// Renders Snapshot's app icon at every size an .iconset needs.
//
// Drawn rather than designed so the icon is reproducible from source and has no
// binary asset to keep in the repository. SF Symbols are deliberately not used:
// Apple's licence forbids them in app icons.
//
// The mark is three cards stacked along a diagonal — successive snapshots of the
// same thing, receding into the past. Cards rather than horizontal bars: three
// equal bars are the universal hamburger-menu glyph and read as a menu, not a
// backup. The front card is fully opaque so it occludes the ones behind cleanly
// instead of blending into mush at small sizes.
import AppKit
import CoreGraphics
import Foundation
import ImageIO
import UniformTypeIdentifiers

let sizes: [(name: String, pixels: Int)] = [
    ("icon_16x16", 16), ("icon_16x16@2x", 32),
    ("icon_32x32", 32), ("icon_32x32@2x", 64),
    ("icon_128x128", 128), ("icon_128x128@2x", 256),
    ("icon_256x256", 256), ("icon_256x256@2x", 512),
    ("icon_512x512", 512), ("icon_512x512@2x", 1024),
]

func draw(pixels: Int) -> CGImage? {
    let side = CGFloat(pixels)
    let scale = side / 1024
    guard
        let context = CGContext(
            data: nil, width: pixels, height: pixels, bitsPerComponent: 8, bytesPerRow: 0,
            space: CGColorSpace(name: CGColorSpace.sRGB)!,
            bitmapInfo: CGImageAlphaInfo.premultipliedLast.rawValue)
    else { return nil }

    context.setShouldAntialias(true)
    context.interpolationQuality = .high

    // macOS icons sit inset within the canvas rather than filling it.
    let inset: CGFloat = 100 * scale
    let plate = CGRect(x: inset, y: inset, width: side - inset * 2, height: side - inset * 2)
    let radius = plate.width * 0.2246  // the Big Sur corner ratio

    context.saveGState()
    context.addPath(CGPath(roundedRect: plate, cornerWidth: radius, cornerHeight: radius, transform: nil))
    context.clip()
    let gradient = CGGradient(
        colorsSpace: CGColorSpace(name: CGColorSpace.sRGB)!,
        colors: [
            CGColor(red: 0.30, green: 0.72, blue: 0.76, alpha: 1),
            CGColor(red: 0.16, green: 0.36, blue: 0.68, alpha: 1),
        ] as CFArray,
        locations: [0, 1])!
    context.drawLinearGradient(
        gradient,
        start: CGPoint(x: plate.midX, y: plate.maxY),
        end: CGPoint(x: plate.midX, y: plate.minY),
        options: [])
    context.restoreGState()

    let card: CGFloat = 400 * scale
    let cornerRadius = card * 0.22
    let step: CGFloat = 78 * scale
    // Three positions along a diagonal, centred as a group on the plate.
    let spread = step * 2
    let originX = plate.midX - card / 2 - spread / 2
    let originY = plate.midY - card / 2 + spread / 2

    // Back to front, so the opaque front card covers the ones behind it.
    for (index, opacity) in [0.45, 0.70, 1.0].enumerated() {
        let offset = CGFloat(2 - index)
        let rect = CGRect(
            x: originX + step * offset,
            y: originY - step * offset,
            width: card,
            height: card)
        context.setFillColor(CGColor(red: 1, green: 1, blue: 1, alpha: opacity))
        context.addPath(
            CGPath(
                roundedRect: rect, cornerWidth: cornerRadius, cornerHeight: cornerRadius,
                transform: nil))
        context.fillPath()
    }

    return context.makeImage()
}

let output = URL(fileURLWithPath: CommandLine.arguments[1], isDirectory: true)
try? FileManager.default.createDirectory(at: output, withIntermediateDirectories: true)

for entry in sizes {
    guard let image = draw(pixels: entry.pixels) else {
        FileHandle.standardError.write(Data("could not render \(entry.name)\n".utf8))
        exit(1)
    }
    let url = output.appendingPathComponent("\(entry.name).png")
    guard
        let destination = CGImageDestinationCreateWithURL(
            url as CFURL, UTType.png.identifier as CFString, 1, nil)
    else { exit(1) }
    CGImageDestinationAddImage(destination, image, nil)
    CGImageDestinationFinalize(destination)
}
print("rendered \(sizes.count) icon sizes")