diff --git a/src/char.rs b/src/char.rs
index bbcda06..b6aa435 100644
--- a/src/char.rs
+++ b/src/char.rs
@@ -1,22 +1,55 @@
 /// Used to get a character with a given brightness.
-pub struct CharSampler {
+pub trait CharSampler {
+    /// Gets a character with the given brightness.
+    /// # Arguments
+    /// * `level`: `0 <= level` and `level < 1`
+    fn sample(&self, level: f32) -> char;
+}
+
+pub struct SimpleCharSampler {
     chars: String
 }
 
-impl CharSampler {
+impl SimpleCharSampler {
     /// # Arguments
     /// * `chars`: The characters ordered by brightness.
     pub fn new(chars: String) -> Self {
         Self { chars }
     }
+}
 
-    /// Gets a character with the given brightness.
-    /// # Arguments
-    /// * `level`: `0 <= level` and `level < 1`
-    pub fn sample(&self, level: f32) -> char {
-        let pos = level * self.chars.chars().count() as f32;
-        let index = pos as usize;
+impl CharSampler for SimpleCharSampler {
+    fn sample(&self, level: f32) -> char {
+        let index = level * self.chars.chars().count() as f32;
 
-        self.chars.chars().nth(index).unwrap()
+        assert!(index >= 0.0);
+
+        self.chars.chars().nth(index as usize).unwrap()
+    }
+}
+
+#[cfg(test)]
+mod test {
+    use super::*;
+
+    #[test]
+    fn sample() {
+        let sampler = SimpleCharSampler::new("abc".to_string());
+
+        assert_eq!('a', sampler.sample(0.1));
+        assert_eq!('b', sampler.sample(0.4));
+        assert_eq!('c', sampler.sample(0.7));
+    }
+
+    #[test]
+    #[should_panic]
+    fn sample_index_negative() {
+        SimpleCharSampler::new("abc".to_string()).sample(-0.1);
+    }
+
+    #[test]
+    #[should_panic]
+    fn sample_index_equals_one() {
+        SimpleCharSampler::new("abc".to_string()).sample(1.0);
     }
 }
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