3 files changed, 216 insertions, 0 deletions
diff --git a/d21/Cargo.toml b/d21/Cargo.toml
new file mode 100644
index 0000000..f06cd6a
--- /dev/null
+++ b/d21/Cargo.toml
@@ -0,0 +1,7 @@
+[package]
+name = "d21"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]
+itertools = "0.10.5"
diff --git a/d21/input.txt b/d21/input.txt
new file mode 100644
index 0000000..b39b884
--- /dev/null
+++ b/d21/input.txt
@@ -0,0 +1,100 @@
+reverse positions 1 through 6
+rotate based on position of letter a
+swap position 4 with position 1
+reverse positions 1 through 5
+move position 5 to position 7
+swap position 4 with position 0
+swap position 4 with position 6
+rotate based on position of letter a
+swap position 0 with position 2
+move position 5 to position 2
+move position 7 to position 1
+swap letter d with letter c
+swap position 5 with position 3
+reverse positions 3 through 7
+rotate based on position of letter d
+swap position 7 with position 5
+rotate based on position of letter f
+swap position 4 with position 1
+swap position 3 with position 6
+reverse positions 4 through 7
+rotate based on position of letter c
+move position 0 to position 5
+swap position 7 with position 4
+rotate based on position of letter f
+reverse positions 1 through 3
+move position 5 to position 3
+rotate based on position of letter g
+reverse positions 2 through 5
+rotate right 0 steps
+rotate left 0 steps
+swap letter f with letter b
+rotate based on position of letter h
+move position 1 to position 3
+reverse positions 3 through 6
+rotate based on position of letter h
+swap position 4 with position 3
+swap letter b with letter h
+swap letter a with letter h
+reverse positions 1 through 6
+swap position 3 with position 6
+swap letter e with letter d
+swap letter e with letter h
+swap position 1 with position 5
+rotate based on position of letter a
+reverse positions 4 through 5
+swap position 0 with position 4
+reverse positions 0 through 3
+move position 7 to position 2
+swap letter e with letter c
+swap position 3 with position 4
+rotate left 3 steps
+rotate left 7 steps
+rotate based on position of letter e
+reverse positions 5 through 6
+move position 1 to position 5
+move position 1 to position 2
+rotate left 1 step
+move position 7 to position 6
+rotate left 0 steps
+reverse positions 5 through 6
+reverse positions 3 through 7
+swap letter d with letter e
+rotate right 3 steps
+swap position 2 with position 1
+swap position 5 with position 7
+swap letter h with letter d
+swap letter c with letter d
+rotate based on position of letter d
+swap letter d with letter g
+reverse positions 0 through 1
+rotate right 0 steps
+swap position 2 with position 3
+rotate left 4 steps
+rotate left 5 steps
+move position 7 to position 0
+rotate right 1 step
+swap letter g with letter f
+rotate based on position of letter a
+rotate based on position of letter b
+swap letter g with letter e
+rotate right 4 steps
+rotate based on position of letter h
+reverse positions 3 through 5
+swap letter h with letter e
+swap letter g with letter a
+rotate based on position of letter c
+reverse positions 0 through 4
+rotate based on position of letter e
+reverse positions 4 through 7
+rotate left 4 steps
+swap position 0 with position 6
+reverse positions 1 through 6
+rotate left 2 steps
+swap position 5 with position 3
+swap letter b with letter d
+swap letter b with letter d
+rotate based on position of letter d
+rotate based on position of letter c
+rotate based on position of letter h
+move position 4 to position 7
diff --git a/d21/src/main.rs b/d21/src/main.rs
new file mode 100644
index 0000000..34d5599
--- /dev/null
+++ b/d21/src/main.rs
@@ -0,0 +1,109 @@
+use std::fs;
+
+use itertools::Itertools;
+
+fn solve_part1(insts: &Vec<&str>, init: &str) -> String {
+    let mut chars: Vec<char> = init.chars().collect();
+
+    for inst in insts {
+        let mut next = chars.clone();
+
+        let words: Vec<&str> = inst.split_ascii_whitespace().collect();
+        if words[0] == "swap" {
+            if words[1] == "position" {
+                // Swap the characters at indices `i' and `j'
+                let i: usize = words[2].parse().unwrap();
+                let j: usize = words[5].parse().unwrap();
+                next[i] = chars[j];
+                next[j] = chars[i];
+            } else if words[1] == "letter" {
+                // Swap all occurrences of characters `x' and `y'
+                let x = words[2].chars().nth(0).unwrap();
+                let y = words[5].chars().nth(0).unwrap();
+                for i in 0..next.len() {
+                    if next[i] == x {
+                        next[i] = y;
+                    } else if next[i] == y {
+                        next[i] = x;
+                    }
+                }
+            }
+        } else if words[0] == "rotate" {
+            if words[1] == "left" {
+                let n: usize = words[2].parse().unwrap();
+                next.rotate_left(n % chars.len());
+            } else if words[1] == "right" {
+                let n: usize = words[2].parse().unwrap();
+                next.rotate_right(n % chars.len());
+            } else if words[1] == "based" {
+                // Get the index `i' of character `x'
+                let x = words[6].chars().nth(0).unwrap();
+                let i = next.iter().position(|&c| c == x).unwrap();
+                // Rotate right by an amount determined by `i'
+                let n = if i < 4 { i + 1 } else { i + 2 };
+                next.rotate_right(n % chars.len());
+            }
+        } else if words[0] == "reverse" {
+            let i: usize = words[2].parse().unwrap();
+            let j: usize = words[4].parse().unwrap();
+            next[i..j + 1].reverse();
+        } else if words[0] == "move" {
+            let i: usize = words[2].parse().unwrap();
+            let j: usize = words[5].parse().unwrap();
+            let c = next.remove(i);
+            next.insert(j, c);
+        }
+
+        chars = next;
+    }
+
+    chars.into_iter().collect()
+}
+
+fn solve_part2(insts: &Vec<&str>, target: &str) -> String {
+    let init = vec!['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h'];
+
+    // The "good" approach would be to reverse the operations in `insts'.
+    // I'm too lazy for that: I just go through all permutations of the
+    // input characters, run `insts' as in part 1, and see if it's the one.
+    for perm in init.into_iter().permutations(8) {
+        let string: String = perm.iter().collect();
+        if solve_part1(insts, &string) == target {
+            return string;
+        }
+    }
+
+    String::new() // shouldn't happen
+}
+
+fn main() {
+    // Read instructions from input text file
+    let input = fs::read_to_string("input.txt").unwrap();
+    let insts = input.lines().collect();
+
+    // Part 1 gives "gfdhebac" for me
+    println!("Part 1 solution: {}", solve_part1(&insts, "abcdefgh"));
+
+    // Part 2 gives "dhaegfbc" for me
+    println!("Part 2 solution: {}", solve_part2(&insts, "fbgdceah"));
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn part1_example1() {
+        let insts = vec![
+            "swap position 4 with position 0",
+            "swap letter d with letter b",
+            "reverse positions 0 through 4",
+            "rotate left 1 step",
+            "move position 1 to position 4",
+            "move position 3 to position 0",
+            "rotate based on position of letter b",
+            "rotate based on position of letter d",
+        ];
+        assert_eq!(solve_part1(&insts, "abcde"), "decab");
+    }
+}