#!/usr/bin/env python3
"""Write a Project Sylpheed save back out — the container's derived fields included.

`savegame.py` parses and re-serializes the *payload* byte-identically. This adds
the rest of the container, which is what a hand-edited save actually needs:

    GDHA header (146 B) ++ zlib.compress(payload, 9)

Three header fields are functions of the payload and must be recomputed, or the
title will read a stale length and reject / mis-read the file. They were found by
diffing two real saves whose payloads differed (see docs/re/structures/savegame-format.md):

    +0x30  u32   len(deflate stream) + 10
    +0x8c  u16   len(payload)                    (545 for every save seen)
    +0x8e  u32   adler32(payload)                (the zlib trailer, duplicated)

Everything else is copied from the donor save. The words that differ between two
saves of the same state are the container FILETIME and uninitialised guest-pointer
padding — they are **not** validated, which is why copying them is safe.

**A hand-written save loads.** Verified 2026-08-11: a save whose 54-byte develop
blob was rewritten by hand booted, loaded, and rendered the edited state in the
Arsenal. Note what the title does on load, though — it **re-derives** which items
are *developable* from its own conditions and announces the difference ("You can
now develop …"), so only the *owned* (`4`) entries are authoritative; writing `2`
is pointless and writing `0` over an owned item is undone for anything whose
conditions are met.

Usage:
    savegame_edit.py <in> <out> --blob i=v[,i=v...]   # patch develop-blob entries
    savegame_edit.py <in> <out> --blob-zero           # clear the whole blob first
    savegame_edit.py <in> <out> --set u32_24=1000     # patch a GHAD field

Always write to a **new slot or a throwaway one**, never over a save you want.
"""
import struct
import sys
import zlib

import savegame as sg

HDR_LEN_OFF = 0x30
HDR_PAYLEN_OFF = 0x8C
HDR_ADLER_OFF = 0x8E


def wrap(header: bytes, payload: bytes) -> bytes:
    """Rebuild the GDHA container around a (possibly edited) payload."""
    comp = zlib.compress(payload, 9)
    if comp[:2] != b"\x78\xda":
        raise ValueError("expected a 78 da zlib stream, got %s" % comp[:2].hex())
    h = bytearray(header)
    struct.pack_into(">I", h, HDR_LEN_OFF, len(comp) + 10)
    struct.pack_into(">H", h, HDR_PAYLEN_OFF, len(payload))
    struct.pack_into(">I", h, HDR_ADLER_OFF, zlib.adler32(payload) & 0xFFFFFFFF)
    return bytes(h) + comp


def main():
    if len(sys.argv) < 3:
        sys.exit(__doc__)
    src, dst = sys.argv[1], sys.argv[2]
    args = sys.argv[3:]

    header, payload = sg.unwrap(open(src, "rb").read())
    parsed = sg.parse(payload)
    if sg.serialize(parsed) != payload:
        sys.exit("refusing to edit: the payload does not round-trip byte-identically")

    blob = bytearray(parsed["ghad"]["blob_68"])
    for i, a in enumerate(args):
        if a == "--blob-zero":
            blob = bytearray(len(blob))
        elif a == "--blob":
            for pair in args[i + 1].split(","):
                k, v = pair.split("=")
                blob[int(k)] = int(v)
        elif a == "--set":
            k, v = args[i + 1].split("=")
            parsed["ghad"][k] = int(v, 0)
        elif a == "--slot":
            # --slot <record>,<field>=<value>, e.g. --slot 0,0=3
            where, v = args[i + 1].split("=")
            rec, fld = (int(x) for x in where.split(","))
            row = list(parsed["slots"][rec])
            row[fld] = int(v, 0)
            parsed["slots"][rec] = tuple(row)
    parsed["ghad"]["blob_68"] = bytes(blob)

    out = wrap(header, sg.serialize(parsed))
    open(dst, "wb").write(out)
    print("wrote %s (%d bytes)" % (dst, len(out)))
    print("blob:", " ".join("%02x" % b for b in blob))


if __name__ == "__main__":
    main()