algorithm.py

# This file is part of python-bidi
#
# python-bidi is free software: you can redistribute it and/or modify
# it under the terms of the GNU Lesser General Public License as published by
# the Free Software Foundation, either version 3 of the License, or
# (at your option) any later version.
#
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
# GNU Lesser General Public License for more details.
#
# You should have received a copy of the GNU Lesser General Public License
# along with this program.  If not, see <http://www.gnu.org/licenses/>.

# Copyright (C) 2008-2010 Yaacov Zamir <kzamir_a_walla.co.il>,
# Copyright (C) 2010-2015 Meir kriheli <mkriheli@gmail.com>.
"bidirectional algorithm implementation"
import inspect
import sys
from collections import deque
from typing import Optional, Union
from unicodedata import bidirectional, mirrored

from .mirror import MIRRORED

StrOrBytes = Union[str, bytes]

# Some definitions
PARAGRAPH_LEVELS = {"L": 0, "AL": 1, "R": 1}
EXPLICIT_LEVEL_LIMIT = 62


def _LEAST_GREATER_ODD(x):
    return (x + 1) | 1


def _LEAST_GREATER_EVEN(x):
    return (x + 2) & ~1


X2_X5_MAPPINGS = {
    "RLE": (_LEAST_GREATER_ODD, "N"),
    "LRE": (_LEAST_GREATER_EVEN, "N"),
    "RLO": (_LEAST_GREATER_ODD, "R"),
    "LRO": (_LEAST_GREATER_EVEN, "L"),
}

# Added 'B' so X6 won't execute in that case and X8 will run it's course
X6_IGNORED = list(X2_X5_MAPPINGS.keys()) + ["BN", "PDF", "B"]
X9_REMOVED = list(X2_X5_MAPPINGS.keys()) + ["BN", "PDF"]


def _embedding_direction(x):
    return ("L", "R")[x % 2]


_IS_UCS2 = sys.maxunicode == 65535
_SURROGATE_MIN, _SURROGATE_MAX = 55296, 56319  # D800, DBFF


def debug_storage(storage, base_info=False, chars=True, runs=False):
    "Display debug information for the storage"

    stderr = sys.stderr

    caller = inspect.stack()[1][3]
    stderr.write(f"in {caller}\n")

    if base_info:
        stderr.write("  base level  : %d\n" % storage["base_level"])
        stderr.write("  base dir    : {}\n".format(storage["base_dir"]))

    if runs:
        stderr.write("  runs        : {}\n".format(list(storage["runs"])))

    if chars:
        output = "  Chars       : "
        for _ch in storage["chars"]:
            if _ch != "\n":
                output += _ch["ch"]
            else:
                output += "C"
        stderr.write(output + "\n")

        output = "  Res. levels : {}\n".format("".join(
            [str(_ch["level"]) for _ch in storage["chars"]]
        ))
        stderr.write(output)

        _types = [_ch["type"].ljust(3) for _ch in storage["chars"]]

        for i in range(3):
            output = "                %s\n" if i else "  Res. types  : %s\n"
            stderr.write(output % "".join([_t[i] for _t in _types]))


def get_base_level(text, upper_is_rtl=False) -> int:
    """Get the paragraph base embedding level. Returns 0 for LTR,
    1 for RTL.

    `text` a unicode object.

    Set `upper_is_rtl` to True to treat upper case chars as strong 'R'
    for debugging (default: False).

    """
    base_level = None

    prev_surrogate = False
    # P2
    for _ch in text:
        # surrogate in case of ucs2
        if _IS_UCS2 and (_SURROGATE_MIN <= ord(_ch) <= _SURROGATE_MAX):
            prev_surrogate = _ch
            continue
        elif prev_surrogate:
            _ch = prev_surrogate + _ch
            prev_surrogate = False

        # treat upper as RTL ?
        if upper_is_rtl and _ch.isupper():
            base_level = 1
            break

        bidi_type = bidirectional(_ch)

        if bidi_type in ("AL", "R"):
            base_level = 1
            break

        elif bidi_type == "L":
            base_level = 0
            break

    # P3
    if base_level is None:
        base_level = 0

    return base_level


def get_embedding_levels(text, storage, upper_is_rtl=False, debug=False):
    """Get the paragraph base embedding level and direction,
    set the storage to the array of chars"""

    prev_surrogate = False
    base_level = storage["base_level"]

    # preset the storage's chars
    for _ch in text:
        if _IS_UCS2 and (_SURROGATE_MIN <= ord(_ch) <= _SURROGATE_MAX):
            prev_surrogate = _ch
            continue
        elif prev_surrogate:
            _ch = prev_surrogate + _ch
            prev_surrogate = False

        bidi_type = "R" if upper_is_rtl and _ch.isupper() else bidirectional(_ch)

        storage["chars"].append(
            {"ch": _ch, "level": base_level, "type": bidi_type, "orig": bidi_type}
        )
    if debug:
        debug_storage(storage, base_info=True)


def explicit_embed_and_overrides(storage, debug=False):
    """Apply X1 to X9 rules of the unicode algorithm.

    See http://unicode.org/reports/tr9/#Explicit_Levels_and_Directions

    """
    overflow_counter = almost_overflow_counter = 0
    directional_override = "N"
    levels = deque()

    # X1
    embedding_level = storage["base_level"]

    for _ch in storage["chars"]:
        bidi_type = _ch["type"]

        level_func, override = X2_X5_MAPPINGS.get(bidi_type, (None, None))

        if level_func:
            # So this is X2 to X5
            # if we've past EXPLICIT_LEVEL_LIMIT, note it and do nothing

            if overflow_counter != 0:
                overflow_counter += 1
                continue

            new_level = level_func(embedding_level)
            if new_level < EXPLICIT_LEVEL_LIMIT:
                levels.append((embedding_level, directional_override))
                embedding_level, directional_override = new_level, override

            elif embedding_level == EXPLICIT_LEVEL_LIMIT - 2:
                # The new level is invalid, but a valid level can still be
                # achieved if this level is 60 and we encounter an RLE or
                # RLO further on.  So record that we 'almost' overflowed.
                almost_overflow_counter += 1

            else:
                overflow_counter += 1
        else:
            # X6
            if bidi_type not in X6_IGNORED:
                _ch["level"] = embedding_level
                if directional_override != "N":
                    _ch["type"] = directional_override

            # X7
            elif bidi_type == "PDF":
                if overflow_counter:
                    overflow_counter -= 1
                elif (
                    almost_overflow_counter
                    and embedding_level != EXPLICIT_LEVEL_LIMIT - 1
                ):
                    almost_overflow_counter -= 1
                elif levels:
                    embedding_level, directional_override = levels.pop()

            # X8
            elif bidi_type == "B":
                levels.clear()
                overflow_counter = almost_overflow_counter = 0
                embedding_level = _ch["level"] = storage["base_level"]
                directional_override = "N"

    # Removes the explicit embeds and overrides of types
    # RLE, LRE, RLO, LRO, PDF, and BN. Adjusts extended chars
    # next and prev as well

    # Applies X9. See http://unicode.org/reports/tr9/#X9
    storage["chars"] = [
        _ch for _ch in storage["chars"] if _ch["type"] not in X9_REMOVED
    ]

    calc_level_runs(storage)

    if debug:
        debug_storage(storage, runs=True)


def calc_level_runs(storage):
    """Split the storage to run of char types at the same level.

    Applies X10. See http://unicode.org/reports/tr9/#X10
    """
    # run level depends on the higher of the two levels on either side of
    # the boundary If the higher level is odd, the type is R; otherwise,
    # it is L

    storage["runs"].clear()
    chars = storage["chars"]

    # empty string ?
    if not chars:
        return

    def calc_level_run(b_l, b_r):
        return ["L", "R"][max(b_l, b_r) % 2]

    first_char = chars[0]

    sor = calc_level_run(storage["base_level"], first_char["level"])
    eor = None

    run_start = run_length = 0

    curr_level, curr_type = 0, ""
    prev_level, prev_type = first_char["level"], first_char["type"]

    for _ch in chars:
        curr_level, curr_type = _ch["level"], _ch["type"]

        if curr_level == prev_level:
            run_length += 1
        else:
            eor = calc_level_run(prev_level, curr_level)
            storage["runs"].append(
                {
                    "sor": sor,
                    "eor": eor,
                    "start": run_start,
                    "type": prev_type,
                    "length": run_length,
                }
            )
            sor = eor
            run_start += run_length
            run_length = 1

        prev_level, prev_type = curr_level, curr_type

    # for the last char/runlevel
    eor = calc_level_run(curr_level, storage["base_level"])
    storage["runs"].append(
        {
            "sor": sor,
            "eor": eor,
            "start": run_start,
            "type": curr_type,
            "length": run_length,
        }
    )


def resolve_weak_types(storage, debug=False):
    """Resolve weak type rules W1 - W3.

    See: http://unicode.org/reports/tr9/#Resolving_Weak_Types

    """

    for run in storage["runs"]:
        prev_strong = prev_type = run["sor"]
        start, length = run["start"], run["length"]
        chars = storage["chars"][start : start + length]
        for _ch in chars:
            # W1. Examine each nonspacing mark (NSM) in the level run, and
            # change the type of the NSM to the type of the previous character.
            # If the NSM is at the start of the level run, it will get the type
            # of sor.
            bidi_type = _ch["type"]

            if bidi_type == "NSM":
                _ch["type"] = bidi_type = prev_type

            # W2. Search backward from each instance of a European number until
            # the first strong type (R, L, AL, or sor) is found. If an AL is
            # found, change the type of the European number to Arabic number.
            if bidi_type == "EN" and prev_strong == "AL":
                _ch["type"] = "AN"

            # update prev_strong if needed
            if bidi_type in ("R", "L", "AL"):
                prev_strong = bidi_type

            prev_type = _ch["type"]

        # W3. Change all ALs to R
        for _ch in chars:
            if _ch["type"] == "AL":
                _ch["type"] = "R"

        # W4. A single European separator between two European numbers changes
        # to a European number. A single common separator between two numbers of
        # the same type changes to that type.
        for idx in range(1, len(chars) - 1):
            bidi_type = chars[idx]["type"]
            prev_type = chars[idx - 1]["type"]
            next_type = chars[idx + 1]["type"]

            if bidi_type == "ES" and (prev_type == next_type == "EN"):
                chars[idx]["type"] = "EN"

            if (
                bidi_type == "CS"
                and prev_type == next_type
                and prev_type in ("AN", "EN")
            ):
                chars[idx]["type"] = prev_type

        # W5. A sequence of European terminators adjacent to European numbers
        # changes to all European numbers.
        for idx in range(len(chars)):
            if chars[idx]["type"] == "EN":
                for et_idx in range(idx - 1, -1, -1):
                    if chars[et_idx]["type"] == "ET":
                        chars[et_idx]["type"] = "EN"
                    else:
                        break
                for et_idx in range(idx + 1, len(chars)):
                    if chars[et_idx]["type"] == "ET":
                        chars[et_idx]["type"] = "EN"
                    else:
                        break

        # W6. Otherwise, separators and terminators change to Other Neutral.
        for _ch in chars:
            if _ch["type"] in ("ET", "ES", "CS"):
                _ch["type"] = "ON"

        # W7. Search backward from each instance of a European number until the
        # first strong type (R, L, or sor) is found. If an L is found, then
        # change the type of the European number to L.
        prev_strong = run["sor"]
        for _ch in chars:
            if _ch["type"] == "EN" and prev_strong == "L":
                _ch["type"] = "L"

            if _ch["type"] in ("L", "R"):
                prev_strong = _ch["type"]

    if debug:
        debug_storage(storage, runs=True)


def resolve_neutral_types(storage, debug):
    """Resolving neutral types. Implements N1 and N2

    See: http://unicode.org/reports/tr9/#Resolving_Neutral_Types

    """

    prev_bidi_type = ""
    for run in storage["runs"]:
        start, length = run["start"], run["length"]
        # use sor and eor
        chars = (
            [{"type": run["sor"]}]
            + storage["chars"][start : start + length]
            + [{"type": run["eor"]}]
        )
        total_chars = len(chars)

        seq_start = None
        for idx in range(total_chars):
            _ch = chars[idx]
            if _ch["type"] in ("B", "S", "WS", "ON"):
                # N1. A sequence of neutrals takes the direction of the
                # surrounding strong text if the text on both sides has the same
                # direction. European and Arabic numbers act as if they were R
                # in terms of their influence on neutrals. Start-of-level-run
                # (sor) and end-of-level-run (eor) are used at level run
                # boundaries.
                if seq_start is None:
                    seq_start = idx
                    prev_bidi_type = chars[idx - 1]["type"]
            else:
                if seq_start is not None:
                    next_bidi_type = chars[idx]["type"]

                    if prev_bidi_type in ("AN", "EN"):
                        prev_bidi_type = "R"

                    if next_bidi_type in ("AN", "EN"):
                        next_bidi_type = "R"

                    for seq_idx in range(seq_start, idx):
                        if prev_bidi_type == next_bidi_type:
                            chars[seq_idx]["type"] = prev_bidi_type
                        else:
                            # N2. Any remaining neutrals take the embedding
                            # direction. The embedding direction for the given
                            # neutral character is derived from its embedding
                            # level: L if the character is set to an even level,
                            # and R if the level is odd.
                            chars[seq_idx]["type"] = _embedding_direction(
                                chars[seq_idx]["level"]
                            )

                    seq_start = None

    if debug:
        debug_storage(storage)


def resolve_implicit_levels(storage, debug):
    """Resolving implicit levels (I1, I2)

    See: http://unicode.org/reports/tr9/#Resolving_Implicit_Levels

    """
    for run in storage["runs"]:
        start, length = run["start"], run["length"]
        chars = storage["chars"][start : start + length]

        for _ch in chars:
            # only those types are allowed at this stage
            assert _ch["type"] in ("L", "R", "EN", "AN"), (
                "{} not allowed here".format(_ch["type"])
            )

            if _embedding_direction(_ch["level"]) == "L":
                # I1. For all characters with an even (left-to-right) embedding
                # direction, those of type R go up one level and those of type
                # AN or EN go up two levels.
                if _ch["type"] == "R":
                    _ch["level"] += 1
                elif _ch["type"] != "L":
                    _ch["level"] += 2
            else:
                # I2. For all characters with an odd (right-to-left) embedding
                # direction, those of type L, EN or AN  go up one level.
                if _ch["type"] != "R":
                    _ch["level"] += 1

    if debug:
        debug_storage(storage, runs=True)


def reverse_contiguous_sequence(
    chars, line_start, line_end, highest_level, lowest_odd_level
):
    """L2. From the highest level found in the text to the lowest odd
    level on each line, including intermediate levels not actually
    present in the text, reverse any contiguous sequence of characters
    that are at that level or higher.

    """
    for level in range(highest_level, lowest_odd_level - 1, -1):
        _start = _end = None

        for run_idx in range(line_start, line_end + 1):
            run_ch = chars[run_idx]

            if run_ch["level"] >= level:
                if _start is None:
                    _start = _end = run_idx
                else:
                    _end = run_idx
            else:
                if _end is not None:
                    chars[_start : +_end + 1] = reversed(chars[_start : +_end + 1])
                    _start = _end = None

        # anything remaining ?
        if _start is not None and _end is not None:
            chars[_start : +_end + 1] = reversed(chars[_start : +_end + 1])


def reorder_resolved_levels(storage, debug):
    """L1 and L2 rules"""

    # Applies L1.

    should_reset = True
    chars = storage["chars"]

    for _ch in chars[::-1]:
        # L1. On each line, reset the embedding level of the following
        # characters to the paragraph embedding level:
        if _ch["orig"] in ("B", "S"):
            # 1. Segment separators,
            # 2. Paragraph separators,
            _ch["level"] = storage["base_level"]
            should_reset = True
        elif should_reset and _ch["orig"] in ("BN", "WS"):
            # 3. Any sequence of whitespace characters preceding a segment
            # separator or paragraph separator
            # 4. Any sequence of white space characters at the end of the
            # line.
            _ch["level"] = storage["base_level"]
        else:
            should_reset = False

    max_len = len(chars)

    # L2 should be per line
    # Calculates highest level and lowest odd level on the fly.

    line_start = line_end = 0
    highest_level = 0
    lowest_odd_level = EXPLICIT_LEVEL_LIMIT

    for idx in range(max_len):
        _ch = chars[idx]

        # calc the levels
        char_level = _ch["level"]
        if char_level > highest_level:
            highest_level = char_level

        if char_level % 2 and char_level < lowest_odd_level:
            lowest_odd_level = char_level

        if _ch["orig"] == "B" or idx == max_len - 1:
            line_end = idx
            # omit line breaks
            if _ch["orig"] == "B":
                line_end -= 1

            reverse_contiguous_sequence(
                chars, line_start, line_end, highest_level, lowest_odd_level
            )

            # reset for next line run
            line_start = idx + 1
            highest_level = 0
            lowest_odd_level = EXPLICIT_LEVEL_LIMIT

    if debug:
        debug_storage(storage)


def apply_mirroring(storage, debug):
    """Applies L4: mirroring

    See: http://unicode.org/reports/tr9/#L4

    """
    # L4. A character is depicted by a mirrored glyph if and only if (a) the
    # resolved directionality of that character is R, and (b) the
    # Bidi_Mirrored property value of that character is true.
    for _ch in storage["chars"]:
        unichar = _ch["ch"]
        if mirrored(unichar) and _embedding_direction(_ch["level"]) == "R":
            _ch["ch"] = MIRRORED.get(unichar, unichar)

    if debug:
        debug_storage(storage)


def get_empty_storage():
    """Return an empty storage skeleton, usable for testing"""
    return {
        "base_level": None,
        "base_dir": None,
        "chars": [],
        "runs": deque(),
    }


def get_display(
    str_or_bytes: StrOrBytes,
    encoding: str = "utf-8",
    upper_is_rtl: bool = False,
    base_dir: Optional[str] = None,
    debug: bool = False,
) -> StrOrBytes:
    """Accepts `str` or `bytes`. In case it's `bytes`, `encoding`
    is needed as the algorithm works on `str` (default:"utf-8").

    Set `upper_is_rtl` to True to treat upper case chars as strong 'R'
    for debugging (default: False).

    Set `base_dir` to 'L' or 'R' to override the calculated base_level.

    Set `debug` to True to display (using sys.stderr) the steps taken with the
    algorithm.

    Returns the display layout, either as unicode or `encoding` encoded
    string.

    """
    storage = get_empty_storage()

    # utf-8 ? we need unicode
    if isinstance(str_or_bytes, bytes):
        text = str_or_bytes.decode(encoding)
        was_decoded = True
    else:
        text = str_or_bytes
        was_decoded = False

    if base_dir is None:
        base_level = get_base_level(text, upper_is_rtl)
    else:
        base_level = PARAGRAPH_LEVELS[base_dir]

    storage["base_level"] = base_level
    storage["base_dir"] = ("L", "R")[base_level]

    get_embedding_levels(text, storage, upper_is_rtl, debug)
    explicit_embed_and_overrides(storage, debug)
    resolve_weak_types(storage, debug)
    resolve_neutral_types(storage, debug)
    resolve_implicit_levels(storage, debug)
    reorder_resolved_levels(storage, debug)
    apply_mirroring(storage, debug)

    chars = storage["chars"]
    display = "".join([_ch["ch"] for _ch in chars])

    if was_decoded:
        display = display.encode(encoding)

    return display