Pick a random problem cycles only through overdue attempts while unseen problems remain #210

Description

Observed behavior

When at least two previously attempted problems are due for review, clicking "Pick a random problem" selects only among those problems. With exactly two due problems, the button alternates between them indefinitely, even when unseen problems exist at the selected difficulty. Changing difficulty may still select an overdue problem outside the selected difficulty.

The random number generator is working. The candidate pool is restricted before the random draw.

Expected behavior

An explicit "Pick a random problem" action should allow new problems at the selected difficulty to be chosen. Automatic spaced-review recommendations can still prioritize overdue attempts, but the explicit random button should not silently inherit a queue that permanently excludes unseen problems.

Reproduction

1. Have two distinct Easy problems with saved scored attempts at least one day old. For example, give each problem a single HIRE result from two days ago. NO_HIRE attempts can become due too.
2. Leave additional Easy problems unattempted and select Easy in the lobby.
3. Click "Pick a random problem" repeatedly without completing another interview.
4. The two due problems alternate; unseen Easy problems are never selected.

Version & environment

OS: Window 10
Browser: Microsoft Edge Version 154.0.4258.48 (Official build) (64-bit)
CodeTrial: latest with commit 417b2c2

Cause

• web/app.js:176: the random button updates roll, sets avoidedProblem to the current problem, and calls recommend().
• web/app.js:880: recommend() calls the same pickProblem() used for review recommendations, with the saved reports.
• web/problem-picker.js:168: due is built from the whole problem bank, including difficulties not selected.
• web/problem-picker.js:171: the picker chooses the first nonempty pool from [due, fresh, eligible], after excluding only the current problem.

With two due problems, the due pool always contains the other problem. The fresh pool is never considered. Clicking does not change the attempt timestamps or due status, so repeated clicks cannot escape the queue.

This test explicitly assert that due reviews take priority over unseen problems and may cross difficulty filters. The behavior is therefore an intentional recommendation policy applied to a button labeled as a random choice, rather than a broken RNG or missing history fetch.

Suggestion Improvement

Give explicit random selection its own policy, either through a separate helper or an explicit picker mode. Keep overdue-review priority for the initial automatic recommendation. For the random button, build the pool from the selected difficulties and exclude the current problem whenever another eligible problem exists. Draw uniformly from that pool so every eligible unseen problem has a chance of selection.

If the intended action is specifically to discover new exercises, offer a clearly labeled "Pick an unattempted problem" action instead. That pool should exclude every previously attempted problem, including NO_HIRE and incomplete attempts; the current fresh pool excludes only HIRE results. Fall back to all eligible problems when no unattempted problem remains and explain that fallback in the recommendation text.