diff --git a/Sources/HTMLStreamer/Tokenizer.swift b/Sources/HTMLStreamer/Tokenizer.swift
index d91fbbf..2dd39d0 100644
--- a/Sources/HTMLStreamer/Tokenizer.swift
+++ b/Sources/HTMLStreamer/Tokenizer.swift
@@ -16,7 +16,12 @@ struct Tokenizer<Chars: IteratorProtocol<Character>>: IteratorProtocol {
     private var returnState: State?
     private var temporaryBuffer: String?
     private var characterReferenceCode: UInt32?
-    private var currentToken: Token?
+    // Optimization: using an enum for the current token means we can't modify the associated values in-place
+    // Separate fields for everything increases the risk of invalid states, but nets us a small perf gain.
+    private var currentStartTag: (String, selfClosing: Bool, attributes: [Attribute])?
+    private var currentEndTag: String?
+    private var currentComment: String?
+    private var currentDoctype: (String, forceQuirks: Bool, publicIdentifier: String?, systemIdentifier: String?)?
     
     init(chars: Chars) {
         self.chars = chars
@@ -187,8 +192,21 @@ struct Tokenizer<Chars: IteratorProtocol<Character>>: IteratorProtocol {
     }
     
     private mutating func takeCurrentToken() -> Token {
-        defer { currentToken = nil }
-        return currentToken!
+        if let currentStartTag {
+            self.currentStartTag = nil
+            return .startTag(currentStartTag.0, selfClosing: currentStartTag.selfClosing, attributes: currentStartTag.attributes)
+        } else if let currentEndTag {
+            self.currentEndTag = nil
+            return .endTag(currentEndTag)
+        } else if let currentComment {
+            self.currentComment = nil
+            return .comment(currentComment)
+        } else if let currentDoctype {
+            self.currentDoctype = nil
+            return .doctype(currentDoctype.0, forceQuirks: currentDoctype.forceQuirks, publicIdentifier: currentDoctype.publicIdentifier, systemIdentifier: currentDoctype.systemIdentifier)
+        } else {
+            preconditionFailure("takeCurrentToken called without current token")
+        }
     }
 }
 
@@ -404,14 +422,9 @@ private extension Tokenizer {
     
     mutating func flushCharacterReference() {
         if case .attributeValue(_) = returnState {
-            if case .startTag(let s, let selfClosing, var attributes) = currentToken {
-                attributes[attributes.count - 1].value.append(temporaryBuffer!)
-                currentToken = .startTag(s, selfClosing: selfClosing, attributes: attributes)
-                temporaryBuffer = nil
-                state = returnState!
-            } else {
-                fatalError("bad current tag")
-            }
+            currentStartTag!.attributes.uncheckedLast.value.append(temporaryBuffer!)
+            temporaryBuffer = nil
+            state = returnState!
         } else {
             state = .flushingTemporaryBuffer(returnState!)
         }
@@ -561,13 +574,8 @@ private extension Tokenizer {
         switch c {
         case .some("0"..."9"), .some("a"..."z"), .some("A"..."Z"):
             if case .attributeValue(_) = returnState {
-                if case .startTag(let s, let selfClosing, var attributes) = currentToken {
-                    attributes[attributes.count - 1].value.append(c!)
-                    currentToken = .startTag(s, selfClosing: selfClosing, attributes: attributes)
-                } else {
-                    fatalError("bad current token")
-                }
-                return next()
+                currentStartTag!.attributes.uncheckedLast.value.append(c!)
+                return tokenizeAmbiguousAmpersand()
             } else {
                 return .character(c!)
             }
@@ -590,7 +598,7 @@ private extension Tokenizer {
             return tokenizeEndTagOpen()
         case "?":
             // parse error: unexpected-question-mark-instead-of-tag-name
-            currentToken = .comment("")
+            currentComment = ""
             state = .bogusComment
             return tokenizeBogusComment()
         case nil:
@@ -598,7 +606,7 @@ private extension Tokenizer {
             state = .endOfFile
             return .character("<")
         case .some("a"..."z"), .some("A"..."Z"):
-            currentToken = .startTag("", selfClosing: false, attributes: [])
+            currentStartTag = ("", selfClosing: false, attributes: [])
             reconsume(c)
             state = .tagName
             return tokenizeTagName()
@@ -614,7 +622,7 @@ private extension Tokenizer {
         let c = nextChar()
         switch c {
         case .some("a"..."z"), .some("A"..."Z"):
-            currentToken = .endTag("")
+            currentEndTag = ""
             reconsume(c)
             state = .tagName
             return tokenizeTagName()
@@ -628,7 +636,7 @@ private extension Tokenizer {
             return .character("<")
         case .some(let c):
             // parse error: invalid-first-character-of-tag-name
-            currentToken = .comment("")
+            currentComment = ""
             reconsume(c)
             state = .bogusComment
             return tokenizeBogusComment()
@@ -636,6 +644,8 @@ private extension Tokenizer {
     }
     
     mutating func tokenizeTagName() -> Token? {
+        // Optimization: this is a hot path where we stay in this state for a while before emitting a token,
+        // and the function call overhead of recursion costs a bit of perf.
         while true {
             switch nextChar() {
             case "\t", "\n", "\u{000C}", " ":
@@ -658,13 +668,11 @@ private extension Tokenizer {
                 } else if ("A"..."Z").contains(c) {
                     c = c.asciiLowercase
                 }
-                if case .startTag(var s, let selfClosing, let attributes) = currentToken {
-                    s.append(c)
-                    currentToken = .startTag(s, selfClosing: selfClosing, attributes: attributes)
+                if currentStartTag != nil {
+                    currentStartTag!.0.append(c)
                     continue
-                } else if case .endTag(var s) = currentToken {
-                    s.append(c)
-                    currentToken = .endTag(s)
+                } else if currentEndTag != nil {
+                    currentEndTag!.append(c)
                     continue
                 } else {
                     fatalError("bad current token")
@@ -676,11 +684,7 @@ private extension Tokenizer {
     mutating func tokenizeSelfClosingStartTag() -> Token? {
         switch nextChar() {
         case ">":
-            if case .startTag(let s, _, let attributes) = currentToken {
-                currentToken = .startTag(s, selfClosing: true, attributes: attributes)
-            } else {
-                fatalError("bad current token")
-            }
+            currentStartTag!.selfClosing = true
             state = .data
             return takeCurrentToken()
         case nil:
@@ -707,22 +711,16 @@ private extension Tokenizer {
             return tokenizeAfterAttributeName()
         case "=":
             // parse error: unexpected-equals-sign-before-attribute-name
-            if case .startTag(let s, let selfClosing, var attributes) = currentToken {
-                attributes.append(Attribute(name: "=", value: ""))
-                currentToken = .startTag(s, selfClosing: selfClosing, attributes: attributes)
-                state = .attributeName
-                return tokenizeAttributeName()
-            } else {
-                fatalError("bad current token")
-            }
+            currentStartTag!.attributes.append(Attribute(name: "=", value: ""))
+            state = .attributeName
+            return tokenizeAttributeName()
         default:
-            if case .startTag(let s, let selfClosing, var attributes) = currentToken {
-                attributes.append(Attribute(name: "", value: ""))
-                currentToken = .startTag(s, selfClosing: selfClosing, attributes: attributes)
+            if currentStartTag != nil {
+                currentStartTag!.attributes.append(Attribute(name: "", value: ""))
                 reconsume(c)
                 state = .attributeName
                 return tokenizeAttributeName()
-            } else if case .endTag(_) = currentToken {
+            } else if currentEndTag != nil {
                 // ignore
                 reconsume(c)
                 state = .attributeName
@@ -753,11 +751,10 @@ private extension Tokenizer {
                     c = "\u{FFFD}"
                 }
                 // if c in ["\"", "'", "<"], parse error: unexpected-character-in-attribute-name
-                if case .startTag(let s, let selfClosing, var attributes) = currentToken {
-                    attributes[attributes.count - 1].name.append(c)
-                    currentToken = .startTag(s, selfClosing: selfClosing, attributes: attributes)
+                if currentStartTag != nil {
+                    currentStartTag!.attributes.uncheckedLast.name.append(c)
                     continue
-                } else if case .endTag(_) = currentToken {
+                } else if currentEndTag != nil {
                     continue
                 } else {
                     fatalError("bad curren token")
@@ -782,13 +779,12 @@ private extension Tokenizer {
             state = .endOfFile
             return nil
         case .some(let c):
-            if case .startTag(let s, let selfClosing, var attributes) = currentToken {
-                attributes.append(Attribute(name: "", value: ""))
-                currentToken = .startTag(s, selfClosing: selfClosing, attributes: attributes)
+            if currentStartTag != nil {
+                currentStartTag!.attributes.append(Attribute(name: "", value: ""))
                 reconsume(c)
                 state = .attributeName
                 return tokenizeAttributeName()
-            } else if case .endTag(_) = currentToken {
+            } else if currentEndTag != nil {
                 reconsume(c)
                 state = .attributeName
                 return tokenizeAttributeName()
@@ -840,11 +836,10 @@ private extension Tokenizer {
                     return nil
                 case .some(let c):
                     // if c in ["\"", "'", "<", "=", "`"], parse error: unexpected-character-in-unquoted-attribute-value
-                    if case .startTag(let s, let selfClosing, var attributes) = currentToken {
-                        attributes[attributes.count - 1].value.append(c)
-                        currentToken = .startTag(s, selfClosing: selfClosing, attributes: attributes)
+                    if currentStartTag != nil {
+                        currentStartTag!.attributes.uncheckedLast.value.append(c)
                         continue
-                    } else if case .endTag(_) = currentToken {
+                    } else if currentEndTag != nil {
                         continue
                     } else {
                         fatalError("bad current token")
@@ -872,14 +867,11 @@ private extension Tokenizer {
                         // parse error: unexpected-null-character
                         c = "\u{FFFD}"
                     }
-                    if case .startTag(let s, let selfClosing, var attributes) = currentToken {
-                        attributes[attributes.count - 1].value.append(c)
-                        currentToken = .startTag(s, selfClosing: selfClosing, attributes: attributes)
+                    if currentStartTag != nil {
+                        currentStartTag!.attributes.uncheckedLast.value.append(c)
                         continue
-                    } else if case .endTag(_) = currentToken {
+                    } else if currentEndTag != nil {
                         continue
-                    } else {
-                        fatalError("bad current token")
                     }
                 }
             }
@@ -922,13 +914,8 @@ private extension Tokenizer {
                 // parse error: unexpected-null-character
                 c = "\u{FFFD}"
             }
-            if case .comment(var s) = currentToken {
-                s.append(c)
-                currentToken = .comment(s)
-                return tokenizeBogusComment()
-            } else {
-                fatalError("bad current token")
-            }
+            currentComment!.append(c)
+            return tokenizeBogusComment()
         }
     }
     
@@ -936,7 +923,7 @@ private extension Tokenizer {
         let peeked = peek(count: 7)
         if peeked.starts(with: "--") {
             consume(count: 2)
-            currentToken = .comment("")
+            currentComment = ""
             state = .commentStart
             return tokenizeCommentStart()
         } else if peeked.lowercased() == "doctype" {
@@ -946,12 +933,12 @@ private extension Tokenizer {
         } else if peeked == "[CDATA[" {
             // TODO: we don't do any of the tree construction stuff yet, so can't really handle this
             // consume(count: 7)
-            currentToken = .comment("")
+            currentComment = ""
             state = .bogusComment
             return tokenizeBogusComment()
         } else {
             // parse error: incorrectly-opened-comment
-            currentToken = .comment("")
+            currentComment = ""
             state = .bogusComment
             return tokenizeBogusComment()
         }
@@ -986,29 +973,19 @@ private extension Tokenizer {
             // parse error: eof-in-comment
             return takeCurrentToken()
         case .some(let c):
-            if case .comment(var s) = currentToken {
-                s.append("-")
-                currentToken = .comment(s)
-                reconsume(c)
-                state = .comment
-                return tokenizeComment()
-            } else {
-                fatalError("bad current token")
-            }
+            currentComment!.append("-")
+            reconsume(c)
+            state = .comment
+            return tokenizeComment()
         }
     }
     
     mutating func tokenizeComment() -> Token? {
         switch nextChar() {
         case "<":
-            if case .comment(var s) = currentToken {
-                s.append("<")
-                currentToken = .comment(s)
-                state = .commentLessThanSign
-                return tokenizeCommentLessThanSign()
-            } else {
-                fatalError("bad current token")
-            }
+            currentComment!.append("<")
+            state = .commentLessThanSign
+            return tokenizeCommentLessThanSign()
         case "-":
             state = .commentEndDash
             return tokenizeCommentEndDash()
@@ -1021,35 +998,20 @@ private extension Tokenizer {
                 // parse error: unexpected-null-character
                 c = "\u{FFFD}"
             }
-            if case .comment(var s) = currentToken {
-                s.append(c)
-                currentToken = .comment(s)
-                return tokenizeComment()
-            } else {
-                fatalError("bad current token")
-            }
+            currentComment!.append(c)
+            return tokenizeComment()
         }
     }
     
     mutating func tokenizeCommentLessThanSign() -> Token? {
         switch nextChar() {
         case "!":
-            if case .comment(var s) = currentToken {
-                s.append("!")
-                currentToken = .comment(s)
-                state = .commentLessThanSignBang
-                return tokenizeCommentLessThanSignBang()
-            } else {
-                fatalError("bad current token")
-            }
+            currentComment!.append("!")
+            state = .commentLessThanSignBang
+            return tokenizeCommentLessThanSignBang()
         case "<":
-            if case .comment(var s) = currentToken {
-                s.append("<")
-                currentToken = .comment(s)
-                return tokenizeComment()
-            } else {
-                fatalError("bad current token")
-            }
+            currentComment!.append("<")
+            return tokenizeComment()
         case let c:
             reconsume(c)
             state = .comment
@@ -1106,15 +1068,10 @@ private extension Tokenizer {
             state = .endOfFile
             return takeCurrentToken()
         case let c:
-            if case .comment(var s) = currentToken {
-                s.append("-")
-                currentToken = .comment(s)
-            } else {
-                fatalError("bad current token")
-            }
+            currentComment!.append("-")
             reconsume(c)
             state = .comment
-            return next()
+            return tokenizeComment()
         }
     }
     
@@ -1127,24 +1084,14 @@ private extension Tokenizer {
             state = .commentEndBang
             return tokenizeCommentEndBang()
         case "-":
-            if case .comment(var s) = currentToken {
-                s.append("-")
-                currentToken = .comment(s)
-                return tokenizeCommentEnd()
-            } else {
-                fatalError("bad current token")
-            }
+            currentComment!.append("-")
+            return tokenizeCommentEnd()
         case nil:
             // parse error: eof-in-comment
             state = .endOfFile
             return takeCurrentToken()
         case .some(let c):
-            if case .comment(var s) = currentToken {
-                s.append("--")
-                currentToken = .comment(s)
-            } else {
-                fatalError("bad current token")
-            }
+            currentComment!.append("--")
             reconsume(c)
             state = .comment
             return tokenizeComment()
@@ -1154,14 +1101,9 @@ private extension Tokenizer {
     mutating func tokenizeCommentEndBang() -> Token? {
         switch nextChar() {
         case "-":
-            if case .comment(var s) = currentToken {
-                s.append("--!")
-                currentToken = .comment(s)
-                state = .commentEndDash
-                return tokenizeCommentEndDash()
-            } else {
-                fatalError("bad current token")
-            }
+            currentComment!.append("--!")
+            state = .commentEndDash
+            return tokenizeCommentEndDash()
         case ">":
             // parse error: incorrectly-closed-comment
             state = .data
@@ -1171,15 +1113,10 @@ private extension Tokenizer {
             state = .endOfFile
             return takeCurrentToken()
         case .some(let c):
-            if case .comment(var s) = currentToken {
-                s.append("--!")
-                currentToken = .comment(s)
-                reconsume(c)
-                state = .comment
-                return tokenizeComment()
-            } else {
-                fatalError("bad current token")
-            }
+            currentComment!.append("--!")
+            reconsume(c)
+            state = .comment
+            return tokenizeComment()
         }
     }
     
@@ -1210,12 +1147,12 @@ private extension Tokenizer {
             // ignore the character
             return tokenizeBeforeDoctypeName()
         case .some(let c) where ("A"..."Z").contains(c):
-            currentToken = .doctype("\(c.asciiLowercase)", forceQuirks: false, publicIdentifier: nil, systemIdentifier: nil)
+            currentDoctype = ("\(c.asciiLowercase)", forceQuirks: false, publicIdentifier: nil, systemIdentifier: nil)
             state = .doctypeName
             return tokenizeDoctypeName()
         case "\0":
             // parse error: unexpected-null-character
-            currentToken = .doctype("\u{FFFD}", forceQuirks: false, publicIdentifier: nil, systemIdentifier: nil)
+            currentDoctype = ("\u{FFFD}", forceQuirks: false, publicIdentifier: nil, systemIdentifier: nil)
             state = .doctypeName
             return tokenizeDoctypeName()
         case ">":
@@ -1227,7 +1164,7 @@ private extension Tokenizer {
             state = .endOfFile
             return .doctype("", forceQuirks: false, publicIdentifier: nil, systemIdentifier: nil)
         case .some(let c):
-            currentToken = .doctype("\(c)", forceQuirks: false, publicIdentifier: nil, systemIdentifier: nil)
+            currentDoctype = ("\(c)", forceQuirks: false, publicIdentifier: nil, systemIdentifier: nil)
             state = .doctypeName
             return tokenizeDoctypeName()
         }
@@ -1243,25 +1180,16 @@ private extension Tokenizer {
             return takeCurrentToken()
         case nil:
             // parse error: eof-in-doctype
-            if case .doctype(let s, _, let publicIdentifier, let systemIdentifier) = currentToken {
-                currentToken = nil
-                return .doctype(s, forceQuirks: true, publicIdentifier: publicIdentifier, systemIdentifier: systemIdentifier)
-            } else {
-                fatalError("bad current token")
-            }
+            currentDoctype!.forceQuirks = true
+            return takeCurrentToken()
         case .some(var c):
             if c == "\0" {
                 c = "\u{FFFD}"
             } else if ("A"..."Z").contains(c) {
                 c = c.asciiLowercase
             }
-            if case .doctype(var s, let forceQuirks, _, _) = currentToken {
-                s.append(c)
-                currentToken = .doctype(s, forceQuirks: forceQuirks, publicIdentifier: nil, systemIdentifier: nil)
-                return tokenizeDoctypeName()
-            } else {
-                fatalError("bad current token")
-            }
+            currentDoctype!.0.append(c)
+            return tokenizeDoctypeName()
         }
     }
     
@@ -1276,12 +1204,8 @@ private extension Tokenizer {
         case nil:
             // parse error: eof-in-doctype
             state = .endOfFile
-            if case .doctype(let s, _, let publicIdentifier, let systemIdentifier) = currentToken {
-                currentToken = nil
-                return .doctype(s, forceQuirks: true, publicIdentifier: publicIdentifier, systemIdentifier: systemIdentifier)
-            } else {
-                fatalError("bad current token")
-            }
+            currentDoctype!.forceQuirks = true
+            return takeCurrentToken()
         case .some(let c):
             reconsume(c)
             let peeked = peek(count: 6).lowercased()
@@ -1295,11 +1219,7 @@ private extension Tokenizer {
                 return tokenizeAfterDoctypeSystemKeyword()
             } else {
                 // parse error: invalid-character-sequence-after-doctype-name
-                if case .doctype(let s, _, let publicIdentifier, let systemIdentifier) = currentToken {
-                    currentToken = .doctype(s, forceQuirks: true, publicIdentifier: publicIdentifier, systemIdentifier: systemIdentifier)
-                } else {
-                    fatalError("bad current token")
-                }
+                currentDoctype!.forceQuirks = true
                 state = .bogusDoctype
                 return tokenizeBogusDoctype()
             }
@@ -1313,39 +1233,23 @@ private extension Tokenizer {
             return tokenizeBeforeDoctypePublicIdentifier()
         case .some(let c) where c == "\"" || c == "'":
             // parse error: missing-whitespace-after-doctype-public-keyword
-            if case .doctype(let s, let forceQuirks, _, _) = currentToken {
-                currentToken = .doctype(s, forceQuirks: forceQuirks, publicIdentifier: "", systemIdentifier: nil)
-                let quotes = c == "\"" ? DoctypeIdentifierQuotation.doubleQuoted : .singleQuoted
-                state = .doctypePublicIdentifier(quotes)
-                return tokenizeDoctypePublicIdentifier(quotes: quotes)
-            } else {
-                fatalError("bad current token")
-            }
+            currentDoctype!.publicIdentifier = ""
+            let quotes = c == "\"" ? DoctypeIdentifierQuotation.doubleQuoted : .singleQuoted
+            state = .doctypePublicIdentifier(quotes)
+            return tokenizeDoctypePublicIdentifier(quotes: quotes)
         case ">":
             // parse error: missing-doctype-public-identifier
             state = .data
-            if case .doctype(let s, _, let publicIdentifier, let systemIdentifier) = currentToken {
-                currentToken = nil
-                return .doctype(s, forceQuirks: true, publicIdentifier: publicIdentifier, systemIdentifier: systemIdentifier)
-            } else {
-                fatalError("bad current token")
-            }
+            currentDoctype!.forceQuirks = true
+            return takeCurrentToken()
         case nil:
             // parse error: eof-in-doctype
             state = .endOfFile
-            if case .doctype(let s, _, let publicIdentifier, let systemIdentifier) = currentToken {
-                currentToken = nil
-                return .doctype(s, forceQuirks: true, publicIdentifier: publicIdentifier, systemIdentifier: systemIdentifier)
-            } else {
-                fatalError("bad current token")
-            }
+            currentDoctype!.forceQuirks = true
+            return takeCurrentToken()
         case .some(let c):
             // parse error: missing-quote-before-doctype-public-identifier
-            if case .doctype(let s, _, let publicIdentifier, let systemIdentifier) = currentToken {
-                currentToken = .doctype(s, forceQuirks: true, publicIdentifier: publicIdentifier, systemIdentifier: systemIdentifier)
-            } else {
-                fatalError("bad current token")
-            }
+            currentDoctype!.forceQuirks = true
             state = .bogusDoctype
             reconsume(c)
             return tokenizeBogusDoctype()
@@ -1358,39 +1262,23 @@ private extension Tokenizer {
             // ignore the character
             return tokenizeBeforeDoctypePublicIdentifier()
         case .some(let c) where c == "\"" || c == "'":
-            if case .doctype(let s, let forceQuirks, _, _) = currentToken {
-                currentToken = .doctype(s, forceQuirks: forceQuirks, publicIdentifier: "", systemIdentifier: nil)
-                let quotes = c == "\"" ? DoctypeIdentifierQuotation.doubleQuoted : .singleQuoted
-                state = .doctypePublicIdentifier(quotes)
-                return tokenizeDoctypePublicIdentifier(quotes: quotes)
-            } else {
-                fatalError("bad current token")
-            }
+            currentDoctype!.publicIdentifier = ""
+            let quotes = c == "\"" ? DoctypeIdentifierQuotation.doubleQuoted : .singleQuoted
+            state = .doctypePublicIdentifier(quotes)
+            return tokenizeDoctypePublicIdentifier(quotes: quotes)
         case ">":
             // parse error: missing-doctype-public-identifier
             state = .data
-            if case .doctype(let s, _, let publicIdentifier, let systemIdentifier) = currentToken {
-                currentToken = nil
-                return .doctype(s, forceQuirks: true, publicIdentifier: publicIdentifier, systemIdentifier: systemIdentifier)
-            } else {
-                fatalError("bad current token")
-            }
+            currentDoctype!.forceQuirks = true
+            return takeCurrentToken()
         case nil:
             // parse error: eof-in-doctype
             state = .endOfFile
-            if case .doctype(let s, _, let publicIdentifier, let systemIdentifier) = currentToken {
-                currentToken = nil
-                return .doctype(s, forceQuirks: true, publicIdentifier: publicIdentifier, systemIdentifier: systemIdentifier)
-            } else {
-                fatalError("bad current token")
-            }
+            currentDoctype!.forceQuirks = true
+            return takeCurrentToken()
         case .some(let c):
             // parse error: missing-quote-before-doctype-public-identifier
-            if case .doctype(let s, _, let publicIdentifier, let systemIdentifier) = currentToken {
-                currentToken = .doctype(s, forceQuirks: true, publicIdentifier: publicIdentifier, systemIdentifier: systemIdentifier)
-            } else {
-                fatalError("bad current token")
-            }
+            currentDoctype!.forceQuirks = true
             reconsume(c)
             state = .bogusDoctype
             return tokenizeBogusDoctype()
@@ -1409,33 +1297,20 @@ private extension Tokenizer {
             // parse error: abrupt-doctype-public-identifier
             reconsume(">")
             state = .data
-            if case .doctype(let s, _, let publicIdentifier, let systemIdentifier) = currentToken {
-                currentToken = nil
-                return .doctype(s, forceQuirks: true, publicIdentifier: publicIdentifier, systemIdentifier: systemIdentifier)
-            } else {
-                fatalError("bad current token")
-            }
+            currentDoctype!.forceQuirks = true
+            return takeCurrentToken()
         case nil:
             // parse error: eof-in-doctype
             state = .endOfFile
-            if case .doctype(let s, _, let publicIdentifier, let systemIdentifier) = currentToken {
-                currentToken = nil
-                return .doctype(s, forceQuirks: true, publicIdentifier: publicIdentifier, systemIdentifier: systemIdentifier)
-            } else {
-                fatalError("bad current token")
-            }
+            currentDoctype!.forceQuirks = true
+            return takeCurrentToken()
         case .some(var c):
             if c == "\0" {
                 // parse error: unexpected-null-character
                 c = "\u{FFFD}"
             }
-            if case .doctype(let s, let forceQuirks, var publicIdentifier, _) = currentToken {
-                publicIdentifier!.append(c)
-                currentToken = .doctype(s, forceQuirks: forceQuirks, publicIdentifier: publicIdentifier, systemIdentifier: nil)
-                return tokenizeDoctypePublicIdentifier(quotes: quotes)
-            } else {
-                fatalError("bad current token")
-            }
+            currentDoctype!.publicIdentifier!.append(c)
+            return tokenizeDoctypePublicIdentifier(quotes: quotes)
         }
     }
     
@@ -1449,30 +1324,18 @@ private extension Tokenizer {
             return takeCurrentToken()
         case .some(let c) where c == "\"" || c == "'":
             // parse error: missing-whitespace-between-doctype-public-and-system-identifiers
-            if case .doctype(let s, let forceQuirks, let publicIdentifier, _) = currentToken {
-                currentToken = .doctype(s, forceQuirks: forceQuirks, publicIdentifier: publicIdentifier, systemIdentifier: "")
-                let quotes = c == "\"" ? DoctypeIdentifierQuotation.doubleQuoted : .singleQuoted
-                state = .doctypeSystemIdentifier(quotes)
-                return tokenizeDoctypeSystemIdentifier(quotes: quotes)
-            } else {
-                fatalError("bad current token")
-            }
+            currentDoctype!.systemIdentifier = ""
+            let quotes = c == "\"" ? DoctypeIdentifierQuotation.doubleQuoted : .singleQuoted
+            state = .doctypeSystemIdentifier(quotes)
+            return tokenizeDoctypeSystemIdentifier(quotes: quotes)
         case nil:
             // parse error: eof-in-doctype
             state = .endOfFile
-            if case .doctype(let s, _, let publicIdentifier, let systemIdentifier) = currentToken {
-                currentToken = nil
-                return .doctype(s, forceQuirks: true, publicIdentifier: publicIdentifier, systemIdentifier: systemIdentifier)
-            } else {
-                fatalError("bad current token")
-            }
+            currentDoctype!.forceQuirks = true
+            return takeCurrentToken()
         case .some(let c):
             // parse error: missing-quote-before-doctype-system-identifier
-            if case .doctype(let s, _, let publicIdentifier, let systemIdentifier) = currentToken {
-                currentToken = .doctype(s, forceQuirks: true, publicIdentifier: publicIdentifier, systemIdentifier: systemIdentifier)
-            } else {
-                fatalError("bad current token")
-            }
+            currentDoctype!.forceQuirks = true
             reconsume(c)
             state = .bogusDoctype
             return tokenizeBogusDoctype()
@@ -1488,30 +1351,18 @@ private extension Tokenizer {
             state = .data
             return takeCurrentToken()
         case .some(let c) where c == "\"" || c == "'":
-            if case .doctype(let s, let forceQuirks, let publicIdentifier, _) = currentToken {
-                currentToken = .doctype(s, forceQuirks: forceQuirks, publicIdentifier: publicIdentifier, systemIdentifier: "")
-                let quotes = c == "\"" ? DoctypeIdentifierQuotation.doubleQuoted : .singleQuoted
-                state = .doctypeSystemIdentifier(quotes)
-                return tokenizeDoctypeSystemIdentifier(quotes: quotes)
-            } else {
-                fatalError("bad current token")
-            }
+            currentDoctype!.systemIdentifier = ""
+            let quotes = c == "\"" ? DoctypeIdentifierQuotation.doubleQuoted : .singleQuoted
+            state = .doctypeSystemIdentifier(quotes)
+            return tokenizeDoctypeSystemIdentifier(quotes: quotes)
         case nil:
             // parse error: eof-in-doctype
             state = .endOfFile
-            if case .doctype(let s, _, let publicIdentifier, let systemIdentifier) = currentToken {
-                currentToken = nil
-                return .doctype(s, forceQuirks: true, publicIdentifier: publicIdentifier, systemIdentifier: systemIdentifier)
-            } else {
-                fatalError("bad current token")
-            }
+            currentDoctype!.forceQuirks = true
+            return takeCurrentToken()
         case .some(let c):
             // parse error: missing-quote-before-doctype-system-identifier
-            if case .doctype(let s, _, let publicIdentifier, let systemIdentifier) = currentToken {
-                currentToken = .doctype(s, forceQuirks: true, publicIdentifier: publicIdentifier, systemIdentifier: systemIdentifier)
-            } else {
-                fatalError("bad current token")
-            }
+            currentDoctype!.forceQuirks = true
             reconsume(c)
             state = .bogusComment
             return tokenizeBogusComment()
@@ -1524,39 +1375,23 @@ private extension Tokenizer {
             state = .beforeDoctypeSystemIdentifier
             return tokenizeBeforeDoctypeSystemIdentifier()
         case .some(let c) where c == "\"" || c == "'":
-            if case .doctype(let s, let forceQuirks, let publicIdentifier, _) = currentToken {
-                currentToken = .doctype(s, forceQuirks: forceQuirks, publicIdentifier: publicIdentifier, systemIdentifier: "")
-                let quotes = c == "\"" ? DoctypeIdentifierQuotation.doubleQuoted : .singleQuoted
-                state = .doctypeSystemIdentifier(quotes)
-                return tokenizeDoctypeSystemIdentifier(quotes: quotes)
-            } else {
-                fatalError("bad current token")
-            }
+            currentDoctype!.systemIdentifier = ""
+            let quotes = c == "\"" ? DoctypeIdentifierQuotation.doubleQuoted : .singleQuoted
+            state = .doctypeSystemIdentifier(quotes)
+            return tokenizeDoctypeSystemIdentifier(quotes: quotes)
         case ">":
             // parse error: missing-doctype-system-identifier
             state = .data
-            if case .doctype(let s, _, let publicIdentifier, let systemIdentifier) = currentToken {
-                currentToken = nil
-                return .doctype(s, forceQuirks: true, publicIdentifier: publicIdentifier, systemIdentifier: systemIdentifier)
-            } else {
-                fatalError("bad current token")
-            }
+            currentDoctype!.forceQuirks = true
+            return takeCurrentToken()
         case nil:
             // parse error: eof-in-doctype:
             state = .endOfFile
-            if case .doctype(let s, _, let publicIdentifier, let systemIdentifier) = currentToken {
-                currentToken = nil
-                return .doctype(s, forceQuirks: true, publicIdentifier: publicIdentifier, systemIdentifier: systemIdentifier)
-            } else {
-                fatalError("bad current token")
-            }
+            currentDoctype!.forceQuirks = true
+            return takeCurrentToken()
         case .some(let c):
             // parse error: missing-quote-before-doctype-system-identifier
-            if case .doctype(let s, _, let publicIdentifier, let systemIdentifier) = currentToken {
-                currentToken = .doctype(s, forceQuirks: true, publicIdentifier: publicIdentifier, systemIdentifier: systemIdentifier)
-            } else {
-                fatalError("bad current token")
-            }
+            currentDoctype!.forceQuirks = true
             reconsume(c)
             state = .bogusDoctype
             return tokenizeBogusDoctype()
@@ -1569,39 +1404,23 @@ private extension Tokenizer {
             // ignore the character
             return tokenizeBeforeDoctypeSystemIdentifier()
         case .some(let c) where c == "\"" || c == "'":
-            if case .doctype(let s, let forceQuirks, let publicIdentifier, _) = currentToken {
-                currentToken = .doctype(s, forceQuirks: forceQuirks, publicIdentifier: publicIdentifier, systemIdentifier: "  ")
-                let quotes = c == "\"" ? DoctypeIdentifierQuotation.doubleQuoted : .singleQuoted
-                state = .doctypeSystemIdentifier(quotes)
-                return tokenizeDoctypeSystemIdentifier(quotes: quotes)
-            } else {
-                fatalError("bad current token")
-            }
+            currentDoctype!.systemIdentifier = ""
+            let quotes = c == "\"" ? DoctypeIdentifierQuotation.doubleQuoted : .singleQuoted
+            state = .doctypeSystemIdentifier(quotes)
+            return tokenizeDoctypeSystemIdentifier(quotes: quotes)
         case ">":
             // parse error: missing-doctype-system-identifier
             state = .data
-            if case .doctype(let s, _, let publicIdentifier, let systemIdentifier) = currentToken {
-                currentToken = nil
-                return .doctype(s, forceQuirks: true, publicIdentifier: publicIdentifier, systemIdentifier: systemIdentifier)
-            } else {
-                fatalError("bad current token")
-            }
+            currentDoctype!.forceQuirks = true
+            return takeCurrentToken()
         case nil:
             // parse error: eof-in-doctype:
             state = .endOfFile
-            if case .doctype(let s, _, let publicIdentifier, let systemIdentifier) = currentToken {
-                currentToken = nil
-                return .doctype(s, forceQuirks: true, publicIdentifier: publicIdentifier, systemIdentifier: systemIdentifier)
-            } else {
-                fatalError("bad current token")
-            }
+            currentDoctype!.forceQuirks = true
+            return takeCurrentToken()
         case .some(let c):
             // parse error: missing-quote-before-doctype-system-identifier
-            if case .doctype(let s, _, let publicIdentifier, let systemIdentifier) = currentToken {
-                currentToken = .doctype(s, forceQuirks: true, publicIdentifier: publicIdentifier, systemIdentifier: systemIdentifier)
-            } else {
-                fatalError("bad current token")
-            }
+            currentDoctype!.forceQuirks = true
             reconsume(c)
             state = .bogusDoctype
             return tokenizeBogusDoctype()
@@ -1619,33 +1438,20 @@ private extension Tokenizer {
         case ">":
             // parse error: abrupt-doctype-system-identifier
             state = .data
-            if case .doctype(let s, _, let publicIdentifier, let systemIdentifier) = currentToken {
-                currentToken = nil
-                return .doctype(s, forceQuirks: true, publicIdentifier: publicIdentifier, systemIdentifier: systemIdentifier)
-            } else {
-                fatalError("bad current token")
-            }
+            currentDoctype!.forceQuirks = true
+            return takeCurrentToken()
         case nil:
             // parse error: eof-in-doctype
             state = .endOfFile
-            if case .doctype(let s, _, let publicIdentifier, let systemIdentifier) = currentToken {
-                currentToken = nil
-                return .doctype(s, forceQuirks: true, publicIdentifier: publicIdentifier, systemIdentifier: systemIdentifier)
-            } else {
-                fatalError("bad current token")
-            }
+            currentDoctype!.forceQuirks = true
+            return takeCurrentToken()
         case .some(var c):
             if c == "\0" {
                 // parse error: unexpected-null-character
                 c = "\u{FFFD}"
             }
-            if case .doctype(let s, let forceQuirks, let publicIdentifier, var systemIdentifier) = currentToken {
-                systemIdentifier!.append(c)
-                currentToken = .doctype(s, forceQuirks: forceQuirks, publicIdentifier: publicIdentifier, systemIdentifier: systemIdentifier)
-                return tokenizeDoctypeSystemIdentifier(quotes: quotes)
-            } else {
-                fatalError("bad current token")
-            }
+            currentDoctype!.systemIdentifier!.append(c)
+            return tokenizeDoctypeSystemIdentifier(quotes: quotes)
         }
     }
     
@@ -1660,12 +1466,8 @@ private extension Tokenizer {
         case nil:
             // parse error: eof-in-doctype
             state = .endOfFile
-            if case .doctype(let s, _, let publicIdentifier, let systemIdentifier) = currentToken {
-                currentToken = nil
-                return .doctype(s, forceQuirks: true, publicIdentifier: publicIdentifier, systemIdentifier: systemIdentifier)
-            } else {
-                fatalError("bad current token")
-            }
+            currentDoctype!.forceQuirks = true
+            return takeCurrentToken()
         case .some(let c):
             // parse error: unexpected-character-after-doctype-system-identifier
             // Note: This does not set the current DOCTYPE token's force-quirks flag to on.
@@ -1699,3 +1501,15 @@ private extension Character {
         return Character(Unicode.Scalar(asciiValue! + 0x20))
     }
 }
+
+private extension Array {
+    // Optimization: allows in-place modification of the last element of the array.
+    var uncheckedLast: Element {
+        _read {
+            yield self[count - 1]
+        }
+        _modify {
+            yield &self[count - 1]
+        }
+    }
+}