From 09a38929d200e46927cd105da0e640945a290224 Mon Sep 17 00:00:00 2001
From: yoshikazu <yoshikazu@yfox.jp>
Date: Mon, 27 Jul 2026 15:38:03 +0000
Subject: [PATCH] Add UnionFind

---
 Sources/Photochk/Core/UnionFind.swift | 50 +++++++++++++++++++++++++++
 1 file changed, 50 insertions(+)
 create mode 100644 Sources/Photochk/Core/UnionFind.swift

diff --git a/Sources/Photochk/Core/UnionFind.swift b/Sources/Photochk/Core/UnionFind.swift
new file mode 100644
index 0000000..b7b9812
--- /dev/null
+++ b/Sources/Photochk/Core/UnionFind.swift
@@ -0,0 +1,50 @@
+import Foundation
+
+/// 経路圧縮 + ランク併合の Union-Find。類似ペアから連結成分（グループ）を作るのに使う。
+struct UnionFind {
+    private var parent: [Int]
+    private var rank: [Int]
+
+    init(count: Int) {
+        parent = Array(0..<count)
+        rank = Array(repeating: 0, count: count)
+    }
+
+    mutating func find(_ x: Int) -> Int {
+        var root = x
+        while parent[root] != root { root = parent[root] }
+        // 経路圧縮
+        var node = x
+        while parent[node] != root {
+            let next = parent[node]
+            parent[node] = root
+            node = next
+        }
+        return root
+    }
+
+    @discardableResult
+    mutating func union(_ a: Int, _ b: Int) -> Bool {
+        let ra = find(a)
+        let rb = find(b)
+        guard ra != rb else { return false }
+        if rank[ra] < rank[rb] {
+            parent[ra] = rb
+        } else if rank[ra] > rank[rb] {
+            parent[rb] = ra
+        } else {
+            parent[rb] = ra
+            rank[ra] += 1
+        }
+        return true
+    }
+
+    /// 連結成分を、根のインデックスをキーにしてまとめて返す。
+    mutating func components() -> [[Int]] {
+        var buckets: [Int: [Int]] = [:]
+        for i in parent.indices {
+            buckets[find(i), default: []].append(i)
+        }
+        return Array(buckets.values)
+    }
+}