From d845ce64a4eef2e1205d88603e708377e697845d Mon Sep 17 00:00:00 2001
From: Felix Martin <mail@felixm.de>
Date: Mon, 16 Nov 2020 21:12:09 -0500
Subject: [PATCH] Implement merge sort that counts inversions

---
 src/main.rs       | 71 +++++++++++++++--------------------------------
 src/merge_sort.rs | 48 ++++++++++++++++++++++++++++++++
 2 files changed, 71 insertions(+), 48 deletions(-)
 create mode 100644 src/merge_sort.rs

diff --git a/src/main.rs b/src/main.rs
index 17baac0..d521d24 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -1,57 +1,32 @@
-use std::clone::Clone;
-use std::cmp::PartialOrd;
-use std::marker::Copy;
+mod merge_sort;
+use crate::merge_sort::merge_sort_inversions;
+use std::fs::File;
+use std::io;
+use std::io::{BufRead, BufReader, Error, ErrorKind};
 use std::vec::Vec;
 
-fn merge<T: Clone + PartialOrd + Copy>(a: Vec<T>, b: Vec<T>) -> Vec<T> {
-    let mut r = vec![];
-    let mut i = 0;
-    let mut j = 0;
-
-    while i < a.len() && j < b.len() {
-        if a[i] < b[j] {
-            r.push(a[i]);
-            i += 1;
-        } else {
-            r.push(b[j]);
-            j += 1;
-        }
+fn read_to_vector(path: &str) -> Result<Vec<i64>, io::Error> {
+    let file = File::open(path)?;
+    let br = BufReader::new(file);
+    let mut v = Vec::new();
+    for line in br.lines() {
+        let line = line?;
+        let n = line
+            .trim()
+            .parse()
+            .map_err(|e| Error::new(ErrorKind::InvalidData, e))?;
+        v.push(n);
     }
-
-    while i < a.len() {
-        r.push(a[i]);
-        i += 1;
-    }
-
-    while j < b.len() {
-        r.push(b[j]);
-        j += 1;
-    }
-
-    return r;
+    Ok(v)
 }
 
-fn merge_sort<T: Clone + PartialOrd + Copy>(v: Vec<T>) -> Vec<T> {
-    if v.len() <= 1 {
-        return v;
-    }
-    let i = v.len() / 2;
-    let a = merge_sort((&v[..i]).to_vec());
-    let b = merge_sort((&v[i..]).to_vec());
-    let r = merge(a, b);
-    return r;
-}
-
-fn mult_bin(a: i64, b: i64) -> i64 {
-    return a * b;
+fn c1a2() {
+    let vec = read_to_vector("data/week_2_assignment_2.txt").unwrap();
+    let (_sorted, inversions) = merge_sort_inversions(vec);
+    // println!("{:?}", sorted.len());
+    println!("course 1 assignment 2: {:?}", inversions);
 }
 
 fn main() {
-    println!("Hello, world!");
-    let l = vec![4, 2, 1, 3];
-    let l = merge_sort(l);
-    println!("{:?}", l);
-
-    let r = mult_bin(42, 21);
-    println!("{:?}", r);
+    c1a2();
 }
diff --git a/src/merge_sort.rs b/src/merge_sort.rs
new file mode 100644
index 0000000..0adb3a9
--- /dev/null
+++ b/src/merge_sort.rs
@@ -0,0 +1,48 @@
+use std::clone::Clone;
+use std::cmp::PartialOrd;
+
+fn merge_count_inversions<T: Clone + PartialOrd + Copy + std::fmt::Debug>(
+    a: Vec<T>,
+    b: Vec<T>,
+) -> (Vec<T>, usize) {
+    let mut r = vec![];
+    let mut i = 0;
+    let mut j = 0;
+    let mut inversions = 0;
+
+    while i < a.len() && j < b.len() {
+        if a[i] < b[j] {
+            r.push(a[i]);
+            i += 1;
+        } else {
+            r.push(b[j]);
+            j += 1;
+            inversions += a.len() - i;
+        }
+    }
+
+    while i < a.len() {
+        r.push(a[i]);
+        i += 1;
+    }
+
+    while j < b.len() {
+        r.push(b[j]);
+        j += 1;
+    }
+
+    return (r, inversions);
+}
+
+pub fn merge_sort_inversions<T: Clone + PartialOrd + Copy + std::fmt::Debug>(
+    v: Vec<T>,
+) -> (Vec<T>, usize) {
+    if v.len() <= 1 {
+        return (v, 0);
+    }
+    let i = v.len() / 2;
+    let (a, n) = merge_sort_inversions((&v[..i]).to_vec());
+    let (b, m) = merge_sort_inversions((&v[i..]).to_vec());
+    let (r, p) = merge_count_inversions(a, b);
+    return (r, n + m + p);
+}