Types

interface FitError {
  /**
   * Machine-readable cause.
   *
   * - `InsufficientData` — too few usable points for this model.
   * - `DegenerateInput` — the x-values carry no variation to fit against.
   * - `InvalidInput` — a non-finite value, or an out-of-domain point.
   * - `MathError` — the fit produced non-finite coefficients.
   * - `NumericalStability` — the normal equations were singular or ill-conditioned.
   */
  type:
    | "InsufficientData"
    | "DegenerateInput"
    | "InvalidInput"
    | "MathError"
    | "NumericalStability";
  /** What went wrong, and what to do about it. */
  message: string;
}

interface Options {
  /** Polynomial degree. @default 2 */
  order: number;
  /** Decimal places for rounding. @default 2 */
  precision: number;
  /** Accepted and ignored, as in `regression`. */
  period: number | null;
}

interface Result {
  /**
   * Fitted coefficients.
   *
   * - `linear` — `[gradient, intercept]`
   * - `exponential` — `[a, b]` for y = ae^(bx)
   * - `logarithmic` — `[a, b]` for y = a + b·ln(x)
   * - `power` — `[a, b]` for y = ax^b
   * - `polynomial` — `[aₙ, …, a₁, a₀]`, highest power first
   *
   * Filled with `NaN` when `ok` is `false`.
   */
  equation: number[];
  /** Human-readable equation. Empty string when `ok` is `false`. */
  string: string;
  /**
   * Predicted `[x, y]` at each input x.
   *
   * Empty when `ok` is `false`. The original returns points full of `NaN`
   * instead, which charting libraries either throw on or render as garbage.
   */
  points: [number, number][];
  /** Coefficient of determination. `NaN` when `ok` is `false`. */
  r2: number;
  /** Predicted `[x, y]` at any x. Returns `[x, NaN]` when `ok` is `false`. */
  predict: (x: number) => [number, number];

  /** `false` when no fit could be produced. Not present in `regression`. */
  ok: boolean;
  /** Why the fit failed. Absent when `ok` is `true`. Not present in `regression`. */
  error?: FitError;
}

type DataPoint = [number, number | null];