS_SIZE + qx] {
+                    let x = x as i16;
+                    let y = y as i16;
                     let Some(line) = lines[Dir::N as usize].get(*line_i) else {
                         continue;
                     };
-                    let dist = x.abs_diff(line.line_offset);
+                    let dist = x.abs_diff(line.line_offset as i16) as i16;
                     if dist > 20 {
                         continue;
                     }
+                    let dist_from_start = line.line_start as i16 - y;
 
-                    let mut line_ns = line.start_ns;
-                    for line_y in (line.line_end..line.line_start + 1).rev() {
-                        let dist = dist + y.abs_diff(line_y);
-                        if dist > 20 {
-                            line_ns += 1;
-                            continue;
-                        }
-                        let diff = ns - line_ns;
-                        if diff >= MIN_CHEAT + dist as u16 {
-                            sum += 1;
-                        }
-                        line_ns += 1;
+                    let ns_at_intersection = line.start_ns as i16 + dist_from_start;
+                    let diff_at_intersection = ns as i16 - ns_at_intersection;
+                    if diff_at_intersection - dist < MIN_CHEAT as i16 {
+                        continue;
                     }
+                    let cheat_left = (diff_at_intersection - dist) - MIN_CHEAT as i16;
 
-                    // let line_len = line.line_start - line.line_end;
-
-                    // let offset_x = line.line_offset.abs_diff(x);
-                    // if offset_x > 20 {
-                    //     continue;
-                    // }
-
-                    // if line.line_start <= y {
-                    //     let offset_y = y - line.line_start;
-                    //     let offset = offset_x + offset_y;
-                    //     if offset > 20 {
-                    //         continue;
-                    //     }
-
-                    //     let diff = ns - line.start_ns;
-                    //     if diff >= MIN_CHEAT + offset as u16 {
-                    //         let diff_left = diff - (MIN_CHEAT + offset as u16);
-                    //         sum += (offset as u32 - 20)
-                    //             .min(line_len as u32)
-                    //             .min((diff_left / 2 + 1) as u32);
-                    //     }
-                    // } else if line.line_end >= y {
-                    //     let offset_y = line.line_end - y;
-                    //     let offset = offset_x + offset_y;
-                    //     if offset > 20 {
-                    //         continue;
-                    //     }
+                    let cheat_start = (y + (20 - dist)).min(line.line_start as i16);
+                    let cheat_end = (y - (20 - dist))
+                        .max(y - cheat_left / 2)
+                        .max(line.line_end as i16);
 
-                    //     let diff = ns - (line.start_ns + line_len as u16);
-
-                    //     if diff >= MIN_CHEAT + offset as u16 {
-                    //         sum += (offset as u32 - 20).min(line_len as u32);
-                    //     }
-                    // } else {
-                    // }
+                    if cheat_start >= cheat_end {
+                        sum += (cheat_start - cheat_end) as u32 + 1;
+                    }
                 }
                 for line_i in &quads[Dir::E as usize][qy * QUADS_SIZE + qx] {
                     let Some(line) = lines[Dir::E as usize].get(*line_i) else {
@@ -342,27 +311,54 @@ fn inner_part2(s: &[u8]) -> u32 {
                     }
                 }
                 for line_i in &quads[Dir::S as usize][qy * QUADS_SIZE + qx] {
+                    let x = x as i16;
+                    let y = y as i16;
                     let Some(line) = lines[Dir::S as usize].get(*line_i) else {
                         continue;
                     };
-                    let dist = x.abs_diff(line.line_offset);
+                    let dist = x.abs_diff(line.line_offset as i16) as i16;
                     if dist > 20 {
                         continue;
                     }
+                    let dist_from_start = y - line.line_start as i16;
 
-                    let mut line_ns = line.start_ns;
-                    for line_y in line.line_start..line.line_end + 1 {
-                        let dist = dist + y.abs_diff(line_y);
-                        if dist > 20 {
-                            line_ns += 1;
-                            continue;
-                        }
-                        let diff = ns - line_ns;
-                        if diff >= MIN_CHEAT + dist as u16 {
-                            sum += 1;
-                        }
-                        line_ns += 1;
+                    let ns_at_intersection = line.start_ns as i16 + dist_from_start;
+                    let diff_at_intersection = ns as i16 - ns_at_intersection;
+                    if diff_at_intersection - dist < MIN_CHEAT as i16 {
+                        continue;
                     }
+                    let cheat_left = (diff_at_intersection - dist) - MIN_CHEAT as i16;
+
+                    let cheat_start = (y - (20 - dist)).max(line.line_start as i16);
+                    let cheat_end = (y + (20 - dist))
+                        .min(y + cheat_left / 2)
+                        .min(line.line_end as i16);
+
+                    if cheat_end >= cheat_start {
+                        sum += (cheat_end - cheat_start) as u32 + 1;
+                    }
+
+                    // let Some(line) = lines[Dir::S as usize].get(*line_i) else {
+                    //     continue;
+                    // };
+                    // let dist = (x as u8).abs_diff(line.line_offset);
+                    // if dist > 20 {
+                    //     continue;
+                    // }
+
+                    // let mut line_ns = line.start_ns;
+                    // for line_y in line.line_start..line.line_end + 1 {
+                    //     let dist = dist + (y as u8).abs_diff(line_y);
+                    //     if dist > 20 {
+                    //         line_ns += 1;
+                    //         continue;
+                    //     }
+                    //     let diff = ns - line_ns;
+                    //     if diff >= MIN_CHEAT + dist as u16 {
+                    //         sum += 1;
+                    //     }
+                    //     line_ns += 1;
+                    // }
                 }
                 for line_i in &quads[Dir::W as usize][qy * QUADS_SIZE + qx] {
                     let Some(line) = lines[Dir::W as usize].get(*line_i) else {