feat: add first-class text layers

core/text-layer.ts

@@ -0,0 +1,44 @@
+import type { BaseLayer } from "./base-layer";
+import type { Size } from "./geometry";
+
+export const builtInTextFonts = ["Arial", "Georgia", "Courier New", "Trebuchet MS"] as const;
+export type TextFontFamily = typeof builtInTextFonts[number];
+export type TextAlignment = "left" | "center" | "right";
+export type TextFontStyle = "normal" | "italic";
+export type TextFontWeight = 400 | 700;
+
+export type TextStyle = {
+  fontFamily: TextFontFamily;
+  fontSize: number;
+  fontWeight: TextFontWeight;
+  fontStyle: TextFontStyle;
+  color: string;
+  alignment: TextAlignment;
+  lineHeight: number;
+};
+
+export type TextLayer = BaseLayer & {
+  type: "text";
+  content: string;
+  style: TextStyle;
+};
+
+/** Deterministic layout box shared by selection, renderer and export. */
+export function measureTextLayer(layer: Pick<TextLayer, "content" | "style">): Size {
+  const lines = layer.content.split("\n");
+  const weightFactor = layer.style.fontWeight === 700 ? 1.04 : 1;
+  const italicFactor = layer.style.fontStyle === "italic" ? 1.03 : 1;
+  const familyFactor = layer.style.fontFamily === "Courier New" ? 0.62 : layer.style.fontFamily === "Georgia" ? 0.56 : 0.54;
+  const longest = Math.max(1, ...lines.map((line) => [...line].length));
+  return {
+    w: Math.max(1, longest * layer.style.fontSize * familyFactor * weightFactor * italicFactor),
+    h: Math.max(1, lines.length * layer.style.fontSize * layer.style.lineHeight),
+  };
+}
+
+export function isValidTextStyle(style: TextStyle): boolean {
+  return builtInTextFonts.includes(style.fontFamily) && Number.isFinite(style.fontSize) && style.fontSize >= 1 && style.fontSize <= 1000
+    && (style.fontWeight === 400 || style.fontWeight === 700) && (style.fontStyle === "normal" || style.fontStyle === "italic")
+    && /^#[0-9a-f]{6}$/i.test(style.color) && (style.alignment === "left" || style.alignment === "center" || style.alignment === "right")
+    && Number.isFinite(style.lineHeight) && style.lineHeight >= 0.5 && style.lineHeight <= 5;
+}