136 lines
3.7 KiB
Python
136 lines
3.7 KiB
Python
from dataclasses import dataclass, field
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from typing import Dict, Optional
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from pyhanko.pdf_utils import generic
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from pyhanko.pdf_utils.writer import BasePdfFileWriter
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__all__ = [
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'ShapeResult',
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'FontEngine',
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'FontSubsetCollection',
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'FontEngineFactory',
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]
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ALPHABET = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ'
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def generate_subset_prefix():
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import random
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return ''.join(ALPHABET[random.randint(0, 25)] for _ in range(6))
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@dataclass(frozen=True)
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class FontSubsetCollection:
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base_postscript_name: str
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"""
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Base postscript name of the font.
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"""
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subsets: Dict[Optional[str], 'FontEngine'] = field(default_factory=dict)
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"""
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Dictionary mapping prefixes to subsets. ``None`` represents the full font.
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"""
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def add_subset(self) -> str:
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while True:
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prefix = generate_subset_prefix()
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if prefix not in self.subsets:
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return prefix
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@dataclass(frozen=True)
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class ShapeResult:
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"""Result of shaping a Unicode string."""
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graphics_ops: bytes
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"""
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PDF graphics operators to render the glyphs.
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"""
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x_advance: float
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"""Total horizontal advance in em units."""
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y_advance: float
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"""Total vertical advance in em units."""
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class FontEngine:
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"""General interface for text shaping and font metrics."""
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def __init__(
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self,
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writer: BasePdfFileWriter,
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base_postscript_name: str,
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embedded_subset: bool,
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obj_stream=None,
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):
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fsc = writer.get_subset_collection(base_postscript_name)
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if embedded_subset:
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self.subset_prefix = prefix = fsc.add_subset()
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else:
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self.subset_prefix = prefix = None
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fsc.subsets[prefix] = self
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self.writer = writer
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self.obj_stream = obj_stream
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@property
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def uses_complex_positioning(self):
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"""
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If ``True``, this font engine expects the line matrix to always be equal
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to the text matrix when exiting and entering :meth:`shape`.
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In other words, the current text position is where ``0 0 Td`` would
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move to.
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If ``False``, this method does not use any text positioning operators,
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and therefore uses the PDF standard's 'natural' positioning rules
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for text showing operators.
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The default is ``True`` unless overridden.
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"""
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return True
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def shape(self, txt: str) -> ShapeResult:
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"""Render a string to a format suitable for inclusion in a content
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stream and measure its total cursor advancement vector in em units.
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:param txt:
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String to shape.
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:return:
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A shaping result.
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"""
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raise NotImplementedError
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def as_resource(self) -> generic.PdfObject:
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"""Convert a :class:`.FontEngine` to a PDF object suitable for embedding
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inside a resource dictionary.
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.. note::
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If the PDF object is an indirect reference, the caller must not
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attempt to dereference it. In other words, implementations can
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use preallocated references to delay subsetting until the last
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possible moment (this is even encouraged, see
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:meth:`prepare_write`).
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:return:
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A PDF dictionary.
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"""
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raise NotImplementedError
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def prepare_write(self):
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"""
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Called by the writer that manages this font resource before the PDF
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content is written to a stream.
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Subsetting operations and the like should be carried out as part of
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this method.
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"""
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pass
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class FontEngineFactory:
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def create_font_engine(
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self, writer: 'BasePdfFileWriter', obj_stream=None
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) -> FontEngine:
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raise NotImplementedError
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