// Values checked at run time and typed at compile time. A descriptor states its property values, directive payloads
// and declaration options with these; parse() returns the value or throws a SchemaError that names where it went
// wrong. Written here rather than taken from a package: mqtt stays the only runtime dependency.

/** A value that does not fit its schema: path is where ("payload.targetSetpoint.scale"), problem is what. */
export class SchemaError extends Error {
  constructor(
    readonly path: string,
    readonly problem: string
  ) {
    super(path ? `${path}: ${problem}` : problem);
    this.name = "SchemaError";
  }
}

export interface Schema<T> {
  /** What the schema accepts, in the words of an error message: "an integer from 0 to 100", "ON | OFF". */
  readonly expects: string;
  /** The checked value. path names it in the error message. */
  parse(input: unknown, path?: string): T;
}

/** A schema that accepts a missing value; s.object() makes its key optional in the inferred type. */
export interface OptionalSchema<T> extends Schema<T | undefined> {
  readonly optional: true;
}

export interface EnumSchema<V> extends Schema<V> {
  readonly values: readonly V[];
}

/** The type a schema produces. */
export type Infer<S> = S extends Schema<infer T> ? T : never;

export type Shape = Record<string, Schema<any>>;
type OptionalKeys<S extends Shape> = { [K in keyof S]: S[K] extends OptionalSchema<any> ? K : never }[keyof S];
export type InferShape<S extends Shape> = { [K in Exclude<keyof S, OptionalKeys<S>>]: Infer<S[K]> } & {
  [K in OptionalKeys<S>]?: Infer<S[K]>;
};

export interface NumberRules {
  min?: number;
  max?: number;
  /** Greater than, the bound itself excluded. */
  gt?: number;
  integer?: boolean;
}

export interface Temperature {
  value: number;
  scale: "CELSIUS" | "FAHRENHEIT" | "KELVIN";
}

export interface TimeInterval {
  start?: string;
  end?: string;
  duration?: string;
}

// The input as an error message shows it: short, and quoted when it is text.
function shown(input: unknown): string {
  if (input === undefined) return "nothing";
  if (typeof input === "function") return "a function";
  const text = JSON.stringify(input) ?? String(input);
  return text.length > 60 ? `${text.slice(0, 57)}...` : text;
}

/** The error of a value that is not what a schema expects. */
export function mismatch(path: string, expects: string, input: unknown): SchemaError {
  return new SchemaError(path, `expected ${expects}, got ${shown(input)}`);
}

/** The path of a key of the object at path. */
export const at = (path: string, key: string): string => (path ? `${path}.${key}` : key);

function isRecord(input: unknown): input is Record<string, unknown> {
  return typeof input === "object" && input !== null && !Array.isArray(input);
}

function schema<T>(expects: string, accepts: (input: unknown) => boolean): Schema<T> {
  return {
    expects,
    parse(input, path = "") {
      if (!accepts(input)) throw mismatch(path, expects, input);
      return input as T;
    },
  };
}

function numberText({ min, max, gt, integer }: NumberRules): string {
  const kind = integer ? "an integer" : "a number";
  if (min !== undefined && max !== undefined) return `${kind} from ${min} to ${max}`;
  if (min !== undefined) return `${kind} of ${min} or more`;
  if (max !== undefined) return `${kind} of ${max} or less`;
  if (gt !== undefined) return `${kind} greater than ${gt}`;
  return kind;
}

function number(rules: NumberRules = {}): Schema<number> {
  const { min = -Infinity, max = Infinity, gt = -Infinity, integer = false } = rules;
  return schema(numberText(rules), (input) =>
    typeof input === "number" && Number.isFinite(input) && input >= min && input <= max && input > gt
      && (!integer || Number.isInteger(input)));
}

function string(rules: { min?: number; max?: number; pattern?: RegExp; expects?: string } = {}): Schema<string> {
  const { min = 0, max = Infinity, pattern } = rules;
  const length = max === Infinity ? (min > 0 ? ` of ${min} or more characters` : "") : ` of ${min} to ${max} characters`;
  return schema(rules.expects ?? `a string${length}`, (input) =>
    typeof input === "string" && input.length >= min && input.length <= max && (!pattern || pattern.test(input)));
}

function boolean(): Schema<boolean> {
  return schema("true or false", (input) => typeof input === "boolean");
}

function literal<V extends string | number | boolean | null>(value: V): Schema<V> {
  return schema(JSON.stringify(value), (input) => input === value);
}

function enumeration<V extends readonly string[]>(...values: V): EnumSchema<V[number]> {
  return oneOf(values, values.join(" | "));
}

/** One of a list too long to print in an error message: expects says what the list is. */
function oneOf<V extends string>(values: readonly V[], expects: string): EnumSchema<V> {
  return { ...schema<V>(expects, (input) => values.includes(input as V)), values };
}

function unknown(): Schema<unknown> {
  return schema("any value", () => true);
}

function optional<T>(inner: Schema<T>): OptionalSchema<T> {
  return {
    expects: inner.expects,
    optional: true,
    parse: (input, path = "") => (input === undefined ? undefined : inner.parse(input, path)),
  };
}

/** A value that is made when none is given: the time of an event, which is now unless the caller says when. */
function defaulted<T>(inner: Schema<T>, make: () => T): Schema<T> {
  return {
    expects: inner.expects,
    parse: (input, path = "") => (input === undefined ? make() : inner.parse(input, path)),
  };
}

function nullable<T>(inner: Schema<T>): Schema<T | null> {
  const expects = `${inner.expects} or null`;
  return {
    expects,
    parse(input, path = "") {
      if (input === null) return null;
      try {
        return inner.parse(input, path);
      } catch (err) {
        // The value itself is of the wrong kind: null was a choice too. An error further in keeps its own path.
        if (err instanceof SchemaError && err.path === path) throw mismatch(path, expects, input);
        throw err;
      }
    },
  };
}

function array<T>(item: Schema<T>, rules: { min?: number; max?: number } = {}): Schema<T[]> {
  const { min = 0, max = Infinity } = rules;
  const count = max === Infinity ? (min > 0 ? ` with ${min} or more entries` : "") : ` with ${min} to ${max} entries`;
  const expects = `a list${count}`;
  return {
    expects,
    parse(input, path = "") {
      if (!Array.isArray(input) || input.length < min || input.length > max) throw mismatch(path, expects, input);
      return input.map((entry, i) => item.parse(entry, `${path}[${i}]`));
    },
  };
}

/**
 * An object with the keys of shape. Keys the shape does not name are kept as they are: a field Alexa adds to a
 * directive reaches the handler. unknownKeys "reject" is for what a developer writes, where such a key is a typo.
 */
function object<S extends Shape>(shape: S, rules: { unknownKeys?: "keep" | "reject" } = {}): Schema<InferShape<S>> {
  const known = Object.keys(shape);
  const expects = known.length > 0 ? `an object with ${known.join(", ")}` : "an object";
  return {
    expects,
    parse(input, path = "") {
      if (!isRecord(input)) throw mismatch(path, expects, input);
      const others = Object.keys(input).filter((key) => !known.includes(key));
      if (others.length > 0 && rules.unknownKeys === "reject") {
        throw new SchemaError(at(path, others[0]), `unknown key, the known ones are ${known.join(", ") || "none"}`);
      }
      const parsed: Record<string, unknown> = {};
      for (const key of known) {
        const value = shape[key].parse(input[key], at(path, key));
        if (value !== undefined) parsed[key] = value;
      }
      for (const key of others) parsed[key] = input[key];
      return parsed as InferShape<S>;
    },
  };
}

// alexa-property-schemas.html "Temperature", "Temperature scales"
function temperature(): Schema<Temperature> {
  return object({ value: number(), scale: enumeration("CELSIUS", "FAHRENHEIT", "KELVIN") });
}

// alexa-property-schemas.html "DateTime": UTC, no offsets. The seconds are optional here because the TimeInterval
// examples on the same page leave them out ("2017-10-04T14:00Z").
const DATE_TIME = /^\d{4}-\d{2}-\d{2}T\d{2}:\d{2}(:\d{2}(\.\d+)?)?Z$/;
function dateTime(): Schema<string> {
  return schema("a UTC time like 2017-08-30T01:18:21Z", (input) =>
    typeof input === "string" && DATE_TIME.test(input) && !Number.isNaN(Date.parse(input)));
}

// alexa-property-schemas.html "Duration": the time portion of ISO 8601, negative for a delta ("PT-30S")
const DURATION = /^PT(?=.)(-?\d+H)?(-?\d+M)?(-?\d+S)?$/;
function duration(): Schema<string> {
  return string({ pattern: DURATION, expects: "a duration like PT3M15S" });
}

// alexa-property-schemas.html "TimeInterval": "Specify one or two of the time interval fields. If you specify all
// three fields, an error occurs."
function timeInterval(): Schema<TimeInterval> {
  const fields = object({ start: optional(dateTime()), end: optional(dateTime()), duration: optional(duration()) });
  const expects = "a time interval with one or two of start, end, duration";
  return {
    expects,
    parse(input, path = "") {
      const interval = fields.parse(input, path);
      const given = [interval.start, interval.end, interval.duration].filter((field) => field !== undefined).length;
      if (given < 1 || given > 2) throw mismatch(path, expects, input);
      return interval;
    },
  };
}

export const s = {
  string,
  number,
  boolean,
  literal,
  enum: enumeration,
  oneOf,
  unknown,
  optional,
  defaulted,
  nullable,
  array,
  object,
  temperature,
  dateTime,
  duration,
  timeInterval,
};