# 13. Replace MDL scoring with Language Size measure

**Date:** 2026-07-11

**Status:** Accepted

## Context

Our ensemble grammar inference uses a scoring function to select the best grammar
when multiple algorithms (CRX, iDRegEx, kORE) produce candidates. The scoring
function determines which grammar wins — so a bad scoring function means the
ensemble picks bad grammars, even when the algorithms produce good ones.

### The concrete problem

On real codebases, the ensemble would pick overly generic grammars like `info+`
over specific ones like `info.file.template.shell.service+`. The generic grammar
accepts an astronomically large language (every permutation of `info` at every
length), while the specific grammar accepts exactly one word of each length.
Any human would pick the specific one. Our scorer picked the generic one.

### Why MDL failed

We used Minimum Description Length (MDL): `score = model_cost + data_cost`,
where `model_cost = len(expr)` and `data_cost = Σ log₂(|L(r)| at seq length)`.

The problem: `model_cost('info+') = 1` (one symbol occurrence), while
`model_cost('info.file.template.shell.service+') = 5`. MDL rewards short
expressions. When `model_cost` is small relative to `data_cost`, short generic
patterns win even though they accept far more spurious words.

### The paper that fixed it

Bex, Goethals, Penninckx, Van Gucht, and Van den Bussche published
*"Learning Deterministic Regular Expressions for the Inference of Schemas
from XML Data"* (arXiv:1004.2372, also VLDB 2007). They proposed the iDRegEx
algorithm and evaluated two scoring measures:

1. **Language Size** (Section 4.3.1): select the expression that accepts the
   **fewest words** up to length `n = 2m + 1`. This directly measures specificity.
2. **MDL** (Section 4.3.2): model cost + data cost, based on Adriaans & Vitányi
   (2006). This rewards short expressions.

Their results (Section 5, line 1470): on a corpus of synthetic regular expressions
with alphabet size 5, **Language Size achieved 98% success rate while MDL achieved
only 21%**. They explicitly abandoned MDL: *"Therefore in the remainder of this
section we only consider iDRegEx with the language size criterion."*

## Decision

Replace MDL with Language Size as the default scoring function. Keep MDL as a
fallback enabled via `scoring_method='mdl'` parameter.

### Implementation

```python
def lang_size_score(expr, sequences):
    """Language Size: Σ |L(r)|_len(seq) — words at each sequence length.

    From Bex et al. (arXiv:1004.2372), Section 4.3.1.
    Lower is better — the grammar that accepts the fewest words wins.
    """
    if not sequences:
        return lang_size(expr, 2 * model_cost(expr) + 1)
    total = 0
    for seq in sequences:
        total += _count_words_fast(expr, len(seq))
    return total
```

Counts words at exactly the lengths present in the input sequences, not all
lengths 0..n. This is a practical adaptation of the paper's measure: the paper
evaluates all candidates at the same fixed n, but our candidates have different
n values. Counting at observed sequence lengths gives the same result — the
grammar accepting the fewest words at the relevant lengths wins.

The existing `_count_words_fast` function already computes the exact word count
needed by this measure. No new algorithms required — just a different aggregation.

## Consequences

**Positive:**
- Ensemble picks specific grammars over generic ones (98% vs 21% on Bex's corpus)
- `_count_words_fast` already exists and is LRU-cached — zero new code for the hard part
- `model_cost` still used for tie-breaking (shortest expression when language sizes equal)

**Negative:**
- `lang_size` can be expensive for expressions with large alphabets (exponential
  in worst case), but `n = 2*model_cost + 1` keeps it bounded in practice
- Old MDL results stored in YAML will have different scores than new runs —
  not a compatibility issue since scores are internal selection criteria, not persisted

**Migration:**
- `mdl_score` preserved as fallback: `scoring_method='mdl'`
- `mdl_score_simple` in ensemble.py updated to use `lang_size_score` by default
- CLI flag `--scoring-method` controls which is used (default: `langsize`)