/**
 * configFile.ts
 * -------------
 * JSON config file loading + strict validation.
 *
 * Default path: ${XDG_CONFIG_HOME:-$HOME/.config}/move/config.json
 *
 * Schema (all keys optional; matching CLI flag names and units):
 *
 *   moveInterval   number   seconds, positive
 *   checkInterval  number   seconds, positive
 *   stepDelay      number   milliseconds, positive
 *   pattern        string   a registered strategy name
 *   verbose        boolean
 *   loop           boolean
 *
 * Unknown keys, wrong types, and non-positive numerics are rejected with a
 * `CliError` so the entry point can exit 2 (user error) with a clear
 * message pointing at the offending file. The removed `stepCount` /
 * `stepSize` keys are the exception: they're tolerated (ignored with a
 * one-line notice) so an older seeded config keeps working after upgrade.
 *
 * Return semantics:
 *   - `null` when no `explicitPath` was passed and the default path does
 *     not exist.  This is the "user has no config" happy path.
 *   - A `ConfigOverrides` when a file was found and validated.
 *   - Throws `CliError` if a problem is detected (missing explicit path,
 *     bad JSON, wrong shape, unknown keys, invalid values).
 */

import { existsSync, readFileSync, statSync } from "node:fs";

import { defaultConfigPath, type ConfigOverrides } from "./config.ts";
import { CliError } from "./errors.ts";
import { PATTERN_NAMES, resolvePatternName } from "./strategies.ts";

const ALLOWED_KEYS: ReadonlySet<string> = new Set<string>([
    "moveInterval",
    "checkInterval",
    "stepDelay",
    "pattern",
    "verbose",
    "loop",
]);

/**
 * Keys that used to be valid but have since been removed. They're tolerated
 * (not rejected like a genuine unknown key) so upgrading doesn't hard-fail a
 * config that was seeded with them — every pre-1.3.0 install has `stepCount`
 * in its file. They no longer do anything: sweep size and step count are now
 * properties of each movement pattern. A one-line notice points the user at
 * the file so they can remove them at leisure.
 */
const DEPRECATED_KEYS: ReadonlySet<string> = new Set<string>([
    "stepCount",
    "stepSize",
]);

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

function requirePositiveNumber(name: string, raw: unknown, path: string): number {
    if (typeof raw !== "number" || !Number.isFinite(raw) || raw <= 0) {
        throw new CliError(
            `invalid value for '${name}' in ${path}: ${JSON.stringify(raw)} (expected a positive number)`,
        );
    }
    return raw;
}

function requireBoolean(name: string, raw: unknown, path: string): boolean {
    if (typeof raw !== "boolean") {
        throw new CliError(
            `invalid value for '${name}' in ${path}: ${JSON.stringify(raw)} (expected a boolean)`,
        );
    }
    return raw;
}

function requirePatternName(name: string, raw: unknown, path: string): string {
    const canonical: string | null = typeof raw === "string" ? resolvePatternName(raw) : null;
    if (canonical === null) {
        throw new CliError(
            `invalid value for '${name}' in ${path}: ${JSON.stringify(raw)} (valid: ${PATTERN_NAMES.join(", ")})`,
        );
    }
    return canonical;
}

/**
 * Load and validate the config file. See module docstring for return
 * semantics.
 *
 * @param explicitPath - If provided (e.g., from `--config`), the file
 *                       must exist and validate. If `undefined`, fall
 *                       back to `defaultConfigPath()`; a missing default
 *                       file is silent (returns `null`).
 */
export function loadConfigFile(explicitPath: string | undefined): ConfigOverrides | null {
    const required: boolean = explicitPath !== undefined;
    const path: string = explicitPath ?? defaultConfigPath();

    if (!existsSync(path)) {
        if (required) {
            throw new CliError(`config file not found: ${path}`);
        }
        return null;
    }

    if (!statSync(path).isFile()) {
        throw new CliError(`config path is not a regular file: ${path}`);
    }

    let raw: string;
    try {
        raw = readFileSync(path, "utf-8");
    } catch (err: unknown) {
        const msg: string = err instanceof Error ? err.message : String(err);
        throw new CliError(`could not read config file ${path}: ${msg}`);
    }

    let parsed: unknown;
    try {
        parsed = JSON.parse(raw);
    } catch (err: unknown) {
        const msg: string = err instanceof Error ? err.message : String(err);
        throw new CliError(`config file ${path} is not valid JSON: ${msg}`);
    }

    if (!isPlainObject(parsed)) {
        throw new CliError(`config file ${path} must contain a JSON object at the root`);
    }

    // Strict mode: reject any key we don't know about (catches typos like
    // 'movInterval'), but tolerate keys we've since removed — collect those
    // and warn once, rather than hard-failing a config seeded by an older
    // install.
    const deprecatedFound: string[] = [];
    for (const key of Object.keys(parsed)) {
        if (ALLOWED_KEYS.has(key)) continue;
        if (DEPRECATED_KEYS.has(key)) {
            deprecatedFound.push(key);
            continue;
        }
        const allowed: string = [...ALLOWED_KEYS].join(", ");
        throw new CliError(`unknown key '${key}' in ${path} (allowed: ${allowed})`);
    }
    if (deprecatedFound.length > 0) {
        const names: string = deprecatedFound.map((k) => `'${k}'`).join(", ");
        process.stderr.write(
            `move: ignoring obsolete key(s) ${names} in ${path}\n` +
                `      (sweep size is now defined by each movement pattern)\n`,
        );
    }

    return {
        moveInterval:
            "moveInterval" in parsed
                ? requirePositiveNumber("moveInterval", parsed.moveInterval, path)
                : undefined,
        checkInterval:
            "checkInterval" in parsed
                ? requirePositiveNumber("checkInterval", parsed.checkInterval, path)
                : undefined,
        stepDelay:
            "stepDelay" in parsed
                ? requirePositiveNumber("stepDelay", parsed.stepDelay, path)
                : undefined,
        pattern:
            "pattern" in parsed
                ? requirePatternName("pattern", parsed.pattern, path)
                : undefined,
        verbose:
            "verbose" in parsed
                ? requireBoolean("verbose", parsed.verbose, path)
                : undefined,
        loop:
            "loop" in parsed
                ? requireBoolean("loop", parsed.loop, path)
                : undefined,
    };
}