ef4e975a47
Answers the backlog's open "first step: diff the two readers across the disc and count disagreements", statically over every IDXD object. Of 7750 objects and 738922 named fields whose true value is numeric, legacy get_f32 is correct 39.42%, returns None (harmless) 43.04%, and returns a WRONG NUMBER 17.54% (129612 fields). The wrongness has an exact predicate: single-record objects 0 of 29822 wrong (0.00%); multi-record objects 129612 of 709100 (18.28%). The mechanism is in get_raw itself -- it flattens the pool to a token list, finds the FIRST occurrence of the key, and returns the preceding token, with no notion of records. So every record after the first inherits record 0's value: Weight truth=1.0 legacy=0.3, Points truth=10000 legacy=4000. Practical rule recorded: a get_f32 number from a single-record object is safe; from a multi-record object only the first record is. Withdrawn in the same document: my first sweep compared against "the string before THIS field's own key" and reported 65.90% -- that is not what get_raw does, so the figure is not the legacy reader's error rate.
64 lines
2.8 KiB
Markdown
64 lines
2.8 KiB
Markdown
# ✅ How wrong the legacy IDXD reader is — measured disc-wide
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[`BACKLOG.md`](BACKLOG.md) left this open: *"the legacy value-before-key
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string-pool reader is an approximation … every number in this corpus that came
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out of `get_f32`/`get_raw` is re-checkable against ground truth but not yet
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re-checked. **First step: diff the two readers across the disc and count
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disagreements.**"* Done, statically, over every IDXD object on the disc.
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## The answer
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**7 750 IDXD objects, 738 922 named fields whose true value is numeric:**
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| legacy `get_f32` outcome | fields | share |
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|---|---|---|
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| correct | 291 307 | 39.42 % |
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| returns `None` — no number produced, so harmless | 318 003 | 43.04 % |
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| **returns a WRONG NUMBER** | **129 612** | **17.54 %** |
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## ✅ And the wrongness has an exact predicate
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| object shape | numeric fields | wrong numbers |
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|---|---|---|
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| **single-record** | 29 822 | **0 — 0.00 %** |
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| **multi-record** | 709 100 | 129 612 — 18.28 % |
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**The legacy reader is exactly correct on single-record objects and unsafe on
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multi-record ones.** Not approximately — *zero* disagreements in 29 822 fields.
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The mechanism is in `get_raw`'s own implementation: it flattens the string pool
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to a token list, finds the **first** occurrence of the key, and returns the token
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before it. It has no notion of records. So in an object with N records that each
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carry a `Weight`, every record's `Weight` resolves to **record 0's** value:
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```
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Weight truth=1.0 legacy=0.3
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Points truth=10000 legacy=4000
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Weight truth=3.3 legacy=0.3
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Points truth=15000 legacy=4000
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```
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`0.3` and `4000` are record 0's values, returned for every record after it.
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## What this means for the corpus
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* A number read with `get_f32` from a **single-record** object is safe — the diff
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found no counterexample in 29 822 fields.
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* A number read from a **multi-record** object is safe only for the **first**
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record; every later record has a ~18 % chance of being record 0's value.
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* The `None` bucket (43 %) is the guard working as designed: `get_f32` rejects a
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non-numeric neighbour rather than inventing a value, so a *defaulted* field
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reads as absent instead of wrong.
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## ⚠️ A first pass that measured the wrong thing
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My first sweep compared the truth against *"the string immediately preceding this
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field's own key in the pool"* and reported **65.90 %** disagreement. That number
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is not the legacy reader's error rate, because it is not what `get_raw` does —
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`get_raw` uses the **first occurrence of the key anywhere in the object**, not
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this field's own key. The 65.90 % figure is withdrawn; 17.54 % is the measured
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one, and the `is_definite_value` guard is why it is so much lower.
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The lesson is the same one this corpus keeps paying for: before measuring how
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wrong a tool is, read what the tool actually does.
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