diff --git a/09/README.md b/09/README.md
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+# [Day 9](https://adventofcode.com/2023/day/9) 
+:gift::gift::gift::gift::gift::gift::gift::gift::gift:
+
+Today's language: **Rust**
+
+Lines of code: **88**
+
+Execution time: **0,003 s**
+
+This solution uses a custom triangle datatype.
+It is a bit overkill code-wise, but efficient with memory and re-allocations.
+
+```shell
+rustc day09.rs
+./day09
+```
+<!-- implemented triangle datatype -->
diff --git a/09/day09.rs b/09/day09.rs
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+use std::io::{self, BufRead};
+use std::fmt::Debug;
+use std::fmt;
+
+// 0 1 3 6 10 ...
+fn triangular_numbers(n: usize) -> usize { (n * (n + 1)) / 2 }
+
+struct Triangle {
+    data: Vec::<i32>,
+    width: usize,
+}
+impl Triangle {
+    fn new(firstrow: &mut Vec<i32>) -> Triangle {
+        let mut triangle = Triangle{data: Vec::new(), width: firstrow.len()};
+        triangle.data = std::mem::take(firstrow);
+        triangle.data.resize(triangular_numbers(triangle.width), 0);
+        return triangle
+    }
+    fn at_mut(&mut self, row: usize, col: usize) -> &mut i32 {
+        let i = self._index(row, col);
+        &mut self.data[i]
+    }
+    fn at(&self, row: usize, col: usize) -> i32 {
+        self.data[self._index(row, col)]
+    }
+// 1 2 3 4
+// 1 2 3
+// 1 2
+// 1
+    fn _index(&self, row: usize, col: usize) -> usize {
+        assert!(col < self.row_len(row), "Invalid indices for Triangle with width {}: row={}, col={}", self.width, row, col);
+        return col + self.data.len() - triangular_numbers(self.width - row);  // subtract  the triangle with width (self.width - row) from total length
+    }
+    // fn row(&mut self, row: usize) -> impl Iterator<Item = &mut i32> {
+    //     assert!(row < self.width, "Invalid row");
+    //     let skip: usize = self.width * row - triangular_numbers(self.width - row);  // skip <row> rows - the triangle with width (self.width - row)
+    //     self.data.iter_mut().skip(skip).take(self.row_len(row));
+    // }
+    // #rows or #colums of first row
+    fn len(&self) -> usize { self.width }
+    fn row_len(&self, row: usize) -> usize { self.width - row }
+}
+impl fmt::Display for Triangle {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        for r in 0..self.len() {
+            for c in 0..self.row_len(r) {
+                let res = write!(f, "{} ", self.at(r, c));
+                if res.is_err() { return res };
+            }
+            if r != self.len() - 1 { println!() }
+        }
+        return Result::Ok(());
+    }
+}
+
+fn predict(numbers: &mut Vec<i32>) -> (i32, i32) {
+    numbers.insert(0, 0);
+    numbers.push(0); 
+    let mut triangle = Triangle::new(numbers);
+    // fill until maxrow is all zero
+    let mut maxrow: usize = triangle.len() - 1;  // skip last row
+    for r in 1..maxrow {
+        maxrow = r;
+        let mut allzero = true;
+        for j in 1..triangle.row_len(r) - 1 {
+            // print!("{} -> ", triangle.at(r, j));
+            let diff = triangle.at(r-1, j+1) - triangle.at(r-1, j); //).abs();
+            allzero &= diff == 0;
+            *triangle.at_mut(r, j) = diff;
+        }
+        if allzero { break }
+        // println!();
+    }
+    assert!(maxrow < triangle.len()- 3, "maxrow={} ", maxrow);
+    for r in (0..maxrow).rev() {
+        // forward prediction
+        let newval = triangle.at(r, triangle.row_len(r) - 2) + triangle.at(r + 1, triangle.row_len(r + 1) - 1);
+        *triangle.at_mut(r, triangle.row_len(r) - 1) = newval;
+
+        let newval = triangle.at(r, 1) - triangle.at(r + 1, 0);
+        *triangle.at_mut(r, 0) = newval;
+    }
+    // println!("{}", triangle);
+    return (triangle.at(0, 0), triangle.at(0, triangle.row_len(0) - 1));
+}
+
+fn main() {
+    let input = "input.txt";
+    // let input = "example.txt";
+    let lines = read_lines(&input);
+    let mut sum_forward_predictions = 0;
+    let mut sum_backward_predictions = 0;
+    for line in lines.map(|r| r.ok().unwrap()) {
+        let preds = predict(&mut split_into_numbers::<i32>(&line).collect());
+        sum_backward_predictions += preds.0;
+        sum_forward_predictions += preds.1;
+    }
+    println!("OASIS: sum of forward  predictions (1): {}", sum_forward_predictions);
+    println!("OASIS: sum of backward predictions (2): {}", sum_backward_predictions);
+}
+
+
+fn split_into_numbers<T: std::str::FromStr>(x: &str) -> impl Iterator<Item = T> + '_ where <T as std::str::FromStr>::Err: Debug {
+    return x.split(' ').filter(|&n| {n != "" && n != " "}).map(|n| n.parse::<T>().unwrap());
+}
+
+
+fn read_lines<P>(filename: P) -> io::Lines<io::BufReader<std::fs::File>>
+where P: AsRef<std::path::Path>, {
+    return match std::fs::File::open(filename) {
+        Err(why) => panic!("Could not open file. {}", why),
+        Ok(file) => std::io::BufReader::new(file).lines()
+    };
+}