PDF comparison helpers with resource limits

FPdfDiff supplies the page alignment, UTF-16 text-change and connected visual-region routines behind CompareWith

AlignPdfDiffPages uses a resource-limited longest common subsequence to pair matching text, geometry and visual fingerprints; it treats remaining exact matches as moves, then pairs unmatched gaps as modifications

Repeated pages receive deterministic pairings; a page moved and edited together need not be recognised as a move

BuildPdfDiffTextChanges supplies zero-based UTF-16 offsets and lengths; character mode never separates a surrogate pair, and word mode groups whitespace apart from other characters

BuildPdfDiffPixelRegions combines connected changed tiles into pixel rectangles with exclusive right and bottom edges; an unchanged pixel inside a tile may still lie within the returned region

Alignment permits 4,096 pages on either side and 16,777,216 cells; text processing permits 65,536 tokens and 16,777,216 UTF-16 units, and explicitly reports truncation when a chosen detail budget runs out

Public declarations

type
  TPdfDiffPageKind= (dpkMatched, dpkModified, dpkMoved, dpkInserted, dpkDeleted);
  TPdfDiffTextMode= (dtmNone, dtmCharacters, dtmWords);
  TPdfDiffPageSignature= record
    Text: WideString;
    VisualHash: UInt64;
    Width, Height: Single;
    Rotation: Integer;
  end;
  TPdfDiffPageSignatures= array of TPdfDiffPageSignature;
  TPdfDiffPagePair= record
    LeftPageIndex, RightPageIndex: Integer;
    Kind: TPdfDiffPageKind;
  end;
  TPdfDiffPagePairs= array of TPdfDiffPagePair;
  // Offsets and lengths count UTF-16 code units, starting at zero
  TPdfDiffTextChange= record
    LeftStart, LeftLength, RightStart, RightLength: Integer;
  end;
  TPdfDiffTextChanges= array of TPdfDiffTextChange;
  TPdfDiffPixelRegion= record
    Left, Top, Right, Bottom: Integer;
  end;
  TPdfDiffPixelRegions= array of TPdfDiffPixelRegion;

function AlignPdfDiffPages(const Left, Right: TPdfDiffPageSignatures;
  MaxCells: Integer): TPdfDiffPagePairs;
function BuildPdfDiffTextChanges(const Left, Right: WideString;
  Mode: TPdfDiffTextMode; MaxCells, MaxTokens: Integer;
  out Truncated: Boolean): TPdfDiffTextChanges;
function BuildPdfDiffPixelRegions(const Mask: TBytes; Width, Height,
  TileSize, MaxRegions: Integer; out Truncated: Boolean): TPdfDiffPixelRegions;