Day 17 2022 bleah

1 files changed, 93 insertions, 0 deletions

diff --git a/2022/17/b.rs b/2022/17/b.rs
new file mode 100644
index 0000000..261dff4
--- /dev/null
+++ b/2022/17/b.rs
@@ -0,0 +1,93 @@
+// Wow this is was terrible. Clearly you have to find out at which
+// exact frequency the shape repeats, but I could not find a good
+// way to estimate this apart from computing the first few (10^5)
+// rows and then manually checking.
+
+mod common;
+use common::*;
+
+fn line_to_u8(line: &[bool]) -> u8 {
+    let mut r = 0;
+    for i in 0..SCREEN_WIDTH {
+        if line[i] { r |= 1 << i as u8; }
+    }
+    r
+}
+
+fn write_lines(screen: &Screen, v: &mut [u8]) {
+    for i in 0..SCREEN_HEIGHT {
+        v[i] = line_to_u8(&screen.cell[i]);
+    }
+}
+
+fn isperiod(v: &[u8], l: usize) -> bool {
+    const PERIOD_GUESS: usize = 10;
+    const TIMES_MATCH: usize = 10;
+
+    for i in 0..PERIOD_GUESS {
+        for j in 1..TIMES_MATCH {
+            if v[i] != v[i+j*l] { return false; }
+        }
+    }
+    true
+}
+
+fn find_period(v: &[u8]) -> (usize, usize) {
+    const PREP: usize = 1000;
+    const PMAX: usize = 10000;
+
+    for start in 0..PREP {
+        for plen in 10..PMAX {
+            if isperiod(&v[start..], plen) { return (start+1, plen); }
+        }
+    }
+    panic!("No period of length < {} found with preperiod < {}",
+        PMAX, PREP);
+}
+
+fn main() {
+    const N: usize = 1000000000000;
+
+    let mut screen = Screen::new();
+    let rocks = get_rocks();
+    let mut line = String::new();
+    let _ = std::io::stdin().read_line(&mut line);
+    let line = &line[0..line.len()-1]; // remove newline
+
+    let mut i = 0;
+    let mut t = 0;
+    let mut vv = [0 as u8; SCREEN_HEIGHT];
+    let mut ii = [0 as usize; SCREEN_HEIGHT];
+    while screen.top < SCREEN_HEIGHT - 10 {
+        screen.drop_rock(&rocks[i%5], &mut t, &line);
+        ii[screen.top] = i;
+        i += 1;
+    }
+    write_lines(&screen, &mut vv);
+    let (prep, plen) = find_period(&vv);
+
+    /* This part shows the period etc...
+    println!("prep = {prep}, plen = {plen}");
+    for j in 0..10 {
+        for k in 0..4 {
+            print!(" | t={:>4} i={:>4}", j+prep+k*plen, ii[j+prep+k*plen]);
+        }
+        println!();
+    }
+    */
+
+    let i0 = ii[prep];
+    let i1 = ii[prep+plen];
+
+    let n = (N - i0) / (i1 - i0); // How many full repeats
+    let t = n * plen + prep; // How tall is the thing after n full repeats
+    // Remainder (unlike in the example case, it does not end with full repeat)
+    let mut rem = 0;
+    for j in 0..SCREEN_HEIGHT {
+        if ii[j] >= i0 + (N - i0) % (i1 - i0) {
+            rem = j-prep;
+            break;
+        }
+    }
+    println!("{}", t + rem - 1);
+}