"""Stamping for the service worker.

The dev server stamps sw.js on every request; the deploy stamps it once into
the uploaded copy. Both import this, because a digest computed two different
ways is a worker that thinks its caches are current when they are not.
"""

from __future__ import annotations

import hashlib
import json
import os

WORKER = "sw.js"

VERSION_LINE = "const VERSION = 'dev';"
LIBRARY_VERSION_LINE = "const LIBRARY_VERSION = 'dev';"
LIBRARY_LINE = "const LIBRARY = [];"

CHUNK = 1 << 20

def walk(roots):
    found = {}
    for root in roots:
        for dirpath, _dirs, names in os.walk(root):
            for name in sorted(names):
                path = os.path.join(dirpath, name)
                rel = os.path.relpath(path, root).replace(os.sep, "/")
                found.setdefault(rel, path)
    return found

def digest(files):
    # Contents, not size and mtime: a checkout, a rebuild and a rsync all move
    # mtimes without changing a byte, and each one of those would otherwise
    # throw away every cache on every device for nothing.
    out = hashlib.sha256()
    for rel in sorted(files):
        out.update(rel.encode("utf-8") + b"\0")
        with open(files[rel], "rb") as fh:
            for chunk in iter(lambda: fh.read(CHUNK), b""):
                out.update(chunk)
        out.update(b"\0")
    return out.hexdigest()[:12]

def page_url(rel):
    if rel == "index.html":
        return "./"
    return "./" + (rel[:-len(".html")] if rel.endswith(".html") else rel)

def stamp_worker(roots):
    chrome = walk(roots[:1])
    library = {rel: path for rel, path in walk(roots).items() if rel not in chrome}

    src = chrome.pop(WORKER, None)
    if src is None:
        return None
    with open(src, encoding="utf-8") as fh:
        body = fh.read()

    for line, value in (
        (VERSION_LINE, "const VERSION = '%s';" % digest(chrome)),
        (LIBRARY_VERSION_LINE, "const LIBRARY_VERSION = '%s';" % digest(library)),
        (LIBRARY_LINE, "const LIBRARY = %s;"
                       % json.dumps(sorted(page_url(rel) for rel in library), indent=2)),
    ):
        if line not in body:
            raise RuntimeError("%s: expected to find %r to stamp" % (WORKER, line))
        body = body.replace(line, value, 1)
    return body.encode("utf-8")