ReadViewSDK/Sources/RDEpubReaderView/EPUBTextRendering/Pagination/RDEPUBPageBreakPolicy.swift

import Foundation
import UIKit

struct RDEPUBPageBreakPolicy {

    private let attributedString: NSAttributedString

    init(attributedString: NSAttributedString) {
        self.attributedString = attributedString
    }

    func lineIsInAvoidPageBreakInsideBlock(_ lineRange: NSRange) -> Bool {
        guard let probeRange = clampedProbeRange(for: lineRange) else {
            return false
        }
        var found = false
        attributedString.enumerateAttributes(in: probeRange) { attributes, _, stop in
            guard shouldTreatAvoidHintAsBlockProtection(attributes) else {
                return
            }
            guard let rawValue = attributes[.rdPageSemanticHints] as? String else { return }
            let hints = rawValue
                .split(separator: ",")
                .compactMap { RDEPUBTextSemanticHint(rawValue: String($0)) }
            if hints.contains(.avoidPageBreakInside) {
                found = true
                stop.pointee = true
            }
        }
        return found
    }

    func lineIsInKeepWithNextBlock(_ lineRange: NSRange) -> Bool {
        guard let probeRange = clampedProbeRange(for: lineRange) else {
            return false
        }
        var found = false
        attributedString.enumerateAttribute(.rdPageSemanticHints, in: probeRange) { value, _, stop in
            guard let rawValue = value as? String else { return }
            let hints = rawValue
                .split(separator: ",")
                .compactMap { RDEPUBTextSemanticHint(rawValue: String($0)) }
            if hints.contains(.keepWithNext) {
                found = true
                stop.pointee = true
            }
        }
        return found
    }

    func adjustedRange(
        from proposedRange: NSRange,
        totalLength: Int,
        lineRanges: [NSRange],
        factory: RDEPUBCoreTextPageFrameFactory
    ) -> (
        range: NSRange,
        breakReason: RDEPUBTextPageBreakReason,
        blockRange: NSRange?,
        attachmentRanges: [NSRange],
        attachmentKinds: [RDEPUBTextAttachmentKind],
        blockKinds: [RDEPUBTextBlockKind],
        semanticHints: [RDEPUBTextSemanticHint],
        attachmentPlacements: [RDEPUBTextAttachmentPlacement],
        diagnostics: [String]
    ) {
        let pageEnd = proposedRange.location + proposedRange.length
        let proposedBlockKinds = factory.blockKinds(in: proposedRange)
        let proposedSemanticHints = factory.semanticHints(in: proposedRange)
        let proposedAttachmentPlacements = factory.attachmentPlacements(in: proposedRange)
        guard pageEnd < totalLength else {
            return (
                range: proposedRange,
                breakReason: .chapterEnd,
                blockRange: factory.blockRange(at: max(proposedRange.location, pageEnd - 1)),
                attachmentRanges: factory.attachmentRanges(in: proposedRange),
                attachmentKinds: factory.attachmentKinds(in: proposedRange),
                blockKinds: proposedBlockKinds,
                semanticHints: proposedSemanticHints,
                attachmentPlacements: proposedAttachmentPlacements,
                diagnostics: factory.diagnostics(
                    reason: .chapterEnd,
                    range: proposedRange,
                    attachmentRanges: factory.attachmentRanges(in: proposedRange),
                    blockRange: factory.blockRange(at: max(proposedRange.location, pageEnd - 1)),
                    blockKinds: proposedBlockKinds,
                    semanticHints: proposedSemanticHints,
                    attachmentPlacements: proposedAttachmentPlacements
                )
            )
        }

        let currentBlockRange = factory.blockRange(at: max(proposedRange.location, pageEnd - 1))
        let currentAttachmentRanges = factory.attachmentRanges(in: proposedRange)
        let currentAttachmentKinds = factory.attachmentKinds(in: proposedRange)
        let currentBlockKinds = proposedBlockKinds
        let currentSemanticHints = proposedSemanticHints
        let currentAttachmentPlacements = proposedAttachmentPlacements

        if let pageRelateBoundary = preferredPageRelateBoundary(
            after: proposedRange,
            minimumEnd: proposedRange.location + 1,
            lineRanges: lineRanges,
            factory: factory
        ) {
            let adjustedRange = NSRange(location: proposedRange.location, length: pageRelateBoundary - proposedRange.location)
            return (
                range: adjustedRange,
                breakReason: .semanticBoundary,
                blockRange: currentBlockRange,
                attachmentRanges: currentAttachmentRanges,
                attachmentKinds: currentAttachmentKinds,
                blockKinds: currentBlockKinds,
                semanticHints: currentSemanticHints,
                attachmentPlacements: currentAttachmentPlacements,
                diagnostics: factory.diagnostics(
                    reason: .semanticBoundary,
                    range: adjustedRange,
                    attachmentRanges: currentAttachmentRanges,
                    blockRange: currentBlockRange,
                    blockKinds: currentBlockKinds,
                    semanticHints: currentSemanticHints,
                    attachmentPlacements: currentAttachmentPlacements,
                    trigger: RDEPUBTextSemanticHint.pageRelate.rawValue
                )
            )
        }

        return (
            range: proposedRange,
            breakReason: .frameLimit,
            blockRange: currentBlockRange,
            attachmentRanges: currentAttachmentRanges,
            attachmentKinds: currentAttachmentKinds,
            blockKinds: currentBlockKinds,
            semanticHints: currentSemanticHints,
            attachmentPlacements: currentAttachmentPlacements,
            diagnostics: factory.diagnostics(
                reason: .frameLimit,
                range: proposedRange,
                attachmentRanges: currentAttachmentRanges,
                blockRange: currentBlockRange,
                blockKinds: currentBlockKinds,
                semanticHints: currentSemanticHints,
                attachmentPlacements: currentAttachmentPlacements
            )
        )
    }

    func preferredPageRelateBoundary(
        after range: NSRange,
        minimumEnd: Int,
        lineRanges: [NSRange],
        factory: RDEPUBCoreTextPageFrameFactory
    ) -> Int? {
        let pageStartOfNext = range.location + range.length
        guard pageStartOfNext > range.location,
              pageStartOfNext < attributedString.length,
              lineRanges.count >= 2,
              factory.semanticHints(at: pageStartOfNext).contains(.pageRelate) else {
            return nil
        }

        let boundaryBlockStart = factory.blockRange(at: pageStartOfNext)?.location
            ?? factory.paragraphRange(containing: pageStartOfNext).location
        guard boundaryBlockStart == pageStartOfNext else { return nil }

        let lastLineStart = lineRanges[lineRanges.count - 1].location
        guard lastLineStart > range.location, lastLineStart >= minimumEnd else {
            return nil
        }
        return lastLineStart
    }

    private func shouldTreatAvoidHintAsBlockProtection(_ attributes: [NSAttributedString.Key: Any]) -> Bool {
        guard let rawValue = attributes[.rdPageSemanticHints] as? String else {
            return false
        }
        let hints = rawValue
            .split(separator: ",")
            .compactMap { RDEPUBTextSemanticHint(rawValue: String($0)) }
        guard hints.contains(.avoidPageBreakInside) else {
            return false
        }

        let placement = (attributes[.rdPageAttachmentPlacement] as? String)
            .flatMap(RDEPUBTextAttachmentPlacement.init(rawValue:))
        let blockKind = (attributes[.rdPageBlockKind] as? String)
            .flatMap(RDEPUBTextBlockKind.init(rawValue:))

        // Inline attachments (footnote icons, rare-character images) flow with
        // the surrounding text, so their avoid hint must not lock the line to
        // the next page. Placement is checked besides blockKind because an
        // enclosing paragraph's semantics overwrite blockKind on the
        // attachment's range, while placement survives.
        if blockKind == .attachment || placement != nil, placement != .centered {
            return false
        }

        return true
    }

    private func clampedProbeRange(for lineRange: NSRange) -> NSRange? {
        clampedRange(NSRange(location: lineRange.location, length: max(lineRange.length, 1)))
    }

    private func clampedRange(_ range: NSRange) -> NSRange? {
        guard range.location >= 0, range.length >= 0 else { return nil }
        guard attributedString.length > 0 else {
            return range.location == 0 ? NSRange(location: 0, length: 0) : nil
        }
        guard range.location < attributedString.length else { return nil }

        let maxLength = attributedString.length - range.location
        return NSRange(location: range.location, length: min(range.length, maxLength))
    }
}