import type { FormFields } from "../contracts/typegen/http/request.ts";

/** Reads the fields of a parsed form body. */
export function toFormFields(body: unknown): FormFields {
	const source =
		body && typeof body === "object" && !Array.isArray(body)
			? (body as Record<string, unknown>)
			: {};
	return {
		get(name: string) {
			const value = source[name];
			if (value === undefined || value === null) return undefined;
			return Array.isArray(value) ? String(value[0]) : String(value);
		},
		all() {
			const result: Record<string, string | string[]> = {};
			for (const [key, value] of Object.entries(source)) {
				result[key] = Array.isArray(value)
					? value.map((entry) => String(entry))
					: String(value);
			}
			return result;
		},
		has(name: string) {
			return name in source;
		},
		values() {
			const result: Record<string, string> = {};
			for (const [key, value] of Object.entries(source)) {
				result[key] = Array.isArray(value)
					? String(value[0] ?? "")
					: String(value);
			}
			return result;
		},
	};
}

/** Rules of a single form field. */
export interface FieldRule {
	/** The field has to be filled in. */
	required?: boolean;
	/** Type of the field. */
	type?: "string" | "number" | "boolean" | "email";
	/** Minimum value of a number field. */
	min?: number;
	/** Maximum value of a number field. */
	max?: number;
	/** Maximum length of a string field. */
	maxLength?: number;
	/** Allowed values. */
	enum?: string[];
	/** Value used when the field is empty. */
	default?: unknown;
}

export type FormSchema = Record<string, FieldRule>;

/** Result of a validation. */
export type ValidationResult<T> =
	| { ok: true; value: T; errors: Record<string, string> }
	| { ok: false; value: undefined; errors: Record<string, string> };

const EMAIL = /^[^\s@]+@[^\s@]+\.[^\s@]+$/;

/** Validates the fields of a form. */
export function validate<T extends Record<string, unknown>>(
	fields: FormFields,
	schema: FormSchema,
): ValidationResult<T> {
	const errors: Record<string, string> = {};
	const value: Record<string, unknown> = {};

	for (const [name, rule] of Object.entries(schema)) {
		const raw = fields.get(name);
		const empty = raw === undefined || raw.trim().length === 0;

		if (empty) {
			if (rule.default !== undefined) {
				value[name] = rule.default;
				continue;
			}
			if (rule.required) {
				errors[name] = `${name} is required`;
			}
			continue;
		}

		let parsed: unknown = raw;
		if (rule.type === "number") {
			const numeric = Number(raw);
			if (Number.isNaN(numeric)) {
				errors[name] = `${name} must be a number`;
				continue;
			}
			if (rule.min !== undefined && numeric < rule.min) {
				errors[name] = `${name} must be at least ${rule.min}`;
				continue;
			}
			if (rule.max !== undefined && numeric > rule.max) {
				errors[name] = `${name} must be at most ${rule.max}`;
				continue;
			}
			parsed = numeric;
		} else if (rule.type === "boolean") {
			parsed = raw === "on" || raw === "true" || raw === "1";
		} else if (rule.type === "email") {
			if (!EMAIL.test(raw)) {
				errors[name] = `${name} must be a valid email address`;
				continue;
			}
		}

		if (
			rule.maxLength !== undefined &&
			String(parsed).length > rule.maxLength
		) {
			errors[name] = `${name} must be at most ${rule.maxLength} characters`;
			continue;
		}
		if (rule.enum && !rule.enum.includes(String(parsed))) {
			errors[name] = `${name} must be one of ${rule.enum.join(", ")}`;
			continue;
		}

		value[name] = parsed;
	}

	if (Object.keys(errors).length > 0) {
		return { ok: false, value: undefined, errors };
	}
	return { ok: true, value: value as T, errors };
}

/**
 * Builds the location of a redirect that carries a message.
 */
export function redirectWithMessage(
	location: string,
	message?: string,
): string {
	if (!message) return location;
	const separator = location.includes("?") ? "&" : "?";
	return `${location}${separator}message=${encodeURIComponent(message)}`;
}