implement recursive binary serach in rust (#93)

01_introduction_to_algorithms/rust/01_binary_search/cargo.lock

@@ -0,0 +1,4 @@
+[[package]]
+name = "binary-search"
+version = "0.1.0"
+

01_introduction_to_algorithms/rust/01_binary_search/cargo.toml

@@ -0,0 +1,6 @@
+[package]
+name = "binary-search"
+version = "0.1.0"
+authors = ["giorgiodelgado <hi@gdelgado.ca>"]
+
+[dependencies]

01_introduction_to_algorithms/rust/01_binary_search/src/main.rs

@@ -0,0 +1,103 @@
+// to run tests, you must have rust and cargo installed
+// then run `cargo test`
+
+use std::cmp;
+
+// assumes that the slice is already sorted
+fn binary_search<T: cmp::PartialOrd>(lst: &[T], item: T) -> Option<usize> {
+    let mid = ((lst.len() / 2) as f32).ceil() as usize;
+
+    match lst.get(mid) {
+        None => None,
+        Some(val) => {
+            if *val == item {
+                Some(mid)
+            } else if *val > item {
+                let sublist = &lst[..mid];
+                binary_search(sublist, item)
+            } else {
+                let sublist = &lst[(mid + 1)..];
+                // mapping is necessary when the item is
+                // to the right of the middle since indices on the
+                // sublist are erased and being at 0, 1, 2, 3, ... etc
+                binary_search(sublist, item).map(|pos| pos + mid + 1)
+            }
+        },
+    }
+}
+
+fn main() {
+    let num_slice = &[2, 4, 5, 12, 15, 30, 32, 33, 34, 40, 45, 51, 55, 57, 60, 66, 70, 71, 90, 99, 100];
+
+    let result = binary_search(num_slice, 70);
+
+    println!("Result: {:?}", result);
+}
+
+
+#[cfg(test)]
+mod test {
+    use super::binary_search;
+
+    #[test]
+    fn finds_number_near_end_of_list() {
+        let num_slice = &[2, 4, 5, 12, 15, 30, 32, 33, 34, 40, 45, 51, 55, 57, 60, 66, 70, 71, 90, 99, 100];
+
+        assert_eq!(binary_search(num_slice, 70), Some(16));
+    }
+
+    #[test]
+    fn finds_number_near_start_of_list() {
+        let num_slice = &[2, 4, 5, 12, 15, 30, 32, 33, 34, 40, 45, 51, 55, 57, 60, 66, 70, 71, 90, 99, 100];
+
+        assert_eq!(binary_search(num_slice, 5), Some(2));
+    }
+
+    #[test]
+    fn returns_none_for_numbers() {
+        let num_slice = &[2, 4, 5, 12, 15, 30, 32, 33, 34, 40, 45, 51, 55, 57, 60, 66, 70, 71, 90, 99, 100];
+
+        assert_eq!(binary_search(num_slice, 1), None);
+    }
+
+    #[test]
+    fn finds_char() {
+        let char_slice = &[
+            'a',
+            'b',
+            'c',
+            'd',
+            'e',
+            'f',
+            'g',
+            'h',
+            'i',
+            'j',
+            'k',
+            'l',
+            'm',
+            'n',
+            'o',
+            'p',
+            'q',
+            'r',
+            's',
+            't',
+            'u',
+            'v',
+            'w',
+            'x',
+            'y',
+            'z',
+        ];
+
+        assert_eq!(binary_search(char_slice, 'l'), Some(11));
+    }
+
+    #[test]
+    fn returns_none_for_chars() {
+        let char_slice = &['a', 'b', 'c'];
+
+        assert_eq!(binary_search(char_slice, 'l'), None);
+    }
+}