Ludovic Marie
81 lines
5.2 KiB
Markdown
81 lines
5.2 KiB
Markdown
# Equivalent position arithmetic in the LOD data lookup
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A live allocation/CPU investigation sampled Distant Horizons 3.3.4's relative
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position conversion on render-loader threads. This is a numeric step within
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`FullDataSourceV2.getDataPointAtBlockPos`; the first candidate is a Java hook at
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that call, with no native transition, cache, new worker or retained per-source
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field. The original library handles containment, column selection and datapoint
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search. No writes, world generation, block mechanics or rendering budgets change.
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Select `terrain-position` for an isolated comparison with `m2-balanced`.
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Graphics, worker and other controller settings match. This hook is off in all
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other profiles, including combined profiles, and requires exactly DH 3.3.4.
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Disable `enabled` in `config/horizon-terrain-position.properties` and restart.
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Reports and both comparison tools capture its actual applied flag.
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The conversion retains the original numeric output, including its unusual
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negative-coordinate bias. A plain coordinate shift failed a differential test
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and was discarded; correcting that apparent rounding is outside this hook's
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scope. The replacement computes truncation toward zero for a positive power-of-two
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divisor using shifts and sign masks. For a negative coordinate, the publisher's
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bias is applied before truncation, and the final relative coordinate retains its
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original remainder result. The zero-shift bias is retained too.
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The decoded coordinate is a signed 28-bit value. For shift values 0 through 30,
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all adjusted negative values and their truncation correction fit a signed 32-bit
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integer. Positive coordinates use the same zero correction. Java's masked shift
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count is retained for larger valid level differences. A sign-bit divisor (masked
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shift 31) and invalid output levels delegate to the original method, preserving
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its result or exception class/message. The hook is numeric and has no mutable
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shared state.
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The actual publisher differential suite checks 1,525,313 cases, including
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1,124,986 invalid-level exceptions. Cases include powers-of-two boundaries,
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positive/negative coordinates, integer extrema passed through the original
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encoding, all output bytes across selected input levels, reserved position bits
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and a million randomized position/level combinations. The observed negative
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counterexample is retained explicitly.
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Private probes rejected a numeric cache, which provided no useful benefit and
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was slower with varying inputs. A version with coordinate-sign branches also
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became slower with a broad input set and was discarded. A subsequent version
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using sign masks matched the randomized results and reduced caller CPU time in
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the local synthetic loop across its tested input-set sizes. These are mechanism
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observations, not whole-game FPS, retained RAM, GPU cost or Mac performance.
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The applied Fabric fixture matched 491,520 public lookups across 30 generated
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data owners and six detail levels, with 327,049 populated-column results. Output
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and source-data fingerprints matched, and both modes verified whether the
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redirect was actually installed.
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Timing remains sensitive to warm-up. The first two-million-lookup observation
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was about 120 ms caller CPU upstream versus 225 ms with the hook. Subsequent
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warmed six-million-lookup sets observed 250/261 ms and, after longer warm-up
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and compilation waits, 331/260 ms. The later set favors the hook, but these
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are fixed-order single forks with three sequential observations, not independent
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repetitions or a statistical speed estimate. Compiler-diagnostic observations
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are kept separate. Live captures and independent platform checks are required
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before recommending it.
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Raw [current applied-hook results](../benchmarks/terrain-position-2026-10-04/local-applied-hooks.json)
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and [earlier exploratory observations](../benchmarks/terrain-position-2026-10-04/exploratory-observations.json)
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retain warm-up, diagnostics and source-provenance limits.
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The independent [Linux CI lookup fixture](../benchmarks/terrain-position-2026-10-04/linux-applied-hooks.json)
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matched the same output/data fingerprints. Its three caller CPU observations
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were about 59.7/52.7/52.5 ms upstream and 56.1/47.9/47.7 ms with the hook,
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for two million checks each. These sequential observations favor the hook in
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that workload, but do not establish whole-game or cross-hardware performance.
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Windows, macOS ARM and Linux compilation and publisher differential checks pass.
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The independent Linux production JAR matches the local binary byte-for-byte.
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The [live software-rendered observations](../benchmarks/terrain-position-2026-10-04/live-observations.json)
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record 221 control frames versus 219 candidate frames in roughly 120 seconds,
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with zero excluded frames and matching captured visual settings. Separate copies
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of the same private world used matched stationary cameras and approximately
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five-minute warm-up. No FPS gain is established. The candidate saved, reloaded,
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saved again and quit; the control saved and quit. Both foreground launchers
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returned zero and left no crash report or game process. The actual JSON/CSV pair
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also passed offline viewer checks, including flag differences and mobile layout.
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The hook remains optional and disabled in combined profiles. These checks do
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not measure Apple Silicon graphics, unified memory or performance.
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