CSV parsing example

Author: chriseidhof

benchmark/single-source/CSVParsing.swift

@@ -0,0 +1,144 @@
+struct ParseError: Error {
+    var message: String
+}
+
+let comma = ",".utf16.first!
+let newline = "\n".utf16.first!
+let carriageReturn = "\n".utf16.first!
+let quote = "\"".utf16.first!
+
+@inline(__always) func parseQuotedField(_ remainder: inout Substring) throws -> Substring? {
+    var result: Substring = "" // we accumulate the result
+    
+    while !remainder.isEmpty {
+        guard let nextQuoteIndex = remainder.index(of: "\"") else {
+            throw ParseError(message: "Expected a closing \"")
+        }
+        
+        // Append until the next quote
+        result += remainder.prefix(upTo: nextQuoteIndex)
+        remainder.remove(upToAndIncluding: nextQuoteIndex)
+        
+        if let peek = remainder.utf16.first {
+            switch peek {
+            case quote: // two quotes after each other is an escaped quote
+                remainder.removeFirst()
+                result.append("\"")
+            case comma: // field ending
+                remainder.removeFirst()
+                return result
+            default:
+                return result
+            }
+        } else {
+            // End of the string
+            return result
+        }
+    }
+    
+    throw ParseError(message: "Expected a closing quote")
+}
+
+// Consume a single field from `remainder`
+@inline(__always) func parseField(_ remainder: inout Substring) throws -> Substring? {
+    guard let start = remainder.utf16.first else { return nil }
+    switch start {
+    case quote:
+        remainder.removeFirst() // remove the first quote
+        return try parseQuotedField(&remainder)
+    case newline:
+        return nil
+    default:
+        // This is the most common case and should ideally be super fast...
+        var index = remainder.utf16.startIndex
+        while index < remainder.utf16.endIndex {
+            switch remainder.utf16[index] {
+            case comma:
+                defer { remainder.remove(upToAndIncluding: index) }
+                return remainder.prefix(upTo: index)
+            case newline:
+                let result = remainder.prefix(upTo: index)
+                remainder.remove(upTo: index)
+                return result
+            default:
+                remainder.utf16.formIndex(after: &index)
+            }
+        }
+        let result = remainder
+        remainder.removeAll()
+        return result
+    }
+}
+
+extension Substring {
+    mutating func remove(upTo index: Index) {
+        self = suffix(from: index)
+    }
+    
+    mutating func remove(upToAndIncluding index: Index) {
+        self = suffix(from: self.index(after: index))
+    }
+}
+
+// Consume a single line from `remainder`
+func parseLine<State>(_ remainder: inout Substring, result: inout State, processField: (inout State, Int, Substring) -> ()) throws -> Bool {
+    var fieldNumber = 0
+    
+    while let field = try parseField(&remainder) {
+        processField(&result, fieldNumber, field)
+        fieldNumber += 1
+    }
+    
+    if !remainder.isEmpty {
+        let next = remainder.utf16[remainder.utf16.startIndex]
+        guard next == carriageReturn || next == newline else {
+            throw ParseError(message: "Expected a newline or CR, got \(next)")
+        }
+        
+        while let x = remainder.utf16.first, x == carriageReturn || x == newline {
+            remainder.utf16.removeFirst()
+        }
+    }
+    
+    return !remainder.isEmpty && fieldNumber > 0
+}
+
+import Foundation
+
+func time<Result>(name: StaticString = #function, line: Int = #line, _ f: () throws -> Result) rethrows -> Result {
+    let startTime = DispatchTime.now()
+    let result = try f()
+    let endTime = DispatchTime.now()
+    let diff = Double(endTime.uptimeNanoseconds - startTime.uptimeNanoseconds) / 1_000_000_000 as Double
+    print("\(name) (line \(line)): \(diff) sec")
+    return result
+}
+
+try time {
+    // URL for this file: http://www.maxmind.com/download/worldcities/worldcitiespop.txt.gz
+    //    let file = URL(fileURLWithPath: "/Users/chris/Downloads/1489325/stops.txt")
+    let file = URL(fileURLWithPath: "worldcitiespop.txt")
+    
+    // The + "\n" is a a trick by Ole Begemann, which forces the String to be a Swift String (not an NSString). It makes it more than twice as fast on my computer...
+    let contents = try String(contentsOf: file, encoding: .isoLatin1) + ""
+    
+    var remainder = contents[...]
+    
+    var result: Int = 0
+    var x: () = ()
+    
+    while !remainder.isEmpty {
+        _ = try parseLine(&remainder, result: &x, processField: { state, _, field in
+            ()
+        })
+        if result < 10 {
+            print("result: \(result)")
+        }
+        result += 1
+        if result % 100_000 == 0 {
+            print(result)
+        }
+    }
+    print(result)
+    
+}