# `Lockstep.Strategy.IDPCT`
[🔗](https://github.com/b-erdem/lockstep/blob/v0.1.0/lib/lockstep/strategy/idpct.ex#L1)

Iterative-Deepening PCT.

PCT's bug-finding probability is `1 / (n · k^(d-1))` per iteration,
where `d` is the bug depth. Most bugs are shallow (`d ≤ 3`). Fixing
a single depth wastes compute either way:

  * **Too shallow** (`d=2`): misses bugs that need 3+ priority swaps.
  * **Too deep** (`d=6`): each schedule is more "scrambled," so
    shallow bugs that PCT-3 would hit on iteration 5 might take
    iteration 50 under PCT-6.

IDPCT cycles depth across iterations: iteration 1 uses depth `d_min`,
iteration 2 uses `d_min+1`, ..., wrapping at `d_max`. A run of N
iterations covers each depth in `[d_min, d_max]` roughly N/(d_max-d_min+1)
times, dominating PCT for the case where bug depth is unknown.

## Options

  * `:seed`        — RNG seed (required, used by underlying PCT)
  * `:max_steps`   — total scheduling steps (required)
  * `:d_min`       — minimum bug depth, default 2
  * `:d_max`       — maximum bug depth, default 6
  * `:high_prio_max` — same as PCT, default 1_000_000

All other options are forwarded to `Lockstep.Strategy.PCT` after
stripping `:d_min` / `:d_max` and computing `:bug_depth`.

## When to use

Reach for IDPCT when:

  * You don't know the bug depth a priori (most cases).
  * You're running many iterations (≥ 1000) and want to amortize
    cost across depths.
  * You want the fastest finds for shallow bugs while still
    retaining a chance at deep ones.

Stick with plain PCT (`:pct`) when:

  * You're doing replay or focused exploration and want a single,
    predictable depth.
  * Iteration count is low (< 100) and you want maximum probability
    per iteration at a known-good depth.

## Implementation

IDPCT is a thin wrapper that derives `bug_depth` from `seed` (so
iterations 1, 2, 3, ... cycle deterministically through the depth
range), then delegates `init/1`, `on_register/2`, and `pick/2` to
`Lockstep.Strategy.PCT`. The strategy state is *literally* a PCT
state — no new struct required.

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*Consult [api-reference.md](api-reference.md) for complete listing*