(a,b)-tree

04-ab_tree/cpp/Makefile

+test: ab_tree_test
+	./$<
+
+CXXFLAGS=-std=c++11 -O2 -Wall -Wextra -g -Wno-sign-compare
+
+ab_tree_test: ab_tree_test.cpp ab_tree.h test_main.cpp
+	$(CXX) $(CXXFLAGS) $^ -o $@
+
+clean:
+	rm -f ab_tree_test
+
+.PHONY: clean test

04-ab_tree/cpp/ab_tree.h

+#include <limits>
+#include <vector>
+#include <iostream>
+
+using namespace std;
+
+// If the condition is not true, report an error and halt.
+#define EXPECT(condition, message) do { if (!(condition)) expect_failed(message); } while (0)
+
+void expect_failed(const string& message);
+
+/*** One node ***/
+
+class ab_node {
+  public:
+    // Keys stored in this node and the corresponding children
+    // The vectors are large enough to accomodate one extra entry
+    // in overflowing nodes.
+    vector<ab_node *> children;
+    vector<int> keys;
+
+    // If this node contains the given key, return true and set i to key's position.
+    // Otherwise return false and set i to the first key greater than the given one.
+    bool find_branch(int key, int &i)
+    {
+        i = 0;
+        while (i < keys.size() && keys[i] <= key) {
+            if (keys[i] == key)
+                return true;
+            i++;
+        }
+        return false;
+    }
+
+    // Insert a new key at posision i and add a new child between keys i and i+1.
+    void insert_branch(int i, int key, ab_node *child)
+    {
+        keys.insert(keys.begin() + i, key);
+        children.insert(children.begin() + i + 1, child);
+    }
+
+    // An auxiliary function for displaying a sub-tree under this node.
+    void show(int indent);
+};
+
+/*** Tree ***/
+
+class ab_tree {
+  public:
+    int a;          // Minimum allowed number of children
+    int b;          // Maximum allowed number of children
+    ab_node *root;  // Root node (even a tree with no keys has a root)
+    int num_nodes;  // We keep track of how many nodes the tree has
+
+    // Create a new node and return a pointer to it.
+    ab_node *new_node()
+    {
+        ab_node *n = new ab_node;
+        n->keys.reserve(b);
+        n->children.reserve(b+1);
+        num_nodes++;
+        return n;
+    }
+
+    // Delete a given node, assuming that its children have been already unlinked.
+    void delete_node(ab_node *n)
+    {
+        num_nodes--;
+        delete n;
+    }
+
+    // Constructor: initialize an empty tree with just the root.
+    ab_tree(int a, int b)
+    {
+        EXPECT(a >= 2 && b >= 2*a - 1, "Invalid values of a,b");
+        this->a = a;
+        this->b = b;
+        num_nodes = 0;
+        // The root has no keys and one null child pointer.
+        root = new_node();
+        root->children.push_back(nullptr);
+    }
+
+    // An auxiliary function for deleting a subtree recursively.
+    void delete_tree(ab_node *n)
+    {
+        for (int i=0; i < n->children.size(); i++)
+            if (n->children[i])
+                delete_tree(n->children[i]);
+        delete_node(n);
+    }
+
+    // Destructor: delete all nodes.
+    ~ab_tree()
+    {
+        delete_tree(root);
+        EXPECT(num_nodes == 0, "Memory leak detected: some nodes were not deleted");
+    }
+
+    // Find a key: returns true if it is present in the tree.
+    bool find(int key)
+    {
+        ab_node *n = root;
+        while (n) {
+            int i;
+            if (n->find_branch(key, i))
+                return true;
+            n = n->children[i];
+        }
+        return false;
+    }
+
+    // Display the tree on standard output in human-readable form.
+    void show();
+
+    // Check that the data structure satisfies all invariants.
+    void audit();
+
+    // Insert: add key to the tree (unless it was already present).
+    void insert(int key)
+    {
+        // FIXME: Implement
+    }
+};

04-ab_tree/cpp/ab_tree_test.cpp

+#include <functional>
+#include <cstdlib>
+#include <vector>
+
+#include "ab_tree.h"
+
+// Debugging output: showing trees prettily on standard output.
+
+void ab_tree::show()
+{
+    root->show(0);
+    for (int i=0; i<70; i++)
+        cout << '=';
+    cout << endl;
+}
+
+void ab_node::show(int indent)
+{
+    for (int i = children.size() - 1; i >= 0 ; i--) {
+        if (i < keys.size()) {
+            for (int j = 0; j < indent; j++)
+                cout << "    ";
+            cout << keys[i] << endl;
+        }
+        if (children[i])
+            children[i]->show(indent+1);
+    }
+}
+
+// Invariant checks
+
+void audit_subtree(ab_tree *tree, ab_node *n, int key_min, int key_max, int depth, int &leaf_depth)
+{
+    if (!n) {
+        // Check that all leaves are on the same level.
+        if (leaf_depth < 0)
+            leaf_depth = depth;
+        else
+            EXPECT(depth == leaf_depth, "Leaves are not on the same level");
+        return;
+    }
+
+    // The number of children must be in the allowed range.
+    if (depth > 0)
+        EXPECT(n->children.size() >= tree->a, "Too few children");
+    EXPECT(n->children.size() <= tree->b, "Too many children");
+
+    // We must have one more children than keys.
+    EXPECT(n->children.size() == n->keys.size() + 1, "Number of keys does not match number of children");
+
+    // Allow degenerate trees with 0 keys in the root.
+    if (n->children.size() == 1)
+        return;
+
+    // Check order of keys: they must be increasing and bounded by the keys on the higher levels.
+    for (int i = 0; i < n->keys.size(); i++) {
+        EXPECT(n->keys[i] >= key_min && n->keys[i] <= key_max, "Wrong key order");
+        EXPECT(i == 0 || n->keys[i-1] < n->keys[i], "Wrong key order");
+    }
+
+    // Call on children recursively.
+    for (int i = 0; i < n->children.size(); i++) {
+        int tmin, tmax;
+        if (i == 0)
+            tmin = key_min;
+        else
+            tmin = n->keys[i-1] + 1;
+        if (i < n->keys.size())
+            tmax = n->keys[i] - 1;
+        else
+            tmax = key_max;
+        audit_subtree(tree, n->children[i], tmin, tmax, depth+1, leaf_depth);
+    }
+}
+
+void ab_tree::audit()
+{
+    EXPECT(root, "Tree has no root");
+    int leaf_depth = -1;
+    audit_subtree(this, root, numeric_limits<int>::min(), numeric_limits<int>::max(), 0, leaf_depth);
+}
+
+// A basic test: insert a couple of keys and show how the tree evolves.
+
+void test_basic()
+{
+    cout << "## Basic test" << endl;
+
+    ab_tree t(2, 3);
+    vector<int> keys = { 3, 1, 4, 5, 9, 2, 6, 8, 7, 0 };
+    for (int k : keys) {
+        t.insert(k);
+        t.show();
+        t.audit();
+        EXPECT(t.find(k), "Inserted key disappeared");
+    }
+
+    for (int k : keys)
+        EXPECT(t.find(k), "Some keys are missing at the end");
+}
+
+// The main test: inserting a lot of keys and checking that they are really there.
+// We will insert num_items keys from the set {1,...,range-1}, where range is a prime.
+
+void test_main(int a, int b, int range, int num_items)
+{
+    // Create a new tree.
+    cout << "## Test: a=" << a << " b=" << b << " range=" << range << " num_items=" << num_items << endl;
+    ab_tree t(a, b);
+
+    int key = 1;
+    int step = (int)(range * 1.618);
+    int audit_time = 1;
+
+    // Insert keys.
+    for (int i=1; i <= num_items; i++) {
+        t.insert(key);
+        // Audit the tree occasionally.
+        if (i == audit_time || i == num_items) {
+            // cout << "== Audit at " << i << endl;
+            // t.show();
+            t.audit();
+            audit_time = (int)(audit_time * 1.33) + 1;
+        }
+        key = (key + step) % range;
+    }
+
+    // Check that the tree contains exactly the items it should contain.
+    key = 1;
+    for (int i=1; i < range; i++) {
+        bool found = t.find(key);
+        // cout << "Step #" << i << ": find(" << key << ") = " << found << endl;
+        EXPECT(found == (i <= num_items), "Tree contains wrong keys");
+        key = (key + step) % range;
+    }
+}
+
+/*** A list of all tests ***/
+
+vector<pair<string, function<void()>>> tests = {
+    { "basic",       [] { test_basic(); } },
+    { "small-2,3",   [] { test_main(2, 3, 997, 700); } },
+    { "small-2,4",   [] { test_main(2, 4, 997, 700); } },
+    { "big-2,3",     [] { test_main(2, 3, 999983, 700000); } },
+    { "big-2,4",     [] { test_main(2, 4, 999983, 700000); } },
+    { "big-10,20",   [] { test_main(10, 20, 999983, 700000); } },
+    { "big-100,200", [] { test_main(100, 200, 999983, 700000); } },
+};

04-ab_tree/cpp/test_main.cpp

+#include <cstdlib>
+#include <functional>
+#include <iostream>
+#include <string>
+#include <utility>
+#include <vector>
+
+using namespace std;
+
+extern vector<pair<string, function<void()>>> tests;
+
+void expect_failed(const string& message) {
+    cerr << "Test error: " << message << endl;
+    exit(1);
+}
+
+int main(int argc, char* argv[]) {
+    vector<string> required_tests;
+
+    if (argc > 1) {
+        required_tests.assign(argv + 1, argv + argc);
+    } else {
+        for (const auto& test : tests)
+            required_tests.push_back(test.first);
+    }
+
+    for (const auto& required_test : required_tests) {
+        bool found = false;
+        for (const auto& test : tests)
+            if (required_test == test.first) {
+                cerr << "Running test " << required_test << endl;
+                test.second();
+                found = true;
+                break;
+            }
+        if (!found) {
+            cerr << "Unknown test " << required_test << endl;
+            return 1;
+        }
+    }
+
+    return 0;
+}

04-ab_tree/python/ab_tree.py

+#!/usr/bin/env python3
+
+class ABNode:
+    """Single node in an ABTree.
+
+    Each node contains keys and childrens
+    (with one more children than there are keys).
+    """
+    def __init__(self, keys = None, children = None):
+        self.keys = keys if keys is not None else []
+        self.children = children if children is not None else []
+
+    def find_branch(self, key):
+        """ Try finding given key in this node.
+
+        If this node contains the given key, returns (True, key_position).
+        If not, returns (False, first_position_with_key_greater_than_the_given).
+        """
+        i = 0
+        while (i < len(self.keys) and self.keys[i] < key):
+            i += 1
+
+        return (i < len(self.keys) and self.keys[i] == key, i)
+
+    def insert_branch(self, i, key, child):
+        """ Insert a new key and a given child between keys i and i+1."""
+        self.keys.insert(i, key)
+        self.children.insert(i + 1, child)
+
+class ABTree:
+    """A class representing the whole ABTree."""
+    def __init__(self, a, b):
+        assert a >= 2 and b >= 2 * a - 1, "Invalid values of a, b: {}, {}".format(a, b)
+        self.a = a
+        self.b = b
+        self.root = ABNode(children=[None])
+
+    def find(self, key):
+        """Find a key in the tree.
+
+        Returns True if the key is present, False otherwise.
+        """
+        node = self.root
+        while node:
+            found, i = node.find_branch(key)
+            if found: return True
+            node = node.children[i]
+        return False
+
+    def insert(self, key):
+        """Add a given key to the tree, unless already present."""
+        # TODO: Implement
+        raise NotImplementedError

04-ab_tree/python/ab_tree_test.py

+#!/usr/bin/env python3
+import math
+import sys
+
+from ab_tree import ABNode, ABTree
+
+def show(tree):
+    """Show a tree."""
+    def show_node(node, indent):
+        for i in reversed(range(len(node.children))):
+            if i < len(node.keys):
+                print("    " * indent, node.keys[i], sep="")
+            if node.children[i]:
+                show_node(node.children[i], indent + 1)
+
+    show_node(tree.root, 0)
+    print("=" * 70)
+
+def audit(tree):
+    """Invariant check for the given tree."""
+    def audit_node(node, key_min, key_max, depth, leaf_depth):
+        if not node:
+            # Check that all leaves are on the same level.
+            if leaf_depth is None:
+                leaf_depth = depth
+            assert depth == leaf_depth, "Leaves are not on the same level"
+
+        else:
+            # The number of children must be in the allowed range.
+            assert depth == 0 or len(node.children) >= tree.a, "Too few children"
+            assert len(node.children) <= tree.b, "Too many children"
+
+            # We must have one more children than keys
+            assert len(node.children) == len(node.keys) + 1, "Number of keys does not match number of children"
+
+            # Check that keys are increasing and in (key_min, key_max) range.
+            for i in range(len(node.keys)):
+                assert node.keys[i] > key_min and node.keys[i] < key_max, "Wrong key order"
+                assert i == 0 or node.keys[i - 1] < node.keys[i], "Wrong key order"
+
+            # Check children recursively
+            for i in range(len(node.children)):
+                child_min = node.keys[i - 1] if i > 0 else key_min
+                child_max = node.keys[i] if i < len(node.keys) else key_max
+                leaf_depth = audit_node(node.children[i], child_min, child_max, depth + 1, leaf_depth)
+
+        return leaf_depth
+
+    assert tree.root, "Tree has no root"
+    audit_node(tree.root, -math.inf, math.inf, 0, None)
+
+def test_basic():
+    """Insert a couple of keys and show how the tree evolves."""
+    print("## Basic test")
+
+    tree = ABTree(2, 3)
+    keys = [3, 1, 4, 5, 9, 2, 6, 8, 7, 0]
+    for key in keys:
+        tree.insert(key)
+        show(tree)
+        audit(tree)
+        assert tree.find(key), "Inserted key disappeared"
+
+    for key in keys:
+        assert tree.find(key), "Some keys are missing at the end"
+
+def test_main(a, b, limit, num_items):
+    print("## Test: a={} b={} range={} num_items={}".format(a, b, limit, num_items))
+
+    tree = ABTree(a, b)
+
+    # Insert keys
+    step = int(limit * 1.618)
+    key, audit_time = 1, 1
+    for i in range(num_items):
+        tree.insert(key)
+        key = (key + step) % limit
+
+        # Audit the tree occasionally
+        if i == audit_time or i + 1 == num_items:
+            audit(tree)
+            audit_time = int(audit_time * 1.33) + 1
+
+    # Check the content of the tree
+    key = 1
+    for i in range(limit):
+        assert tree.find(key) == (i < num_items), "Tree contains wrong keys"
+        key = (key + step) % limit
+
+tests = [
+    ("basic", test_basic),
+    ("small-2,3", lambda: test_main(2, 3, 997, 700)),
+    ("small-2,4", lambda: test_main(2, 4, 997, 700)),
+    ("big-2,3", lambda: test_main(2, 3, 99991, 70000)),
+    ("big-2,4", lambda: test_main(2, 4, 99991, 70000)),
+    ("big-10,20", lambda: test_main(10, 20, 99991, 70000)),
+    ("big-100,200", lambda: test_main(100, 200, 99991, 70000)),
+]
+
+if __name__ == "__main__":
+    for required_test in sys.argv[1:] or [name for name, _ in tests]:
+        for name, test in tests:
+            if name == required_test:
+                print("Running test {}".format(name), file=sys.stderr)
+                test()
+                break
+        else:
+            raise ValueError("Unknown test {}".format(name))

04-ab_tree/task.md

+You are given a representation of _(a, b)-tree_ with a `find` operation,
+and a representation of an _(a, b)-tree node_.
+
+Your goal is to implement an `insert` operation, which inserts the given
+key in the tree (or does nothing if the key is already present).
+
+You should submit the `ab_tree.*` file (but not `ab_tree_test.*` files).