diff --git a/src/lib.rs b/src/lib.rs
index 7d2b089..c5802d2 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -95,3 +95,5 @@ pub(crate) mod optimization;
 pub mod svm;
 /// Supervised tree-based learning methods
 pub mod tree;
+/// Preprocessing utilities
+pub mod preprocessing;
diff --git a/src/preprocessing/mod.rs b/src/preprocessing/mod.rs
new file mode 100644
index 0000000..e4b5190
--- /dev/null
+++ b/src/preprocessing/mod.rs
@@ -0,0 +1 @@
+pub mod target_encoders;
\ No newline at end of file
diff --git a/src/preprocessing/target_encoders.rs b/src/preprocessing/target_encoders.rs
new file mode 100644
index 0000000..1894361
--- /dev/null
+++ b/src/preprocessing/target_encoders.rs
@@ -0,0 +1,209 @@
+
+#![allow(clippy::ptr_arg)]
+//! # Encode categorical features as a one-hot or multi-class numeric array.
+//! 
+
+use std::hash::Hash;
+use std::collections::HashMap;
+
+use crate::math::num::RealNumber;
+use crate::error::Failed;
+
+
+/// Turn a collection of label types into a one-hot vectors.
+/// This struct encodes single class per exmample
+pub struct OneHotEncoder<T> {
+    label_to_idx: HashMap<T, usize>,
+    labels: Vec<T>,
+    num_classes: usize
+
+}
+
+enum LabelDefinition<T> {
+    LabelToClsNumMap(HashMap<T, usize>),
+    PositionalLabel(Vec<T>),
+}
+
+/// Crearte a vector of size num_labels with zeros everywhere and 1 at label_idx (one-hot vector)
+pub fn make_one_hot<T: RealNumber>(label_idx: usize, num_labels: usize) -> Vec<T> {
+    let (pos, neg) = (T::from_f64(1f64).unwrap(), T::from_f64(0f64).unwrap());
+    (0..num_labels).map(|idx| if idx == label_idx {pos.clone()} else {neg.clone()}).collect()
+      
+}
+
+impl<'a, T: Hash + Eq + Clone> OneHotEncoder<T> 
+{
+    
+    /// Fit an encoder to a lable list
+    ///
+    /// Label numbers will be assigned in the order they are encountered
+    /// Example:
+    /// ```
+    /// let fake_labels: Vec<usize> = vec![1,2,3,4,5,3,5,3,1,2,4];
+    /// let enc = OneHotEncoder::<usize>::fit(&fake_labels[0..]);
+    /// let oh_vec = enc.transform_one(&1); // notice that 1 is actually a zero-th positional label
+    /// assert_eq!(oh_vec, vec![1f64,0f64,0f64,0f64,0f64]);
+    /// ```
+    pub fn fit(labels: &[T]) -> Self {
+        
+        let mut label_map: HashMap<T, usize> = HashMap::new();
+        let mut class_num = 0usize;
+        let mut unique_lables: Vec<T> = Vec::new();
+
+        for l in labels
+         {
+            if !label_map.contains_key(&l) {
+                label_map.insert(l.clone(), class_num);
+                unique_lables.push(l.clone());
+                class_num += 1;
+            }
+        }
+        Self {label_to_idx: label_map, num_classes: class_num, labels:unique_lables}
+    }
+
+        
+    /// Build an encoder from a predefined (label -> class number) map
+    ///
+    /// Definition example:
+    /// ```
+    /// let fake_label_map: HashMap<&str, u32> = vec![("background",0), ("dog", 1), ("cat", 2)]
+    /// .into_iter()
+    /// .collect();
+    /// let enc = OneHotEncoder::<&str>::from_label_map(fake_label_map);
+    /// ```
+    pub fn from_label_map(labels: HashMap<T, usize>) -> Self {
+        Self::from_label_def(LabelDefinition::LabelToClsNumMap(labels))
+    }
+    /// Build an encoder from a predefined positional label-class num vector
+    ///
+    /// Definition example:
+    /// ```
+    /// let fake_label_pos = vec!["background","dog", "cat"];
+    /// let enc = OneHotEncoder::<&str>::from_positional_label_vec(fake_label_pos);
+    /// ```
+    pub fn from_positional_label_vec(labels: Vec<T>) -> Self {
+        Self::from_label_def(LabelDefinition::PositionalLabel(labels))
+    }
+    
+    /// Transform a slice of label types into one-hot vectors
+    /// None is returned if unknown label is encountered 
+    pub fn transform(&self, labels: &[T]) -> Vec<Option<Vec<f64>>> {
+        labels
+        .into_iter()
+        .map(|l| self.transform_one(l))
+        .collect()
+    }
+
+    /// Transform a single label type into a one-hot vector
+    pub fn transform_one(&self, label: &T) -> Option<Vec<f64>> {
+        match self.label_to_idx.get(label) {
+            None => None,
+            Some(&idx) => Some(make_one_hot(idx, self.num_classes))
+        }
+    }
+
+    /// Invert one-hot vector, back to the label
+    ///```
+    /// let lab = enc.invert_one(res)?; // e.g. res = [0,1,0,0...] "dog" == class 1
+    /// assert_eq!(lab, "dog")
+    /// ```
+    pub fn invert_one<U: RealNumber>(&self, one_hot: Vec<U>) -> Result<T, Failed> {
+        let pos = U::from_f64(1f64).unwrap();
+
+        let s: Vec<usize> = one_hot
+        .into_iter()
+        .enumerate()
+        .filter_map(|(idx, v)| if v == pos {Some(idx)} else {None})
+        .collect();
+        
+        if s.len() == 1 {
+            let idx = s[0];
+            return Ok(self.labels[idx].clone())
+        }
+        let pos_entries = format!("Expected a single positive entry, {} entires found", s.len());
+        Err(Failed::transform(&pos_entries[..]))
+    }
+
+    
+    fn from_label_def(labels: LabelDefinition<T>) -> Self {
+        
+        let (label_map, class_num, unique_lables) = match labels {
+            LabelDefinition::LabelToClsNumMap(h) => {
+                let mut _unique_lab: Vec<(T, usize)> = h.iter().map(|(k,v)| (k.clone(), v.clone())).collect();
+                _unique_lab.sort_by(|a,b| a.1.cmp(&b.1));
+                let unique_lab: Vec<T> = _unique_lab.into_iter().map(|a| a.0).collect();
+                (h, unique_lab.len(), unique_lab)
+            },
+            LabelDefinition::PositionalLabel(unique_lab) => {
+                let h: HashMap<T, usize> = unique_lab.iter().enumerate().map(|(v, k)| (k.clone(),v)).collect();
+                (h, unique_lab.len(), unique_lab)
+            }
+        };
+        Self {label_to_idx: label_map, num_classes: class_num, labels:unique_lables}
+
+    }
+}
+
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn from_labels() {
+        let fake_labels: Vec<usize> = vec![1,2,3,4,5,3,5,3,1,2,4];
+        let enc = OneHotEncoder::<usize>::fit(&fake_labels[0..]);
+        let oh_vec = match enc.transform_one(&1) {
+            None => panic!("Wrong labels"),
+            Some(v) => v
+        };
+        let res: Vec<f64> = vec![1f64,0f64,0f64,0f64,0f64];
+        assert_eq!(oh_vec, res);
+    }
+
+
+    fn build_fake_str_enc<'a>() -> OneHotEncoder<&'a str>{
+        let fake_label_pos = vec!["background","dog", "cat"];
+        let enc = OneHotEncoder::<&str>::from_positional_label_vec(fake_label_pos);
+        enc
+    }
+
+    #[test]
+    fn label_map_and_vec() {
+            let fake_label_map: HashMap<&str, usize> = vec![("background",0), ("dog", 1), ("cat", 2)].into_iter().collect();
+            let enc = OneHotEncoder::<&str>::from_label_map(fake_label_map);
+            let oh_vec = match enc.transform_one(&"dog") {
+                None => panic!("Wrong labels"),
+                Some(v) => v
+            };
+            let res: Vec<f64> = vec![0f64, 1f64,0f64];
+            assert_eq!(oh_vec, res);
+        }
+    
+    #[test]
+    fn positional_labels_vec() {
+            let enc = build_fake_str_enc();
+            let oh_vec = match enc.transform_one(&"dog") {
+                None => panic!("Wrong labels"),
+                Some(v) => v
+            };
+            let res: Vec<f64> = vec![0f64, 1f64,0f64];
+            assert_eq!(oh_vec, res);
+    }
+
+    #[test]
+    fn invert_label_test() {
+        let enc = build_fake_str_enc();
+        let res: Vec<f64> = vec![0f64, 1f64,0f64];
+        let lab = enc.invert_one(res).unwrap();
+        assert_eq!(lab, "dog");
+            
+        if let Err(e) = enc.invert_one(vec![0.0, 0.0,0.0]) {
+            let pos_entries = format!("Expected a single positive entry, 0 entires found");
+            assert_eq!(e, Failed::transform(&pos_entries[..]));
+        };
+    }
+
+    
+
+}
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