"""
Apollo Ranging Source -- streams PRN ranging chips.

Outputs a continuous stream of bytes (0 or 1) representing the composite
PRN ranging code. The code repeats every 5,456,682 chips (~5.49 seconds
at the nominal 993,963 chip/s rate).

The full code period is pre-generated and cycled through, so startup cost
is paid once and streaming is zero-allocation.

Reference: Ken Shirriff's Apollo ranging analysis
    http://www.righto.com/2022/04/the-digital-ranging-system-that.html
"""

import numpy as np

from apollo.ranging import RANGING_CODE_LENGTH, RangingCodeGenerator

# ---------------------------------------------------------------------------
# GNU Radio block (optional -- only if gnuradio is available)
# ---------------------------------------------------------------------------

try:
    from gnuradio import gr

    class ranging_source(gr.sync_block):
        """GNU Radio source: continuous PRN ranging chip stream.

        Outputs bytes (0 or 1) at the chip rate. Pre-generates the full
        code period and cycles through it.
        """

        def __init__(self):
            gr.sync_block.__init__(
                self,
                name="apollo_ranging_source",
                in_sig=None,
                out_sig=[np.byte],
            )

            self._gen = RangingCodeGenerator()
            self._code = self._gen.generate_sequence()
            self._pos = 0

        def work(self, input_items, output_items):
            out = output_items[0]
            n = len(out)
            produced = 0

            while produced < n:
                remaining = RANGING_CODE_LENGTH - self._pos
                chunk = min(n - produced, remaining)
                out[produced : produced + chunk] = self._code[self._pos : self._pos + chunk]
                self._pos = (self._pos + chunk) % RANGING_CODE_LENGTH
                produced += chunk

            return produced

except ImportError:
    pass