lineprior

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lineprior is a Rust library and CLI for building domain-agnostic action priors from historical action sequences. Given a state, it answers:

What actions have historically worked well from here?

It is not a shogi opening book library, a chess-specific book format, a planner, a solver, or a game engine. It is a small, reusable component that turns a log of past (state, action, outcome) steps into a ranked list of candidate actions per state — useful for games, search, automation, agents, optimization, and any other domain where past successful sequences can guide future decisions.

What it is not

lineprior does not decide the best action by itself. It is a prior, not an oracle:

• It suggests candidate actions with a count, rate, and confidence attached.
• The caller is expected to combine this with search, evaluation, rules, or verification before acting.
• When data is sparse or a state is unseen, it returns no candidates rather than inventing one.

If historical data is biased, the prior will be biased too. lineprior can improve candidate ordering when historical sequences are relevant and representative — it does not guarantee better decisions.

Building a prior book

lineprior build observations.jsonl \
  --out prior.jsonl \
  --min-count 1 \
  --smoothing-alpha 5.0

Useful flags: --max-step (drop observations past a given step), --max-actions-per-state (keep only the top N candidates), --tags (keep only observations carrying at least one of the given tags, comma-separated), --confidence-k (tune how fast confidence grows with sample size), --confidence-mode (heuristic (default), wilson-lower-bound, or hybrid — see "Confidence modes" below), --confidence-z (z-score for the Wilson lower bound, default 1.96, ignored under heuristic), --min-weighted-count / --min-confidence (filter on the weighted count or confidence directly, instead of just the raw --min-count), --draw-value (success credit for a draw outcome — default 0.5, since a draw is a genuine partial outcome in adversarial games, not a loss), --time-decay-half-life-days / --time-decay-reference-unix-seconds / --missing-timestamp-policy (age-based weight decay — see "Time decay and source reliability" below), --source-weights / --default-source-weight (per-source reliability multipliers, same section), --config <path.json> (load the whole BuildConfig from a file instead of individual flags, e.g. one saved by lineprior tune --save-best-config — see "Tuning" below; errors if combined with any flag above), --strict (fail on the first invalid record instead of skipping it with a warning).

--min-confidence's meaning depends on --confidence-mode: under heuristic it's a pure sample-size floor, blind to outcome. Under wilson-lower-bound/hybrid it's success-rate-aware, so a high-count but mostly-failing action that used to pass the filter can now be dropped by it — switching --confidence-mode on an existing --min-confidence threshold is a real behavior change, not just an additive one.

Confidence modes

• heuristic (default): weighted_count / (weighted_count + confidence_k) — a sample-size heuristic, blind to outcome. Not a statistical guarantee, but works even for score-only datasets with no outcome labels at all.
• wilson-lower-bound: the Wilson score interval lower bound on the action's success rate — an actual statistical lower bound, useful once outcome labels are meaningful. Falls back to heuristic for an action with no decisive-outcome observations (nothing to bound).
• hybrid: heuristic * wilson-lower-bound, so both low sample size and a weak success rate pull confidence down. Same fallback as wilson-lower-bound when there's no outcome data.

Weighted/fractional observations (--weight, draw outcomes under --draw-value) feed the Wilson bound through an effective sample size (sum(weight)^2 / sum(weight^2), Kish's formula) rather than the raw weighted count — an engineering approximation, exact for uniform weight 1.0 observations.

Time decay and source reliability

Not every observation deserves equal trust. build/eval can compute an effective_weight per observation — weight * time_decay_multiplier * source_reliability_multiplier — feeding everything downstream (prior, confidence, eval calibration) automatically. Both factors default to a no-op, so this is entirely opt-in.

Stale data, decayed by age:

lineprior build observations.jsonl \
  --out prior.jsonl \
  --time-decay-half-life-days 30 \
  --time-decay-reference-unix-seconds 1783540000

--time-decay-reference-unix-seconds is required whenever --time-decay-half-life-days is set — there's no implicit "now," since that would make identical build/eval invocations produce different priors (and a different build_config_fingerprint) depending on when you happened to run them. An observation's weight decays as 0.5 ^ (age_days / half_life_days); a future-dated observation (observed_at_unix_seconds after the reference) clamps to age 0, silently. --missing-timestamp-policy (keep-base-weight, the default, or drop) decides what happens to an observation with no observed_at_unix_seconds — inert when decay is disabled.

Multiple sources of differing reliability:

lineprior build observations.jsonl \
  --out prior.jsonl \
  --source-weights engine_v012=1.0,engine_v010=0.6,human=0.8 \
  --default-source-weight 1.0

An observation's source field looks itself up in --source-weights; an absent or unrecognized source falls back to --default-source-weight (default 1.0, i.e. trust it same as any other). This is independent of time decay — you can use either, both, or neither.

Caveat: Kish's effective sample size (the same formula the Wilson bound above uses) is invariant to uniformly scaling every one of an action's own weights by the same factor. So when every observation behind an action shares the same age/source, pure wilson-lower-bound confidence does not reflect decay at all — only weighted_count (and therefore prior, and heuristic/hybrid confidence) does. Use hybrid, not bare wilson-lower-bound, if you want the confidence number itself to drop for stale or unreliable data.

You could always precompute weight yourself before feeding it to lineprior — this feature exists so the common case (decay by age, discount by source) is reproducible and folded into the config fingerprint, not as a replacement for custom weighting logic.

build also prints a one-line summary of what its filters actually did, e.g. stats: 950/1000 observations kept, 42/50 candidates kept (5 by min_count, ...) — useful for sanity-checking your own pre-filtering (e.g. a domain-specific ply/depth cutoff) against --min-count/etc. without re-deriving the numbers by hand. As a library, this is BuildOutput.stats (a BuildStats) returned alongside the book by build_prior_book_from_reader.

Querying a prior book

lineprior query prior.jsonl --state state_a --top-k 5

An unseen state prints nothing and still exits 0 — that's the expected fallback behavior, not an error.

As a library, PriorBook::candidates() gives you every (state, action) candidate across the whole book as a flat Vec<(String, PriorAction)>, for callers filtering or sampling candidates directly (e.g. building a domain-specific "opening suite") instead of working through the nested per-state structure entries_sorted() returns.

Other commands

lineprior summary prior.jsonl      # coverage, average confidence, per-state entropy
lineprior validate observations.jsonl   # parse and report issues without building

Input schema

One JSON object per line:

{"sequence_id":"case-001","step":0,"state":"state_a","action":"action_x","outcome":"success","score":0.8,"weight":1.0,"tags":["trusted"],"observed_at_unix_seconds":1783540000,"source":"engine_v012"}

Required: sequence_id, step, state, action. Optional, with defaults: outcome (unknown), score (null), weight (1.0), tags ([]), observed_at_unix_seconds (null, only consulted when time decay is enabled — see "Time decay and source reliability" above), source (null, only consulted via --source-weights).

Output schema

One JSON object per state, actions ranked by descending prior:

{"state":"state_a","actions":[{"action":"action_x","count":3,"weighted_count":3.0,"success_rate":0.667,"mean_score":0.633,"prior":0.557,"confidence":0.130}]}

success_rate and mean_score are the raw, unsmoothed observed rates (for transparency); prior is the smoothed, normalized ranking score; confidence is a heuristic sample-size indicator by default, or a real Wilson-bound statistical lower bound under --confidence-mode wilson-lower-bound/hybrid (see "Confidence modes" above). success_rate credits a success outcome as 1.0, a draw as --draw-value (default 0.5), and a failure as 0.0.

lineprior build's CLI output (and the library's save_prior_book_with_config) prepends a header line carrying a fingerprint of the BuildConfig used to build it, e.g. {"build_config_fingerprint":7592859384087124328}. load_prior_book/lineprior query/lineprior summary all skip this line transparently — it doesn't change how you read a prior book day to day.

Detecting a stale cached prior book

If you cache a prior book on disk and rebuild it later under different BuildConfig values (a different --smoothing-alpha, --confidence-k, etc.), the raw confidence/prior numbers in the old file were computed under the old config's semantics — reusing it silently can be misleading. As a library:

// When saving, embed the config that produced it:
save_prior_book_with_config(&book, &config, writer)?;

// Later, check a cached file against your current config before trusting it:
match load_prior_book_with_config(reader, &config) {
    Ok(book) => { /* config matches (or the file predates this check) */ }
    Err(Error::BuildConfigMismatch { .. }) => { /* stale -- rebuild */ }
    Err(e) => { /* other error */ }
}

A file saved via plain save_prior_book (or by a version of lineprior that predates this) has no fingerprint to compare against, so load_prior_book_with_config accepts it unconditionally — there's nothing to detect drift against. The fingerprint is stable within a given lineprior version, not guaranteed forever-stable across upgrades (it hashes a JSON encoding of BuildConfig, and floats' exact byte layout isn't itself a cross-version guarantee) — it's meant to catch a stale cache within one project's lifetime, not serve as a long-term archival checksum.

Upgrading to a lineprior version that adds new BuildConfig fields (like confidence_mode/confidence_z, or time_decay_half_life_days/source_weights) changes the fingerprint for every config, even when the new fields are at their inert defaults (heuristic mode, decay disabled, no source weights) — so a prior book cached before upgrading will trip BuildConfigMismatch once after upgrading. That's the fingerprint mechanism working as intended, not a regression.

Limitations

• By default (--confidence-mode heuristic), confidence is a sample-size heuristic (weighted_count / (weighted_count + k)), not a statistical confidence interval. This remains the default for backward compatibility and for score-only datasets with no outcome labels. --confidence-mode wilson-lower-bound/hybrid give an actual statistical lower bound on the success rate when outcome data is meaningful (see "Confidence modes" above) — but they're still a lower bound on the observed rate, not a guarantee about future actions if the underlying data is biased or non-stationary.
• A low-sample action does not get reported as certain just because it has a 100% success rate from one observation — smoothing pulls it toward the dataset's overall rate.
• lineprior never invents actions: an unseen state or a state with no candidates above threshold returns an empty result.
• The library does not parse any domain-specific format (SFEN, CSA, USI, FEN, PGN, etc.) — that mapping is the caller's job.

Examples for two domains

The same observations.jsonl shape works whether the "state" is a board position or a UI screen:

Automation:
  state  = "checkout_page"
  action = "click_pay_button"

Optimization:
  state  = "partial_solution_hash_42"
  action = "branch_left"

Domain-specific mappings (e.g. a chess/shogi position as state, a UCI/USI move as action) belong in adapters outside this crate, not in lineprior itself.

For a real domain example: examples/shogi_opening.jsonl uses state = an SFEN string and action = a USI move, the mapping described in AGENTS.md's Sekirei integration notes. Its generated prior (examples/shogi_prior.jsonl) ranks 7g7f above 2g2f despite 2g2f's raw observed rate being higher (100% vs. 83%) — 7g7f has one more supporting observation, and smoothing correctly refuses to let 2g2f's smaller sample outrank it on a single-observation-driven perfect record.

Performance

Measured on an Apple M4 (macOS 26.5.1), release build, 1,000,000 observations across 50,000 unique (state, action) pairs (1,000 states × 50 actions):

wall-clock:        1.71s
peak RSS:          ~15.4 MB

Reproduce with:

awk 'BEGIN{
  for (s=0; s<1000; s++) for (a=0; a<50; a++) for (i=0; i<20; i++)
    printf "{\"sequence_id\":\"seq_%d_%d_%d\",\"step\":0,\"state\":\"state_%05d\",\"action\":\"action_%03d\",\"outcome\":\"%s\",\"score\":%.2f,\"weight\":1.0}\n", \
      s, a, i, s, a, (i % 3 == 0 ? "failure" : "success"), 0.5 + (i % 10) * 0.01
}' > large.jsonl
cargo build --release
time ./target/release/lineprior build large.jsonl --out /dev/null --min-count 1

Memory is now genuinely bounded by unique (state, action) pairs rather than total observation count, matching AGENTS.md's MVP performance goal: the CLI's build command streams straight from the input file into the prior book via build_prior_book_from_reader, folding each observation into a bounded accumulator as it's parsed instead of collecting a Vec<Observation> first. Peak RSS on the measurement above dropped from ~199MB (the old, fully-materializing path) to ~15.4MB — about 13x less, for the same 1,000,000-observation input and identical output.

Smaller, checked-in benchmarks live in crates/lineprior/benches/scoring.rs (run with cargo bench -p lineprior), covering both the eager build_prior_book and the streaming build_prior_book_from_reader at 1k/10k/50k-observation scales. A dedicated regression test (crates/lineprior/tests/streaming_memory.rs, Linux-only, runs in CI) fails if peak memory ever creeps back up toward the old per-observation scaling.

Evaluating a prior

A prior is only useful if it actually ranks the right action highly on data it wasn't built from. lineprior eval holds out part of the observation log, builds a prior from the rest, and reports ranking-quality metrics on the held-out slice:

lineprior eval observations.jsonl \
  --split-by sequence --train-ratio 0.8 --top-k 1,3,5 --out eval.json

The split is by sequence_id, not by individual observation, so every step of the same sequence lands on the same side — otherwise later steps could leak information about earlier ones across the train/test boundary. The split is a deterministic hash of the id, so re-running eval with the same --train-ratio reproduces the same split.

Headline fields in the JSON report:

• top1_hit_rate / topk_hit_rate: how often the actual action taken was the prior's #1 pick (or within its top-k), among test observations where the prior had any candidate at all.
• mean_reciprocal_rank: the same idea averaged over rank (1/rank, 0 if the action wasn't among the candidates), a softer signal than a hard hit/miss cutoff.
• coverage vs. fallback_rate: these intentionally do not sum to 1. coverage is state-weighted (the fraction of distinct test states for which the prior returned any candidate); fallback_rate is observation-weighted (the fraction of test observations whose state had none). One rarely-seen state with no candidates barely moves fallback_rate but still costs a full point of coverage — the report also includes the raw counts each rate is computed from, so either framing can be double-checked directly.

lineprior eval --help lists the full set of build-equivalent tuning flags (--min-count, --smoothing-alpha, --confidence-mode, --time-decay-half-life-days, --source-weights, etc.) — eval builds its train-side prior under the same knobs a real build run would use, so the two stay comparable.

Confidence calibration and threshold sweep

--calibration-bins/--thresholds turn eval into a selective-prediction tool: instead of just "how good is the prior overall," they answer "if I only trust the prior above confidence X, how much of my data can I still act on, and how accurate is it?"

lineprior eval observations.jsonl \
  --confidence-mode wilson-lower-bound \
  --calibration-bins 10 \
  --thresholds 0.3,0.5,0.7,0.9

• confidence_calibration (from --calibration-bins N): N equal-width bins over [0, 1], always exactly N entries regardless of how many observations landed in each. Each bin reports top1_hit_rate/mean_reciprocal_rank among evaluated test observations whose #1 candidate's confidence fell in that bin — a well-calibrated confidence mode should show hit rate tracking bin confidence roughly 1:1.
• threshold_sweep (from --thresholds): one entry per requested threshold, always in the requested order. covered_fraction is the fraction of all test observations where the state had a candidate and its #1 confidence was >= min_confidence; abstained_fraction = 1.0 - covered_fraction. These are a different weighting convention than the top-level coverage/fallback_rate above — both are observation-weighted here and sum to 1 by construction, whereas the top-level pair deliberately doesn't. top1_hit_rate/ mean_reciprocal_rank in each entry are computed among covered observations only (accuracy given a prediction was actually made), the same "conditioned on evaluated" convention the headline metrics already use.

Both are omitted (empty arrays) unless explicitly requested, so existing eval usage is unaffected.

Tuning: choosing a BuildConfig automatically

eval scores one config at a time; tune grid-searches many and picks the best one, using the same deterministic train/test split for every candidate so they're directly comparable:

lineprior tune observations.jsonl \
  --split-by sequence --train-ratio 0.8 \
  --param confidence-mode=heuristic,wilson-lower-bound,hybrid \
  --param min-confidence=0.0,0.3,0.5,0.7 \
  --param smoothing-alpha=1.0,5.0,10.0 \
  --param time-decay-half-life-days=none,30,90 \
  --time-decay-reference-unix-seconds 1783540000 \
  --objective covered-mrr --min-covered-fraction 0.4 \
  --out tune.json --save-best-config best_config.json

Each --param key=v1,v2,... sweeps one BuildConfig field (repeat --param for more than one); any field never named in a --param stays at its BuildConfig::default() for every candidate. Supported keys: confidence-mode, min-confidence, smoothing-alpha, confidence-k, confidence-z, min-count, min-weighted-count, draw-value, time-decay-half-life-days (accepts none), default-source-weight. --time-decay-reference-unix-seconds is a single value applied to every candidate (never swept) — required whenever a swept time-decay-half-life-days value isn't none, same reproducibility rule build/eval already use.

--objective (default covered-mrr) is what candidates are ranked by:

objective	meaning
mrr	mean_reciprocal_rank, among covered test observations only
top1	top1_hit_rate, among covered test observations only
covered-mrr (default)	covered_fraction * mean_reciprocal_rank — MRR averaged across all test observations, an uncovered one contributing 0
top1-at-min-coverage	same as top1, but requires --min-covered-fraction also be set

covered-mrr is the default because optimizing mrr alone tends to pick configs that abstain (report no candidate) except when very confident, while optimizing coverage alone tolerates a sloppy prior — covered-mrr penalizes both.

--min-covered-fraction / --max-fallback-rate / --min-top1-hit-rate reject a candidate from being best, but it still shows up in the JSON report's all_results (with meets_constraints: false) so you can see what got excluded and why, rather than it silently vanishing.

The JSON report's pareto_front is the non-dominated set over (mrr, covered_fraction) — every config on it is the best some MRR/coverage tradeoff, independent of --objective, in case you'd rather eyeball the tradeoff yourself than trust the single best pick.

--save-best-config best_config.json writes the winning candidate's BuildConfig as JSON; build and eval both accept it back via --config best_config.json (errors if combined with any individual build-config flag like --min-count, since it's a whole-config replacement, not an overlay) — so a config chosen once by tune is reused exactly, not re-typed by hand:

lineprior build observations.jsonl --out prior.jsonl --config best_config.json

tune is exactly as domain-agnostic as the rest of lineprior (it only ever sees state/ action/sequence_id/outcome data) and doesn't change what lineprior fundamentally is — a prior, not an oracle. It automates what you'd otherwise do by hand-sweeping eval; it doesn't make the resulting prior any less something the caller should verify before acting on.

Academic positioning

lineprior is an engineering-oriented Rust implementation inspired by existing ideas in case-based planning, plan reuse, sequence prediction, variable-order Markov models, and policy-guided search. It is not a new theoretical algorithm.

Development

cargo fmt --all -- --check
cargo clippy --all-targets --all-features -- -D warnings
cargo test --all-features