CSharpHttpServer/SimpleHttpServer/Types/PathTree.cs

using System.Data;
using System.Diagnostics.CodeAnalysis;

namespace SimpleHttpServer.Types;

internal class PathTree<T> where T : class {
    private readonly Node? rootNode = null;

    public PathTree() : this(new()) { }
    public PathTree(Dictionary<string, T> dict) {
        if (dict == null || dict.Count == 0)
            return;

        rootNode = new();
        var currNode = rootNode;
        var unpackedPaths = dict.Keys.Select(p => p.Split('/').ToArray()).ToArray();
        var unpackedLeafData = dict.Values.ToArray();
        for (int i = 0; i < unpackedPaths.Length; i++) {
            var path = unpackedPaths[i];
            var catchallidx = Array.IndexOf(path, "$*");
            if (catchallidx != -1 && catchallidx != path.Length - 1) {
                throw new Exception($"Found illegal catchall-wildcard in path: '{string.Join('/', path)}'");
            }

            var leafdata = unpackedLeafData[i];
            rootNode.AddSuccessor(path, leafdata);
        }
    }

    internal bool TryGetPath(string reqPath, [MaybeNullWhen(false)] out T? endpoint) {
        if (rootNode == null) {
            endpoint = null;
            return false;
        }

        // try to find path-match
        Node currNode = rootNode;
        Assert(reqPath[0] == '/');
        var splittedPath = reqPath[1..].Split("/");
        Node? lastCatchallNode = null;
        for (int i = 0; i < splittedPath.Length; i++) {

            // keep track of the current best catchallNode
            if (currNode.catchAllNext != null) {
                lastCatchallNode = currNode.catchAllNext;
            }

            var seg = splittedPath[i];
            if (currNode.next?.TryGetValue(seg, out var next) == true) { // look for an explicit path to follow greedily
                currNode = next;
            } else if (currNode.pathWildcardNext != null) { // otherwise look for a single-wildcard to follow
                currNode = currNode.pathWildcardNext;
            } else { // otherwise we are done, there is no valid path --> fall back to the most specific catchall
                endpoint = lastCatchallNode?.leafData;
                return lastCatchallNode != null;
            }
        }

        // return found path
        endpoint = currNode.leafData;
        return true;
    }

    private class Node {
        public T? leafData = null;
        public Dictionary<string, Node>? next = null;
        public Node? pathWildcardNext = null; // path wildcard
        public Node? catchAllNext = null; // trailing-catchall wildcard

        public void AddSuccessor(string[] segments, T? newLeafData) {
            if (segments.Length == 0) { // actually add the data to this node
                Assert(leafData == null);
                leafData = newLeafData;
                return;
            }

            var seg = segments[0];
            bool newIsWildcard = seg.Length > 1 && seg[0] == '$';
            if (newIsWildcard) {
                bool newIsCatchallWildcard = newIsWildcard && seg.Length == 2 && seg[1] == '*';
                if (newIsCatchallWildcard) { // this is a catchall wildcard
                    Assert(catchAllNext == null);
                    catchAllNext = new();
                    catchAllNext.AddSuccessor(segments[1..], newLeafData);
                    return;
                } else { // must be single wildcard otherwise
                    Assert(pathWildcardNext == null);
                    pathWildcardNext = new();
                    pathWildcardNext.AddSuccessor(segments[1..], newLeafData);
                    return;
                }
            }

            // otherwise we want to add a new constant path successor
            if (next == null) {
                next = new();
            }

            if (next.TryGetValue(seg, out var existingNode)) {
                existingNode.AddSuccessor(segments[1..], newLeafData);
            } else {
                var newNode = next[seg] = new();
                newNode.AddSuccessor(segments[1..], newLeafData);
            }
        }
    }
}