import argparse
import collections
import configparser
from datetime import datetime
from fnmatch import fnmatch
import hashlib
from math import ceil
import os
import re
import sys
import zlib

argparser = argparse.ArgumentParser(description="The stupidest content tracker")

argsubparsers = argparser.add_subparsers(title="Commands", dest="command")
argsubparsers.required = True

def main(argv=sys.argv[1:]):
    args = argparser.parse_args(argv)
    match args.command:
        case "add"          : cmd_add(args)
        case "cat-file"     : cmd_cat_file(args)
        case "check-ignore" : cmd_check_ignore(args)
        case "checkout"     : cmd_checkout(args)
        case "commit"       : cmd_commit(args)
        case "hash-object"  : cmd_hash_object(args)
        case "init"         : cmd_init(args)
        case "log"          : cmd_log(args)
        case "ls-files"     : cmd_ls_files(args)
        case "ls-tree"      : cmd_ls_tree(args)
        case "rev-parse"    : cmd_rev_parse(args)
        case "rm"           : cmd_rm(args)
        case "show-ref"     : cmd_show_ref(args)
        case "status"       : cmd_status(args)
        case "tag"          : cmd_tag(args)
        case _              : print("Bad command.")

class GitRepository (object):
    """A git repo"""

    worktree = None
    gitdir = None
    conf = None

    def __init__(self, path, force=False):
        self.worktree = path
        self.gitdir = os.path.join(path, ".git")

        if not (force or os.path.isdir(self.gitdir)):
            raise Exception(f"Not a git repository {path}")

        # Read configuration file in .git/config
        self.conf = configparser.ConfigParser()
        cf = GitRepository.repo_file(self, "config")

        if cf and os.path.exists(cf):
            self.conf.read([cf])
        elif not force:
            raise Exception("Configuration file missing")

        if not force:
            vers = int(self.conf.get("core", "repositoryformatversion"))
            if vers != 0:
                raise Exception(f"Unsupported repositoryformatversion {vers}")

    def repo_path(repo, *path):
        """Compute path under repo's gitdir."""
        return os.path.join(repo.gitdir, *path)

    def repo_file(repo, *path, mkdir=False):
        """Same as repo_path, but create dirname(*path) if absent. For
        example, repo_file(r, \"refs\", \"remotes\", \"origin\", \"HEAD\") will create
        .git/refs/remotes/origin."""

        if GitRepository.repo_dir(repo, *path[:-1], mkdir=mkdir):
            return GitRepository.repo_path(repo, *path)

    def repo_dir(repo, *path, mkdir=False):
        """Same as repo_path, bt mkdir *path if absent if mkdir"""

        path = GitRepository.repo_path(repo, *path)

        if (os.path.exists(path)):
            if (os.path.isdir(path)):
                return path
            else:
                raise Exception(f"Not a directory {path}")

        if mkdir:
            os.makedirs(path)
            return path
        else:
            return None

def repo_create(path):
    """Create a new repository at path."""

    repo = GitRepository(path, True)

    # First, we make sure the path either doesn't exist or is an
    # empty dir.

    if os.path.exists(repo.worktree):
        if not os.path.isdir(repo.worktree):
            raise Exception(f"{path} is not a directory!")
        if os.path.exists(repo.gitdir) and os.listdir(repo.gitdir):
            raise Exception(f"{path} is not empty!")
    else:
        os.makedirs(repo.worktree)

    assert GitRepository.repo_dir(repo, "branches", mkdir=True)
    assert GitRepository.repo_dir(repo, "objects", mkdir=True)
    assert GitRepository.repo_dir(repo, "refs", "tags", mkdir=True)
    assert GitRepository.repo_dir(repo, "refs", "heads", mkdir=True)

    # .git/description
    with open(GitRepository.repo_file(repo, "description"), "w") as f:
        f.write("Unnamed repository; edit this file 'description' to name the repository")

    with open(GitRepository.repo_file(repo, "HEAD"), "w") as f:
        f.write("ref: refs/heads/master\n")

    with open(GitRepository.repo_file(repo, "config"), "w") as f:
        config = repo_default_config()
        config.write(f)

    return repo

def repo_default_config():
    ret = configparser.ConfigParser()

    ret.add_section("core")
    ret.set("core", "repositoryformatVersion", "0")
    ret.set("core", "filemode", "false")
    ret.set("core", "bare", "false")

    return ret

argsp = argsubparsers.add_parser("init", help="Initialize a new, empty repository.")

argsp.add_argument("path",
                   metavar="directory",
                   nargs="?",
                   default=".",
                   help="Where to create the repository.")

def cmd_init(args):
    repo_create(args.path)

def repo_find(path=".", required=True):
    path = os.path.realpath(path)

    if os.path.isdir(os.path.join(path, ".git")):
        return GitRepository(path)

    # If we haven't returned, recurse in parent
    parent = os.path.realpath(os.path.join(path, ".."))

    match os.name:
        case "nt":
            is_root = os.path.splitdrive(path)[1] == "\\"
        case "posix":
            # If parent==path, then path is root.
            is_root = parent == path
        case _:
            raise Exception(f"Unsupported os {os.name}")

    if is_root:
        if required:
            raise Exception("No git directory.")
        else:
            return None

    # Recursive case
    return repo_find(parent, required)

argsp = argsubparsers.add_parser("status", help="Get the status of the repo")

argsp.add_argument("path",
                   metavar="directory",
                   nargs="?",
                   default=".",
                   help="Which directory")

def cmd_status(args):
    # TODO: actually get status
    print(repo_find(args.path).worktree)

class GitObject (object):
    def __init__(self, data=None):
        if data != None:
            self.deserialize(data)
        else:
            self.init()

    def serialize(self, repo):
        """This function MUST be implemented by subclasses.

It must read the object's contents from self.data, a byte string, and
do whatever it takes to convert it into a meaningful representation. What exactly that means depends on each subclass."""
        raise Exception("Unimplemented!")

    def deserialize(self, repo):
        raise Exception("Unimplemented!")

    def init(self):
        pass # other implementations might do something here

def object_read(repo, sha):
    """Read object sha from Git repository repo. Return a
    GitObject whose exact type depends on the object."""

    path = GitRepository.repo_file(repo, "objects", sha[0:2], sha[2:])

    if not os.path.isfile(path):
        return None

    with open(path, "rb") as f:
        raw = zlib.decompress(f.read())

        # Read object type
        x = raw.find(b' ')
        fmt = raw[0:x]

        # Read and validate object size
        y = raw.find(b'\x00', x)
        size = int(raw[x:y].decode("ascii"))
        if size != len(raw)-y-1:
            raise Exception(f"Malformed object {sha}: bad length")

        match fmt:
            case b'commit' : c=GitCommit
            case b'tree'   : c=GitTree
            case b'tag'    : c=GitTag
            case b'blob'   : c=GitBlob
            case _:
                raise Exception(f"Unknown type {fmt.decode('ascii')} for object {sha}")

        return c(raw[y+1:])

def object_write(obj, repo=None):
    data = obj.serialize()
    # Add header
    result = obj.fmt + b' ' + str(len(data)).encode() + b'\x00' + data
    # Compute hash
    sha = hashlib.sha1(result).hexdigest()

    if repo:
        # Compute path
        path=GitRepository.repo_file(repo, "objects", sha[0:2], sha[2:], mkdir=True)

        if not os.path.exists(path):
            with open(path, 'wb') as f:
                f.write(zlib.compress(result))
    return sha

class GitBlob(GitObject):
    fmt=b'blob'

    def serialize(self):
        return self.blobdata

    def deserialize(self, data):
        self.blobdata = data

argsp = argsubparsers.add_parser("cat-file",
                                 help="Provide content of repository objects")

argsp.add_argument("type",
                   metavar="type",
                   choices=["blob", "commit", "tag", "tree"],
                   help="Specify the type")

argsp.add_argument("object",
                   metavar="object",
                   help="The object to display")

def cmd_cat_file(args):
    repo = repo_find()
    cat_file(repo, args.object, fmt=args.type.encode())

def cat_file(repo, obj, fmt=None):
    obj = object_read(repo, object_find(repo, obj, fmt=fmt))
    sys.stdout.buffer.write(obj.serialize())

def object_find(repo, name, fmt=None, follow=True):
    return name

argsp = argsubparsers.add_parser(
    "hash-object",
    help="Compute object ID an optionally creates a blob from a file")

argsp.add_argument("-t",
                   metavar="type",
                   dest="type",
                   choices=["blob", "commit", "tag", "tree"],
                   default="blob",
                   help="Specify the type")

argsp.add_argument("-w",
                   dest="write",
                   action="store_true",
                   help="Actually write the object into the database")

argsp.add_argument("path",
                   help="Read object from <file>")

def cmd_hash_object(args):
    if args.write:
        repo = repo_find()
    else:
        repo = None

    with open(args.path, "rb") as fd:
        sha = object_hash(fd, args.type.encode(), repo)
        print(sha)

def object_hash(fd, fmt, repo=None):
    """Hash object, writing it to repo if provided"""
    data = fd.read()

    match fmt:
        case b'commit' : obj=GitCommit(data)
        case b'tree'   : obj=GitTree(data)
        case b'tag'    : obj=GitTag(data)
        case b'blob'   : obj=GitBlob(data)
        case _         : raise Exception(f"Unknown type {fmt}")

    return object_write(obj, repo)

def kvlm_parse(raw, start=0, dct=None):
    if not dct:
        dct = collections.OrderedDict()
        # You CANNOT declare the argument as dct=OrderedDict() or all
        # call to the functions will endlessly grow the same dict.

    # This function is recursive: it reads a key/value pair, then call
    # itself back with the new position. So we first need to know
    # where we are: at a keyword, or already in the messageQ

    # We search for the next space and the next newline.
    spc = raw.find(b' ', start)
    nl = raw.find(b'\n', start)

    # If space appears before newline, we have a keyword. Otherwise,
    # it's the final message, which we just read to the end of the file.

    # Base case
    # =========
    #
    # If newline appears first (or there's no space at all), we asume
    # a blank line. A blank line means the remainder of the data is the
    # message. We store it in the dictionary, with None as the key, and
    # return.
    if (spc < 0) or (nl < spc):
        assert nl == start
        dct[None] = raw[start+1:]
        return dct

    # Recursive case
    # ==============
    #
    # We read a key-value pair and recurse for the next.
    key = raw[start:spc]

    # Find the end of the value. Continuation lines begin with a
    # space, so we loop until we find a "\n" not followed by a space.
    end = start
    while True:
        end = raw.find(b'\n', end+1)
        if raw[end+1] != ord(' '): break

    value = raw[spc+1:end].replace(b'\n ', b'\n')

    # Don't overwrite existing data contents
    if key in dct:
        if type(dct[key]) == list:
            dct[key].append(value)
        else:
            dct[key] = [ dct[key], value ]
    else:
        dct[key] = value

    return kvlm_parse(raw, start=end+1, dct=dct)

def kvlm_serialize(kvlm):
    ret = b''

    # Output fiels
    for k in kvlm.keys():
        # Skip the message itself
        if k == None: continue
        val = kvlm[k]
        # Normalize to a list
        if type(val) != list:
            val = [ val ]

        for v in val:
            ret += k + b' ' + (v.replace(b'\n', b'\n ')) + b'\n'

    ret += b'\n' + kvlm[None] + b'\n'

    return ret

class GitCommit(GitObject):
    fmt = b'commit'

    def deserialize(self, data):
        self.kvlm = kvlm_parse(data)

    def serialize(self):
        return kvlm_serialize(self.kvlm)

    def init(self):
        self.kvlm = dict()

argsp = argsubparsers.add_parser("log", help="Display history of a given commit.")
argsp.add_argument("commit",
                   default="HEAD",
                   nargs="?",
                   help="Commit to start at.")

def cmd_log(args):
    repo = repo_find()

    print("digraph wyaglog{")
    print("  node[shape=rect]")
    log_graphviz(repo, object_find(repo, args.commit), set())
    print("}")

def log_graphviz(repo, sha, seen):
    if sha in seen:
        return
    seen.add(sha)

    commit = object_read(repo, sha)
    short_hash = sha[0:8]
    message = commit.kvlm[None].decode("utf8").strip()
    message = message.replace("\\", "\\\\")
    message = message.replace("\"", "\\\"")

    if "\n" in message: # keep only the first line
        message = message[:message.index("\n")]
    print(f"  c_{sha} [label=\"{sha[0:7]}: {message}\"]")
    assert commit.fmt==b'commit'

    if not b'parent' in commit.kvlm.keys():
        # Base case: the initial commit.
        return

    parents = commit.kvlm[b'parent']

    if type(parents) != list:
        parents = [ parents ]

    for p in parents:
        p = p.decode("ascii")
        print (f"  c_{sha} -> c_{p}")
        log_graphviz(repo, p, seen)

class GitTreeLeaf (object):
    def __init__(self, mode, path, sha):
        self.mode = mode
        self.path = path
        self.sha = sha

def tree_parse_one(raw, start=0):
    # Find the space terminator of the mode
    x = raw.find(b' ', start)
    assert x-start == 5 or x-start == 6

    mode = raw[start:x]
    if len(mode) == 5:
        # Normalize to six bytes.
        mode = b"0" + mode

    # Find the NULL terminator of the path
    y = raw.find(b'\x00', x)
    path = raw[x+1:y]

    sha = format(int.from_bytes(raw[y+1:y+21], "big"), "040x")
    return y+21, GitTreeLeaf(mode, path.decode('utf8'), sha)

def tree_parse(raw):
    pos = 0
    max = len(raw)
    ret = list()
    while pos < max:
        pos, data = tree_parse_one(raw, pos)
        ret.append(data)

    return ret

# Notice: this isn't a comparison function, but a conversion function.
# Python's default sort doesn't accept a custom comparison function,
# like in most languages, but a 'key' argument that returns a new
# value, which is compared using the default rules. So we just return
# the leaf name, with an extra '/' if it's a directory.
def tree_leaf_sort_key(leaf):
    if leaf.mode.startswith(b"10"):
        return leaf.path
    else:
        return leaf.path + "/"

def tree_serialize(obj):
    obj.items.sort(key=tree_leaf_sort_key)
    ret = b''
    for i in obj.items:
        ret += i.mode
        ret += b' '
        ret += i.path.encode("utf8")
        ret += b'\x00'
        sha = int(i.sha, 16)
        ret += sha.to_bytes(20, byteorder="big")
    return ret

class GitTree(GitObject):
    fmt=b'tree'

    def deserialize(self, data):
        self.items = tree_parse(data)

    def serialize(self):
        return tree_serialize(self)

    def init(self):
        self.items = list()

argsp = argsubparsers.add_parser("ls-tree", help="Pretty-print a tree object.")
argsp.add_argument("-r",
                   dest="recursive",
                   action="store_true",
                   help="Recurse into sub-trees")

argsp.add_argument("tree",
                   help="A tree-ish object.")

def cmd_ls_tree(args):
    repo = repo_find()
    ls_tree(repo, args.tree, args.recursive)

def ls_tree(repo, ref, recursive=None, prefix=""):
    sha = object_find(repo, ref, fmt=b"tree")
    obj = object_read(repo, sha)
    for item in obj.items:
        if len(item.mode) == 5:
            type = item.mode[0:1]
        else:
            type = item.mode[0:2]

        match type:
            case b'04': type = "tree"
            case b'10': type = "blob"
            case b'12': type = "blob" # a symlink
            case b'16': type = "commit" # a submodule
            case _: raise Exception(f"Weird tree leaf mode {item.mode}")

        if not (recursive and type=='tree'): # This is a leaf
            print("{0} {1} {2}\t{3}".format(
                "0" * (6 - len(item.mode)) + item.mode.decode("ascii"),
                # Git's ls-tree displays the type
                # of the object pointed to.
                type,
                item.sha,
                os.path.join(prefix, item.path)
            ))
        else: # This is a branch (vs. leaf), recurse
            ls_tree(repo, item.sha, recursive, os.path.join(prefix, item.path))

argsp = argsubparsers.add_parser("checkout", help="Checkout a commit inside of a directory.")

argsp.add_argument("commit",
                   help="The commit or tree to checkout.")

argsp.add_argument("path",
                   help="The EMPTY directory to checkout on.")

def cmd_checkout(args):
    repo = repo_find()

    obj = object_read(repo, object_find(repo, args.commit))

    # If the object is a commit, we grab its tree
    if obj.fmt == b'commit':
        obj = object_read(repo, obj.kvlm[b'tree'].decode("ascii"))

    # Verify the path is an empty directory
    if os.path.exists(args.path):
        if not os.path.isdir(args.path):
            raise Exception(f"This is not a directory: '{ags.path}'")
        if os.listdir(args.path):
            raise Exception(f"Directory '{args.path}' is not empty")
    else:
        os.makedirs(args.path)

    tree_checkout(repo, obj, os.path.realpath(args.path))

def tree_checkout(repo, tree, path):
    for item in tree.items:
        obj = object_read(repo, item.sha)
        dest = os.path.join(path, item.path)

        if obj.fmt == b'tree':
            os.mkdir(dest)
            tree_checkout(repo, obj, dest)
        elif obj.fmt == b'blob':
            # TODO: Support symlinks (identified by mode 12*****)
            with open(dest, 'wb') as f:
                f.write(obj.blobdata)