chore: initial commit

src/lib/layout.ts

@@ -0,0 +1,109 @@
+import { Png2PdfError } from "./errors.js";
+
+export const PAGE_SIZES = ["fit", "a4", "letter"] as const;
+export type PageSize = (typeof PAGE_SIZES)[number];
+
+export type PageLayoutOptions = {
+  pixelWidth: number;
+  pixelHeight: number;
+  dpi?: number;
+  pageSize?: PageSize;
+  margin?: number;
+};
+
+export type PageLayout = {
+  pageWidth: number;
+  pageHeight: number;
+  imageX: number;
+  imageY: number;
+  imageWidth: number;
+  imageHeight: number;
+};
+
+const POINTS_PER_INCH = 72;
+const DEFAULT_DPI = 96;
+const PAPER_DIMENSIONS = {
+  a4: { width: 595.28, height: 841.89 },
+  letter: { width: 612, height: 792 },
+} as const satisfies Record<
+  Exclude<PageSize, "fit">,
+  { width: number; height: number }
+>;
+
+export function isPageSize(value: string): value is PageSize {
+  return (PAGE_SIZES as readonly string[]).includes(value);
+}
+
+export function calculatePageLayout(options: PageLayoutOptions): PageLayout {
+  const {
+    pixelWidth,
+    pixelHeight,
+    dpi = DEFAULT_DPI,
+    pageSize = "fit",
+    margin = 0,
+  } = options;
+
+  assertPositiveFinite(pixelWidth, "Image width");
+  assertPositiveFinite(pixelHeight, "Image height");
+  assertPositiveFinite(dpi, "DPI");
+  assertNonNegativeFinite(margin, "Margin");
+
+  const imageWidth = pixelsToPoints(pixelWidth, dpi);
+  const imageHeight = pixelsToPoints(pixelHeight, dpi);
+
+  if (pageSize === "fit") {
+    return {
+      pageWidth: imageWidth + margin * 2,
+      pageHeight: imageHeight + margin * 2,
+      imageX: margin,
+      imageY: margin,
+      imageWidth,
+      imageHeight,
+    };
+  }
+
+  const paper = PAPER_DIMENSIONS[pageSize];
+  const landscape = imageWidth > imageHeight;
+  const pageWidth = landscape ? paper.height : paper.width;
+  const pageHeight = landscape ? paper.width : paper.height;
+  const availableWidth = pageWidth - margin * 2;
+  const availableHeight = pageHeight - margin * 2;
+
+  if (availableWidth <= 0 || availableHeight <= 0) {
+    throw new Png2PdfError(
+      `Margin ${margin} pt is too large for ${pageSize.toUpperCase()} paper.`,
+    );
+  }
+
+  const scale = Math.min(
+    availableWidth / imageWidth,
+    availableHeight / imageHeight,
+  );
+  const drawnWidth = imageWidth * scale;
+  const drawnHeight = imageHeight * scale;
+
+  return {
+    pageWidth,
+    pageHeight,
+    imageX: (pageWidth - drawnWidth) / 2,
+    imageY: (pageHeight - drawnHeight) / 2,
+    imageWidth: drawnWidth,
+    imageHeight: drawnHeight,
+  };
+}
+
+function pixelsToPoints(pixels: number, dpi: number): number {
+  return (pixels / dpi) * POINTS_PER_INCH;
+}
+
+function assertPositiveFinite(value: number, label: string): void {
+  if (!Number.isFinite(value) || value <= 0) {
+    throw new Png2PdfError(`${label} must be a positive number.`);
+  }
+}
+
+function assertNonNegativeFinite(value: number, label: string): void {
+  if (!Number.isFinite(value) || value < 0) {
+    throw new Png2PdfError(`${label} must be zero or greater.`);
+  }
+}