From 9a8ffa64e1d948066be3f53b7b29a81de394b213 Mon Sep 17 00:00:00 2001
From: Felix Martin <mail@felixm.de>
Date: Sat, 28 Nov 2020 12:59:14 -0500
Subject: [PATCH] Finish assignment for course 1 week 4

---
 Cargo.toml     |  1 +
 src/main.rs    | 14 +++++++++++++-
 src/min_cut.rs | 51 ++++++++++++++++++++++++++++++++++++++++++++++++++
 src/util.rs    | 24 +++++++++++++++---------
 4 files changed, 80 insertions(+), 10 deletions(-)

diff --git a/Cargo.toml b/Cargo.toml
index 6839c06..0936d0f 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -7,3 +7,4 @@ edition = "2018"
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
 
 [dependencies]
+rand = "0.7.3"
\ No newline at end of file
diff --git a/src/main.rs b/src/main.rs
index 3b6a633..be80f40 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -1,9 +1,11 @@
 mod merge_sort;
 mod quick_sort;
+mod min_cut;
 mod util;
 
 use crate::util::read_to_graph;
 use crate::util::read_to_vector;
+use crate::min_cut::min_cut;
 use crate::quick_sort::quick_sort;
 use crate::merge_sort::merge_sort_inversions;
 
@@ -27,9 +29,19 @@ fn c1a3() {
     */
 }
 
+#[allow(dead_code)]
 fn c1a4() {
 	let g = read_to_graph("data/course_1_assignment_4.txt").unwrap();
-	println!("{:?}", g);
+	let mut smalles_min_cut = u32::MAX;
+	let iterations = g.nodes.len().pow(1);
+	for _ in 0..iterations {
+		let new_min_cut = min_cut(g.clone());
+		if new_min_cut < smalles_min_cut {
+			smalles_min_cut = new_min_cut;
+		}
+	}
+	/* 17 */
+	println!("course 1 assignment 4: {:?}", smalles_min_cut);
 }
 
 fn main() {
diff --git a/src/min_cut.rs b/src/min_cut.rs
index fd40910..db45f02 100644
--- a/src/min_cut.rs
+++ b/src/min_cut.rs
@@ -1,4 +1,55 @@
+use rand::Rng; 
+use std::convert::TryInto;
 
+#[derive(Debug, Clone)]
+pub struct Graph {
+    pub nodes: Vec<u32>,
+    pub edges: Vec<(u32, u32)>,
+}
 
+pub fn min_cut(mut g: Graph) -> u32 {
+	let mut rng = rand::thread_rng();
+	// println!("min_cut({:?})\n", g);
+	while g.nodes.len() > 2 {
+		// pick random edge (u, v)
+		let edge_index = rng.gen_range(0, g.edges.len());
+		let removed_edge = g.edges.swap_remove(edge_index);
 
+		// merge u and v into a single node u
+		let mut self_loops = vec![];
+		let u = removed_edge.0;
+		let v = removed_edge.1;
+		for i in 0..g.edges.len() {
+			// replace all v with u
+			if g.edges[i].0 == v {
+				g.edges[i].0 = u;
+			}
+			if g.edges[i].1 == v {
+				g.edges[i].1 = u;
+			}
+			// make sure lower node comes first
+			if g.edges[i].0 > g.edges[i].1 {
+				g.edges[i] = (g.edges[i].1, g.edges[i]. 0);
+			}
 
+			if g.edges[i].0 == g.edges[i].1 {
+				self_loops.push(i);
+			}
+		}
+
+		// remove v from nodes
+		g.nodes.pop(); // we only use the nodes.len()
+		// if let Some(i) = g.nodes.iter().position(|x| *x == v) {
+	 	//   	g.nodes.swap_remove(i);
+		// }
+
+		// remove self-loops
+		let mut self_loops_removed: usize = 0;
+		for i in self_loops {
+			g.edges.remove(i - self_loops_removed);
+			self_loops_removed += 1;
+		}
+	}
+	// return cut represented by the final two nodes
+	g.edges.len().try_into().unwrap()
+}
diff --git a/src/util.rs b/src/util.rs
index b4c5bfc..543a9f1 100644
--- a/src/util.rs
+++ b/src/util.rs
@@ -1,16 +1,10 @@
+use crate::min_cut::Graph;
 use std::fs::File;
 use std::io;
 use std::io::{BufRead, BufReader, Error, ErrorKind};
 use std::vec::Vec;
 
 
-#[derive(Debug)]
-pub struct Graph {
-    nodes: Vec<u32>,
-    edges: Vec<(u32, u32)>,
-}
-
-
 pub fn read_to_vector(path: &str) -> Result<Vec<i64>, io::Error> {
     let file = File::open(path)?;
     let br = BufReader::new(file);
@@ -34,8 +28,20 @@ pub fn read_to_graph(path: &str) -> Result<Graph, io::Error> {
 
     for line in br.lines() {
         let line = line?;
-        println!("{:?}", line);
-    }
+        let mut nodes = line.split_whitespace();
+        let current_node = nodes.next().unwrap().parse().unwrap();
+        g.nodes.push(current_node);
 
+        for neightbor in nodes {
+            let neightbor: u32 = neightbor.parse().unwrap();
+            // If the neighbor is smaller than the current node that means we
+            // have already added it when we iterated over that smaller node.
+            // Hence, we do not add the edge again.
+            if neightbor > current_node {
+                let edge = (current_node, neightbor);
+                g.edges.push(edge);
+            } 
+        }
+    }
     Ok(g)
 }
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