/**
 * config.ts
 * ---------
 * Runtime configuration types, defaults, the layered resolver, and the
 * default config-file path.
 *
 * The keeper is driven by a single `Config` object that carries every
 * tunable it cares about, including the `verbose` flag. Defaults live in
 * `DEFAULT_CONFIG`; CLI and config-file overrides are layered on top by
 * `resolveConfig` rather than mutating the defaults, so the defaults stay
 * genuinely constant and the resolved config stays structurally typed.
 *
 * All numeric `Config` fields are in their internal units (ms, pixels).
 * The CLI and config file expose the time-valued fields in seconds for
 * ergonomics; `resolveConfig` performs the seconds->ms conversion at the
 * boundary so downstream code never has to think about it.
 *
 * Layering precedence (highest wins):
 *   CLI overrides  >  file overrides  >  DEFAULT_CONFIG
 */

import { join } from "node:path";

import { CliError } from "./errors.ts";

// Single source of truth for default values. The same file ships in the
// install tree and is copied to $XDG_CONFIG_HOME/move/config.json on a
// fresh install (only if no config exists there yet). Values use the CLI
// units (seconds for time fields, ms for stepDelay, pixels for stepCount);
// the seconds->ms conversion happens below where DEFAULT_CONFIG is built.
import seedRaw from "../scripts/config.default.json" with { type: "json" };

/**
 * The shape of a resolved runtime configuration. `readonly` to make
 * accidental mutation a type error.
 *
 * - `moveInterval`  — how long the mouse must be idle (no real movement)
 *                     before a synthetic sweep is triggered. Milliseconds.
 * - `checkInterval` — cadence of the idleness poll in the main loop.
 *                     Milliseconds.
 * - `stepDelay`     — pause between individual synthetic mouse steps inside
 *                     a sweep. Also the window in which the user can
 *                     "interrupt" by moving the cursor. Milliseconds.
 * - `stepCount`     — number of pixel-steps in a single sweep. Pixels.
 * - `verbose`       — whether per-sweep / interrupt / bounds events are
 *                     logged. The startup banner is always printed.
 */
export interface Config {
    readonly moveInterval: number;
    readonly checkInterval: number;
    readonly stepDelay: number;
    readonly stepCount: number;
    readonly verbose: boolean;
}

/**
 * Shape of `scripts/config.default.json` after parsing. The cast below
 * trusts the file's structure; `assertSeedShape` performs a small runtime
 * sanity check at import time so a corrupted seed file fails loudly
 * instead of silently producing `NaN` or `undefined` defaults.
 */
interface SeedShape {
    moveInterval: number;   // seconds
    checkInterval: number;  // seconds
    stepDelay: number;      // milliseconds
    stepCount: number;      // pixels
    verbose: boolean;
}

function assertSeedShape(raw: unknown): asserts raw is SeedShape {
    if (typeof raw !== "object" || raw === null) {
        throw new Error("scripts/config.default.json: root must be an object");
    }
    const r = raw as Record<string, unknown>;
    for (const key of ["moveInterval", "checkInterval", "stepDelay", "stepCount"] as const) {
        const v = r[key];
        if (typeof v !== "number" || !Number.isFinite(v) || v <= 0) {
            throw new Error(`scripts/config.default.json: '${key}' must be a positive finite number (got ${JSON.stringify(v)})`);
        }
    }
    if (typeof r.verbose !== "boolean") {
        throw new Error(`scripts/config.default.json: 'verbose' must be a boolean (got ${JSON.stringify(r.verbose)})`);
    }
}

assertSeedShape(seedRaw);
const seed: SeedShape = seedRaw;

/**
 * Built-in defaults used when neither the CLI nor the config file supplies
 * a value for a given field. Derived from `scripts/config.default.json`
 * (the single source of truth); time-valued fields are converted from
 * seconds to milliseconds here so the rest of the codebase works in
 * internal units.
 */
export const DEFAULT_CONFIG: Config = {
    moveInterval:  seed.moveInterval * 1000,
    checkInterval: seed.checkInterval * 1000,
    stepDelay:     seed.stepDelay,
    stepCount:     seed.stepCount,
    verbose:       seed.verbose,
};

/**
 * Common shape for override layers (CLI args and config-file content).
 *
 * `undefined` means "this layer doesn't supply a value"; the next layer
 * down (file overrides, then DEFAULT_CONFIG) is consulted in that case.
 *
 * Numeric fields are in CLI / config-file units:
 *   moveInterval, checkInterval — seconds
 *   stepDelay                   — milliseconds
 *   stepCount                   — pixels
 *
 * `verbose` is `boolean | undefined` like the numeric fields, so all five
 * fields share the same "first defined value wins" precedence logic.
 *
 * For the CLI specifically, `verbose` is `undefined` when `-V/--verbose`
 * was not passed and `true` when it was. There is no CLI off-switch
 * today, so CLI `false` doesn't occur — a file-set `verbose: true` cannot
 * be overridden back to false from the command line (see the Configuration
 * section of the README).
 */
export interface ConfigOverrides {
    readonly moveInterval: number | undefined;
    readonly checkInterval: number | undefined;
    readonly stepDelay: number | undefined;
    readonly stepCount: number | undefined;
    readonly verbose: boolean | undefined;
}

/**
 * Resolve the default config-file path per the XDG Base Directory Spec.
 * Honors `$XDG_CONFIG_HOME` if set; otherwise falls back to
 * `$HOME/.config`. The file itself is always `move/config.json` under
 * that base.
 *
 * Computed at call time (not at module load) so tests can override
 * `XDG_CONFIG_HOME` after import.
 */
export function defaultConfigPath(): string {
    const xdg = process.env.XDG_CONFIG_HOME;
    if (xdg && xdg.length > 0) {
        return join(xdg, "move", "config.json");
    }
    const home = process.env.HOME;
    if (!home || home.length === 0) {
        // Neither var is set; we don't have a sensible fallback. Throwing
        // CliError lets the entry-point's normal handler surface this as
        // 'move: cannot resolve default config path: ...' + exit 2 instead
        // of silently producing '/.config/move/config.json' and bewildering
        // the user with a downstream 'file not found' message.
        throw new CliError(
            "cannot resolve default config path: neither $XDG_CONFIG_HOME nor $HOME is set",
        );
    }
    return join(home, ".config", "move", "config.json");
}

/**
 * Overlay file overrides (lowest priority) and CLI overrides (highest)
 * on top of `DEFAULT_CONFIG` and return a resolved `Config`. Time-valued
 * numeric inputs are in seconds; this is where they're converted to
 * milliseconds for internal use.
 *
 * For each field, the first layer that supplies a defined value wins:
 *   CLI -> file -> DEFAULT_CONFIG
 */
export function resolveConfig(file: ConfigOverrides | null, cli: ConfigOverrides): Config {
    const pickSeconds = (
        cliVal: number | undefined,
        fileVal: number | undefined,
        fallbackMs: number,
    ): number => {
        if (cliVal !== undefined) return cliVal * 1000;
        if (fileVal !== undefined) return fileVal * 1000;
        return fallbackMs;
    };

    const pickRaw = <T>(
        cliVal: T | undefined,
        fileVal: T | undefined,
        fallback: T,
    ): T => {
        if (cliVal !== undefined) return cliVal;
        if (fileVal !== undefined) return fileVal;
        return fallback;
    };

    return {
        moveInterval:  pickSeconds(cli.moveInterval,  file?.moveInterval,  DEFAULT_CONFIG.moveInterval),
        checkInterval: pickSeconds(cli.checkInterval, file?.checkInterval, DEFAULT_CONFIG.checkInterval),
        stepDelay:     pickRaw(cli.stepDelay,         file?.stepDelay,     DEFAULT_CONFIG.stepDelay),
        stepCount:     pickRaw(cli.stepCount,         file?.stepCount,     DEFAULT_CONFIG.stepCount),
        verbose:       pickRaw(cli.verbose,           file?.verbose,       DEFAULT_CONFIG.verbose),
    };
}