88 lines
2.7 KiB
Python
88 lines
2.7 KiB
Python
from pathlib import Path
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import numpy as np
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import polars as pl
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from pipeline.local_temp import local_tmp_dir
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from .memory import release_memory
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def load_uprns(uprn_path: Path) -> tuple[pl.DataFrame, dict[str, tuple[int, int]]]:
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"""Load UPRNs as a sorted polars DataFrame with OA offset lookup.
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Returns (df, offsets) where offsets[oa_code] = (start_row, end_row).
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Peak ~5GB during sort, steady state ~1.5GB (Arrow columnar with compact strings).
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"""
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import tempfile
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print("Loading UPRN lookup...")
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# Sort via streaming sink to avoid polars doubling memory during in-memory sort
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with tempfile.NamedTemporaryFile(
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suffix=".parquet", delete=False, dir=local_tmp_dir()
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) as tmp:
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tmp_path = Path(tmp.name)
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(
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pl.scan_parquet(uprn_path)
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.select("GRIDGB1E", "GRIDGB1N", "PCDS", "OA21CD")
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.filter(~pl.col("OA21CD").str.starts_with("S"))
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.filter(pl.col("GRIDGB1E").is_not_null() & pl.col("GRIDGB1N").is_not_null())
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.with_columns(pl.col("PCDS").str.strip_chars())
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.filter(pl.col("PCDS").is_not_null() & (pl.col("PCDS") != ""))
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.sort("OA21CD")
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.sink_parquet(tmp_path)
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)
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release_memory()
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# Read the sorted data — only one copy in memory (~2GB)
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df = pl.read_parquet(tmp_path)
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tmp_path.unlink()
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n = len(df)
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print(f" Loaded {n:,} UPRNs (England & Wales)")
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# Compute OA group offsets using polars (avoids 37M Python string creation)
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boundary_df = (
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df.lazy()
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.with_row_index("_i")
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.filter(
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pl.col("OA21CD").shift(1).is_null()
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| (pl.col("OA21CD") != pl.col("OA21CD").shift(1))
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)
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.select("_i", "OA21CD")
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.collect()
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)
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starts_list = boundary_df["_i"].to_list()
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oa_list = boundary_df["OA21CD"].to_list()
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del boundary_df
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offsets: dict[str, tuple[int, int]] = {}
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for j in range(len(starts_list)):
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end = starts_list[j + 1] if j + 1 < len(starts_list) else n
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offsets[oa_list[j]] = (starts_list[j], end)
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del starts_list, oa_list
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# Drop OA column (no longer needed) to save ~400MB
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df = df.select("GRIDGB1E", "GRIDGB1N", "PCDS")
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release_memory()
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print(f" Grouped into {len(offsets)} OAs")
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return df, offsets
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def get_oa_uprns(
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df: pl.DataFrame, offsets: dict[str, tuple[int, int]], oa_code: str
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) -> tuple[np.ndarray, list[str]]:
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"""Get UPRN coordinates and postcodes for a single OA.
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Returns (points_nx2, postcodes_list).
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"""
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s, e = offsets[oa_code]
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sub = df[s:e]
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points = np.column_stack(
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[
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sub["GRIDGB1E"].to_numpy(),
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sub["GRIDGB1N"].to_numpy(),
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]
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)
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postcodes = sub["PCDS"].to_list()
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return points, postcodes
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