add HTTPHeaders.first(name:) (#37)

* add `HTTPHeaders.first(name:)`
* use explicit `UTF8View` instead of generic sequence of bytes

Author: MahdiBM

Sources/AWSLambdaEvents/Utils/HTTP.swift

@@ -12,11 +12,68 @@
 //
 //===----------------------------------------------------------------------===//
 
-// MARK: HTTPMethod
+// MARK: HTTPHeaders
 
 public typealias HTTPHeaders = [String: String]
 public typealias HTTPMultiValueHeaders = [String: [String]]
 
+extension HTTPHeaders {
+    /// Retrieves the first value for a given header-field / dictionary-key (`name`) from the block.
+    /// This method uses case-insensitive comparisons.
+    ///
+    /// - Parameter name: The header field name whose first value should be retrieved.
+    /// - Returns: The first value for the header field name.
+    public func first(name: String) -> String? {
+        guard !self.isEmpty else {
+            return nil
+        }
+
+        return self.first { header in header.0.isEqualCaseInsensitiveASCIIBytes(to: name) }?.1
+    }
+}
+
+extension String {
+    internal func isEqualCaseInsensitiveASCIIBytes(to: String) -> Bool {
+        self.utf8.compareCaseInsensitiveASCIIBytes(to: to.utf8)
+    }
+}
+
+extension String.UTF8View {
+    /// Compares the collection of `UInt8`s to a case insensitive collection.
+    ///
+    /// This collection could be get from applying the `UTF8View`
+    ///   property on the string protocol.
+    ///
+    /// - Parameter bytes: The string constant in the form of a collection of `UInt8`
+    /// - Returns: Whether the collection contains **EXACTLY** this array or no, but by ignoring case.
+    internal func compareCaseInsensitiveASCIIBytes(to: String.UTF8View) -> Bool {
+        // fast path: we can get the underlying bytes of both
+        let maybeMaybeResult = self.withContiguousStorageIfAvailable { lhsBuffer -> Bool? in
+            to.withContiguousStorageIfAvailable { rhsBuffer in
+                if lhsBuffer.count != rhsBuffer.count {
+                    return false
+                }
+
+                for idx in 0 ..< lhsBuffer.count {
+                    // let's hope this gets vectorised ;)
+                    if lhsBuffer[idx] & 0xDF != rhsBuffer[idx] & 0xDF {
+                        return false
+                    }
+                }
+                return true
+            }
+        }
+
+        if let maybeResult = maybeMaybeResult, let result = maybeResult {
+            return result
+        } else {
+            return self.elementsEqual(to, by: { ($0 & 0xDF) == ($1 & 0xDF) })
+        }
+    }
+}
+
+// MARK: HTTPMethod
+
 public struct HTTPMethod: RawRepresentable, Equatable {
     public var rawValue: String
 

Tests/AWSLambdaEventsTests/Utils/HTTPHeadersTests.swift

@@ -0,0 +1,31 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the SwiftAWSLambdaRuntime open source project
+//
+// Copyright (c) 2017-2020 Apple Inc. and the SwiftAWSLambdaRuntime project authors
+// Licensed under Apache License v2.0
+//
+// See LICENSE.txt for license information
+// See CONTRIBUTORS.txt for the list of SwiftAWSLambdaRuntime project authors
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+//===----------------------------------------------------------------------===//
+
+import AWSLambdaEvents
+import XCTest
+
+class HTTPHeadersTests: XCTestCase {
+    func first() throws {
+        let headers: HTTPHeaders = [
+            ":method": "GET",
+            "foo": "bar",
+            "custom-key": "value-1,value-2",
+        ]
+
+        XCTAssertEqual(headers.first(name: ":method"), "GET")
+        XCTAssertEqual(headers.first(name: "Foo"), "bar")
+        XCTAssertEqual(headers.first(name: "custom-key"), "value-1,value-2")
+        XCTAssertNil(headers.first(name: "not-present"))
+    }
+}