import Foundation

/// xxHash64 (seed 0) — pure Swift, streaming. Spec: github.com/Cyan4973/xxHash.
public struct XXHash64 {
    private static let p1: UInt64 = 0x9E3779B185EBCA87
    private static let p2: UInt64 = 0xC2B2AE3D27D4EB4F
    private static let p3: UInt64 = 0x165667B19E3779F9
    private static let p4: UInt64 = 0x85EBCA77C2B2AE63
    private static let p5: UInt64 = 0x27D4EB2F165667C5

    private var v1: UInt64
    private var v2: UInt64
    private var v3: UInt64
    private var v4: UInt64
    private var buffer = [UInt8]()
    private var totalLength: UInt64 = 0
    private let seed: UInt64

    public init(seed: UInt64 = 0) {
        self.seed = seed
        v1 = seed &+ Self.p1 &+ Self.p2
        v2 = seed &+ Self.p2
        v3 = seed
        v4 = seed &- Self.p1
    }

    @inline(__always)
    private static func rotl(_ x: UInt64, _ r: UInt64) -> UInt64 {
        (x << r) | (x >> (64 - r))
    }

    @inline(__always)
    private static func round(_ acc: UInt64, _ input: UInt64) -> UInt64 {
        rotl(acc &+ input &* p2, 31) &* p1
    }

    @inline(__always)
    private static func mergeRound(_ acc: UInt64, _ val: UInt64) -> UInt64 {
        (acc ^ round(0, val)) &* p1 &+ p4
    }

    @inline(__always)
    private static func readLE64(_ bytes: [UInt8], _ offset: Int) -> UInt64 {
        var v: UInt64 = 0
        for i in 0..<8 {
            v |= UInt64(bytes[offset + i]) << (8 * UInt64(i))
        }
        return v
    }

    @inline(__always)
    private static func readLE32(_ bytes: [UInt8], _ offset: Int) -> UInt64 {
        var v: UInt64 = 0
        for i in 0..<4 {
            v |= UInt64(bytes[offset + i]) << (8 * UInt64(i))
        }
        return v
    }

    public mutating func update(_ data: Data) {
        totalLength += UInt64(data.count)
        buffer.append(contentsOf: data)
        // Consume full 32-byte stripes, keep remainder buffered.
        var offset = 0
        while buffer.count - offset >= 32 {
            v1 = Self.round(v1, Self.readLE64(buffer, offset))
            v2 = Self.round(v2, Self.readLE64(buffer, offset + 8))
            v3 = Self.round(v3, Self.readLE64(buffer, offset + 16))
            v4 = Self.round(v4, Self.readLE64(buffer, offset + 24))
            offset += 32
        }
        if offset > 0 {
            buffer.removeFirst(offset)
        }
    }

    public func finalize() -> UInt64 {
        var h: UInt64
        if totalLength >= 32 {
            h = Self.rotl(v1, 1) &+ Self.rotl(v2, 7) &+ Self.rotl(v3, 12) &+ Self.rotl(v4, 18)
            h = Self.mergeRound(h, v1)
            h = Self.mergeRound(h, v2)
            h = Self.mergeRound(h, v3)
            h = Self.mergeRound(h, v4)
        } else {
            h = seed &+ Self.p5
        }
        h &+= totalLength

        var offset = 0
        let tail = buffer
        while tail.count - offset >= 8 {
            h ^= Self.round(0, Self.readLE64(tail, offset))
            h = Self.rotl(h, 27) &* Self.p1 &+ Self.p4
            offset += 8
        }
        if tail.count - offset >= 4 {
            h ^= Self.readLE32(tail, offset) &* Self.p1
            h = Self.rotl(h, 23) &* Self.p2 &+ Self.p3
            offset += 4
        }
        while offset < tail.count {
            h ^= UInt64(tail[offset]) &* Self.p5
            h = Self.rotl(h, 11) &* Self.p1
            offset += 1
        }

        h ^= h >> 33
        h &*= Self.p2
        h ^= h >> 29
        h &*= Self.p3
        h ^= h >> 32
        return h
    }

    /// One-shot.
    public static func hash(_ data: Data, seed: UInt64 = 0) -> UInt64 {
        var h = XXHash64(seed: seed)
        h.update(data)
        return h.finalize()
    }

    public static func hexString(_ value: UInt64) -> String {
        String(format: "%016lx", value)
    }
}