Add getNeighbors method (#4)

.vscode/launch.json

@@ -0,0 +1,22 @@
+{
+    "configurations": [
+        {
+            "type": "lldb",
+            "request": "launch",
+            "args": [],
+            "cwd": "${workspaceFolder:GeoHashSwift}",
+            "name": "Debug geohash",
+            "program": "${workspaceFolder:GeoHashSwift}/.build/debug/geohash",
+            "preLaunchTask": "swift: Build Debug geohash"
+        },
+        {
+            "type": "lldb",
+            "request": "launch",
+            "args": [],
+            "cwd": "${workspaceFolder:GeoHashSwift}",
+            "name": "Release geohash",
+            "program": "${workspaceFolder:GeoHashSwift}/.build/release/geohash",
+            "preLaunchTask": "swift: Build Release geohash"
+        }
+    ]
+}

.vscode/settings.json

@@ -13,4 +13,14 @@
     "editor.defaultFormatter": "vknabel.vscode-apple-swift-format",
     "cSpell.words": [],
     "makefile.configureOnOpen": false,
+    "swift.sourcekit-lsp.serverArguments": [
+        "-Xswiftc",
+        "-sdk",
+        "-Xswiftc",
+        "/Applications/Xcode-16.1.0.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneOS.sdk",
+        "-Xswiftc",
+        "-target",
+        "-Xswiftc",
+        "arm64-apple-ios18.1-simulator",
+    ]
 }

Sources/GeoHashFramework/GeoHash.swift

@@ -154,7 +154,7 @@ extension GeoHash {
     }
 }
 
-// MARK: Public getter
+// MARK: Public properties and methods
 extension GeoHash {
     public var latitudeBits: String {
         var lat: String = ""
@@ -209,6 +209,60 @@ extension GeoHash {
         return hash
     }
 
+    public func getNeighbors() -> [GeoHash] {
+        let latitudeBits = self.latitudeBits
+        let longitudeBits = self.longitudeBits
+
+        let north = add(bits: latitudeBits, by: 1)
+        let south = add(bits: latitudeBits, by: -1)
+        let east = add(bits: longitudeBits, by: 1)
+        let west = add(bits: longitudeBits, by: -1)
+
+        let northEast = combineBits(latitude: north, longitude: east)
+        let northWest = combineBits(latitude: north, longitude: west)
+        let southEast = combineBits(latitude: south, longitude: east)
+        let southWest = combineBits(latitude: south, longitude: west)
+
+        return [
+            GeoHash(binary: combineBits(latitude: north, longitude: longitudeBits), precision: precision),
+            GeoHash(binary: northEast, precision: precision),
+            GeoHash(binary: combineBits(latitude: latitudeBits, longitude: east), precision: precision),
+            GeoHash(binary: southEast, precision: precision),
+            GeoHash(binary: combineBits(latitude: south, longitude: longitudeBits), precision: precision),
+            GeoHash(binary: southWest, precision: precision),
+            GeoHash(binary: combineBits(latitude: latitudeBits, longitude: west), precision: precision),
+            GeoHash(binary: northWest, precision: precision)
+        ]
+    }
+
+    /// Add `delta` to `bits`
+    private func add(bits: String, by delta: Int) -> String {
+        if let decimal = Int(bits, radix: 2) {
+            let moved = decimal + delta
+            // 11 -> 1110
+            return String(moved, radix: 2)
+        }
+        return bits
+    }
+
+    /// Combine `latitude` and `longitude` in bits.
+    private func combineBits(latitude: String, longitude: String) -> String {
+        var result = ""
+        let latArray = Array(latitude)
+        let lngArray = Array(longitude)
+        let maxLength = max(latArray.count, lngArray.count)
+
+        for i in 0..<maxLength {
+            if i < lngArray.count {
+                result.append(lngArray[i])
+            }
+            if i < latArray.count {
+                result.append(latArray[i])
+            }
+        }
+        return result
+    }
+
     public static func getBound(with precision: GeoHashBitsPrecision) -> [GeoHashCoordinate2D] {
         // Initial: BottomLeft in zoom-level 0
         let baseGeoCoordinate = GeoHash(

Tests/GeoHashFrameworkTests/GeoHashTests.swift

@@ -85,4 +85,26 @@ struct GeoHashTests {
         }
         print(count == 131072)  // 2^15 * 4
     }
+
+    @Test
+    func getNeighbors() async throws {
+        // Tokyo Station
+        let lat = 35.681382
+        let lng = 139.766084
+
+        let expected = [
+            "xn76urws",
+            "xn76urwu",
+            "xn76urwg",
+            "xn76urwf",
+            "xn76urwd",
+            "xn76urw6",
+            "xn76urw7",
+            "xn76urwk"
+        ]
+
+        let geoHash = GeoHash(latitude: lat, longitude: lng)
+        let neighbors = geoHash.getNeighbors()
+        #expect(neighbors.map(\.geoHash) == expected)
+    }
 }