Add homework 3 solution for CS325

code/cs325-langs/sols/hw3.lang

@@ -0,0 +1,95 @@
+function qselect(xs, k, c) {
+    if xs == [] {
+        return 0;
+    }
+
+    traverser bisector(list: xs, span: (0,len(xs)));
+    traverser pivot(list: xs, random: true);
+
+    let pivotE = pop!(pivot);
+    let (leftList, rightList) = bisect!(bisector, (x) -> c(x) < c(pivotE));
+    
+    if k > len(leftList) + 1 {
+        return qselect(rightList, k - len(leftList) - 1, c);
+    } elsif k == len(leftList) + 1 {
+        return pivotE;
+    } else {
+        return qselect(leftList, k, c);
+    }
+}
+
+function closestUnsorted(xs, k, n) {
+    let min = qselect(list(xs), k, (x) -> abs(x - n));
+    let out = [];
+    let countEqual = k;
+
+    traverser iter(list: xs, span: (0, len(xs)));
+    while valid!(iter) {
+        if abs(at!(iter)-n) < abs(min-n) {
+            let countEqual = countEqual - 1;
+        }
+        step!(iter);
+    }
+
+    traverser iter(list: xs, span: (0, len(xs)));
+    while valid!(iter) {
+        if abs(at!(iter)-n) == abs(min-n) and countEqual > 0 {
+            let countEqual = countEqual - 1;
+            let out = out + [at!(iter)];
+        } elsif abs(at!(iter)-n) < abs(min-n) {
+            let out = out + [at!(iter)];
+        }
+        step!(iter);
+    }
+
+    return out;
+}
+
+function closestSorted(xs, k, n) {
+    let start = bisect(xs, n);
+    let counter = 0;
+    traverser left(list: xs, span: (0, start), reverse: true);
+    traverser right(list: xs, span: (start, len(xs)));
+
+    while counter != k and canstep!(left) and valid!(right) {
+        if abs(at!(left, 1) - n) < abs(at!(right) - n) {
+            step!(left);
+        } else {
+            step!(right);
+        }
+        let counter = counter + 1;
+    }
+
+    while counter != k and (canstep!(left) or valid!(right)) {
+        if canstep!(left) { step!(left); }
+        else { step!(right); }
+        let counter = counter + 1;
+    }
+
+    return subset!(left, right);
+}
+
+sorted function xyz(xs, k) {
+    traverser x(list: xs, span: (0,len(xs)));
+    let dest = [];
+
+    while valid!(x) {
+        traverser z(list: xs, span: (pos!(x)+2,len(xs)));
+        traverser y(list: xs, span: (pos!(x)+1,pos!(z)));
+
+        while valid!(y) and valid!(z) {
+            if at!(x) + at!(y) == at!(z) {
+                let dest = dest + [(at!(x), at!(y), at!(z))];
+                step!(z);
+            } elsif at!(x) + at!(y) > at!(z) {
+                step!(z);
+            } else {
+                step!(y);
+            }
+        }
+
+        step!(x);
+    }
+
+    return dest;
+}