/**
 * What the progress bar on a download row should say.
 *
 * Extracted from `DownloadItem.svelte` so the three cases can be unit-tested:
 * an exact total (the server sent `Content-Length`), an estimated one (a
 * transcode sends none, so the backend predicted it from the source), and no
 * total at all — which used to render as an empty bar reading "0%" for the
 * whole of a transcode download. Progress with no denominator is not 0%; it
 * is unknown, and the bar says so.
 *
 * TRACES: UR-071 | DR-290 | UT-254
 */

export interface ProgressSource {
  status: "pending" | "downloading" | "completed" | "failed" | "paused";
  /** 0..1 as the backend reports it; already capped short of 1 when estimated. */
  progress: number;
  bytesDownloaded: number;
  fileSize?: number;
  /** `fileSize` is the backend's prediction, not the server's word. */
  fileSizeEstimated?: boolean;
}

export type ProgressKind = "exact" | "estimated" | "indeterminate";

export interface ProgressView {
  kind: ProgressKind;
  /** Whole percent for the bar width; `null` when there is nothing to measure against. */
  percent: number | null;
  /** "42%" / "~42%"; empty when indeterminate. */
  percentLabel: string;
  /** "512 B / 1.0 KB", "420 B / ~1000 B", or just the bytes so far. */
  label: string;
}

export function formatBytes(bytes: number): string {
  if (bytes === 0) return "0 B";
  const k = 1024;
  const sizes = ["B", "KB", "MB", "GB"];
  const i = Math.min(Math.floor(Math.log(bytes) / Math.log(k)), sizes.length - 1);
  return `${(bytes / Math.pow(k, i)).toFixed(i === 0 ? 0 : 1)} ${sizes[i]}`;
}

export function describeProgress(d: ProgressSource): ProgressView {
  const downloaded = formatBytes(d.bytesDownloaded);
  if (!d.fileSize) {
    return { kind: "indeterminate", percent: null, percentLabel: "", label: downloaded };
  }
  const percent = Math.round(Math.min(Math.max(d.progress, 0), 1) * 100);
  if (d.fileSizeEstimated) {
    return {
      kind: "estimated",
      percent,
      percentLabel: `~${percent}%`,
      label: `${downloaded} / ~${formatBytes(d.fileSize)}`,
    };
  }
  return {
    kind: "exact",
    percent,
    percentLabel: `${percent}%`,
    label: `${downloaded} / ${formatBytes(d.fileSize)}`,
  };
}