Implement Papadimitriou's algorithm

README.md

@@ -1,3 +1,7 @@
 # algos
 
-My solutions for the Coursera specialization algorithms in Rust.
+This repository contains my solutions to the Coursera Algorithms specialization
+by Tim Roughgarden from the Stanford University. I implemented the assignments
+in Rust to learn more about the language. I took the class to refresh my
+knowledge about algorithms.
+

data/c4a4_sat.txt

@@ -0,0 +1,5 @@
+4
+1 2
+-1 3
+3 4
+-2 -4

src/main.rs

@@ -9,6 +9,7 @@ mod knapsack;
 mod merge_sort;
 mod min_cut;
 mod mwis;
+mod papadimitriou;
 mod prims;
 mod quick_sort;
 mod ssc;
@@ -25,6 +26,7 @@ use crate::knapsack::knapsack;
 use crate::merge_sort::merge_sort_inversions;
 use crate::min_cut::min_cut;
 use crate::mwis::mwis;
+use crate::papadimitriou::sat2;
 use crate::prims::prims;
 use crate::quick_sort::quick_sort;
 use crate::ssc::ssc;
@@ -172,7 +174,12 @@ fn c4a3() {
 
 #[allow(dead_code)]
 fn c4a4() {
-    println!("we are done soon!");
+    for i in 1..7 {
+        let f = format!("data/c4a4_sat{}.txt", i);
+        let s = util::read_sat2(&f).unwrap();
+        let r = sat2(s);
+        println!("{} = {}", f, r);
+    }
 }
 
 fn main() {
@@ -188,6 +195,6 @@ fn main() {
     // c3a3();
     // c3a4();
     // c4a1();
-    c4a3();
-    // c4a4();
+    // c4a3();
+    c4a4();
 }

src/papadimitriou.rs

@@ -0,0 +1,57 @@
+use rand::Rng;
+
+#[derive(Debug)]
+pub struct Sat2 {
+    pub n_clauses: usize,
+    pub clauses: Vec<Clause>,
+}
+
+#[derive(Debug)]
+pub struct Clause {
+    pub v1: usize,
+    pub n1: bool,
+    pub v2: usize,
+    pub n2: bool,
+}
+
+fn get_random_assignment(n_clauses: usize) -> Vec<bool> {
+    let mut rng = rand::thread_rng();
+    let mut assignment = vec![false; n_clauses];
+    for i in 0..n_clauses {
+        assignment[i] = rng.gen_bool(0.5);
+    }
+    assignment
+}
+
+pub fn sat2(s: Sat2) -> bool {
+    let n_clauses: f64 = s.n_clauses as f64;
+    let outer_repeats: usize =  n_clauses.log(2.0).ceil() as usize;
+    let inner_repeats: usize = 2 * s.n_clauses * s.n_clauses;
+    let mut rng = rand::thread_rng();
+
+    for _ in 0..outer_repeats {
+        let mut a = get_random_assignment(s.n_clauses);
+        for _ in 0..inner_repeats {
+            let mut valid = true;
+            for c in &s.clauses {
+                if a[c.v1] == c.n1 || a[c.v2] == c.n2 {
+                    // println!("{:?} matches", c);
+                } else {
+                    // println!("{:?} doesn't match", c);
+                    let first = rng.gen_bool(0.5);
+                    if first {
+                        a[c.v1] = !a[c.v1];
+                    } else {
+                        a[c.v2] = !a[c.v2];
+                    }
+                    valid = false;
+                    break;
+                }
+            }
+            if valid {
+                return true;
+            }
+        }
+    }
+    false
+}

src/util.rs

@@ -6,6 +6,7 @@ use crate::k_clustering_big;
 use crate::knapsack;
 use crate::min_cut;
 use crate::mwis;
+use crate::papadimitriou;
 use crate::prims;
 use crate::ssc;
 use crate::tsp;
@@ -382,3 +383,35 @@ pub fn read_tsp_graph(path: &str) -> Result<tsp::Graph, io::Error> {
 
     Ok(g)
 }
+
+pub fn read_sat2(path: &str) -> Result<papadimitriou::Sat2, io::Error> {
+    let file = File::open(path)?;
+    let mut lines = BufReader::new(file).lines();
+    let line = lines.next().unwrap()?;
+    let n_clauses = line.parse().unwrap();
+
+    let mut s = papadimitriou::Sat2 {
+        n_clauses: n_clauses,
+        clauses: Vec::new(),
+    };
+
+    for line in lines {
+        let line = line?;
+        let mut it = line.split_whitespace();
+        let var1: i32 = it.next().unwrap().parse().unwrap();
+        let neg1 = var1 < 0;
+        let var2: i32 = it.next().unwrap().parse().unwrap();
+        let neg2 = var2 < 0;
+        let var1: usize = var1.abs() as usize;
+        let var2: usize = var2.abs() as usize;
+        let c = papadimitriou::Clause {
+            v1: var1 - 1,
+            n1: !neg1,
+            v2: var2 - 1,
+            n2: !neg2,
+        };
+        s.clauses.push(c);
+    }
+
+    Ok(s)
+}