#!/usr/bin/env python
"""look through the files given for datetime-like numbers. if found, add a
comment to the end of the line with a human readable version. filename and
linenumber are written to stderr.

"""
from datetime import datetime
import argparse
import fileinput
import re
import sys

COMMENT = {
    "": ("#", None),  # default
    "py": ("#", None),
    "hs": ("--", None),
    "c": ("/*", "*/"),
    "nix": ("#", None),
    "js": ("/*", "*/"),
}

REGEX = re.compile(r"(\d{10})")


def find_datetime(line):
    """return the date if the line contains what looks like a second-encoded date,
    otherwise None

    """
    m = REGEX.match(line)
    if m:
        return int(m.group(1))
    else:
        return None


def empty(s):
    return "" if s is None else s


cli = argparse.ArgumentParser(description=__doc__)
cli.add_argument(
    "files", nargs="*", metavar="FILE", help="file(s) to operate on, or omit for stdin"
)
cli.add_argument(
    "-d",
    "--dry",
    action="store_true",
    help="dry run, don't actually write to files, but print what would happen",
)
args = cli.parse_args()

for line in fileinput.input(files=args.files, inplace=True):
    # python3.8: use := operator here
    found = find_datetime(line)
    if found:
        sys.stderr.write(f"{fileinput.filename()}:{fileinput.lineno()}\n")
        human = datetime.fromtimestamp(found).strftime("%Y-%m-%d %H:%M")
        ext = fileinput.filename().split(".")[-1]
        (left, right) = COMMENT[ext]
        if args.dry:
            print(line, end="")
        else:
            print(f"{line.strip()}  {left} {human} {empty(right)}", end="")