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Optimize two-pages spread computation #527

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Jan 6, 2025
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Optimize two-pages spread computation #527

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104 changes: 52 additions & 52 deletions Sources/Navigator/EPUB/EPUBSpread.swift
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Original file line number Diff line number Diff line change
Expand Up @@ -10,17 +10,17 @@ import ReadiumShared
/// A list of EPUB resources to be displayed together on the screen, as one-page or two-pages spread.
struct EPUBSpread: Loggable {
/// Indicates whether two pages are displayed side by side.
let spread: Bool
var spread: Bool

/// Links for the resources displayed in the spread, in reading order.
/// Note: it's possible to have less links than the amount of `pageCount` available, because a single page might be displayed in a two-page spread (eg. with Properties.Page center, left or right)
let links: [Link]
var links: [Link]

/// Spread reading progression direction.
let readingProgression: ReadingProgression
var readingProgression: ReadingProgression

/// Rendition layout of the links in the spread.
let layout: EPUBLayout
var layout: EPUBLayout

init(spread: Bool, links: [Link], readingProgression: ReadingProgression, layout: EPUBLayout) {
precondition(!links.isEmpty, "A spread must have at least one page")
Expand Down Expand Up @@ -143,64 +143,40 @@ struct EPUBSpread: Loggable {
/// Builds a list of two-page spreads for the given Publication.
private static func makeTwoPagesSpreads(
for publication: Publication,
readingOrder: [Link],
readingOrder links: [Link],
readingProgression: ReadingProgression
) -> [EPUBSpread] {
/// Builds two-pages spreads from a list of links and a spread accumulator.
func makeSpreads(for links: [Link], index: Int, in spreads: [EPUBSpread] = []) -> [EPUBSpread] {
var links = links
var spreads = spreads
guard !links.isEmpty else {
return spreads
}
var spreads: [EPUBSpread] = []

let first = links.removeFirst()
var index = 0
while index < links.count {
let first = links[index]
let layout = publication.metadata.presentation.layout(of: first)
// To be displayed together, the two pages must have a fixed layout, and have consecutive position hints (Properties.Page).
if let second = links.first,
layout == .fixed,
layout == publication.metadata.presentation.layout(of: second),
areConsecutive(first, second, index: index)
{
spreads.append(EPUBSpread(
spread: true,
links: [first, second],
readingProgression: readingProgression, layout: layout
))
links.removeFirst() // Removes the consumed "second" page
} else {
spreads.append(EPUBSpread(
spread: true,
links: [first],
readingProgression: readingProgression, layout: layout
))
}

return makeSpreads(for: links, index: index + 1, in: spreads)
}
var spread = EPUBSpread(
spread: true,
links: [first],
readingProgression: readingProgression,
layout: layout
)

/// Two resources are consecutive if their position hint (Properties.Page) are paired according to the reading progression.
func areConsecutive(_ first: Link, _ second: Link, index: Int) -> Bool {
guard index > 0 || first.properties.page != nil else {
return false
// To be displayed together, the two pages must have a fixed layout,
// and have consecutive position hints (Properties.Page).
if
let second = links.getOrNil(index + 1),
layout == .fixed,
layout == publication.metadata.presentation.layout(of: second),
publication.areConsecutive(first, second, index: index)
{
spread.links.append(second)
index += 1 // Skips the consumed "second" page
}

// Here we use the default publication reading progression instead
// of the custom one provided, otherwise the page position hints
// might be wrong, and we could end up with only one-page spreads.
switch publication.metadata.readingProgression {
case .ltr, .ttb, .auto:
let firstPosition = first.properties.page ?? .left
let secondPosition = second.properties.page ?? .right
return firstPosition == .left && secondPosition == .right
case .rtl, .btt:
let firstPosition = first.properties.page ?? .right
let secondPosition = second.properties.page ?? .left
return firstPosition == .right && secondPosition == .left
}
spreads.append(spread)
index += 1
}

return makeSpreads(for: readingOrder, index: 0)
return spreads
}
}

Expand All @@ -213,3 +189,27 @@ extension Array where Element == EPUBSpread {
}
}
}

private extension Publication {
/// Two resources are consecutive if their position hint (Properties.Page)
/// are paired according to the reading progression.
func areConsecutive(_ first: Link, _ second: Link, index: Int) -> Bool {
guard index > 0 || first.properties.page != nil else {
return false
}

// Here we use the default publication reading progression instead
// of the custom one provided, otherwise the page position hints
// might be wrong, and we could end up with only one-page spreads.
switch metadata.readingProgression {
case .ltr, .ttb, .auto:
let firstPosition = first.properties.page ?? .left
let secondPosition = second.properties.page ?? .right
return firstPosition == .left && secondPosition == .right
case .rtl, .btt:
let firstPosition = first.properties.page ?? .right
let secondPosition = second.properties.page ?? .left
return firstPosition == .right && secondPosition == .left
}
}
}