HtmlParser.h

// Copyright 2026 Stream.IO, Inc. All Rights Reserved.
 
#pragma once
 
#include "CoreMinimal.h"
 
namespace HtmlTag
{
const FName Anchor{TEXT("a")};
const FName Strong{TEXT("strong")};
const FName Emphasis{TEXT("em")};
const FName InlineCode{TEXT("code")};
const FName DeletedText{TEXT("del")};
const FName Heading1{TEXT("h1")};
const FName Heading2{TEXT("h2")};
const FName Heading3{TEXT("h3")};
const FName Heading4{TEXT("h4")};
const FName Heading5{TEXT("h5")};
const FName Heading6{TEXT("h6")};
const FName UnorderedList{TEXT("ul")};
const FName OrderedList{TEXT("ol")};
const FName ListItem{TEXT("li")};
const FName Paragraph{TEXT("p")};
const FName LineBreak{TEXT("br")};
const FName HorizontalRule{TEXT("hr")};
}    // namespace HtmlTag
 
class FHtmlScanner
{
public:
    explicit FHtmlScanner(const FString& InSource);
 
    enum class ETokenType : uint8
    {
        AngleOpen,
        AngleClose,
        Slash,
        Equal,
        String,
        Identifier,
        Content,
        Eof,
        Error
    };
 
    struct FToken
    {
        ETokenType Type;
        FStringView Lexeme;
    };
 
    FToken ScanToken();
    const FString& GetOutput() const;
    int32 PrevStart = 0;
    int32 Start = 0;
    int32 Current = 0;
 
private:
    TCHAR Advance();
    void SkipWhitespace();
 
    bool IsAtEnd() const;
    TCHAR Peek() const;
 
    FToken MakeToken(ETokenType Type) const;
 
    FToken String();
    FToken Identifier();
    FToken Content();
 
    FString Source;
    bool bInTag = false;
};
 
class FHtmlParser
{
public:
    struct FElement
    {
        FStringView Name;
        TMap<FStringView, FStringView> Attributes;
        int32 OpeningTagStart;
    };
 
    using FCallbackFn = TFunction<void(const FHtmlParser& Parser)>;
 
    // Initialize with source string. Doesn't take ownership of string, so caller must ensure it stays in memory.
    // Callback is called on each content chunk as it is found, along with the stack of surrounding element names
    explicit FHtmlParser(const FString& Source, FCallbackFn InCallback);
 
    // Parse the source string. Returns success.
    bool Parse();
 
    // The range of the content in the current run
    // <strong>Hello<em> world</em</strong>
    //         ^^^^^
    FTextRange GetContentRange() const;
    // The range from the start of the current element, to the end of the current content
    // <strong>Hello<em> world</em</strong>
    // ^^^^^^^^^^^^^
    FTextRange GetOriginalRange() const;
    FStringView GetContent() const;
    const FString& GetOutput() const;
 
    uint32 Line = 0;
    int32 ParagraphStartIndex = 0;
    TArray<FElement> ElementStack;
 
private:
    void Advance();
    bool AdvanceMatching(FHtmlScanner::ETokenType TokenType);
 
    bool Element();
    bool Attribute();
    bool Content();
    void CloseElement();
    void Newline(uint32 Index);
    FCallbackFn Callback;
    FHtmlScanner::FToken Current;
    FHtmlScanner Scanner;
};