Add dHash tests

Tests/PhotochkTests/DHashTests.swift

@@ -0,0 +1,58 @@
+import Foundation
+import Testing
+
+@testable import Photochk
+
+@Suite("dHash")
+struct DHashTests {
+    @Test("同じ値のハミング距離は 0")
+    func identical() {
+        #expect(DHash.hammingDistance(0xDEAD_BEEF_1234_5678, 0xDEAD_BEEF_1234_5678) == 0)
+    }
+
+    @Test("違うビットの数がそのまま距離になる")
+    func differingBits() {
+        #expect(DHash.hammingDistance(0, 0b1011) == 3)
+        #expect(DHash.hammingDistance(.max, 0) == 64)
+        #expect(DHash.hammingDistance(0b1100, 0b0110) == 2)
+    }
+
+    @Test("生成した画像から 64bit のハッシュが取れる")
+    func computesFromImage() throws {
+        let directory = try TestImages.makeTemporaryDirectory()
+        defer { try? FileManager.default.removeItem(at: directory) }
+
+        let url = directory.appendingPathComponent("base.jpg")
+        try TestImages.writeJPEG(TestImages.pattern(seed: 1), to: url, quality: 0.9)
+
+        let basics = try #require(DHash.compute(for: url))
+        #expect(basics.pixelWidth == TestImages.width)
+        #expect(basics.pixelHeight == TestImages.height)
+
+        // 同じファイルからは必ず同じ値が出る
+        let again = try #require(DHash.compute(for: url))
+        #expect(basics.dHash == again.dHash)
+    }
+
+    @Test("リサイズしてもハッシュはほとんど変わらず、別の絵柄とは大きく離れる")
+    func robustToResize() throws {
+        let directory = try TestImages.makeTemporaryDirectory()
+        defer { try? FileManager.default.removeItem(at: directory) }
+
+        let base = directory.appendingPathComponent("base.jpg")
+        let small = directory.appendingPathComponent("small.jpg")
+        let other = directory.appendingPathComponent("other.jpg")
+
+        let pattern = TestImages.pattern(seed: 7)
+        try TestImages.writeJPEG(pattern, to: base, quality: 0.9)
+        try TestImages.writeJPEG(TestImages.scaled(pattern, factor: 0.5), to: small, quality: 0.8)
+        try TestImages.writeJPEG(TestImages.pattern(seed: 99), to: other, quality: 0.9)
+
+        let baseHash = try #require(DHash.compute(for: base)).dHash
+        let smallHash = try #require(DHash.compute(for: small)).dHash
+        let otherHash = try #require(DHash.compute(for: other)).dHash
+
+        #expect(DHash.hammingDistance(baseHash, smallHash) <= 12)
+        #expect(DHash.hammingDistance(baseHash, otherHash) > 12)
+    }
+}