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diff --git a/s4_prob/p01/Cargo.toml b/s4_prob/p01/Cargo.toml
new file mode 100644
index 0000000..030f301
--- /dev/null
+++ b/s4_prob/p01/Cargo.toml
@@ -0,0 +1,12 @@
+[package]
+name = "p01"
+version = "0.1.0"
+edition = "2024"
+
+[dependencies]
+fern = "0.7.1"
+humantime = "2.1.0"
+log = "0.4.26"
+num-traits = "0.2.19"
+ordered-float = "5.0.0"
+tabled = "0.18.0"
diff --git a/s4_prob/p01/src/main.rs b/s4_prob/p01/src/main.rs
new file mode 100644
index 0000000..80ec31c
--- /dev/null
+++ b/s4_prob/p01/src/main.rs
@@ -0,0 +1,170 @@
+fn setup_logger() -> Result<(), fern::InitError> {
+    fern::Dispatch::new()
+        .format(|out, message, record| {
+            out.finish(format_args!(
+                "[{} {} {}:{}] {}",
+                humantime::format_rfc3339_seconds(std::time::SystemTime::now()),
+                record.level(),
+                record.file().unwrap(),
+                record.line().unwrap(),
+                message
+            ))
+        })
+        .level(log::LevelFilter::Debug)
+        .chain(std::io::stdout())
+        .apply()?;
+    Ok(())
+}
+
+pub trait IntoOrd {
+    type Output: Ord;
+    fn into_ord(self) -> Self::Output;
+}
+
+impl IntoOrd for i32 {
+    type Output = i32;
+    fn into_ord(self) -> Self::Output {
+        self
+    }
+}
+
+impl IntoOrd for f32 {
+    type Output = ordered_float::OrderedFloat<f32>;
+    fn into_ord(self) -> Self::Output {
+        self.into()
+    }
+}
+
+pub struct Variation<T: IntoOrd>(std::collections::BTreeMap<T::Output, usize>);
+
+impl<T: IntoOrd> Variation<T> {
+    pub fn get(&self, value: T) -> Option<&usize> {
+        self.0.get(&value.into_ord())
+    }
+
+    // TODO: https://en.wikipedia.org/wiki/Histogram#Number_of_bins_and_width
+    pub fn get_count(&self, total: usize) -> usize {
+        let count = total.isqrt();
+        log::debug!("n = {count}");
+        count
+    }
+
+    pub fn get_total(&self) -> usize {
+        let total = self.0.values().sum();
+        log::debug!("n = {total}");
+        total
+    }
+}
+
+impl<T: IntoOrd> Variation<T>
+where
+    T::Output: Copy,
+{
+    pub fn get_first_key(&self) -> Option<T::Output> {
+        self.0.first_key_value().map(|(key, _)| *key)
+    }
+
+    pub fn get_last_key(&self) -> Option<T::Output> {
+        self.0.last_key_value().map(|(key, _)| *key)
+    }
+
+    pub fn get_step(&self, count: usize) -> T::Output
+    where
+        T::Output: num_traits::cast::FromPrimitive
+            + std::fmt::Display
+            + std::ops::Div<Output = T::Output>
+            + std::ops::Sub<Output = T::Output>,
+    {
+        let first = self.get_first_key().unwrap();
+        let last = self.get_last_key().unwrap();
+        let step = (last - first) / num_traits::FromPrimitive::from_usize(count).unwrap();
+        log::debug!("h = (x_k - x_1) / m = ({last} - {first}) / {count} = {step}");
+        step
+    }
+}
+
+impl<T: IntoOrd + Copy> From<&[T]> for Variation<T> {
+    fn from(items: &[T]) -> Self {
+        Self(items.iter().fold(Default::default(), |mut map, &item| {
+            *map.entry(item.into_ord()).or_insert(0) += 1;
+            map
+        }))
+    }
+}
+
+impl<T: IntoOrd> std::fmt::Display for Variation<T>
+where
+    T::Output: std::fmt::Display,
+{
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        let mut builder = tabled::builder::Builder::new();
+        for (i, (key, value)) in self.0.iter().enumerate() {
+            builder.push_column([format!("x_{i}"), key.to_string(), value.to_string()]);
+        }
+        let mut table = builder.build();
+        table.with(tabled::settings::Style::modern());
+        write!(f, "{table}")
+    }
+}
+
+fn main() {
+    setup_logger().unwrap();
+
+    #[rustfmt::skip]
+    const ITEMS: &[i32] = &[
+        75, 85, 84, 81, 84, 80, 82, 76, 75, 77,
+        80, 82, 81, 84, 85, 77, 76, 84, 83, 87,
+        78, 77, 88, 86, 87, 79, 80, 79, 78, 87,
+        76, 81, 83, 85, 78, 76, 83, 81, 84, 88,
+    ];
+    let series = Variation::from(ITEMS);
+    let total = series.get_total();
+    let count = series.get_count(total);
+    let step = series.get_step(count);
+    println!("{series}");
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    macro_rules! assert_delta {
+        ($x:expr, $y:expr, $d:expr) => {
+            if !($x - $y < $d || $y - $x < $d) {
+                panic!();
+            }
+        };
+    }
+
+    #[test]
+    fn it_works() {
+        #[rustfmt::skip]
+        const ITEMS: &[f32] = &[
+            0.87, 0.94, 0.99, 0.90, 0.90, 0.87, 0.85, 0.87,
+            0.90, 0.94, 0.87, 0.87, 0.82, 0.90, 0.94, 0.90,
+            0.85, 0.85, 0.87, 0.94, 0.81, 0.82, 0.87, 0.97,
+            0.90, 0.94, 0.85, 0.81, 0.87, 0.85, 0.90, 0.82,
+            0.99, 0.90, 0.94, 0.82, 0.97, 0.81, 0.85, 0.87,
+        ];
+        let series = Variation::from(ITEMS);
+        const EXPECTED: &[(f32, usize)] = &[
+            (0.81, 3),
+            (0.82, 4),
+            (0.85, 6),
+            (0.87, 9),
+            (0.90, 8),
+            (0.94, 6),
+            (0.97, 2),
+            (0.99, 2),
+        ];
+        for (value, count) in EXPECTED.iter() {
+            assert_eq!(*series.get(*value).unwrap(), *count);
+        }
+        let total = series.get_total();
+        assert_eq!(total, 40);
+        let count = series.get_count(total);
+        assert_eq!(count, 6);
+        let step = series.get_step(count);
+        assert_delta!(step.0, 0.03, 1e-3);
+    }
+}