diff --git a/src/fenwicktree.rs b/src/fenwicktree.rs
index e69de29..2bd56c2 100644
--- a/src/fenwicktree.rs
+++ b/src/fenwicktree.rs
@@ -0,0 +1,60 @@
+// Reference: https://en.wikipedia.org/wiki/Fenwick_tree
+pub struct FenwickTree<T> {
+    n: usize,
+    ary: Vec<T>,
+    e: T,
+}
+
+impl<T: Clone + std::ops::AddAssign<T>> FenwickTree<T> {
+    pub fn new(n: usize, e: T) -> Self {
+        FenwickTree {
+            n: n,
+            ary: vec![e.clone(); n],
+            e: e,
+        }
+    }
+    pub fn accum(&self, mut idx: usize) -> T {
+        let mut sum = self.e.clone();
+        while idx > 0 {
+            sum += self.ary[idx - 1].clone();
+            idx &= idx - 1;
+        }
+        sum
+    }
+    /// performs data[idx] += val;
+    pub fn add<U: Clone>(&mut self, mut idx: usize, val: U)
+    where
+        T: std::ops::AddAssign<U>,
+    {
+        let n = self.n;
+        idx += 1;
+        while idx <= n {
+            self.ary[idx - 1] += val.clone();
+            idx += idx & idx.wrapping_neg();
+        }
+    }
+    /// Returns data[l] + ... + data[r - 1].
+    pub fn sum(&self, l: usize, r: usize) -> T
+    where
+        T: std::ops::Sub<Output = T>,
+    {
+        self.accum(r) - self.accum(l)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn fenwick_tree_works() {
+        let mut bit = FenwickTree::new(5, 0i64);
+        // [1, 2, 3, 4, 5]
+        for i in 0..5 {
+            bit.add(i, i as i64 + 1);
+        }
+        assert_eq!(bit.sum(0, 5), 15);
+        assert_eq!(bit.sum(0, 4), 10);
+        assert_eq!(bit.sum(1, 3), 5);
+    }
+}
diff --git a/src/lib.rs b/src/lib.rs
index d11bcef..ce39049 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -10,3 +10,5 @@ mod scc;
 mod segtree;
 mod string;
 mod twosat;
+
+pub use fenwicktree::FenwickTree;