/**
 * The four path functions the include code needs, for slash-separated paths. They exist so code the extension carries does not import node:path
 * (the extension has no Node). Behavior is held to node's `path.posix` by test/unit/posixPath.test.ts.
 */
export const SEP = "/";

export const isAbsolute = (p: string): boolean => p.startsWith("/");

const normalize = (segments: string[]): string[] => {
  const out: string[] = [];
  for (const s of segments) {
    if (s === "" || s === ".") continue;
    if (s === "..") out.pop();
    else out.push(s);
  }
  return out;
};

/** Joins right to left until a path is absolute, then cleans it. A relative result is taken from "/" (the callers only pass absolute bases). */
export function resolve(...parts: string[]): string {
  let joined = "";
  for (let i = parts.length - 1; i >= 0 && !isAbsolute(joined); i--) {
    const p = parts[i]!;
    if (p !== "") joined = joined === "" ? p : `${p}/${joined}`;
  }
  return `/${normalize(joined.split("/")).join("/")}`;
}

export function dirname(p: string): string {
  const end = p.replace(/\/+$/, "");
  if (end === "") return p.startsWith("/") ? "/" : ".";
  const i = end.lastIndexOf("/");
  if (i === -1) return ".";
  if (i === 0) return "/";
  return end.slice(0, i).replace(/\/+$/, "") || "/";
}

export function relative(from: string, to: string): string {
  const a = normalize(resolve(from).split("/"));
  const b = normalize(resolve(to).split("/"));
  let i = 0;
  while (i < a.length && i < b.length && a[i] === b[i]) i++;
  return [...a.slice(i).map(() => ".."), ...b.slice(i)].join("/");
}