Declare the register steps and the schema slice each one collects

web/frontend/app/features/auth/registerSteps.ts

@@ -0,0 +1,49 @@
+import type { z } from 'zod';
+import { registerFields, type RegisterSchema } from './schemas';
+
+interface RegisterStep {
+    name: string;
+    title: string;
+    subtitle: string;
+    /** The slice of the register schema this step collects. */
+    schema: z.ZodObject;
+    fields: readonly (keyof RegisterSchema)[];
+}
+
+const defineStep = (step: Omit<RegisterStep, 'fields'>): RegisterStep => ({
+    ...step,
+    fields: Object.keys(step.schema.shape) as (keyof RegisterSchema)[],
+});
+
+export const registerSteps: readonly RegisterStep[] = [
+    defineStep({
+        name: 'Account',
+        title: 'Create your account',
+        subtitle: 'Choose how you will sign in.',
+        schema: registerFields.pick({ username: true, email: true }),
+    }),
+    defineStep({
+        name: 'Personal details',
+        title: 'About you',
+        subtitle: 'Tell us a little about yourself.',
+        schema: registerFields.pick({ firstName: true, lastName: true, dateOfBirth: true }),
+    }),
+    defineStep({
+        name: 'Password',
+        title: 'Secure your account',
+        subtitle: 'Pick a strong password.',
+        schema: registerFields.pick({ password: true, confirmPassword: true }),
+    }),
+];
+
+export const lastRegisterStepIndex = registerSteps.length - 1;
+
+/** The step a field belongs to, so an error can open the step that owns it. */
+export const stepIndexForField = (field: keyof RegisterSchema) =>
+    registerSteps.findIndex((step) => step.fields.includes(field));
+
+/** The earliest step owning any of `fields`, or `null` when none of them do. */
+export const earliestStepForFields = (fields: readonly (keyof RegisterSchema)[]) => {
+    const indexes = fields.map(stepIndexForField).filter((index) => index >= 0);
+    return indexes.length ? Math.min(...indexes) : null;
+};